iptables-extensions(8)

📦

version 1.8.13 · 9 versions

💖 NAME 💖

okay so welcome to iptables-extensions(8) from iptables 1.8.13 💅. This is literally the master list of extensions in the standard iptables distribution. No cap, if you're trying to remember which match module does what, this is your girl.

✨ SYNOPSIS ✨

Two invocation shapes, bestie:

ip6tables [-m name [module-options...]]  [-j target-name [target-
options...]

iptables [-m name [module-options...]]  [-j target-name [target-
options...]

🌸 MATCH EXTENSIONS 🌸

iptables can use extended packet matching modules with the -m or --match options, followed by the matching module name. After these, various extra command line options become available, depending on the specific module. You can specify multiple extended match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific to that module. The extended match modules are evaluated in the order they are specified in the rule.

If the -p or --protocol was specified and if and only if an unknown option is encountered, iptables will try load a match module of the same name as the protocol, to try making the option available. (iykyk)

🦋 addrtype

This module matches packets based on their address type. Address types are used within the kernel networking stack and categorize addresses into various groups. The exact definition of that group depends on the specific layer three protocol.

The following address types are possible:

UNSPEC — an unspecified address (i.e. 0.0.0.0)
UNICAST — an unicast address
LOCAL — a local address
BROADCAST — a broadcast address
ANYCAST — an anycast packet
MULTICAST — a multicast address
BLACKHOLE — a blackhole address
UNREACHABLE — an unreachable address
PROHIBIT — a prohibited address
THROW — FIXME
NAT — FIXME
XRESOLVE
[!] --src-type type — Matches if the source address is of given type
[!] --dst-type type — Matches if the destination address is of given type
--limit-iface-in — The address type checking can be limited to the interface the packet is coming in. This option is only valid in the PREROUTING, INPUT and FORWARD chains. It cannot be specified with the --limit-iface-out option.
--limit-iface-out — The address type checking can be limited to the interface the packet is going out. This option is only valid in the POSTROUTING, OUTPUT and FORWARD chains. It cannot be specified with the --limit-iface-in option.

🐚 ah (IPv6-specific)

This module matches the parameters in Authentication header of IPsec packets.

[!] --ahspi spi[:spi] — Matches SPI.
[!] --ahlen length — Total length of this header in octets.
--ahres — Matches if the reserved field is filled with zero.

🐚 ah (IPv4-specific)

This module matches the SPIs in Authentication header of IPsec packets.

[!] --ahspi spi[:spi]

🧚 bpf

Match using Linux Socket Filter. Expects a path to an eBPF object or a cBPF program in decimal format.

--object-pinned path — Pass a path to a pinned eBPF object.

Applications load eBPF programs into the kernel with the bpf() system call and BPF_PROG_LOAD command and can pin them in a virtual filesystem with BPF_OBJ_PIN. To use a pinned object in iptables, mount the bpf filesystem using

mount -t bpf bpf ${BPF_MOUNT}

then insert the filter in iptables by path:

iptables -A OUTPUT -m bpf --object-pinned
${BPF_MOUNT}/{PINNED_PATH} -j ACCEPT

--bytecode code — Pass the BPF byte code format as generated by the nfbpf_compile utility.

The code format is similar to the output of the tcpdump -ddd command: one line that stores the number of instructions, followed by one line for each instruction. Instruction lines follow the pattern 'u16 u8 u8 u32' in decimal notation. Fields encode the operation, jump offset if true, jump offset if false and generic multiuse field 'K'. Comments are not supported.

For example, to read only packets matching 'ip proto 6', insert the following, without the comments or trailing whitespace:

4               # number of instructions
48 0 0 9        # load byte  ip->proto
21 0 1 6        # jump equal IPPROTO_TCP
6 0 0 1         # return     pass (non-zero)
6 0 0 0         # return     fail (zero)

You can pass this filter to the bpf match with the following command:

iptables -A OUTPUT -m bpf --bytecode '4,48 0 0 9,21 0 1 6,6 0 0 1,6 0 0 0' -j ACCEPT

Or instead, you can invoke the nfbpf_compile utility.

iptables -A OUTPUT -m bpf --bytecode "`nfbpf_compile RAW 'ip proto 6'`" -j ACCEPT

Or use tcpdump -ddd. In that case, generate BPF targeting a device with the same data link type as the xtables match. Iptables passes packets from the network layer up, without mac layer. Select a device with data link type RAW, such as a tun device:

ip tuntap add tun0 mode tun
ip link set tun0 up
tcpdump -ddd -i tun0 ip proto 6

See tcpdump -L -i $dev for a list of known data link types for a given device.

You may want to learn more about BPF from FreeBSD's bpf(4) manpage.

🎀 cgroup

[!] --path path — Match cgroup2 membership.

Each socket is associated with the v2 cgroup of the creating process. This matches packets coming from or going to all sockets in the sub-hierarchy of the specified path. The path should be relative to the root of the cgroup2 hierarchy.

[!] --cgroup classid — Match cgroup net_cls classid.

classid is the marker set through the cgroup net_cls controller. This option and --path can't be used together.

Example:

iptables -A OUTPUT -p tcp --sport 80 -m cgroup ! --path service/http-server -j DROP

iptables -A OUTPUT -p tcp --sport 80 -m cgroup ! --cgroup 1 -j DROP

IMPORTANT: when being used in the INPUT chain, the cgroup matcher is currently only of limited functionality, meaning it will only match on packets that are processed for local sockets through early socket demuxing. Therefore, general usage on the INPUT chain is not advised unless the implications are well understood.

Available since Linux 3.14.

🔥 cluster

Allows you to deploy gateway and back-end load-sharing clusters without the need of load-balancers.

This match requires that all the nodes see the same packets. Thus, the cluster match decides if this node has to handle a packet given the following options:

--cluster-total-nodes num — Set number of total nodes in cluster.
[!] --cluster-local-node num — Set the local node number ID.
[!] --cluster-local-nodemask mask — Set the local node number ID mask. You can use this option instead of --cluster-local-node.
--cluster-hash-seed value — Set seed value of the Jenkins hash.

Example:

iptables -A PREROUTING -t mangle -i eth1 -m cluster --cluster-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff

iptables -A PREROUTING -t mangle -i eth2 -m cluster --cluster-total-nodes 2 --cluster-local-node 1 --cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff

iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff -j DROP

iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff -j DROP

And the following commands to make all nodes see the same packets:

ip maddr add 01:00:5e:00:01:01 dev eth1

ip maddr add 01:00:5e:00:01:02 dev eth2

arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:01

arptables -A INPUT -i eth1 --h-length 6 --destination-mac 01:00:5e:00:01:01 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27

arptables -A OUTPUT -o eth2 --h-length 6 -j mangle --mangle-mac-s 01:00:5e:00:01:02

arptables -A INPUT -i eth2 --h-length 6 --destination-mac 01:00:5e:00:01:02 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27

NOTE: the arptables commands above use mainstream syntax. If you are using arptables-jf included in some RedHat, CentOS and Fedora versions, you will hit syntax errors. Therefore, you'll have to adapt these to the arptables-jf syntax to get them working.

In the case of TCP connections, pickup facility has to be disabled to avoid marking TCP ACK packets coming in the reply direction as valid.

echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose

💬 comment

Allows you to add comments (up to 256 characters) to any rule.

--comment comment

Example:

iptables -A INPUT -i eth1 -m comment --comment "my local LAN"

📊 connbytes

Match by how many bytes or packets a connection (or one of the two flows constituting the connection) has transferred so far, or by average bytes per packet.

The counters are 64-bit and are thus not expected to overflow ;)

The primary use is to detect long-lived downloads and mark them to be scheduled using a lower priority band in traffic control.

The transferred bytes per connection can also be viewed through conntrack -L and accessed via ctnetlink.

NOTE that for connections which have no accounting information, the match will always return false. The "net.netfilter.nf_conntrack_acct" sysctl flag controls whether new connections will be byte/packet counted. Existing connection flows will not be gaining/losing a/the accounting structure when the sysctl flag is flipped.

[!] --connbytes from[:to] — match packets from a connection whose packets/bytes/average packet size is more than FROM and less than TO bytes/packets. if TO is omitted only FROM check is done. "!" is used to match packets not falling in the range.
--connbytes-dir {original|reply|both} — which packets to consider
--connbytes-mode {packets|bytes|avgpkt} — whether to check the amount of packets, number of bytes transferred or the average size (in bytes) of all packets received so far. Note that when "both" is used together with "avgpkt", and data is going (mainly) only in one direction (for example HTTP), the average packet size will be about half of the actual data packets.

Example:

iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both --connbytes-mode bytes ...

🔗 connlimit

Allows you to restrict the number of parallel connections to a server per client IP address (or client address block).

--connlimit-upto n — Match if the number of existing connections is below or equal n.
--connlimit-above n — Match if the number of existing connections is above n.
--connlimit-mask prefix_length — this is how you group the hosts: by prefix length. For IPv4 it's gotta be a number between (including) 0 and 32; for IPv6, between 0 and 128. Leave it out and it just uses the max prefix length for whatever protocol you're on 🎀
--connlimit-saddr — apply the limit to the source group. This is the default when --connlimit-daddr isn't specified, so it's giving main character energy by just being the vibe you get for free ✨
--connlimit-daddr — apply the limit to the destination group instead 💅

Examples: (okay these are lowkey iconic, pay attention 👀)

allow 2 telnet connections per client host:

iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT

you can also match the other way around:

iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-upto 2 -j ACCEPT

limit the number of parallel HTTP requests to 16 per class C sized source network (24 bit netmask):

iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16 --connlimit-mask 24 -j REJECT

limit the number of parallel HTTP requests to 16 for the link local network (IPv6):

ip6tables -p tcp --syn --dport 80 -s fe80::/64 -m connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT

Limit the number of connections to a particular host:

ip6tables -p tcp --syn --dport 49152:65535 -d 2001:db8::1 -m connlimit --connlimit-above 100 -j REJECT

🦋 connmark

This module matches the netfilter mark field that's attached to a connection (and yes, you can set that bad boy with the CONNMARK target below 💖).

