IPVSADM(8)

IPVSADM(8) — Linux Virtual Server administration 💅

okay so bestie, this is the ipvsadm(8) page from the Linux Administrator's Guide, and it's literally the tool for running the Linux Virtual Server show. No sign-off here — this is part 1, and we're just getting started. 🫶

💖 NAME 💖

ipvsadm — Linux Virtual Server administration. That's the name, that's the vibe. ✨

✨ SYNOPSIS ✨

ipvsadm -A|E virtual-service [-s scheduler]
        [-p [timeout]] [-M netmask] [-b sched-flags]
ipvsadm -D virtual-service
ipvsadm -C
ipvsadm -R
ipvsadm -S [-n]
ipvsadm -a|e virtual-service -r server-address
        [-g|i|m] [-w weight] [-x upper] [-y lower]
ipvsadm -d virtual-service -r server-address
ipvsadm -L|l [virtual-service] [options]
ipvsadm -Z [virtual-service]
ipvsadm --set tcp tcpfin udp
ipvsadm --start-daemon state [daemon-options]
        [--syncid syncid]
ipvsadm --stop-daemon state
ipvsadm -h

🌸 DESCRIPTION 🌸

Ipvsadm(8) is used to set up, maintain or inspect the virtual server table in the Linux kernel. The Linux Virtual Server can be used to build scalable network services based on a cluster of two or more nodes. The active node of the cluster redirects service requests to a collection of server hosts that will actually perform the services.

Supported features include three protocols (TCP, UDP and SCTP), three packet-forwarding methods (NAT, tunneling, and direct routing), and eight load balancing algorithms (round robin, weighted round robin, least-connection, weighted least-connection, locality-based least-connection, locality-based least-connection with replication, destination-hashing, and source-hashing). It's giving very "cluster main character energy," ngl. 👑

The command has two basic formats for execution:

ipvsadm COMMAND virtual-service
        [scheduling-method] [persistence options]

ipvsadm command virtual-service
        server-address [packet-forwarding-method]
        [weight options]

The first format manipulates a virtual service and the algorithm for assigning service requests to real servers. Optionally, a persistent timeout and network mask for the granularity of a persistent service and a persistence engine may be specified.

The second format manipulates a real server that is associated with an existing virtual service. When specifying a real server, the packet-forwarding method and the weight of the real server, relative to other real servers for the virtual service, may be specified, otherwise defaults will be used.

🎀 COMMANDS 🎀

ipvsadm(8) recognises the commands described below. Upper-case commands maintain virtual services. Lower-case commands maintain real servers that are associated with a virtual service. yes bestie, the capitalization is doing real work here. 💅

• -A, --add-service — Add a virtual service. A service address is uniquely defined by a triplet: IP address, port number, and protocol. Alternatively, a virtual service may be defined by a firewall-mark.
• -E, --edit-service — Edit a virtual service.
• -D, --delete-service — Delete a virtual service, along with any associated real servers.
• -C, --clear — Clear the virtual server table.
• -R, --restore — Restore Linux Virtual Server rules from stdin. Each line read from stdin will be treated as the command line options to a separate invocation of ipvsadm. Lines read from stdin can optionally begin with "ipvsadm". This option is useful to avoid executing a large number of ipvsadm commands when constructing an extensive routing table.
• -S, --save — Dump the Linux Virtual Server rules to stdout in a format that can be read by -R|--restore.
• -a, --add-server — Add a real server to a virtual service.
• -e, --edit-server — Edit a real server in a virtual service.
• -d, --delete-server — Remove a real server from a virtual service.
• -L, -l, --list — List the virtual server table if no argument is specified. If a service-address is selected, list this service only. If the -c option is selected, then display the connection table. The exact output is affected by the other arguments given.
• -Z, --zero — Zero the packet, byte and rate counters in a service or all services.
• --set tcp tcpfin udp — Change the timeout values used for IPVS connections. This command always takes 3 parameters, representing the timeout values (in seconds) for TCP sessions, TCP sessions after receiving a FIN packet, and UDP packets, respectively. A timeout value 0 means that the current timeout value of the corresponding entry is preserved.
• --start-daemon state — Start the connection synchronization daemon. The state is to indicate that the daemon is started as master or backup. The connection synchronization daemon is implemented inside the Linux kernel. The master daemon running at the primary load balancer multicasts changes of connections periodically, and the backup daemon running at the backup load balancers receives multicast message and creates corresponding connections. Then, in case the primary load balancer fails, a backup load balancer will takeover, and it has state of almost all connections, so that almost all established connections can continue to access the service.

