IP(4)

💖 IP(4) — Internet Protocol 💖

from the Kernel Interfaces Manual — okay bestie, this is the whole ip situation, and we are gonna get every single crumb of it. 🫶

🎀 NAME 🎀

ip — Internet Protocol

✨ SYNOPSIS ✨

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int
socket(AF_INET, SOCK_RAW, proto);

🌈 DESCRIPTION 🌈

Okay so IP is the transport layer protocol used by the Internet protocol family. Options may be set at the IP level when using higher-level protocols that are based on IP (such as TCP and UDP). It may also be accessed through a “raw socket” when developing new protocols, or special-purpose applications. 💅

There are several IP-level setsockopt(2) and getsockopt(2) options. Let's go through them, because we are not skipping a single one:

• IP_OPTIONS — may be used to provide IP options to be transmitted in the IP header of each outgoing packet or to examine the header options on incoming packets. IP options may be used with any socket type in the Internet family. The format of IP options to be sent is that specified by the IP protocol specification (RFC-791), with one exception: the list of addresses for Source Route options must include the first-hop gateway at the beginning of the list of gateways. The first-hop gateway address will be extracted from the option list and the size adjusted accordingly before use. To disable previously specified options, use a zero-length buffer:

setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);

• IP_TOS — may be used to set the differential service codepoint (DSCP) and the explicit congestion notification (ECN) codepoint. Setting the ECN codepoint — the two least significant bits — on a socket using a transport protocol implementing ECN has no effect.
• IP_TTL — configures the time-to-live (TTL) field in the IP header for SOCK_STREAM, SOCK_DGRAM, and certain types of SOCK_RAW sockets. For example:

int tos = IPTOS_DSCP_EF;       /* see <netinet/ip.h> */
setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));

int ttl = 60;                   /* max = 255 */
setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

• IP_IPSEC_POLICY — controls IPSec policy for sockets. For example:

const char *policy = "in ipsec ah/transport//require";
char *buf = ipsec_set_policy(policy, strlen(policy));
setsockopt(s, IPPROTO_IP, IP_IPSEC_POLICY, buf, ipsec_get_policylen(buf));

• IP_MINTTL — may be used to set the minimum acceptable TTL a packet must have when received on a socket. All packets with a lower TTL are silently dropped. This option is only really useful when set to 255, preventing packets from outside the directly connected networks reaching local listeners on sockets.
• IP_DONTFRAG — may be used to set the Don't Fragment flag on IP packets. Currently this option is respected only on udp(4) and raw ip sockets, unless the IP_HDRINCL option has been set. On tcp(4) sockets, the Don't Fragment flag is controlled by the Path MTU Discovery option. Sending a packet larger than the MTU size of the egress interface, determined by the destination address, returns an EMSGSIZE error.
• IP_ORIGDSTADDR — if this option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the destination IP address and destination port for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the sockaddr_in structure. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(struct sockaddr_in))
cmsg_level = IPPROTO_IP
cmsg_type = IP_ORIGDSTADDR

• IP_RECVDSTADDR — if this option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the destination IP address for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the IP address. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR

• IP_SENDSRCADDR — the source address to be used for outgoing UDP datagrams on a socket can be specified as ancillary data with a type code of IP_SENDSRCADDR. The msg_control field in the msghdr structure should point to a buffer that contains a cmsghdr structure followed by the IP address. The cmsghdr fields should have the following values:

cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_SENDSRCADDR

The socket should be either bound to INADDR_ANY and a local port, and the address supplied with IP_SENDSRCADDR shouldn't be INADDR_ANY, or the socket should be bound to a local address and the address supplied with IP_SENDSRCADDR should be INADDR_ANY. In the latter case bound address is overridden via generic source address selection logic, which would choose IP address of interface closest to destination.