[!] --mark value[/mask] — matches packets in connections rockin' the given mark value. If you throw a mask in there, it gets logically ANDed with the mark before the comparison happens. Math but make it firewall 🧮✨

🌸 conntrack

Okay so when this module teams up with connection tracking, it hands you the keys to the connection tracking state for this packet/connection. Bestie duo fr fr 🫶

[!] --ctstate statelist — statelist is a comma separated list of the connection states you wanna match. All the possible states are listed below, keep scrolling 👇
[!] --ctproto l4proto — the Layer-4 protocol to match (by number or by name)
[!] --ctorigsrc address[/mask]
[!] --ctorigdst address[/mask]
[!] --ctreplsrc address[/mask]
[!] --ctrepldst address[/mask] — match against the original/reply source/destination address 🎯
[!] --ctorigsrcport port[:port]
[!] --ctorigdstport port[:port]
[!] --ctreplsrcport port[:port]
[!] --ctrepldstport port[:port] — match against the original/reply source/destination port (TCP/UDP/etc.) or the GRE key. Heads up tho: matching against port ranges is only supported in kernel versions above 2.6.38 ⚠️
[!] --ctstatus statelist — statuslist is a comma separated list of the connection statuses to match. Possible statuses are listed below 💜
[!] --ctexpire time[:time] — match the remaining lifetime in seconds against the given value or a range of values (inclusive)
--ctdir {ORIGINAL|REPLY} — matches packets flowing in the specified direction. If you don't specify this flag at all, it matches packets in both directions, no cap 🌈

States for --ctstate:

INVALID — the packet is associated with no known connection 🙈
NEW — the packet has started a new connection, or is otherwise associated with a connection that hasn't seen packets in both directions yet
ESTABLISHED — the packet is associated with a connection that has seen packets in both directions. grown-up connection energy 👑
RELATED — the packet is starting a new connection but is associated with an existing connection, like an FTP data transfer or an ICMP error
UNTRACKED — the packet isn't tracked at all, which happens if you explicitly untrack it by using -j CT --notrack in the raw table 💀
SNAT — a virtual state, matching if the original source address differs from the reply destination
DNAT — a virtual state, matching if the original destination differs from the reply source

Statuses for --ctstatus:

NONE — none of the below. literally nothing, a clean slate 🎀
EXPECTED — this is an expected connection (i.e. a conntrack helper set it up)
SEEN_REPLY — conntrack has seen packets in both directions
ASSURED — conntrack entry should never be early-expired
CONFIRMED — connection is confirmed: the originating packet has left the box 🚪

⭐ cpu

[!] --cpu number — match the cpu handling this packet. cpus are numbered from 0 to NR_CPUS-1. You can use this in combination with RPS (Remote Packet Steering) or multiqueue NICs to spread network traffic across different queues 🧚

Example:

iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j REDIRECT --to-ports 8080

iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j REDIRECT --to-ports 8081

Available since Linux 2.6.36 ✨

🍓 dccp

[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --dccp-types mask — matches when the DCCP packet type is one of 'mask'. 'mask' is a comma-separated list of packet types. The packet types are: REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ CLOSE RESET SYNC SYNCACK INVALID. 💅
[!] --dccp-option number — matches if the DCCP option is set

🎀 devgroup

Match the device group of a packet's incoming/outgoing interface.

[!] --src-group name — match the device group of the incoming device
[!] --dst-group name — match the device group of the outgoing device

💖 dscp

This module matches the 6 bit DSCP field sitting inside the TOS field in the IP header. DSCP has superseded TOS within the IETF, so it's the newer icon 👑

[!] --dscp value — match against a numeric (decimal or hex) value in the range 0–63
[!] --dscp-class class — match the DiffServ class. This value can be any of the BE, EF, AFxx or CSx classes. It then gets converted into its according numeric value 🔥

🦋 dst (IPv6-specific)

This module matches the parameters in the Destination Options header.

[!] --dst-len length — total length of this header in octets
--dst-opts type[:length][,type[:length]...] — the numeric type of the option and the length of the option data in octets

🌈 ecn

This lets you match the ECN bits of the IPv4/IPv6 header and the TCP header. ECN is the Explicit Congestion Notification mechanism as specified in RFC3168 📜

[!] --ecn-tcp-cwr — matches if the TCP ECN CWR (Congestion Window Received) bit is set
[!] --ecn-tcp-ece — matches if the TCP ECN ECE (ECN Echo) bit is set
[!] --ecn-ip-ect num — matches a particular IPv4/IPv6 ECT (ECN-Capable Transport). You have to specify a number between 0 and 3 🎯

💜 esp

This module matches the SPIs in the ESP header of IPsec packets.

[!] --espspi spi[:spi]

🧚 eui64 (IPv6-specific)

This module matches the EUI-64 part of a stateless autoconfigured IPv6 address. It compares the EUI-64 derived from the source MAC address in the Ethernet frame against the lower 64 bits of the IPv6 source address. But the "Universal/Local" bit is not compared, just so you know 👀. Also this module doesn't match other link layer frames, and it's only valid in the PREROUTING, INPUT and FORWARD chains — so don't come for it anywhere else, it will not show up 💀

🔥 frag (IPv6-specific)

This module matches the parameters in the Fragment header.

[!] --fragid id[:id] — matches the given Identification or a range of it
[!] --fraglen length — ⚠️ this option cannot be used with kernel version 2.6.10 or later. The length of the Fragment header is static and this option doesn't make sense anymore. rip to a fallen icon 😭
--fragres — matches if the reserved fields are filled with zero
--fragfirst — matches on the first fragment
--fragmore — matches if there are more fragments
--fraglast — matches if this is the last fragment

🌸 hashlimit

hashlimit uses hash buckets to express a rate limiting match (like the limit match) for a group of connections using a single iptables rule. Grouping can be done per-hostgroup (source and/or destination address) and/or per-port. It gives you the ability to express "N packets per time quantum per group" or "N bytes per seconds" (see below for some examples). It's giving scalability, it's giving organized 🫶

A hash limit option (--hashlimit-upto, --hashlimit-above) and --hashlimit-name are required. No exceptions bestie 💅

--hashlimit-upto amount[/second|/minute|/hour|/day] — match if the rate is below or equal to amount/quantum. It's specified either as a number, with an optional time quantum suffix (the default is 3/hour), or as amountb/second (number of bytes per second)
--hashlimit-above amount[/second|/minute|/hour|/day] — match if the rate is above amount/quantum
--hashlimit-burst amount — maximum initial number of packets to match: this number gets recharged by one every time the limit specified above isn't reached, up to this number; the default is 5. When byte-based rate matching is requested, this option specifies the amount of bytes that can exceed the given rate. ⚠️ This option should be used with caution — if the entry expires, the burst value is reset too. careful with her 🙈
--hashlimit-mode {srcip|srcport|dstip|dstport},... — a comma-separated list of objects to take into consideration. If no --hashlimit-mode option is given, hashlimit acts like limit, but at the expensive of doing the hash housekeeping 😭
--hashlimit-srcmask prefix — when --hashlimit-mode srcip is used, all source addresses encountered get grouped according to the given prefix length and the so-created subnet is subject to hashlimit. prefix must be between (inclusive) 0 and 32. Note that --hashlimit-srcmask 0 is basically doing the same thing as not specifying srcip for --hashlimit-mode, but is technically more expensive 💸
--hashlimit-dstmask prefix — like --hashlimit-srcmask, but for destination addresses
--hashlimit-name foo — the name for the /proc/net/ipt_hashlimit/foo entry
--hashlimit-htable-size buckets — the number of buckets of the hash table
--hashlimit-htable-max entries — maximum entries in the hash
--hashlimit-htable-expire msec — after how many milliseconds do hash entries expire
--hashlimit-htable-gcinterval msec — how many milliseconds between garbage collection intervals
--hashlimit-rate-match — classify the flow instead of rate-limiting it. This acts like a true/false match on whether the rate is above/below a certain number ✅
--hashlimit-rate-interval sec — can be used with --hashlimit-rate-match to specify the interval at which the rate should be sampled

Examples:

matching on source host 🌸

"1000 packets per second for every host in 192.168.0.0/16" — like, every single host gets its own lil bucket, bestie:

-s 192.168.0.0/16 --hashlimit-mode srcip --hashlimit-upto 1000/sec

matching on source port 🎀

"100 packets per second for every service of 192.168.1.1" — same energy but per-port:

-s 192.168.1.1 --hashlimit-mode srcport --hashlimit-upto 100/sec

matching on subnet 🦋

"10000 packets per minute for every /28 subnet (groups of 8 addresses) in 10.0.0.0/8" — yes the mask trick is doing the absolute most here and we love her:

-s 10.0.0.0/8 --hashlimit-mask 28 --hashlimit-upto 10000/min

matching bytes per second 💜

"flows exceeding 512kbyte/s" — bytes not packets, okurr:

--hashlimit-mode srcip,dstip,srcport,dstport --hashlimit-above 512kb/s

matching bytes per second (yes, again, different flavor) ⭐

"hosts that exceed 512kbyte/s, but permit up to 1Megabytes without matching" — the burst is the vibe check:

--hashlimit-mode dstip --hashlimit-above 512kb/s --hashlimit-burst 1mb

💫 hbh (IPv6-specific)

This module matches the parameters in the Hop-by-Hop Options header. Very IPv6 of her.

[!] --hbh-len length — total length of this header in octets.
--hbh-opts type[:length][,type[:length]...] — the numeric type of the option, plus the length of the option data in octets. 🫶

🧚 helper

This module matches packets related to a specific conntrack helper. Like, it knows who's helping.

[!] --helper string — matches packets related to the specified conntrack helper.

So string can literally be "ftp" for packets related to an FTP session on the default port. For other ports, you append --portnr to the value, ie. "ftp-2121".

Same rules apply for other conntrack helpers. No cap. 💅

🌈 hl (IPv6-specific)

This module matches the Hop Limit field in the IPv6 header.

[!] --hl-eq value — matches if Hop Limit equals value.
--hl-lt value — matches if Hop Limit is less than value.
--hl-gt value — matches if Hop Limit is greater than value.

🍓 icmp (IPv4-specific)

This extension can be used if --protocol icmp is specified. It provides the following option:

[!] --icmp-type {type[/code]|typename} — this lets you specify the ICMP type, which can be a numeric ICMP type, a type/code pair, or one of the ICMP type names shown by the command:

iptables -p icmp -h

🔥 icmp6 (IPv6-specific)

This extension can be used if --protocol ipv6-icmp or --protocol icmpv6 is specified. It provides the following option:

[!] --icmpv6-type type[/code]|typename — this lets you specify the ICMPv6 type, which can be a numeric ICMPv6 type, type and code, or one of the ICMPv6 type names shown by the command:

ip6tables -p ipv6-icmp -h

👑 iprange

This matches on a given arbitrary range of IP addresses. Pick your faves.

[!] --src-range from[-to] — match source IP in the specified range.
[!] --dst-range from[-to] — match destination IP in the specified range.

🌟 ipv6header (IPv6-specific)

This module matches IPv6 extension headers and/or upper layer header. So many headers, so little time.

--soft — matches if the packet includes any of the headers specified with --header. Chill mode.
[!] --header header[,header...] — matches the packet which EXACTLY includes all specified headers. Like, all of them, no substitutions. The headers encapsulated with ESP header are out of scope, so don't come for her about those.

Possible header types can be:

hop|hop-by-hop — Hop-by-Hop Options header
dst — Destination Options header
route — Routing header
frag — Fragment header
auth — Authentication header
esp — Encapsulating Security Payload header
none — No Next header, which matches 59 in the 'Next Header field' of IPv6 header or any IPv6 extension headers
prot — which matches any upper layer protocol header. A protocol name from /etc/protocols and a numeric value are also allowed. The number 255 is equivalent to prot. ✨

💀 ipvs

Match IPVS connection properties. Niche? Maybe. Iconic? Absolutely.

[!] --ipvs — packet belongs to an IPVS connection.

Any of the following options implies --ipvs (even negated). Yes, even the negated ones, she said what she said:

[!] --vproto protocol — VIP protocol to match; by number or name, e.g. "tcp"
[!] --vaddr address[/mask] — VIP address to match
[!] --vport port — VIP port to match; by number or name, e.g. "http"
--vdir {ORIGINAL|REPLY} — flow direction of packet
[!] --vmethod {GATE|IPIP|MASQ} — IPVS forwarding method used
[!] --vportctl port — VIP port of the controlling connection to match, e.g. 21 for FTP 🙈

🎀 length

This module matches the length of the layer-3 payload (e.g. layer-4 packet) of a packet against a specific value or range of values.

