--- name: "ip" section: "4" platform: "openbsd" package: "man" version: "8.0-current-20261101" release: "OpenBSD 8.0-current (2026-11-01)" platforms: ["linux", "freebsd", "openbsd", "macos"] url: "https://new.carta.tech/ip/4" --- # ip(4) Internet Protocol ## NAME **ip** – Internet Protocol ## SYNOPSIS **#include \** **#include \** **#include \** *int*\ **socket**(*AF\_INET*, *SOCK\_RAW*, *proto*); ## DESCRIPTION IP is the network 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)](/setsockopt/2.md)/[getsockopt(2)](/getsockopt/2.md) options. 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 and IP\_TTL may be used to set the type-of-service and time-to-live fields in the IP header for SOCK\_STREAM, SOCK\_DGRAM and SOCK\_RAW sockets. For example, ``` int tos = IPTOS_LOWDELAY; /* see */ setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos)); int ttl = 60; /* max = 255 */ setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)); ``` IP\_IPDEFTTL can be used to retrieve the system wide default TTL. If the IP\_RECVDSTADDR option is enabled on a SOCK\_DGRAM socket, the [recvmsg(2)](/recvmsg/2.md) 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 ``` If the IP\_RECVDSTPORT option is enabled on a SOCK\_DGRAM socket, the [recvmsg(2)](/recvmsg/2.md) call will return the 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 port in 16-bit network byte order. The *cmsghdr* fields have the following values: ``` cmsg_len = CMSG_LEN(sizeof(u_int16_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTPORT ``` If the IP\_RECVTTL option is enabled on a SOCK\_DGRAM or SOCK\_RAW socket, the [recvmsg(2)](/recvmsg/2.md) call will return the TTL of the received datagram. The *msg\_control* field in the *msghdr* structure points to a buffer that contains a *cmsghdr* structure followed by the TTL value. The *cmsghdr* fields have the following values: ``` cmsg_len = CMSG_LEN(sizeof(u_int8_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVTTL ``` The IP\_MINTTL option may be used on TCP and UDP sockets to discard packets with a TTL lower than the option value. This can be used to implement the *Generalized* *TTL* *Security* *Mechanism* *(GTSM)* according to RFC 5082. To discard all packets with a TTL lower than 255: ``` int minttl = 255; setsockopt(s, IPPROTO_IP, IP_MINTTL, &minttl, sizeof(minttl)); ``` If the IP\_RECVIF option is enabled on a SOCK\_DGRAM or SOCK\_RAW socket, the [recvmsg(2)](/recvmsg/2.md) 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 ``` If the IP\_IPSECFLOWINFO option is enabled on a SOCK\_DGRAM socket, the [recvmsg(2)](/recvmsg/2.md) call will return information identifying the incoming IPsec SA for a UDP datagram. The *msg\_control* field in the *msghdr* structure points to a buffer that contains a *cmsghdr* structure followed by flow information in 32-bit network byte order. When this information is passed to a [sendmsg(2)](/sendmsg/2.md) call, the ID of the incoming SA will be used for looking up the outgoing SA for the UDP datagram. The *cmsghdr* fields for [recvmsg(2)](/recvmsg/2.md) and [sendmsg(2)](/sendmsg/2.md) have the following values: ``` cmsg_len = CMSG_LEN(sizeof(u_int32_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_IPSECFLOWINFO ``` The IP\_PORTRANGE option causes the default allocation policy for when the kernel is asked to choose a free port number. Three choices are available: - IP\_PORTRANGE\_DEFAULT The regular range of non-reserved ports. - IP\_PORTRANGE\_HIGH A high range, for fun. - IP\_PORTRANGE\_LOW Reserved ports; between 600 and 1023. If the IP\_RECVRTABLE option is enabled on a SOCK\_DGRAM socket, the [recvmsg(2)](/recvmsg/2.md) call will return the source routing domain for a UDP datagram. The *msg\_control* field in the *msghdr* structure points to a buffer that contains a *cmsghdr* structure followed by the routing table ID. The *cmsghdr* fields have the following values: ``` cmsg_len = CMSG_LEN(sizeof(u_int)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVRTABLE ``` When sending on a SOCK\_DGRAM socket with [sendmsg(2)](/sendmsg/2.md), the source address to be used can be passed 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 requested source 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 same checks and restrictions as for [bind(2)](/bind/2.md) apply, unless the socket is bound to INADDR\_ANY. In this case, there is no source address overlap check. ### 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. For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The IP\_MULTICAST\_IF option overrides the 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 desired interface or INADDR\_ANY to specify the default interface. An interface's local IP address and multicast capability can be obtained via the SIOCGIFCONF and SIOCGIFFLAGS [ioctl(2)](/ioctl/2.md)'s. Normal applications should not need to use this option. If a multicast datagram is sent to a group to which the sending host itself belongs (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 are 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 router 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). A multicast datagram sent with an initial TTL greater than 1 may be delivered to the sending host on a different interface from that on which it was sent, if the host belongs to the destination group on that other interface. The loopback control option has no effect on such delivery. 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_mreq mreq; setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)); ``` where *mreq* is either of the following structures: ``` struct ip_mreq { struct in_addr imr_multiaddr; /* multicast group to join */ struct in_addr imr_interface; /* interface to join on */ } struct ip_mreqn { struct in_addr imr_multiaddr; /* multicast group to join */ struct in_addr imr_address; /* local IP address of interface */ int imr_ifindex; /* interface index to join */ }; ``` *imr\_interface* should be INADDR\_ANY to choose the default multicast interface, or the IP address of a particular multicast-capable interface if the host is multihomed. The *imr\_ifindex* element of *struct* *ip\_mreqn* can be set to the interface index instead of specifying the IP address of a particular multicast-capable interface. Membership is associated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. Up to IP\_MAX\_MEMBERSHIPS (currently 4095) memberships may be added on a single socket. 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. ### Raw IP Sockets Raw IP sockets are connectionless, and are normally used with the [sendto(2)](/sendto/2.md) and [recvfrom(2)](/recvfrom/2.md) calls, though the [connect(2)](/connect/2.md) call may also be used to fix the destination for future packets (in which case the [read(2)](/read/2.md) or [recv(2)](/recv/2.md) and [write(2)](/write/2.md) or [send(2)](/send/2.md) system calls may be used). 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. 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. Incoming packets are received with IP header and options intact. IP\_HDRINCL indicates the complete IP header is included with the data and may be used only with the SOCK\_RAW type. ``` #include 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); ``` Additionally note that starting with OpenBSD 2.1, the *ip\_off* and *ip\_len* fields are in network byte order. If the header source address is set to INADDR\_ANY, the kernel will choose an appropriate address. ## DIAGNOSTICS A socket operation may fail with one of the following errors returned: - \[EISCONN\] when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected; - \[ENOTCONN\] when trying to send a datagram, but no destination address is specified, and the socket hasn't been connected; - \[ENOBUFS\] when the system runs out of memory for an internal data structure; - \[EADDRNOTAVAIL\] when an attempt is made to create a socket with a network address for which no network interface exists. - \[EACCES\] when an attempt is made to create a raw IP socket by a non-privileged process. The following errors specific to IP may occur when setting or getting IP options: - \[EINVAL\] An unknown socket option name was given. - \[EINVAL\] The IP option field was improperly formed; an option field was shorter than the minimum value or longer than the option buffer provided. ## SEE ALSO [getsockopt(2)](/getsockopt/2.md), [ioctl(2)](/ioctl/2.md), [recv(2)](/recv/2.md), [send(2)](/send/2.md), [icmp(4)](/icmp/4.md), [inet(4)](/inet/4.md), [netintro(4)](/netintro/4.md) ## HISTORY The **ip** protocol appeared in 4.2BSD.