For convenience, IP_SENDSRCADDR is defined to have the same value as IP_RECVDSTADDR, so the IP_RECVDSTADDR control message from recvmsg(2) can be used directly as a control message for sendmsg(2). ✨

• IP_ONESBCAST — if this option is enabled on a SOCK_DGRAM or a SOCK_RAW socket, the destination address of outgoing broadcast datagrams on that socket will be forced to the undirected broadcast address, INADDR_BROADCAST, before transmission. This is in contrast to the default behavior of the system, which is to transmit undirected broadcasts via the first network interface with the IFF_BROADCAST flag set.

This option allows applications to choose which interface is used to transmit an undirected broadcast datagram. For example, the following code would force an undirected broadcast to be transmitted via the interface configured with the broadcast address 192.168.2.255:

char msg[512];
struct sockaddr_in sin;
int onesbcast = 1;      /* 0 = disable (default), 1 = enable */

setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
sin.sin_addr.s_addr = inet_addr("192.168.2.255");
sin.sin_port = htons(1234);
sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));

It is the application's responsibility to set the IP_TTL option to an appropriate value in order to prevent broadcast storms. The application must have sufficient credentials to set the SO_BROADCAST socket level option, otherwise the IP_ONESBCAST option has no effect. 😭

• IP_BINDANY — if this option is enabled on a SOCK_STREAM, SOCK_DGRAM or a SOCK_RAW socket, one can bind(2) to any address, even one not bound to any available network interface in the system. This functionality (in conjunction with special firewall rules) can be used for implementing a transparent proxy. The PRIV_NETINET_BINDANY privilege is needed to set this option.
• IP_RECVTTL — if this option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the IP TTL (time to live) field for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the TTL. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(u_char))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTTL

• IP_RECVTOS — if this option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the IP TOS (type of service) field for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the TOS. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(u_char))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTOS

• IP_RECVIF — if this option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call returns a struct sockaddr_dl corresponding to the interface on which the packet was received. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the struct sockaddr_dl. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(struct sockaddr_dl))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF

• IP_PORTRANGE — may be used to set the port range used for selecting a local port number on a socket with an unspecified (zero) port number. It has the following possible values:
  • IP_PORTRANGE_DEFAULT — use the default range of values, normally IPPORT_HIFIRSTAUTO through IPPORT_HILASTAUTO. This is adjustable through the sysctl setting: net.inet.ip.portrange.first and net.inet.ip.portrange.last.
  • IP_PORTRANGE_HIGH — use a high range of values, normally IPPORT_HIFIRSTAUTO and IPPORT_HILASTAUTO. This is adjustable through the sysctl setting: net.inet.ip.portrange.hifirst and net.inet.ip.portrange.hilast.
  • IP_PORTRANGE_LOW — use a low range of ports, which are normally restricted to privileged processes on Unix systems. The range is normally from IPPORT_RESERVED - 1 down to IPPORT_RESERVEDSTART in descending order. This is adjustable through the sysctl setting: net.inet.ip.portrange.lowfirst and net.inet.ip.portrange.lowlast.

The range of privileged ports which only may be opened by root-owned processes may be modified by the net.inet.ip.portrange.reservedlow and net.inet.ip.portrange.reservedhigh sysctl settings. The values default to the traditional range, 0 through IPPORT_RESERVED - 1 (0 through 1023), respectively. Note that these settings do not affect and are not accounted for in the use or calculation of the other net.inet.ip.portrange values above. Changing these values departs from Unix tradition and has security consequences that the administrator should carefully evaluate before modifying these settings. 🚨

Ports are allocated at random within the specified port range in order to increase the difficulty of random spoofing attacks. In scenarios such as benchmarking, this behavior may be undesirable. In these cases, net.inet.ip.portrange.randomized can be used to toggle randomization off.