[!] --length length[:length]

🦋 limit

This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is reached. It can be used in combination with the LOG target to give limited logging, for example. Very demure, very mindful logging. 💅

Heads up bestie: xt_limit has no negation support — you will have to use -m hashlimit ! --hashlimit rate in this case whilst omitting --hashlimit-mode.

--limit rate[/second|/minute|/hour|/day] — maximum average matching rate: specified as a number, with an optional /second, /minute, /hour, or /day suffix; the default is 3/hour.
--limit-burst number — maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not reached, up to this number; the default is 5.

🌸 mac

[!] --mac-source address — match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets coming from an Ethernet device and entering the PREROUTING, FORWARD or INPUT chains. So like, don't try it elsewhere, it'll just be awkward. 😭

🧚 mark

This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).

[!] --mark value[/mask] — matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mask before the comparison).

⭐ mh (IPv6-specific)

This extension is loaded if --protocol ipv6-mh or --protocol mh is specified. It provides the following option:

[!] --mh-type type[:type] — this allows specification of the Mobility Header(MH) type, which can be a numeric MH type, type or one of the MH type names shown by the command:

ip6tables -p mh -h

💜 multiport

This module matches a set of source or destination ports. Up to 15 ports can be specified. A port range (port:port) counts as two ports. It can only be used in conjunction with one of the following protocols: tcp, udp, udplite, dccp and sctp. Rules are rules, bestie.

[!] --source-ports,--sports port[,port|,port:port]... — match if the source port is one of the given ports. The flag --sports is a convenient alias for this option. Multiple ports or port ranges are separated using a comma, and a port range is specified using a colon. 53,1024:65535 would therefore match ports 53 and all from 1024 through 65535. Slay.
[!] --destination-ports,--dports port[,port|,port:port]... — match if the destination port is one of the given ports. The flag --dports is a convenient alias for this option.
[!] --ports port[,port|,port:port]... — match if either the source or destination ports are equal to one of the given ports.

🌈 nfacct

The nfacct match provides the extended accounting infrastructure for iptables. You have to use this match together with the standalone user-space utility nfacct(8). They're a package deal, fr fr.

The only option available for this match is the following:

--nfacct-name name — this allows you to specify the existing object name that will be use for accounting the traffic that this rule-set is matching.

To use this extension, you have to create an accounting object:

nfacct add http-traffic

Then, you have to attach it to the accounting object via iptables:

iptables -I INPUT -p tcp --sport 80 -m nfacct --nfacct-name http-traffic

iptables -I OUTPUT -p tcp --dport 80 -m nfacct --nfacct-name http-traffic

Then, you can check for the amount of traffic that the rules match:

nfacct get http-traffic

{ pkts = 00000000000000000156, bytes = 00000000000000151786 } = http-traffic;

You can obtain nfacct(8) from https://www.netfilter.org or, alternatively, from the git.netfilter.org repository. 🫶

🔥 osf

The osf module does passive operating system fingerprinting. This module compares some data (Window Size, MSS, options and their order, TTL, DF, and others) from packets with the SYN bit set. She's reading your packets' aura, basically.

[!] --genre string — match an operating system genre by using a passive fingerprinting.
--ttl level — do additional TTL checks on the packet to determine the operating system. level can be one of the following values:
- 0 — True IP address and fingerprint TTL comparison. This generally works for LANs.
- 1 — Check if the IP header's TTL is less than the fingerprint one. Works for globally-routable addresses.
- 2 — Do not compare the TTL at all.
--log level — log determined genres into dmesg even if they do not match the desired one. level can be one of the following values:
- 0 — Log all matched or unknown signatures
- 1 — Log only the first one
- 2 — Log all known matched signatures

You may find something like this in syslog:

Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> 11.22.33.44:139 hops=3 Linux [2.5-2.6:] : 1.2.3.4:42624 -> 1.2.3.5:22 hops=4

OS fingerprints are loadable using the nfnl_osf program. To load fingerprints from a file, use:

nfnl_osf -f /usr/share/xtables/pf.os

To remove them again,

nfnl_osf -f /usr/share/xtables/pf.os -d

The fingerprint database can be downloaded from http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os. 😭 (yes, http, we know)

🎀 owner

This module attempts to match various characteristics of the packet creator, for locally generated packets. This match is only valid in the OUTPUT and POSTROUTING chains. Forwarded packets do not have any socket associated with them. Packets from kernel threads do have a socket, but usually no owner. So like, she can't always tell you who did it. 💀

[!] --uid-owner username
[!] --uid-owner userid[-userid] — matches if the packet socket's file structure (if it has one) is owned by the given user. You may also specify a numerical UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid] — matches if the packet socket's file structure is owned by the given group. You may also specify a numerical GID, or a GID range. 🫶
--suppl-groups — makes the group(s) you named with --gid-owner also get checked in the supplementary groups of a process. so like, all of them, not just the primary one 💅
[!] --socket-exists — matches if the packet is associated with a socket. that's it, that's the whole vibe.

physdev 🦋

okay so this module matches on the bridge port input and output devices that are enslaved to a bridge device. it's part of the whole infrastructure that makes a transparent bridging IP firewall happen, and heads up: it's only useful for kernel versions above version 2.5.44. yes, that's a real number, no i'm not kidding.

[!] --physdev-in name — the name of the bridge port a packet is received on. this only applies to packets entering the INPUT, FORWARD and PREROUTING chains. if the interface name ends in a "+", then any interface that begins with that name will match — little wildcard moment. if the packet didn't arrive through a bridge device, it just won't match this option, unless you throw a '!' at it.
[!] --physdev-out name — the name of a bridge port a packet is going to be sent out of (for bridged packets entering the FORWARD and POSTROUTING chains). same "+" deal as above: if the name ends in "+", any interface beginning with that name matches.
[!] --physdev-is-in — matches if the packet has entered through a bridge interface.
[!] --physdev-is-out — matches if the packet will leave through a bridge interface.
[!] --physdev-is-bridged — matches if the packet is being bridged and therefore is not being routed. only useful in the FORWARD and POSTROUTING chains, bestie.

pkttype 🎀

this one matches the link-layer packet type. simple, iconic, does exactly what it says.

[!] --pkt-type {unicast|broadcast|multicast} — pick your fighter. literally those three values, nothing else.

policy 👑

this module matches the policy that IPsec is using to handle a packet. very serious, very secure, very main character.

--dir {in|out} — selects whether you're matching the policy used for decapsulation or the policy that will be used for encapsulation. in is valid in the PREROUTING, INPUT and FORWARD chains; out is valid in the POSTROUTING, OUTPUT and FORWARD chains.
--pol {none|ipsec} — matches if the packet is subject to IPsec processing. 🚨 important: --pol none cannot be combined with --strict.
--strict — picks whether you match the exact policy, or match if any rule of the policy matches the given policy.

so for each policy element you wanna describe, you can use one or more of these next options. when --strict is in effect, at least one of them must be used per element. rules are rules 💅

[!] --reqid id — matches the reqid of the policy rule. fun fact: the reqid can be specified with setkey(8) using unique:id as the level.
[!] --spi spi — matches the SPI of the SA.
[!] --proto {ah|esp|ipcomp} — matches the encapsulation protocol.
[!] --mode {tunnel|transport} — matches the encapsulation mode.
[!] --tunnel-src addr[/mask] — matches the source end-point address of a tunnel mode SA. only valid with --mode tunnel.
[!] --tunnel-dst addr[/mask] — matches the destination end-point address of a tunnel mode SA. also only valid with --mode tunnel.
--next — starts the next element in the policy specification. can only be used with --strict.

quota 🍓

this one implements network quotas by decrementing a byte counter with each packet. the condition matches until the byte counter hits zero. and because everything in life is reversible, negation flips the behaviour: the condition does not match until the byte counter reaches zero.

[!] --quota bytes — the quota, in bytes. that's the whole option, she's lowkey simple.

rateest 🔥

the rate estimator can match on estimated rates as collected by the RATEEST target. she supports matching on absolute bps/pps values, comparing two rate estimators, and matching on the difference between two rate estimators. versatile queen.

so you can actually understand the options, here are literally all the possible combinations:

Absolute:

rateest operator rateest-bps
rateest operator rateest-pps

Absolute + Delta:

(rateest minus rateest-bps1) operator rateest-bps2
(rateest minus rateest-pps1) operator rateest-pps2

Relative:

rateest1 operator rateest2 rateest-bps (without rate!)
rateest1 operator rateest2 rateest-pps (without rate!)

Relative + Delta:

(rateest1 minus rateest-bps1) operator (rateest2 minus rateest-bps2)
(rateest1 minus rateest-pps1) operator (rateest2 minus rateest-pps2)
--rateest-delta — for each estimator (absolute or relative mode), calculates the difference between the estimator-determined flow rate and the static value you picked with the BPS/PPS options. if the flow rate is higher than the specified BPS/PPS, 0 gets used instead of a negative value. in other words, it's "max(0, rateest#_rate - rateest#_bps)". math slay 💅
[!] --rateest-lt — match if the rate is less than the given rate/estimator.
[!] --rateest-gt — match if the rate is greater than the given rate/estimator.
[!] --rateest-eq — match if the rate is equal to the given rate/estimator.

in the so-called "absolute mode", only one rate estimator is used and it gets compared against a static value. in "relative mode", two rate estimators get compared against each other. two icons going head to head, honestly.

--rateest name — name of the one rate estimator for absolute mode.
--rateest1 name, --rateest2 name — the names of the two rate estimators for relative mode.
--rateest-bps [value], --rateest-pps [value], --rateest-bps1 [value], --rateest-bps2 [value], --rateest-pps1 [value], --rateest-pps2 [value] — compare the estimator(s) by bytes or packets per second, and compare against the chosen value. peep the bullet list above for which option to use in which case. a unit suffix may be used — the available ones are: bit, [kmgt]bit, [KMGT]ibit, Bps, [KMGT]Bps, [KMGT]iBps.

Example: here's what you can use to route outgoing data connections from an FTP server over two lines based on the available bandwidth at the time the data connection was started. genuinely so cool:

# Estimate outgoing rates

iptables -t mangle -A POSTROUTING -o eth0 -j RATEEST --rateest-name eth0 --rateest-interval 250ms --rateest-ewma 0.5s

iptables -t mangle -A POSTROUTING -o ppp0 -j RATEEST --rateest-name ppp0 --rateest-interval 250ms --rateest-ewma 0.5s

# Mark based on available bandwidth

iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest --rateest-delta --rateest1 eth0 --rateest-bps1 2.5mbit --rateest-gt --rateest2 ppp0 --rateest-bps2 2mbit -j CONNMARK --set-mark 1

iptables -t mangle -A balance -m conntrack --ctstate NEW -m helper --helper ftp -m rateest --rateest-delta --rateest1 ppp0 --rateest-bps1 2mbit --rateest-gt --rateest2 eth0 --rateest-bps2 2.5mbit -j CONNMARK --set-mark 2

iptables -t mangle -A balance -j CONNMARK --restore-mark

realm (IPv4-specific) 🌈

this matches the routing realm. routing realms are used in complex routing setups involving dynamic routing protocols like BGP. very "i have a complicated network topology and i'm not sorry" of them.