The sync daemon supports IPv4 and IPv6 connections. 🦋

• --stop-daemon — Stop the connection synchronization daemon.
• -h, --help — Display a description of the command syntax.

🦋 virtual-service 🦋

Specifies the virtual service based on protocol/addr/port or firewall mark.

• -t, --tcp-service service-address — Use TCP service. The service-address is of the form host[:port]. Host may be one of a plain IP address or a hostname. Port may be either a plain port number or the service name of port. The Port may be omitted, in which case zero will be used. A Port of zero is only valid if the service is persistent as the -p|--persistent option, in which case it is a wild-card port, that is connections will be accepted to any port.
• -u, --udp-service service-address — Use UDP service. See the -t|--tcp-service for the description of the service-address.
• --sctp-service service-address — Use SCTP service. See the -t|--tcp-service for the description of the service-address.
• -f, --fwmark-service integer — Use a firewall-mark, an integer value greater than zero, to denote a virtual service instead of an address, port and protocol (UDP, TCP or SCTP). The marking of packets with a firewall-mark is configured using the -m|--mark option to iptables(8), the meta mark set value option to nft(8) or via an eBPF program. It can be used to build a virtual service associated with the same real servers, covering multiple IP address, port and protocol triplets. If IPv6 addresses are used, the -6 option must be used.

Using firewall-mark virtual services provides a convenient method of grouping together different IP addresses, ports and protocols into a single virtual service. This is useful for both simplifying configuration if a large number of virtual services are required and grouping persistence across what would otherwise be multiple virtual services. lowkey iconic. 🎀

💅 PARAMETERS 💅

The commands above accept or require zero or more of the following parameters.

• -s, --scheduler scheduling-method — scheduling-method Algorithm for allocating TCP connections and UDP datagrams to real servers. Scheduling algorithms are implemented as kernel modules. Ten are shipped with the Linux Virtual Server:
  • rr — Round Robin: distributes jobs equally amongst the available real servers.
  • wrr — Weighted Round Robin: assigns jobs to real servers proportionally to their real servers' weight. Servers with higher weights receive new jobs first and get more jobs than servers with lower weights. Servers with equal weights get an equal distribution of new jobs.
  • lc — Least-Connection: assigns more jobs to real servers with fewer active jobs.
  • wlc — Weighted Least-Connection: assigns more jobs to servers with fewer jobs and relative to the real servers' weight (Ci/Wi). This is the default.
  • lblc — Locality-Based Least-Connection: assigns jobs destined for the same IP address to the same server if the server is not overloaded and available; otherwise assign jobs to servers with fewer jobs, and keep it for future assignment.
  • lblcr — Locality-Based Least-Connection with Replication: assigns jobs destined for the same IP address to the least-connection node in the server set for the IP address. If all the node in the server set are over loaded, it picks up a node with fewer jobs in the cluster and adds it in the server set for the target. If the server set has not been modified for the specified time, the most loaded node is removed from the server set, in order to avoid high degree of replication.
  • dh — Destination Hashing: assigns jobs to servers through looking up a statically assigned hash table by their destination IP addresses.
  • sh — Source Hashing: assigns jobs to servers through looking up a statically assigned hash table by their source IP addresses. This scheduler has two flags: sh-fallback, which enables fallback to a different server if the selected server was unavailable, and sh-port, which adds the source port number to the hash computation.
  • sed — Shortest Expected Delay: assigns an incoming job to the server with the shortest expected delay. The expected delay that the job will experience is (Ci + 1) / Ui if sent to the ith server, in which Ci is the number of jobs on the ith server and Ui is the fixed service rate (weight) of the ith server.
  • nq — Never Queue: assigns an incoming job to an idle server if there is, instead of waiting for a fast one; if all the servers are busy, it adopts the Shortest Expected Delay policy to assign the job.
  • fo — Weighted Failover: assigns an incoming job to the server with the highest weight that is currently available.
  • ovf — Weighted Overflow: assigns an incoming job to the server with the highest weight that is currently available and overflows to the next when active connections exceed the node's weight. Note that this scheduler might not be suitable for UDP because it only uses active connections.
• mh — Maglev Hashing 💅 assigns incoming jobs based on Google's Maglev hashing algorithm, giving each real server an almost equal share of jobs while keeping disruption minimal. When the set of real servers changes, a connection will likely still get sent to the same real server as it was before. This scheduler has two flags: mh-fallback, which enables fallback to a different server if the selected server was unavailable, and mh-port, which adds the source port number to the hash computation.
• -p, --persistent [timeout] — says this virtual service is persistent ✨. Multiple requests from a client get redirected to the same real server that was picked for the first request. You can optionally give the timeout of persistent sessions in seconds; otherwise the default of 300 seconds is used. This option may be used with protocols like SSL or FTP where it's important that clients consistently connect with the same real server.