🦋 Multicast Options 🦋

IP multicasting is supported only on AF_INET sockets of type SOCK_DGRAM and SOCK_RAW, and only on networks where the interface driver supports multicasting. 🌈

The IP_MULTICAST_TTL option changes the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts:

u_char ttl;     /* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));

Datagrams with a TTL of 1 are not forwarded beyond the local network. Multicast datagrams with a TTL of 0 will not be transmitted on any network, but may be delivered locally if the sending host belongs to the destination group and if multicast loopback has not been disabled on the sending socket (see below). Multicast datagrams with TTL greater than 1 may be forwarded to other networks if a multicast router is attached to the local network. 💜

Okay so picking right back up where we were, bestie 💖

For hosts rockin' multiple interfaces, where an interface has not been specified for a multicast group membership, every multicast transmission gets sent from the primary network interface. The IP_MULTICAST_IF option overrides that default for subsequent transmissions from a given socket:

struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));

where "addr" is the local IP address of the interface you want, or INADDR_ANY to say "default interface, pls" ✨

Wanna specify an interface by index instead? Pass an instance of ip_mreqn and the imr_ifindex member should be set to the index of the desired interface, or 0 to specify the default interface. The kernel tells these two structs apart by their size — iykyk 🫶

Ngl, IP_MULTICAST_IF is not recommended. Multicast memberships are scoped to each individual interface, so it's only supported for legacy use by applications like routing daemons, which expect to be able to transmit link-local IPv4 multicast datagrams (224.0.0.0/24) on multiple interfaces without requesting an individual membership for each interface. It's giving "we keep it around for the oldheads" 💅

An interface's local IP address and multicast capability can be obtained via the SIOCGIFCONF and SIOCGIFFLAGS ioctls. Normal applications should not need to use this option. Periodt.

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If a multicast datagram is sent to a group that the sending host itself belongs to (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. The IP_MULTICAST_LOOP option gives the sender explicit control over whether or not subsequent datagrams get looped back:

u_char loop;    /* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));

This option improves performance for applications that may have no more than one instance on a single host (such as a routing daemon), by eliminating the overhead of receiving their own transmissions. It should generally not be used by applications for which there may be more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time querying program). Like, read the room, bestie 🙈

The sysctl setting net.inet.ip.mcast.loop controls the default setting of the IP_MULTICAST_LOOP socket option for new sockets. Cute default, right? 💜

A multicast datagram sent with an initial TTL greater than 1 may be delivered to the sending host on a different interface from the one it was sent on, if the host belongs to the destination group on that other interface. The loopback control option has no effect on such delivery. So don't come for it when it shows up somewhere else 🌈

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A host must become a member of a multicast group before it can receive datagrams sent to the group. To join a multicast group, use the IP_ADD_MEMBERSHIP option:

struct ip_mreqn mreqn;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreqn, sizeof(mreqn));

where mreqn is the following structure:

struct ip_mreqn {
    struct in_addr imr_multiaddr; /* IP multicast address of group */
    struct in_addr imr_interface; /* local IP address of interface */
    int            imr_ifindex;   /* interface index */
}

imr_ifindex should be set to the index of a particular multicast-capable interface if the host is multihomed. If imr_ifindex is non-zero, the value of imr_interface is ignored. Otherwise, if imr_ifindex is 0, the kernel will use the IP address from imr_interface to look up the interface. The value of imr_interface may be set to INADDR_ANY to choose the default interface, although this is not recommended; this is considered to be the first interface corresponding to the default route. Otherwise, the first multicast-capable interface configured in the system will be used. Slay, we love a fallback chain 💅

The legacy struct ip_mreq, which lacks the imr_ifindex field, is also supported by the IP_ADD_MEMBERSHIP setsockopt. In this case the kernel would behave as if imr_ifindex was set to zero: imr_interface will be used to look up the interface. Old school but still valid, no cap 🎀

History lesson time 👑 Prior to FreeBSD 7.0, if the imr_interface member is within the network range 0.0.0.0/8, it is treated as an interface index in the system interface MIB, as per the RIP Version 2 MIB Extension (RFC-1724). In versions of FreeBSD since 7.0, this behavior is no longer supported. Developers should instead use the RFC 3678 multicast source filter APIs; in particular, MCAST_JOIN_GROUP.