[!] --realm value[/mask] — matches a given realm number (and optionally mask). if it's not a number, value can be a named realm from /etc/iproute2/rt_realms — but heads up, the mask cannot be used in that case. both value and mask are four byte unsigned integers and may be specified in decimal, hex (by prefixing with "0x") or octal (if a leading zero is given). three whole number formats, she contains multitudes.

recent 💀

this one lets you dynamically create a list of IP addresses and then match against that list in a few different ways. it's giving "i remember what you did" and honestly? iconic.

for example, you can create a "badguy" list out of people attempting to connect to port 139 on your firewall, and then DROP all future packets from them without even considering them. bye bestie 👋

🚨 --set, --rcheck, --update and --remove are mutually exclusive. pick one, you cannot have all four.

--name name — specify the list to use for the commands. if no name is given, then DEFAULT will be used.
[!] --set — adds the source address of the packet to the list. if the source address is already in the list, this updates the existing entry. this will always return success (or failure if ! is passed in).
--rsource — match/save the source address of each packet in the recent list table. this is the default. 💅
--rdest — match/save the destination address of each packet in the recent list table.
--mask netmask — the netmask that will be applied to this recent list.
[!] --rcheck — check if the source address of the packet is currently in the list.
[!] --update — like --rcheck, except it'll update the "last seen" timestamp if it matches. glow-up AND a timestamp.
[!] --remove — check if the source address of the packet is currently in the list, and if so, that address gets removed from the list and the rule returns true. if the address is not found, false is returned.
--seconds seconds — this option must be used in conjunction with one of --rcheck or --update. when used, it narrows the match so it only happens when the address is in the list and was seen within the last given number of seconds.
--reap — this option can only be used in conjunction with --seconds. when used, it causes entries older than the last given number of seconds to be purged. spring cleaning, but for IPs 🧹
--hitcount hits — this option must be used in conjunction with one of --rcheck or --update. when used, it narrows the match so it only happens when the address is in the list and packets had been received greater than or equal to the given value. this option may be used along with --seconds to create an even narrower match requiring a certain number of hits within a specific time frame. stacking conditions, we love to see it.
--rttl — this option may only be used in conjunction with one of --rcheck or --update. when used, it narrows the match so it only happens when the address is in the list and the TTL of the current packet matches that of the packet which hit the --set rule. this may be useful if you have problems with people faking their source address in order to DoS you via this module, by disallowing others access to your site by sending bogus packets to you. rude behaviour, honestly, block them.

Examples:

iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP

iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j DROP

/proc/net/xt_recent/* are the current lists of addresses and information about each entry of each list.

each file in /proc/net/xt_recent/ can be read from to see the current list, or written to using the following commands to modify the list:

echo +addr >/proc/net/xt_recent/DEFAULT

that one adds addr to the DEFAULT list 💖

To remove addr from the DEFAULT list, you do this lil thing:

echo -addr >/proc/net/xt_recent/DEFAULT

And to flush the DEFAULT list (aka remove all entries, bye bestie):

echo / >/proc/net/xt_recent/DEFAULT

The module itself accepts parameters, defaults shown:

ip_list_tot=100 — number of addresses remembered per table.
ip_pkt_list_tot=0 — number of packets per address remembered. This parameter is obsolete since kernel version 3.19, which started calculating the table size based on the given --hitcount parameter.
ip_list_hash_size=0 — hash table size. 0 means calculate it based on ip_list_tot by rounding it up to the next power of two (with ip_list_tot defaulting to 100, ip_list_hash_size calculates to 128 by default).
ip_list_perms=0644 — permissions for /proc/net/xt_recent/* files.
ip_list_uid=0 — numerical UID for ownership of /proc/net/xt_recent/* files.
ip_list_gid=0 — numerical GID for ownership of /proc/net/xt_recent/* files.

💫 rpfilter 💫

This one does a reverse path filter test on a packet. If a reply to the packet would be sent via the same interface that the packet arrived on, the packet matches. Note that, unlike the in-kernel rp_filter, packets protected by IPSec are not treated specially. Combine this match with the policy match if you want that. Also, packets arriving via the loopback interface are always permitted. This match can only be used in the PREROUTING chain of the raw or mangle table.

--loose — used to specify that the reverse path filter test should match even if the selected output device is not the expected one.
--validmark — also use the packets' nfmark value when performing the reverse path route lookup.
--accept-local — this permits packets arriving from the network with a source address that is also assigned to the local machine.
--invert — inverts the sense of the match. Instead of matching packets that passed the reverse path filter test, match those that failed it.

Example to log and drop packets failing the reverse path filter test:

iptables -t raw -N RPFILTER

iptables -t raw -A RPFILTER -m rpfilter -j RETURN

iptables -t raw -A RPFILTER -m limit --limit 10/minute -j NFLOG
--nflog-prefix "rpfilter drop"

iptables -t raw -A RPFILTER -j DROP

iptables -t raw -A PREROUTING -j RPFILTER

Example to drop failed packets, without logging:

iptables -t raw -A RPFILTER -m rpfilter --invert -j DROP

🦋 rt (IPv6-specific) 🦋

Match on IPv6 routing header.

[!] --rt-type type — match the type (numeric).
[!] --rt-segsleft num[:num] — match the segments left field (range).
[!] --rt-len length — match the length of this header.
--rt-0-res — match the reserved field, too (type=0).
--rt-0-addrs addr[,addr...] — match type=0 addresses (list).
--rt-0-not-strict — list of type=0 addresses is not a strict list.

🎀 sctp 🎀

This module matches Stream Control Transmission Protocol headers.

[!] --source-port,--sport port[:port]
[!] --destination-port,--dport port[:port]
[!] --chunk-types {all|any|only} chunktype[:flags] [...] — the flag letter in upper case indicates that the flag is to match if set; in lower case it indicates match if unset.

Match types:

all — match if all given chunk types are present and flags match.
any — match if any of the given chunk types is present with given flags.
only — match if only the given chunk types are present with given flags and none are missing.

Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE I_DATA RE_CONFIG PAD ASCONF ASCONF_ACK FORWARD_TSN I_FORWARD_TSN

Available flags per chunk type:

DATA — available flags: I U B E i u b e
I_DATA — available flags: I U B E i u b e
ABORT — available flags: T t
SHUTDOWN_COMPLETE — available flags: T t

(lowercase means the flag should be "off", uppercase means "on" 💅)

Examples:

iptables -A INPUT -p sctp --dport 80 -j DROP

iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP

iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT

🌈 set 🌈

This module matches IP sets which can be defined by ipset(8).

[!] --match-set setname flag[,flag]... — where flags are the comma separated list of src and/or dst specifications, and there can be no more than six of them. So the command

iptables -A FORWARD -m set --match-set test src,dst

will match packets for which (if the set type is ipportmap) the source address and destination port pair can be found in the specified set. If the set type of the specified set is single dimension (for example ipmap), then the command will match packets for which the source address can be found in the specified set.

--return-nomatch — if this option is specified and the set type supports the nomatch flag, then the matching is reversed: a match with an element flagged with nomatch returns true, while a match with a plain element returns false.
! --update-counters — if this flag is negated, then the packet and byte counters of the matching element in the set won't be updated. Default is that the packet and byte counters are updated.
! --update-subcounters — if this flag is negated, then the packet and byte counters of the matching element in the member set of a list type of set won't be updated. Default is that the packet and byte counters are updated.
[!] --packets-eq value — if the packet is matched to an element in the set, match only if the packet counter of the element matches the given value too.
--packets-lt value — if the packet is matched to an element in the set, match only if the packet counter of the element is less than the given value as well.
--packets-gt value — if the packet is matched to an element in the set, match only if the packet counter of the element is greater than the given value as well.
[!] --bytes-eq value — if the packet is matched to an element in the set, match only if the byte counter of the element matches the given value too.
--bytes-lt value — if the packet is matched to an element in the set, match only if the byte counter of the element is less than the given value as well.
--bytes-gt value — if the packet is matched to an element in the set, match only if the byte counter of the element is greater than the given value as well.

The packet and byte counters related options and flags are ignored when the set was defined without counter support.

The option --match-set can be replaced by --set if that does not clash with an option of other extensions.

Use of -m set requires that ipset kernel support is provided, which, for standard kernels, is the case since Linux 2.6.39.

🫶 socket 🫶

This matches if an open TCP/UDP socket can be found by doing a socket lookup on the packet. It matches if there is an established or non-zero bound listening socket (possibly with a non-local address). The lookup is performed using the packet tuple of TCP/UDP packets, or the original TCP/UDP header embedded in an ICMP/ICPMv6 error packet.

--transparent — ignore non-transparent sockets.
--nowildcard — do not ignore sockets bound to 'any' address. The socket match won't accept zero-bound listeners by default, since then local services could intercept traffic that would otherwise be forwarded. This option therefore has security implications when used to match traffic being forwarded to redirect such packets to local machine with policy routing. When using the socket match to implement fully transparent proxies bound to non-local addresses, it is recommended to use the --transparent option instead.
--restore-skmark — set the packet mark to the matching socket's mark. Can be combined with the --transparent and --nowildcard options to restrict the sockets to be matched when restoring the packet mark.

Example (assuming packets with mark 1 are delivered locally):

-t mangle -A PREROUTING -m socket --transparent -j MARK
--set-mark 1

Example: an application opens 2 transparent (IP_TRANSPARENT) sockets and sets a mark on them with the SO_MARK socket option. We can filter matching packets like this:

-t mangle -I PREROUTING -m socket --transparent --restore-skmark
-j action

-t mangle -A action -m mark --mark 10 -j action2

-t mangle -A action -m mark --mark 11 -j action3

🌸 state 🌸

The "state" extension is a subset of the "conntrack" module. "state" allows access to the connection tracking state for this packet.

[!] --state state — where state is a comma separated list of the connection states to match. Only a subset of the states understood by "conntrack" are recognized: INVALID, ESTABLISHED, NEW, RELATED or UNTRACKED. For their description, see the "conntrack" heading in this manpage.

⭐ statistic ⭐

This module matches packets based on some statistic condition. It supports two distinct modes settable with the --mode option.

Supported options:

--mode mode — set the matching mode of the matching rule; supported modes are random and nth.
[!] --probability p — set the probability for a packet to be randomly matched. It only works with the random mode. p must be within 0.0 and 1.0. The supported granularity is in 1/2147483648th increments.
[!] --every n — match one packet every nth packet. It works only with the nth mode (see also the --packet option).
--packet p — set the initial counter value (0 <= p <= n-1, default 0) for the nth mode.

💜 string 💜

This module matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.

--algo {bm|kmp} — select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
--from offset — set the offset from which it starts looking for any matching. If not passed, default is 0.
--to offset — caps off how far into the packet we're willing to scan. So like, byte offset (counting from 0) is the last byte that gets looked at, and it's also the max position the last character of your pattern can land on. Don't pass it? Default is just the packet size, no cap. 🎀
[!] --string pattern — matches the pattern you give it, fr.
[!] --hex-string pattern — same deal but the pattern's written in hex notation.
--icase — ignore case while searching. basic but essential 💅

Examples ✨

# The string pattern can be used for simple text characters.
iptables -A INPUT -p tcp --dport 80 -m string --algo bm --string
'GET /index.html' -j LOG

# The hex string pattern can be used for non-printable
characters, like |0D 0A| or |0D0A|.
iptables -p udp --dport 53 -m string --algo bm --from 40 --to 57
--hex-string '|03|www|09|netfilter|03|org|00|'

Note: okay so this is important — Boyer-Moore (BM) searches for matches going right-to-left, and the kernel might store a packet in several discontiguous blocks. Which means a match could be spread across multiple blocks, and in that case this algorithm straight up won't find it. 😭

If you wanna make sure that never ever happens, use the Knuth-Pratt-Morris (KMP) algorithm instead. Bottom line: pick the right string search algorithm for your use-case, bestie.