🚨 Note: if a virtual service is going to handle FTP connections, then persistence must be set for the virtual service if Direct Routing or Tunnelling is used as the forwarding mechanism. If Masquerading is used with an FTP service, then persistence is not necessary, but the ip_vs_ftp kernel module must be used. This module may be manually inserted into the kernel using insmod(8).

• -M, --netmask netmask — sets the granularity for grouping clients on persistent virtual services 🫶. The source address of the request gets masked with this netmask so all clients from a network go to the same real server. The default is 255.255.255.255, meaning the persistence granularity is per client host. Less specific netmasks may be used to resolve problems with non-persistent cache clusters on the client side. IPv6 netmasks should be specified as a prefix length between 1 and 128. The default prefix length is 128.
• --pe persistence-engine — picks an alternative persistence engine. RN the only alternative persistence engine available is sip. 🍓
• -b, --sched-flags sched-flags — sets scheduler flags for this virtual server. sched-flags is a comma-separated list of flags. See the scheduler descriptions for valid scheduler flags. 💜
• -r, --real-server server-address — the real server that an associated request for service may be assigned to. The server-address is the host address of a real server, and may plus port. Host can be either a plain IP address or a hostname. Port can be either a plain port number or the service name of port. With the masquerading method, the host address is usually an RFC 1918 private IP address, and the port can be different from that of the associated service. With the tunneling and direct routing methods, port must be equal to that of the service address. For normal services, the port specified in the service address will be used if port is not specified. For fwmark services, port may be omitted, in which case the destination port on the real server will be the destination port of the request sent to the virtual service. 🔥

🌈 [packet-forwarding-method] 🌈

• -g, --gatewaying — use gatewaying (direct routing). This is the default. 👑
• -i, --ipip — use ipip encapsulation (tunneling). 🧚
• --tun-type tun-type — tun-type is one of ipip|gue|gre. The default value of tun-type is ipip.
• --tun-port tun-port — tun-port is an integer specifying the destination port. Only valid for tun-type gue.
• --tun-nocsum — specify that tunnel checksums are disabled. This is the default. Only valid for tun-type gue and gre.
• --tun-csum — specify that tunnel checksums are enabled. Only valid for tun-type gue and gre.
• --tun-remcsum — specify that Remote Checksum Offload is enabled. Only valid for tun-type gue.
• -m, --masquerading — use masquerading (network access translation, or NAT). 💅

👀 Note: regardless of the packet-forwarding mechanism specified, real servers for addresses for which there are interfaces on the local node will use the local forwarding method, then packets for the servers will be passed to upper layer on the local node. This cannot be specified by ipvsadm; rather, it is set by the kernel as real servers are added or modified.