Up to IP_MAX_MEMBERSHIPS memberships may be added on a single socket. Membership is associated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. Multitasking queen behavior 💖

To drop a membership, use:

struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));

where mreq contains the same values as used to add the membership. Memberships are dropped when the socket is closed or the process exits. It's giving "you can leave whenever, we won't be dramatic about it" 🦋

The IGMP protocol uses the primary IP address of the interface as its identifier for group membership. This is the first IP address configured on the interface. If this address is removed or changed, the results are undefined, as the IGMP membership state will then be inconsistent. If multiple IP aliases are configured on the same interface, they will be ignored. Lowkey chaotic, so pls be careful 😭

This shortcoming was addressed in IPv6; MLDv2 requires that the unique link-local address for an interface is used to identify an MLDv2 listener. Glow-up of the century tbh ⭐

✨ Source-Specific Multicast Options ✨

Since FreeBSD 8.0, the use of Source-Specific Multicast (SSM) is supported. These extensions require an IGMPv3 multicast router in order to make best use of them. If a legacy multicast router is present on the link, FreeBSD will simply downgrade to the version of IGMP spoken by the router, and the benefits of source filtering on the upstream link will not be present, although the kernel will continue to squelch transmissions from blocked sources. Adaptive icon behavior, honestly 👑

Each group membership on a socket now has a filter mode:

• MCAST_EXCLUDE — Datagrams sent to this group are accepted, unless the source is in a list of blocked source addresses.
• MCAST_INCLUDE — Datagrams sent to this group are accepted only if the source is in a list of accepted source addresses.

Groups joined using the legacy IP_ADD_MEMBERSHIP option are placed in exclusive-mode, and are able to request that certain sources are blocked or allowed. This is known as the delta-based API. Fancy name, simple vibe 🧚

To block a multicast source on an existing group membership:

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_BLOCK_SOURCE, &mreqs, sizeof(mreqs));

where mreqs is the following structure:

struct ip_mreq_source {
    struct in_addr imr_multiaddr; /* IP multicast address of group */
    struct in_addr imr_sourceaddr; /* IP address of source */
    struct in_addr imr_interface; /* local IP address of interface */
}

imr_sourceaddr should be set to the address of the source to be blocked. Bye bestie, blocked 🚫💅

To unblock a multicast source on an existing group:

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_UNBLOCK_SOURCE, &mreqs, sizeof(mreqs));

Heads up: the IP_BLOCK_SOURCE and IP_UNBLOCK_SOURCE options are not permitted for inclusive-mode group memberships. Different mode, different rules, fr fr.

To join a multicast group in MCAST_INCLUDE mode with a single source, or add another source to an existing inclusive-mode membership:

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_ADD_SOURCE_MEMBERSHIP, &mreqs, sizeof(mreqs));

To leave a single source from an existing group in inclusive mode:

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_DROP_SOURCE_MEMBERSHIP, &mreqs, sizeof(mreqs));

If this is the last accepted source for the group, the membership will be dropped. Last one out, turn off the lights 🕯️

The IP_ADD_SOURCE_MEMBERSHIP and IP_DROP_SOURCE_MEMBERSHIP options are not accepted for exclusive-mode group memberships. However, both exclusive and inclusive mode memberships support the use of the full-state API documented in RFC 3678. For management of source filter lists using this API, please refer to sourcefilter(3). Go read it, it's good for you 🫶

The sysctl settings net.inet.ip.mcast.maxsocksrc and net.inet.ip.mcast.maxgrpsrc are used to specify an upper limit on the number of per-socket and per-group source filter entries which the kernel may allocate. Budget your filter entries accordingly, bestie 💀

🌸 Raw IP Sockets 🌸

Raw IP sockets are connectionless, and are normally used with the sendto(2) and recvfrom(2) calls, though the connect(2) call may also be used to fix the destination for future packets (in which case the read(2) or recv(2) and write(2) or send(2) system calls may be used). Two paths, both iconic ✨

If proto is 0, the default protocol IPPROTO_RAW is used for outgoing packets, and only incoming packets destined for that protocol are received. If proto is non-zero, that protocol number will be used on outgoing packets and to filter incoming packets. Simple as that 🎀

Outgoing packets automatically have an IP header prepended to them (based on the destination address and the protocol number the socket is created with), unless the IP_HDRINCL option has been set. Unlike in previous BSD releases, incoming packets are received with the IP header and options intact, leaving all fields in network byte order. We love a glow-up 🌈

IP_HDRINCL indicates the complete IP header is included with the data and may be used only with the SOCK_RAW type.