So like, for example — if you're using the module for filtering, NIDS, or any similar security-focused purpose, choose KMP. But on the other hand, if you really care about performance — say you're classifying packets to apply Quality of Service (QoS) policies — and you don't mind about missing possible matches spread over multiple fragments, then choose BM. 🦋

tcp 🧚

These extensions can be used if --protocol tcp is specified. It provides the following options:

[!] --source-port,--sport port[:port] — source port or port range spec. Can be a service name or a port number. You can also give an inclusive range using the format first:last. If the first port is omitted, "0" is assumed; if the last is omitted, "65535" is assumed. And --sport is the convenient alias for this one, iykyk.
[!] --destination-port,--dport port[:port] — destination port or port range spec. The flag --dport is the convenient alias for this option.
[!] --tcp-flags mask comp — matches when the TCP flags are as specified. First arg mask is the flags we should examine, written as a comma-separated list, and second arg comp is a comma-separated list of flags which must be set. The flags are: SYN ACK FIN RST URG PSH ALL NONE. So hence the command

iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN

will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset. periodt.

[!] --syn — only matches TCP packets with the SYN bit set and the ACK, RST and FIN bits cleared. Those packets are used to request TCP connection initiation — so like, blocking such packets coming in an interface will prevent incoming TCP connections, but outgoing TCP connections will be unaffected. It's equivalent to --tcp-flags SYN,RST,ACK,FIN SYN. If the "!" flag comes before the --syn, the sense of the option is inverted. 🔥
[!] --tcp-option number — matches if the TCP option is set.

tcpmss 🌸

This one matches the TCP MSS (maximum segment size) field of the TCP header. Heads up: you can only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP handshake at connection startup time.

[!] --mss value[:value] — match a given TCP MSS value or range. If you give a range, the second value must be greater than or equal to the first value. no cap 🫶

time ⭐

Matches if the packet arrival time/date is within a given range. All the options are optional, but they're ANDed together when specified. And all times are interpreted as UTC by default.

--datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]
--datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]] — only match during the given time, which has to be in ISO 8601 "T" notation. The possible time range is 1970-01-01T00:00:00 to 2038-01-19T04:17:07.

If --datestart or --datestop aren't specified, it defaults to 1970-01-01 and 2038-01-19, respectively.

--timestart hh:mm[:ss]
--timestop hh:mm[:ss] — only match during the given daytime. Possible time range is 00:00:00 to 23:59:59. Leading zeroes are allowed (e.g. "06:03") and correctly interpreted as base-10. 💖
[!] --monthdays day[,day...] — only match on the given days of the month. Possible values are 1 to 31. Note that specifying 31 will of course not match on months which don't have a 31st day — same goes for 28- or 29-day February. yeah. 🙈
[!] --weekdays day[,day...] — only match on the given weekdays. Possible values are Mon, Tue, Wed, Thu, Fri, Sat, Sun, or values from 1 to 7, respectively. You may also use the two-character variants (Mo, Tu, etc.).
--contiguous — when --timestop is smaller than the --timestart value, match this as a single time period instead of distinct intervals. See EXAMPLES. 🎀
--kerneltz — use the kernel timezone instead of UTC to figure out whether a packet meets the time regulations.

About kernel timezones: so Linux keeps the system time in UTC, and always does so. On boot, system time is initialized from a referential time source. Where this time source has no timezone info — like the x86 CMOS RTC — UTC will be assumed. If the time source isn't in UTC though, userspace should provide the correct system time and timezone to the kernel once it has that info.

Local time is a feature layered on top of the (timezone independent) system time. Every process has its own idea of local time, specified via the TZ environment variable. The kernel also has its own timezone offset variable. The TZ userspace environment variable specifies how the UTC-based system time is displayed, e.g. when you run date(1), or what you see on your desktop clock. The TZ string may resolve to different offsets at different dates, which is what enables the automatic time-jumping in userspace when DST changes. The kernel's timezone offset variable is used when it has to convert between non-UTC sources, like FAT filesystems, to UTC (since the latter is what the rest of the system uses).

The caveat with the kernel timezone is that Linux distributions may ignore setting the kernel timezone, and instead only set the system time. Even if a particular distribution does set the timezone at boot, it usually does not keep the kernel timezone offset — which is what changes on DST — up to date. ntpd will not touch the kernel timezone, so running it will not resolve the issue. As such, one may encounter a timezone that is always +0000, or one that is wrong half of the time of the year. As such, using --kerneltz is highly discouraged. 💀

EXAMPLES. To match on weekends, use:

-m time --weekdays Sa,Su

Or, to match (once) on a national holiday block:

-m time --datestart 2007-12-24 --datestop 2007-12-27

Since the stop time is actually inclusive, you'd need the following stop time to not match the first second of the new day:

-m time --datestart 2007-01-01T17:00 --datestop
2007-01-01T23:59:59

During lunch hour:

-m time --timestart 12:30 --timestop 13:30

The fourth Friday in the month:

-m time --weekdays Fr --monthdays 22,23,24,25,26,27,28

(Note that this exploits a certain mathematical property. It's not possible to say "fourth Thursday OR fourth Friday" in one rule. It is possible with multiple rules, though.)

Matching across days might not do what's expected. For instance,

-m time --weekdays Mo --timestart 23:00  --timestop 01:00 Will
match Monday, for one hour from midnight to 1 a.m., and then
again for another hour from 23:00 onwards.  If this is unwanted,
e.g. if you would like 'match for two hours from Montay 23:00
onwards' you need to also specify the --contiguous option in the
example above.

tos 🌈

This module matches the 8-bit Type of Service field in the IPv4 header (i.e. including the "Precedence" bits) or the (also 8-bit) Priority field in the IPv6 header.

[!] --tos value[/mask] — matches packets with the given TOS mark value. If a mask is specified, it's logically ANDed with the TOS mark before the comparison.
[!] --tos symbol — you can specify a symbolic name when using the tos match for IPv4. The list of recognized TOS names can be obtained by calling iptables with -m tos -h. Note that this implies a mask of 0x3F, i.e. all but the ECN bits. 👑

ttl (IPv4-specific) 💜

This module matches the time to live field in the IP header.

[!] --ttl-eq ttl — matches the given TTL value.
--ttl-gt ttl — matches if TTL is greater than the given TTL value.
--ttl-lt ttl — matches if TTL is less than the given TTL value.

u32 🦋

U32 tests whether quantities of up to 4 bytes extracted from a packet have specified values. The specification of what to extract is general enough to find data at given offsets from tcp headers or payloads. it's giving low-level sorcery, ngl ✨

[!] --u32 tests — the argument amounts to a program in a small language described below.

tests := location "=" value | tests "&&" location "=" value

value := range | value "," range

range := number | number ":" number

a single number, n, is interpreted the same as n:n. n:m is interpreted as the range of numbers >=n and <=m.

location := number | location operator number

operator := "&" | "<<" | ">>" | "@"

The operators &, <<, >> and && mean the same as in C. The = is really a set membership operator and the value syntax describes a set. The @ operator is what allows moving to the next header and is described further below.

There are currently some artificial implementation limits on the size of the tests:

no more than 10 of "=" (and 9 "&&"s) in the u32 argument
no more than 10 ranges (and 9 commas) per value
no more than 10 numbers (and 9 operators) per location

To describe the meaning of location, imagine the following machine that interprets it. There are three registers:

A is of type char *, initially the address of the IP header

B and C are unsigned 32 bit integers, initially zero

The instructions are:

number B = number;

C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)

&number — C = C & number
<< number — C = C << number
>> number — C = C >> number
@number — A = A + C; then jump to instruction number number

Any memory access outside [skb->data,skb->end]? The match just fails, no drama. Otherwise the result of the computation is the final value of C. 💅

Whitespace is allowed but not required in the tests. But — heads up bestie — the characters that show up there are probably gonna need shell quoting, so do yourself a favor and wrap the args in quotes. 🫶

Example: match IP packets with total length >= 256

The IP header has a total length field sitting in bytes 2–3.

--u32 "0 & 0xFFFF = 0x100:0xFFFF"

read bytes 0–3
AND that with 0xFFFF (giving bytes 2–3), and test whether that is in the range [0x100:0xFFFF]

Example: (more realistic, hence more complicated — buckle up 🎀)

match ICMP packets with icmp type 0

First test that it is an ICMP packet — true iff byte 9 (protocol) = 1

--u32 "6 & 0xFF = 1 && ...

read bytes 6–9, use & to throw away bytes 6–8 and compare the result to 1. Next test that it is not a fragment. (If so, it might be part of such a packet but we cannot always tell.) N.B.: This test is generally needed if you want to match anything beyond the IP header. The last 6 bits of byte 6 and all of byte 7 are 0 iff this is a complete packet (not a fragment). Alternatively, you can allow first fragments by only testing the last 5 bits of byte 6.

 ... 4 & 0x3FFF = 0 && ...

Last test: the first byte past the IP header (the type) is 0. This is where we have to use the @ syntax. The length of the IP header (IHL) in 32 bit words is stored in the right half of byte 0 of the IP header itself.

 ... 0 >> 22 & 0x3C @ 0 >> 24 = 0"

The first 0 means read bytes 0–3, >>22 means shift that 22 bits to the right. Shifting 24 bits would give the first byte, so only 22 bits is four times that plus a few more bits. &3C then eliminates the two extra bits on the right and the first four bits of the first byte. For instance, if IHL=5, then the IP header is 20 (4 x 5) bytes long. In this case, bytes 0–1 are (in binary) xxxx0101 yyzzzzzz, >>22 gives the 10 bit value xxxx0101yy and &3C gives 010100. @ means to use this number as a new offset into the packet, and read four bytes starting from there. This is the first 4 bytes of the ICMP payload, of which byte 0 is the ICMP type. Therefore, we simply shift the value 24 to the right to throw out all but the first byte and compare the result with 0. 🧚

Example: TCP payload bytes 8–12 is any of 1, 2, 5 or 8

First we test that the packet is a tcp packet (similar to ICMP).

--u32 "6 & 0xFF = 6 && ...

Next, test that it is not a fragment (same as above).

 ... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8"

0>>22&3C as above computes the number of bytes in the IP header. @ makes this the new offset into the packet, which is the start of the TCP header. The length of the TCP header (again in 32 bit words) is the left half of byte 12 of the TCP header. The 12>>26&3C computes this length in bytes (similar to the IP header before). "@" makes this the new offset, which is the start of the TCP payload. Finally, 8 reads bytes 8–12 of the payload and = checks whether the result is any of 1, 2, 5 or 8. Slay. ✨

🌸 udp 🌸

These extensions can be used if --protocol udp is specified. It provides the following options:

[!] --source-port,--sport port[:port] — Source port or port range specification. See the description of the --source-port option of the TCP extension for details.
[!] --destination-port,--dport port[:port] — Destination port or port range specification. See the description of the --destination-port option of the TCP extension for details.

👑 TARGET EXTENSIONS 👑

iptables can use extended target modules: the following are included in the standard distribution. (yes, there's even more where this came from 💀)

🕵️ AUDIT

This target creates audit records for packets hitting the target. It can be used to record accepted, dropped, and rejected packets. See auditd(8) for additional details.