• -w, --weight weight — weight is an integer specifying the capacity of a server relative to the others in the pool. Valid values of weight are 0 through to 65535. The default is 1. Quiescent servers are specified with a weight of zero. A quiescent server will receive no new jobs but still serve the existing jobs, for all scheduling algorithms distributed with the Linux Virtual Server. Setting a quiescent server may be useful if the server is overloaded or needs to be taken out of service for maintenance. 🛠️
• -x, --u-threshold uthreshold — uthreshold is an integer specifying the upper connection threshold of a server. Valid values of uthreshold are 0 through to 65535. The default is 0, which means the upper connection threshold is not set. If uthreshold is set with other values, no new connections will be sent to the server when the number of its connections exceeds its upper connection threshold. 🚦
• -y, --l-threshold lthreshold — lthreshold is an integer specifying the lower connection threshold of a server. Valid values of lthreshold are 0 through to 65535. The default is 0, which means the lower connection threshold is not set. If lthreshold is set with other values, the server will receive new connections when the number of its connections drops below its lower connection threshold. If lthreshold is not set but uthreshold is set, the server will receive new connections when the number of its connections drops below three forth of its upper connection threshold. 🌸
• -c, --connection — connection output. The list command with this option will list current IPVS connections.
• --timeout — timeout output. The list command with this option will display the timeout values (in seconds) for TCP sessions, TCP sessions after receiving a FIN packet, and UDP packets. ⏳
• --daemon — daemon information output. The list command with this option will display the daemon status and its multicast interface.
• --stats — output of statistics information. The list command with this option will display the statistics information of services and their servers. 📊
• --rate — output of rate information. The list command with this option will display the rate information (such as connections/second, bytes/second and packets/second) of services and their servers. ⚡
• --thresholds — output of thresholds information. The list command with this option will display the upper/lower connection threshold information of each server in service listing.
• --persistent-conn — output of persistent connection information. The list command with this option will display the persistent connection counter information of each server in service listing. The persistent connection is used to forward the actual connections from the same client/network to the same server. The list command with the -c, --connection option and this option will include persistence engine data, if any is present, when listing connections. 🔗
• --tun-info — output of tunneling information. The list command with this option will display the tunneling information of services and their servers.
• --sort — sort the list of virtual services and real servers. Virtual service entries are sorted in ascending order by <protocol, address, port>. Real server entries are sorted in ascending order by <address, port>. (default) ✨
• --nosort — do not sort the list of virtual services and real servers.
• -n, --numeric — numeric output. IP addresses and port numbers will be printed in numeric format rather than as host names and services respectively, which is the default. 🔢
• --exact — expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (multiples of 1000) M's (multiples of 1000K) or G's (multiples of 1000M). This option is only relevant for the -L command.
• -6, --ipv6 — use with -f to signify fwmark rule uses IPv6 addresses. 🌐
• -o, --ops — one-packet scheduling. Used with a UDP virtual service or a fwmark virtual service that handles only UDP packets. All connections are created such that they only schedule one packet. 🎀

🔄 PARAMETERS FOR SYNCHRONIZATION DAEMON 🔄

The --start-daemon requires zero or more of the following parameters.

• --syncid syncid — specify the syncid that the sync master daemon fills in the SyncID header while sending multicast messages, or that the sync backup daemon uses to filter out multicast messages not matched with the SyncID value. Valid values of syncid are 0 through to 255. The default is 0, which means no filtering at all. 🦋
• --sync-maxlen length — specify the desired length of sync messages (UDP payload size). It is expected that the backup server will use a value not less than the used value in the master server. Valid values of length are in the 1 .. (65535 - 20 - 8) range, but the kernel ensures a space for at least one sync message. If the value is lower than MTU, the sync messages will be fragmented by the IP layer. The default value is derived from the MTU value when the daemon is started, but the master daemon will not default to a value above 1500 for compatibility reasons. 📦
• --mcast-interface interface — specify the multicast interface that the sync master daemon sends outgoing multicasts through, or that the sync backup daemon listens to for multicasts. 🌈
• --mcast-group address — which multicast address the sync messages go to, IPv4 or IPv6. Default is 224.0.0.81 ✨
• --mcast-port port — which UDP port the sync messages use. Default is 8848 💌
• --mcast-ttl ttl — the TTL for sync messages, valid range is 1 .. 255. Default is 1 🎀