#include <netinet/in_systm.h>
#include <netinet/ip.h>

int hincl = 1;                  /* 1 = on, 0 = off */
setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));

Unlike previous BSD releases, the program must set all the fields of the IP header, including the following:

ip->ip_v = IPVERSION;
ip->ip_hl = hlen >> 2;
ip->ip_id = 0;  /* 0 means kernel set appropriate value */
ip->ip_off = htons(offset);
ip->ip_len = htons(len);

The packet should be provided as is to be sent over wire. This implies all fields, including ip_len and ip_off, to be in network byte order. See byteorder(3) for more information on network byte order. If the ip_id field is set to 0 then the kernel will choose an appropriate value. If the header source address is set to INADDR_ANY, the kernel will choose an appropriate address. So like... set your fields, bestie, the kernel's not gonna do your homework for you 😭💖

💀 ERRORS

okay so sometimes a socket operation just fails, and here's the whole receipt of why. each one of these bad boys can come back at you:

• [EISCONN] — this hits when you're trying to establish a connection on a socket that literally already has one (bestie, it's taken 💅), or when you're trying to send a datagram with the destination address specified and the socket is already connected.
• [ENOTCONN] — the opposite vibe 😭 you're trying to send a datagram, but no destination address is specified, and the socket has not been connected. like, where is it even supposed to go??
• [ENOBUFS] — the system ran out of memory for an internal data structure. not you, it's them. still a fail tho.
• [EADDRNOTAVAIL] — you tried to create a socket with a network address for which no network interface exists. the address is giving imaginary friend energy 🫠
• [EACCES] — a non-privileged process tried to create a raw IP socket. nope, that's a privileged-only club, sorry bestie 🚫

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then there's the IP-specific crew 👑 — these can pop off when you're setting or getting IP options:

• [EINVAL] — an unknown socket option name was given. didn't recognize it, didn't respect it.
• [EINVAL] — the IP option field was improperly formed ✨ an option field was shorter than the minimum value or longer than the option buffer provided. it's giving "close but not quite."

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and the final boss 😤 these can happen when you're attempting to send IP datagrams via a “raw socket” with the IP_HDRINCL option set:

• [EINVAL] — the user-supplied ip_len field was not equal to the length of the datagram written to the socket. the math did NOT math, and the kernel noticed 💀

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🌈 SEE ALSO

go read these next, they're literally all connected, iykyk:

• getsockopt(2)
• recv(2)
• send(2)
• byteorder(3)
• CMSG_DATA(3)
• sourcefilter(3)
• icmp(4)
• igmp(4)
• inet(4)
• intro(4)
• multicast(4)

also this certified banger of a paper 📄:

D. Thaler, B. Fenner, and B. Quinn, Socket Interface Extensions for Multicast Source Filters, RFC 3678, Jan 2004.

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🕰️ HISTORY

the ip protocol showed up way back in 4.2BSD — genuine elder, iconic, no notes 👑 and the ip_mreqn structure appeared in Linux 2.4. glow-up timeline fr fr.

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🐛 BUGS

okay so the lore 🫶 — before FreeBSD 10.0, packets received on raw IP sockets had the ip_hl subtracted from the ip_len field. yeah. that was a whole thing.

and before FreeBSD 11.0, packets received on raw IP sockets had the ip_len and ip_off fields converted to host byte order 😭 and packets written to raw IP sockets were expected to have ip_len and ip_off in host byte order. wild times, we don't live there anymore.

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and that's the whole page bestie!! 🎀 this one's from Debian, dated August 9, 2021, and it covers IP(4) 💜 go forth and packet responsibly ✨🦋