--type {accept|drop|reject} — Set type of audit record. Starting with linux-4.12, this option has no effect on generated audit messages anymore. It is still accepted by iptables for compatibility reasons, but ignored.

ngl this one's a little vintage — the --type flag is literally just there for vibes on modern kernels 🙈

Example:

iptables -N AUDIT_DROP

iptables -A AUDIT_DROP -j AUDIT

iptables -A AUDIT_DROP -j DROP

✅ CHECKSUM

This target selectively works around broken/old applications. It can only be used in the mangle table.

--checksum-fill — Compute and fill in the checksum in a packet that lacks a checksum. This is particularly useful, if you need to work around old applications such as dhcp clients, that do not work well with checksum offloads, but don't want to disable checksum offload in your device. 🫶

🏷️ CLASSIFY

This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).

--set-class major:minor — Set the major and minor class value. The values are always interpreted as hexadecimal even if no 0x prefix is given. 👀

🧚 CLUSTERIP (IPv4-specific)

This module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load balancer in front of them. Connections are statically distributed between the nodes in this cluster.

Please note that CLUSTERIP target is considered deprecated in favour of cluster match which is more flexible and not limited to IPv4. 💀

--new — Create a new ClusterIP. You always have to set this on the first rule for a given ClusterIP.
--hashmode mode — Specify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sourceip-sourceport-destport.
--clustermac mac — Specify the ClusterIP MAC address. Has to be a link-layer multicast address
--total-nodes num — Number of total nodes within this cluster.
--local-node num — Local node number within this cluster.
--hash-init rnd — Specify the random seed used for hash initialization.

🔖 CONNMARK

This module sets the netfilter mark value associated with a connection. The mark is 32 bits wide.

--set-xmark value[/mask] — Zero out the bits given by mask and XOR value into the ctmark.
--save-mark [--nfmask nfmask] [--ctmask ctmask] — Copy the packet mark (nfmark) to the connection mark (ctmark) using the given masks. The new nfmark value is determined as follows:

ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)

i.e. ctmask defines what bits to clear and nfmask what bits of the nfmark to XOR into the ctmark. ctmask and nfmask default to 0xFFFFFFFF.

--restore-mark [--nfmask nfmask] [--ctmask ctmask] — Copy the connection mark (ctmark) to the packet mark (nfmark) using the given masks. The new ctmark value is determined as follows:

nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);

i.e. nfmask defines what bits to clear and ctmask what bits of the ctmark to XOR into the nfmark. ctmask and nfmask default to 0xFFFFFFFF.

--restore-mark is only valid in the mangle table. 🚨

The following mnemonics are available for --set-xmark:

--and-mark bits — Binary AND the ctmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits is the binary negation of bits.)
--or-mark bits — Binary OR the ctmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits — Binary XOR the ctmark with bits. (Mnemonic for --set-xmark bits/0.)
--set-mark value[/mask] — Set the connection mark. If a mask is specified then only those bits set in the mask are modified.
--save-mark [--mask mask] — Copy the nfmark to the ctmark. If a mask is specified, only those bits are copied.
--restore-mark [--mask mask] — Copy the ctmark to the nfmark. If a mask is specified, only those bits are copied. This is only valid in the mangle table. 🚨

🔐 CONNSECMARK

This module copies security markings from packets to connections (if unlabeled), and from connections back to packets (also only if unlabeled). Typically used in conjunction with SECMARK, it is valid in the security table (for backwards compatibility with older kernels, it is also valid in the mangle table).

--save — If the packet has a security marking, copy it to the connection if the connection is not marked.
--restore — If the packet does not have a security marking, and the connection does, copy the security marking from the connection to the packet.

🌀 CT

The CT target sets parameters for a packet or its associated connection. The target attaches a "template" connection tracking entry to the packet, which is then used by the conntrack core when initializing a new ct entry. This target is thus only valid in the "raw" table. 🎀

--notrack — Disables connection tracking for this packet.
--helper name — Use the helper identified by name for the connection. This is more flexible than loading the conntrack helper modules with preset ports.
--ctevents event[,...] — Only generate the specified conntrack events for this connection. Possible event types are: new, related, destroy, reply, assured, protoinfo, helper, mark (this refers to the ctmark, not nfmark), natseqinfo, secmark (ctsecmark).
--expevents event[,...] — Only generate the specified expectation events for this connection. Possible event types are: new.
--zone-orig {id|mark} — For traffic coming from ORIGINAL direction, assign this packet to zone id and only have lookups done in that zone. If mark is used instead of id, the zone is derived from the packet nfmark.
--zone-reply {id|mark} — For traffic coming from REPLY direction, assign this packet to zone id and only have lookups done in that zone. If mark is used instead of id, the zone is derived from the packet nfmark.
--zone {id|mark} — Assign this packet to zone id and only have lookups done in that zone. If mark is used instead of id, the zone is derived from the packet nfmark. By default, packets have zone 0. This option applies to both directions. 💅
--timeout name — bestie, this makes the connection use the timeout policy identified by name. It's giving way more flexible timeout policy definition than the global timeout values you'd get at /proc/sys/net/netfilter/nf_conntrack_*_timeout_*. ✨

💅 DNAT 💅

Okay so this target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and in user-defined chains which are only called from those chains. No cap. What it does: it specifies that the destination address of the packet should be modified (and all future packets in this connection will also be mangled 😭), and rules should cease being examined. It takes the following options:

--to-destination [ipaddr[-ipaddr]][:port[-port[/baseport]]] — this one can specify a single new destination IP address, or an inclusive range of IP addresses. Optionally a port range too, if the rule also specifies one of these protocols: tcp, udp, dccp or sctp. If no port range is specified, then the destination port will never be modified. If no IP address is specified then only the destination port will be modified. If baseport is given, the difference of the original destination port and its value is used as offset into the mapping port range — this lets you create shifted portmap ranges and it's available since kernel version 4.18. For a single port or baseport, a service name as listed in /etc/services may be used. ⚠️ If ipaddr is an IPv4 loopback address (i.e. 127.0.0.0/8) the net.ipv4.conf.*.route_localnet sysctl for the input interface needs to be set to 1. Otherwise packets will be dropped by the routing code as "martians". 🛸
--random — randomize source port mapping (kernel >= 2.6.22). 🎲
--persistent — gives a client the same source-/destination-address for each connection. This supersedes the SAME target. Support for persistent mappings is available from 2.6.29-rc2. 🫶

IPv6 support available since Linux kernels >= 3.7. 🦋

🌈 DNPT (IPv6-specific) 🌈

This one provides stateless destination IPv6-to-IPv6 Network Prefix Translation (as described by RFC 6296). Periodt.

You have to use this target in the mangle table, not in the nat table. It takes the following options:

--src-pfx [prefix/length] — set source prefix that you want to translate and length.
--dst-pfx [prefix/length] — set destination prefix that you want to use in the translation and length.

You have to use the SNPT target to undo the translation. Example, bestie:

ip6tables -t mangle -I POSTROUTING -s fd00::/64 ! -o vboxnet0 -j SNPT --src-pfx fd00::/64 --dst-pfx 2001:e20:2000:40f::/64

ip6tables -t mangle -I PREROUTING -i wlan0 -d 2001:e20:2000:40f::/64 -j DNPT --src-pfx 2001:e20:2000:40f::/64 --dst-pfx fd00::/64

You may need to enable IPv6 neighbor proxy:

sysctl -w net.ipv6.conf.all.proxy_ndp=1

You also have to use the NOTRACK target to disable connection tracking for translated flows. Don't forget it, iykyk 💀

🎀 DSCP 🎀

This target alters the value of the DSCP bits within the TOS header of the IPv4 packet. As this manipulates a packet, it can only be used in the mangle table.

--set-dscp value — set the DSCP field to a numerical value (can be decimal or hex).
--set-dscp-class class — set the DSCP field to a DiffServ class.

🧚 ECN (IPv4-specific) 🧚

This target selectively works around known ECN blackholes. It can only be used in the mangle table.

--ecn-tcp-remove — remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.

💀 HL (IPv6-specific) 💀

This is used to modify the Hop Limit field in the IPv6 header. The Hop Limit field is similar to what is known as the TTL value in IPv4. Setting or incrementing the Hop Limit field can potentially be very dangerous, so it should be avoided at any cost. This target is only valid in the mangle table.

🚨 Don't ever set or increment the value on packets that leave your local network! Like, seriously. Not me warning you twice. 🚨

--hl-set value — set the Hop Limit to value.
--hl-dec value — decrement the Hop Limit value times.
--hl-inc value — increment the Hop Limit value times.

⭐ HMARK ⭐

Like MARK, i.e. it sets the fwmark, but the mark is calculated from hashing the packet selector of your choice. You have also to specify the mark range and, optionally, the offset to start from. ICMP error messages are inspected and used to calculate the hashing. Obsessed with the attention to detail tbh.

Existing options are:

--hmark-tuple tuple — possible tuple members are: src meaning source address (IPv4, IPv6 address), dst meaning destination address (IPv4, IPv6 address), sport meaning source port (TCP, UDP, UDPlite, SCTP, DCCP), dport meaning destination port (TCP, UDP, UDPlite, SCTP, DCCP), spi meaning Security Parameter Index (AH, ESP), and ct meaning the usage of the conntrack tuple instead of the packet selectors.
--hmark-mod value (must be > 0) — modulus for hash calculation (to limit the range of possible marks).
--hmark-offset value — offset to start marks from.

For advanced usage, instead of using --hmark-tuple, you can specify custom prefixes and masks:

--hmark-src-prefix cidr — the source address mask in CIDR notation.
--hmark-dst-prefix cidr — the destination address mask in CIDR notation.
--hmark-sport-mask value — a 16 bit source port mask in hexadecimal.
--hmark-dport-mask value — a 16 bit destination port mask in hexadecimal.
--hmark-spi-mask value — a 32 bit field with spi mask.
--hmark-proto-mask value — an 8 bit field with layer 4 protocol number.
--hmark-rnd value — a 32 bit random custom value to feed hash calculation.

Examples, slay:

iptables -t mangle -A PREROUTING -m conntrack --ctstate NEW -j HMARK --hmark-tuple ct,src,dst,proto --hmark-offset 10000 --hmark-mod 10 --hmark-rnd 0xfeedcafe

iptables -t mangle -A PREROUTING -j HMARK --hmark-offset 10000 --hmark-tuple src,dst,proto --hmark-mod 10 --hmark-rnd 0xdeafbeef

🌸 IDLETIMER 🌸

This target can be used to identify when interfaces have been idle for a certain period of time. Timers are identified by labels and are created when a rule is set with a new label. The rules also take a timeout value (in seconds) as an option. If more than one rule uses the same timer label, the timer will be restarted whenever any of the rules get a hit. One entry for each timer is created in sysfs. This attribute contains the timer remaining for the timer to expire. The attributes are located under the xt_idletimer class:

/sys/class/xt_idletimer/timers/<label>

When the timer expires, the target module sends a sysfs notification to the userspace, which can then decide what to do (eg. disconnect to save power). Very main character energy of it. 🔋

--timeout amount — this is the time in seconds that will trigger the notification.
--label string — this is a unique identifier for the timer. The maximum length for the label string is 27 characters.