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🌸 EXAMPLE 1 - Simple Virtual Service 🌸

ok so these commands set up a Linux Director to spread incoming requests to port 80 on 207.175.44.110 equally across port 80 on five real servers 💅 the forwarding method here is NAT, and each real server gets masqueraded by the Linux Director. iconic behavior fr.

ipvsadm -A -t 207.175.44.110:80 -s rr
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.1:80 -m
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.2:80 -m
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.3:80 -m
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.4:80 -m
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.5:80 -m

Alternatively, this could be achieved in a single ipvsadm command 🫶

echo "
-A -t 207.175.44.110:80 -s rr
-a -t 207.175.44.110:80 -r 192.168.10.1:80 -m
-a -t 207.175.44.110:80 -r 192.168.10.2:80 -m
-a -t 207.175.44.110:80 -r 192.168.10.3:80 -m
-a -t 207.175.44.110:80 -r 192.168.10.4:80 -m
-a -t 207.175.44.110:80 -r 192.168.10.5:80 -m
" | ipvsadm -R

since masquerading is the forwarding mechanism in this example, the default route of the real servers has to be set to the linux director, and that director needs to be configured to forward and masquerade packets. you can do both of those with:

echo "1" > /proc/sys/net/ipv4/ip_forward

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🔥 EXAMPLE 2 - Firewall-Mark Virtual Service 🔥

these commands set up a Linux Director to distribute incoming requests addressed to any port on 207.175.44.110 or 207.175.44.111 equally to the corresponding port on five real servers 💖 same as the last example, the forwarding method is NAT, with each real server being masqueraded by the Linux Director.

ipvsadm -A -f 1  -s rr
ipvsadm -a -f 1 -r 192.168.10.1:0 -m
ipvsadm -a -f 1 -r 192.168.10.2:0 -m
ipvsadm -a -f 1 -r 192.168.10.3:0 -m
ipvsadm -a -f 1 -r 192.168.10.4:0 -m
ipvsadm -a -f 1 -r 192.168.10.5:0 -m

again, since masquerading is the forwarding mechanism, the real servers' default route must point at the linux director, which needs to be configured to forward and masquerade packets. also the real server should be set up to mark incoming packets addressed to any port on 207.175.44.110 and 207.175.44.111 with firewall-mark 1 🎯 and if FTP traffic is gonna be handled by this virtual service, then the ip_vs_ftp kernel module needs to be inserted into the kernel. all of that, in these commands:

echo "1" > /proc/sys/net/ipv4/ip_forward
modprobe ip_tables
iptables  -A PREROUTING -t mangle -d 207.175.44.110/31 -j MARK --set-mark 1
modprobe ip_vs_ftp

---

🦋 EXAMPLE 3 - Virtual Service with GUE Tunneling 🦋

these commands set up a Linux Director to distribute incoming requests to port 80 on 207.175.44.110 equally to port 80 on five real servers 🌈 the forwarding method here is tunneling with gue encapsulation. love that for us.

ipvsadm -A -t 207.175.44.110:80 -s rr
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.1:80 -i --tun-type gue --tun-port 6080 --tun-nocsum
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.2:80 -i --tun-type gue --tun-port 6080 --tun-csum
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.3:80 -i --tun-type gue --tun-port 6080 --tun-remcsum
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.4:80 -i --tun-type gue --tun-port 6078
ipvsadm -a -t 207.175.44.110:80 -r 192.168.10.5:80 -i --tun-type gue --tun-port 6079

Alternatively, this could be achieved in a single ipvsadm command ⭐

echo "
-A -t 207.175.44.110:80 -s rr
-a -t 207.175.44.110:80 -r 192.168.10.1:80 -i --tun-type gue --tun-port 6080 --tun-nocsum
-a -t 207.175.44.110:80 -r 192.168.10.2:80 -i --tun-type gue --tun-port 6080 --tun-csum
-a -t 207.175.44.110:80 -r 192.168.10.3:80 -i --tun-type gue --tun-port 6080 --tun-remcsum
-a -t 207.175.44.110:80 -r 192.168.10.4:80 -i --tun-type gue --tun-port 6078
-a -t 207.175.44.110:80 -r 192.168.10.5:80 -i --tun-type gue --tun-port 6079
" | ipvsadm -R