🦋 LED 🦋

This creates an LED-trigger that can then be attached to system indicator lights, to blink or illuminate them when certain packets pass through the system. One example might be to light up an LED for a few minutes every time an SSH connection is made to the local machine. The following options control the trigger behavior:

--led-trigger-id name — this is the name given to the LED trigger. The actual name of the trigger will be prefixed with "netfilter-".
--led-delay ms — this indicates how long (in milliseconds) the LED should be left illuminated when a packet arrives before being switched off again. The default is 0 (blink as fast as possible.) The special value inf can be given to leave the LED on permanently once activated. (In this case the trigger will need to be manually detached and reattached to the LED device to switch it off again.) 🙈
--led-always-blink — always make the LED blink on packet arrival, even if the LED is already on. This allows notification of new packets even with long delay values (which otherwise would result in a silent prolonging of the delay time.)

Example:

Create an LED trigger for incoming SSH traffic:

iptables -A INPUT -p tcp --dport 22 -j LED --led-trigger-id ssh

Then attach the new trigger to an LED:

echo netfilter-ssh >/sys/class/leds/ledname/trigger

💜 LOG 💜

Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on all matching packets (like most IP/IPv6 header fields) via the kernel log (where it can be read with dmesg(1) or read in the syslog).

This is a "non-terminating target", i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two separate rules with the same matching criteria, first using target LOG then DROP (or REJECT). Two-step, bestie. 💅

--log-level level — level of logging, which can be (system-specific) numeric or a mnemonic. Possible values are (in decreasing order of priority): emerg, alert, crit, error, warning, notice, info or debug.
--log-prefix prefix — prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.
--log-tcp-sequence — log TCP sequence numbers. ⚠️ This is a security risk if the log is readable by users.
--log-tcp-options — log options from the TCP packet header.
--log-ip-options — log options from the IP/IPv6 packet header.
--log-uid — log the userid of the process which generated the packet.
--log-macdecode — log MAC addresses and protocol.

👑 MARK 👑

This target is used to set the Netfilter mark value associated with the packet. It can, for example, be used in conjunction with routing based on fwmark (needs iproute2). If you plan on doing so, note that the mark needs to be set in either the PREROUTING or the OUTPUT chain of the mangle table to affect routing. The mark field is 32 bits wide.

--set-xmark value[/mask] — zeroes out the bits given by mask and XORs value into the packet mark ("nfmark"). If mask is omitted, 0xFFFFFFFF is assumed.
--set-mark value[/mask] — zeroes out the bits given by mask and ORs value into the packet mark. If mask is omitted, 0xFFFFFFFF is assumed.

The following mnemonics are available:

--and-mark bits — binary AND the nfmark with bits. (Mnemonic for --set-xmark 0/invbits, where invbits is the binary negation of bits.)

okay bestie, we’re still mid-MARK and these are the last two bit-twiddling girlies:

--or-mark bits — bitwise-ORs the nfmark with bits. (Mnemonic for --set-xmark bits/bits.)
--xor-mark bits — bitwise-XORs the nfmark with bits. (Mnemonic for --set-xmark bits/0.)

💅 MASQUERADE 💅

okay so MASQUERADE is nat-table-only, POSTROUTING-chain-only, and honestly it’s for the dialup girlies. It should only be used with dynamically assigned IP (dialup) connections — if you have a static IP address, you should use the SNAT target instead. Masquerading is equivalent to specifying a mapping to the IP address of the interface the packet is going out, but it also has the extra effect that connections are forgotten when the interface goes down. That is the correct behavior when the next dialup is unlikely to have the same interface address (and hence any established connections are lost anyway). 💖

--to-ports port[-port] — specifies a range of source ports to use, overriding the default SNAT source port selection heuristics (see above). This is only valid if the rule also specifies one of these protocols: tcp, udp, dccp or sctp.
--random — randomize source port mapping (kernel >= 2.6.21). Since kernel 5.0, --random is identical to --random-fully.
--random-fully — fully randomize source port mapping (kernel >= 3.13).

IPv6 support available since Linux kernels >= 3.7. 🦋

🌈 NETMAP 🌈

This target lets you statically map a whole network of addresses onto another network of addresses. It can only be used from rules in the nat table.

--to address[/mask] — network address to map to. The resulting address will be constructed like this: all the 'one' bits in the mask are filled in from the new address; all the bits that are zero in the mask are filled in from the original address.

IPv6 support available since Linux kernels >= 3.7. ✨

🎀 NFLOG 🎀

This target provides logging of matching packets. When this target is set for a rule, the Linux kernel will pass the packet to the loaded logging backend to log the packet. This is usually used in combination with nfnetlink_log as the logging backend, which will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may subscribe to the group to receive the packets. Like LOG, this is a non-terminating target, i.e. rule traversal continues at the next rule. 💜

--nflog-group nlgroup — the netlink group (0–2^16-1) to which packets are sent (only applicable for nfnetlink_log). The default value is 0.
--nflog-prefix prefix — a prefix string to include in the log message, up to 64 characters long, useful for distinguishing messages in the logs.
--nflog-range size — this option has never worked, use --nflog-size instead.
--nflog-size size — the number of bytes to be copied to userspace (only applicable for nfnetlink_log). nfnetlink_log instances may specify their own range; this option overrides it.
--nflog-threshold size — number of packets to queue inside the kernel before sending them to userspace (only applicable for nfnetlink_log). Higher values result in less overhead per packet, but increase delay until the packets reach userspace. The default value is 1.

🔥 NFQUEUE 🔥

This target passes the packet to userspace using the nfnetlink_queue handler. The packet is put into the queue identified by its 16-bit queue number. Userspace can inspect and modify the packet if desired. Userspace must then drop or reinject the packet into the kernel. Please see libnetfilter_queue for details. nfnetlink_queue was added in Linux 2.6.14. The queue-balance option was added in Linux 2.6.31, queue-bypass in 2.6.39. 👑

--queue-num value — specifies the QUEUE number to use. Valid queue numbers are 0 to 65535. The default value is 0.
--queue-balance value:value — specifies a range of queues to use. Packets are then balanced across the given queues. This is useful for multicore systems: start multiple instances of the userspace program on queues x, x+1, .. x+n and use --queue-balance x:x+n. Packets belonging to the same connection are put into the same nfqueue. Due to implementation details, a lower range value of 0 limits the higher range value to 65534, i.e. one can only balance between at most 65535 queues.
--queue-bypass — by default, if no userspace program is listening on an NFQUEUE, then all packets that are to be queued are dropped. When this option is used, the NFQUEUE rule behaves like ACCEPT instead, and the packet will move on to the next table.
--queue-cpu-fanout — available starting Linux kernel 3.10. When used together with --queue-balance this will use the CPU ID as an index to map packets to the queues. The idea is that you can improve performance if there’s a queue per CPU. This requires --queue-balance to be specified.

🦋 NOTRACK 🦋

This extension disables connection tracking for all packets matching that rule. It is equivalent with -j CT --notrack. Like CT, NOTRACK can only be used in the raw table.

🌸 RATEEST 🌸

The RATEEST target collects statistics, performs rate estimation calculation and saves the results for later evaluation using the rateest match.

--rateest-name name — count matched packets into the pool referred to by name, which is freely choosable.
--rateest-interval amount{s|ms|us} — rate measurement interval, in seconds, milliseconds or microseconds.
--rateest-ewmalog value — rate measurement averaging time constant.

🍓 REDIRECT 🍓

This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It redirects the packet to the machine itself by changing the destination IP to the primary address of the incoming interface — locally-generated packets are mapped to the localhost address, 127.0.0.1 for IPv4 and ::1 for IPv6, and packets arriving on interfaces that don’t have an IP address configured are dropped. 😭

--to-ports port[-port] — specifies a destination port or range of ports to use. Without this, the destination port is never altered. This is only valid if the rule also specifies one of these protocols: tcp, udp, dccp or sctp. For a single port, a service name as listed in /etc/services may be used.
--random — randomize source port mapping (kernel >= 2.6.22).

IPv6 support available starting Linux kernels >= 3.7. 🌈

🧚 REJECT (IPv6-specific) 🧚

This is used to send back an error packet in response to the matched packet; otherwise it is equivalent to DROP, so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:

--reject-with type — the type given can be icmp6-no-route, no-route, icmp6-adm-prohibited, adm-prohibited, icmp6-addr-unreachable, addr-unreach, or icmp6-port-unreachable, which return the appropriate ICMPv6 error message (icmp6-port-unreachable is the default). Finally, the option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won’t accept your mail otherwise). tcp-reset can only be used with kernel versions 2.6.14 or later.

⚠️ Warning: You should not indiscriminately apply the REJECT target to packets whose connection state is classified as INVALID; instead, you should only DROP these.

Consider a source host transmitting a packet P, with P experiencing so much delay along its path that the source host issues a retransmission, P_2, with P_2 being successful in reaching its destination and advancing the connection state normally. It is conceivable that the late-arriving P may be considered not to be associated with any connection tracking entry. Generating a reject response for a packet so classed would then terminate the healthy connection.

So, instead of:

-A INPUT ... -j REJECT

do consider using:

-A INPUT ... -m conntrack --ctstate INVALID -j DROP
-A INPUT ... -j REJECT

💀 REJECT (IPv4-specific) 💀

--reject-with type — the type given can be icmp-net-unreachable, icmp-host-unreachable, icmp-port-unreachable, icmp-proto-unreachable, icmp-net-prohibited, icmp-host-prohibited, or icmp-admin-prohibited (*), which return the appropriate ICMP error message (icmp-port-unreachable is the default). The option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won’t accept your mail otherwise).

(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT.

⚠️ Warning: You should not indiscriminately apply the REJECT target to packets whose connection state is classified as INVALID; instead, you should only DROP these.

So, instead of:

-A INPUT ... -j REJECT

do consider using:

-A INPUT ... -m conntrack --ctstate INVALID -j DROP
-A INPUT ... -j REJECT

⭐ SECMARK ⭐

This is used to set the security mark value associated with the packet for use by security subsystems such as SELinux. It is valid in the security table (for backwards compatibility with older kernels, it is also valid in the mangle table). The mark is 32 bits wide.

--selctx security_context — sets the security context value.

SET 🎯

Okay so the SET module is the one that literally adds and/or deletes entries from IP sets, and those sets can be defined by ipset(8). It’s giving “let the set handle the drama,” and I’m obsessed. 💅

--add-set setname flag[,flag...] — add the address(es)/port(s) of the packet to the set ✨
--del-set setname flag[,flag...] — delete the address(es)/port(s) of the packet from the set 🫶
--map-set setname flag[,flag...] — with optional [--map-mark] [--map-prio] [--map-queue], this maps packet properties (firewall mark, tc priority, hardware queue) 💖
--timeout value — when adding an entry, the timeout value to use instead of the default one from the set definition ⏳
--exist — when adding an entry, if it already exists, reset the timeout value to the specified one or to the default from the set definition 🔁
--map-set set-name — the set-name should be created with the --skbinfo option. Then:
- --map-mark — map firewall mark to packet by lookup of value in the set
- --map-prio — map traffic control priority to packet by lookup of value in the set
- --map-queue — map hardware NIC queue to packet by lookup of value in the set

Oh and real quick: where flag(s) are src and/or dst specifications, and there can be no more than six of them. No cap, six max. 🙈

The --map-set option can be used from the mangle table only. The --map-prio and --map-queue flags can be used in the OUTPUT, FORWARD and POSTROUTING chains.

Use of -j SET requires that ipset kernel support is provided, which, for standard kernels, is the case since Linux 2.6.39. 💫

SNAT 🌈

Heads up, bestie: SNAT is only valid in the nat table, in the POSTROUTING and INPUT chains, and user-defined chains which are only called from those chains.

It specifies that the source address of the packet should be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes the following options:

--to-source [ipaddr[-ipaddr]][:port[-port]] — which can specify a single new source IP address, an inclusive range of IP addresses. Optionally a port range, if the rule also specifies one of the following protocols: tcp, udp, dccp or sctp. If no port range is specified, then source ports below 512 will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be mapped to 1024 or above. Where possible, no port alteration will occur. 🎀
--random — randomize source port mapping through a hash-based algorithm (kernel >= 2.6.21) 🎲
--random-fully — fully randomize source port mapping through a PRNG (kernel >= 3.14) ✨
--persistent — gives a client the same source-/destination-address for each connection. This supersedes the SAME target. Support for persistent mappings is available from 2.6.29-rc2. 💖

Kernels prior to 2.6.36-rc1 don’t have the ability to SNAT in the INPUT chain. 😭

IPv6 support available since Linux kernels >= 3.7. 🦋

SNPT (IPv6-specific) 💜

Provides stateless source IPv6-to-IPv6 Network Prefix Translation (as described by RFC 6296).

You have to use this target in the mangle table, not in the nat table. It takes the following options:

--src-pfx [prefix/length] — set source prefix that you want to translate and length 🌸
--dst-pfx [prefix/length] — set destination prefix that you want to use in the translation and length 🎀

You have to use the DNPT target to undo the translation. Example:

ip6tables -t mangle -I POSTROUTING -s fd00::/64 ! -o vboxnet0 -j SNPT --src-pfx fd00::/64 --dst-pfx 2001:e20:2000:40f::/64
ip6tables -t mangle -I PREROUTING -i wlan0 -d 2001:e20:2000:40f::/64 -j DNPT --src-pfx 2001:e20:2000:40f::/64 --dst-pfx fd00::/64

You may need to enable IPv6 neighbor proxy:

sysctl -w net.ipv6.conf.all.proxy_ndp=1

You also have to use the NOTRACK target to disable connection tracking for translated flows. 🔥

SYNPROXY 🧚

This target will process TCP three-way-handshake parallel in netfilter context to protect either local or backend system. This target requires connection tracking because sequence numbers need to be translated. The kernels ability to absorb SYNFLOOD was greatly improved starting with Linux 4.4, so this target should not be needed anymore to protect Linux servers. 💀

--mss maximum segment size — maximum segment size announced to clients. This must match the backend. 📏
--wscale window scale — window scale announced to clients. This must match the backend. 🪟
--sack-perm — pass client selective acknowledgement option to backend (will be disabled if not present) ✅
--timestamps — pass client timestamp option to backend (will be disabled if not present, also needed for selective acknowledgement and window scaling) ⏱️

Example:

Determine tcp options used by backend, from an external system

tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)' port 80 &
telnet 192.0.2.42 80
18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
    Flags [S.], seq 360414582, ack 788841994, win 14480,
    options [mss 1460,sackOK,
    TS val 1409056151 ecr 9690221,
    nop,wscale 9],
    length 0

Switch tcp_loose mode off, so conntrack will mark out-of-flow packets as state INVALID.

echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose

Make SYN packets untracked

iptables -t raw -A PREROUTING -i eth0 -p tcp --dport 80 --syn -j CT --notrack

Catch UNTRACKED (SYN packets) and INVALID (3WHS ACK packets) states and send them to SYNPROXY. This rule will respond to SYN packets with SYN+ACK syncookies, create ESTABLISHED for valid client response (3WHS ACK packets) and drop incorrect cookies. Flags combinations not expected during 3WHS will not match and continue (e.g. SYN+FIN, SYN+ACK).

iptables -A INPUT -i eth0 -p tcp --dport 80 -m state --state UNTRACKED,INVALID -j SYNPROXY --sack-perm --timestamp --mss 1460 --wscale 9

Drop invalid packets, this will be out-of-flow packets that were not matched by SYNPROXY.

iptables -A INPUT -i eth0 -p tcp --dport 80 -m state --state INVALID -j DROP

TCPMSS 🍓

This target alters the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing interface’s MTU minus 40 for IPv4 or 60 for IPv6, respectively). Of course, it can only be used in conjunction with -p tcp. 🧵

This target is used to overcome criminally braindead ISPs or servers which block “ICMP Fragmentation Needed” or “ICMPv6 Packet Too Big” packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets:

Web browsers connect, then hang with no data received. 😭
Small mail works fine, but large emails hang. 📧
ssh works fine, but scp hangs after initial handshaking. 💀

Workaround: activate this option and add a rule to your firewall configuration like:

iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

--set-mss value — explicitly sets MSS option to specified value. If the MSS of the packet is already lower than value, it will not be increased (from Linux 2.6.25 onwards) to avoid more problems with hosts relying on a proper MSS. 📐
--clamp-mss-to-pmtu — automatically clamp MSS value to (path_MTU - 40 for IPv4; -60 for IPv6). This may not function as desired where asymmetric routes with differing path MTU exist — the kernel uses the path MTU which it would use to send packets from itself to the source and destination IP addresses. Prior to Linux 2.6.25, only the path MTU to the destination IP address was considered by this option; subsequent kernels also consider the path MTU to the source IP address. 🛰️

These options are mutually exclusive. Periodt. 💅

TCPOPTSTRIP ✂️

This target will strip TCP options off a TCP packet. (It will actually replace them by NO-OPs.) As such, you will need to add the -p tcp parameters. 🧼

--strip-options option[,option...] — strip the given option(s). The options may be specified by TCP option number or by symbolic name. The list of recognized options can be obtained by calling iptables with -j TCPOPTSTRIP -h. 👑

TEE 🪞

The TEE target will clone a packet and redirect this clone to another machine on the local network segment. In other words, the nexthop must be the target, or you will have to configure the nexthop to forward it further if so desired. ✨

--gateway ipaddr — send the cloned packet to the host reachable at the given IP address. Use of 0.0.0.0 (for IPv4 packets) or :: (IPv6) is invalid. 🚫

To forward all incoming traffic on eth0 to an Network Layer logging box:

-t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1

TOS 🎚️

This module sets the Type of Service field in the IPv4 header (including the “precedence” bits) or the Priority field in the IPv6 header. Note that TOS shares the same bits as DSCP and ECN. The TOS target is only valid in the mangle table. 💖

--set-tos value[/mask] — zeroes out the bits given by mask (see NOTE below) and XORs value into the TOS/Priority field. If mask is omitted, 0xFF is assumed. 🎀
--set-tos symbol — you can specify a symbolic name when using the TOS target for IPv4. It implies a mask of 0xFF (see NOTE below). The list of recognized TOS names can be obtained by calling iptables with -j TOS -h. 🌟

The following mnemonics are available:

--and-tos bits — binary AND the TOS value with bits. (Mnemonic for --set-tos 0/invbits, where invbits is the binary negation of bits. See NOTE below.) 🔗
--or-tos bits — binary OR the TOS value with bits. (Mnemonic for --set-tos bits/bits. See NOTE below.) ➕
--xor-tos bits — binary XOR the TOS value with bits. (Mnemonic for --set-tos bits/0. See NOTE below.) 🔀

NOTE: In Linux kernels up to and including 2.6.38, with the exception of longterm releases 2.6.32 (>=.42), 2.6.33 (>=.15), and 2.6.35 (>=.14), there is a bug whereby IPv6 TOS mangling does not behave as documented and differs from the IPv4 version. The TOS mask indicates the bits one wants to zero out, so it needs to be inverted before applying it to the original TOS field. However, the aforementioned kernels forgo the inversion which breaks --set-tos and its mnemonics. 😭

TPROXY 💅

okay so this one is lowkey picky about where it lives: it's only valid in the mangle table, in the PREROUTING chain — and in user-defined chains that are only called from that chain. anywhere else? she will not show up. periodt.

what she does: redirects the packet to a local socket without changing the packet header in any way ✨. she can also change the mark value, which you can then use in advanced routing rules. she takes three options:

--on-port port — tells her which destination port to use. this one is a required option, and 0 means the new destination port is the same as the original. only valid if the rule also specifies -p tcp or -p udp 🫶
--on-ip address — tells her which destination address to use. by default the address is the IP address of the incoming interface. also only valid if the rule also specifies -p tcp or -p udp
--tproxy-mark value[/mask] — marks packets with the given value/mask. the fwmark value set here can be used by advanced routing. (Required for transparent proxying to work: otherwise these packets will get forwarded, which is probably not what you want.) 😭 girl, set the mark. set it.

TRACE 🔍

this target marks packets so that the kernel will log every rule which match the packets as those traverse the tables, chains, rules. she's your little stalker cam for a packet's whole journey ✨. only usable in the raw table, no exceptions.

with iptables-legacy, a logging backend — such as ip(6)t_LOG or nfnetlink_log — must be loaded for this to be visible 👀. the packets get logged with the string prefix:

"TRACE: tablename:chainname:type:rulenum "

where type can be "rule" for plain rule, "return" for implicit rule at the end of a user defined chain, and "policy" for the policy of the built in chains. iykyk 🧚

with iptables-nft, the target is translated into nftables' meta nftrace expression. hence the kernel sends trace events via netlink to userspace, where they may be displayed using the xtables-monitor --trace command 🦋. for details, refer to xtables-monitor(8).

TTL (IPv4-specific) ⏳

this one modifies the IPv4 TTL header field. the TTL field determines how many hops (routers) a packet can traverse until it's time to live is exceeded, aka the packet's little countdown timer 💀.

⚠️ Setting or incrementing the TTL field can potentially be very dangerous, so it should be avoided at any cost. this target is only valid in mangle table.

🚨 Don't ever set or increment the value on packets that leave your local network! like, ever. i'm so serious. no cap.

--ttl-set value — Set the TTL value to value.
--ttl-dec value — Decrement the TTL value value times.
--ttl-inc value — Increment the TTL value value times.

ULOG (IPv4-specific) 🪦

okay bestie, this is the deprecated IPv4-only predecessor of the NFLOG target 🙈. she's giving legacy. she provides userspace logging of matching packets — when this target is set for a rule, the Linux kernel will multicast this packet through a netlink socket. one or more userspace processes may then subscribe to various multicast groups and receive the packets 📬. like LOG, this is a "non-terminating target", i.e. rule traversal continues at the next rule.

--ulog-nlgroup nlgroup — specifies the netlink group (1–32) to which the packet is sent. Default value is 1.
--ulog-prefix prefix — Prefix log messages with the specified prefix; up to 32 characters long, and useful for distinguishing messages in the logs 🏷️
--ulog-cprange size — Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default is 0.
--ulog-qthreshold size — Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility). 🍓

and that's the whole page, bestie!! 🎀 you made it to the end, i'm so proud, genuinely 🫶 this is iptables 1.8.13, and the date is right there in the footer with the iptables-extensions(8) tag — go look, she's stamped and everything 👑✨ slay, go filter some packets responsibly 💖

📦 where this page comes from

alpine 3.24: iptables-doc 1.8.13-r0 · GPL-2.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-06: a language model's rewrite of the original, under its terms