---

💜 EXAMPLE 4 - Virtual Service with GRE Tunneling 💜

these commands tell a Linux Director to use GRE encapsulation 👑

ipvsadm -A -t 10.0.0.1:80 -s rr
ipvsadm -a -t 10.0.0.1:80 -r 192.168.11.1:80 -i --tun-type gre --tun-csum

---

🌈 IPv6 🌈

IPv6 addresses should be surrounded by square brackets ([ and ]) — non-negotiable bestie 🎀

ipvsadm -A -t [2001:db8::80]:80 -s rr
ipvsadm -a -t [2001:db8::80]:80 -r [2001:db8::a0a0]:80 -m

fwmark IPv6 services require the -6 option 🫡

---

💀 NOTES 💀

ok so the Linux Virtual Server implements three defense strategies against some types of denial of service (DoS) attacks. here's the situation: the Linux Director creates an entry for each connection to keep its state, and each entry occupies 128 bytes effective memory. LVS's vulnerability to a DoS attack lies in the potential to increase the number of entries as much as possible until the linux director runs out of memory 😭

the three defense strategies against the attack are:

1. Randomly drop some entries in the table.
2. Drop 1/rate packets before forwarding them.
3. And use secure tcp state transition table and short timeouts.

the strategies are controlled by sysctl variables and corresponding entries in the /proc filesystem:

/proc/sys/net/ipv4/vs/drop_entry /proc/sys/net/ipv4/vs/drop_packet
/proc/sys/net/ipv4/vs/secure_tcp

valid values for each variable are 0 through to 3 ✨ the default value is 0, which disables the respective defense strategy. 1 and 2 are automatic modes — when there is no enough available memory, the respective strategy will be enabled and the variable is automatically set to 2, otherwise the strategy is disabled and the variable is set to 1. a value of 3 denotes that the respective strategy is always enabled 👑

the available memory threshold and secure TCP timeouts can be tuned using the sysctl variables and corresponding entries in the /proc filesystem:

/proc/sys/net/ipv4/vs/amemthresh /proc/sys/net/ipv4/vs/timeout_*

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🍓 FILES 🍓

these are the files, bestie, all of them, no cap:

/proc/net/ip_vs
/proc/net/ip_vs_app
/proc/net/ip_vs_conn
/proc/net/ip_vs_stats
/proc/sys/net/ipv4/vs/am_droprate
/proc/sys/net/ipv4/vs/amemthresh
/proc/sys/net/ipv4/vs/drop_entry
/proc/sys/net/ipv4/vs/drop_packet
/proc/sys/net/ipv4/vs/secure_tcp
/proc/sys/net/ipv4/vs/timeout_close
/proc/sys/net/ipv4/vs/timeout_closewait
/proc/sys/net/ipv4/vs/timeout_established
/proc/sys/net/ipv4/vs/timeout_finwait
/proc/sys/net/ipv4/vs/timeout_icmp
/proc/sys/net/ipv4/vs/timeout_lastack
/proc/sys/net/ipv4/vs/timeout_listen
/proc/sys/net/ipv4/vs/timeout_synack
/proc/sys/net/ipv4/vs/timeout_synrecv
/proc/sys/net/ipv4/vs/timeout_synsent
/proc/sys/net/ipv4/vs/timeout_timewait
/proc/sys/net/ipv4/vs/timeout_udp

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🧚 SEE ALSO 🧚

the LVS web site (http://www.linuxvirtualserver.org/) for more documentation about LVS 🌸

also go read ipvsadm-save(8), ipvsadm-restore(8), iptables(8), insmod(8), modprobe(8) — the whole friend group 💅

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👑 AUTHORS 👑

ipvsadm — Wensong Zhang <wensong@linuxvirtualserver.org> Peter Kese <peter.kese@ijs.si>

man page — Mike Wangsmo <wanger@redhat.com> Wensong Zhang <wensong@linuxvirtualserver.org> Horms <horms@verge.net.au>

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and that's the whole page, bestie, you ate that 💖 this one's from the 4th Berkeley Distribution, dated 5th July 2003, filed under IPVSADM(8) — go forth and load balance, ily 🎀✨🦋