ip(7)

NAME
ip - Linux IPv4 protocol implementation

SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h> /* superset of previous */

tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
raw_socket = socket(AF_INET, SOCK_RAW, protocol);

DESCRIPTION
Linux implements the Internet Protocol, version 4, described in RFC 791 and RFC 1122. ip contains a level 2 multicasting implementation conforming to RFC 1112. It also contains an IP router including a packet filter.

The programming interface is BSD-sockets compatible. For more information on sockets, see socket(7).

An IP socket is created using socket(2):

    socket(AF_INET, socket_type, protocol);

Valid socket types include SOCK_STREAM to open a stream socket, SOCK_DGRAM to open a datagram socket, and SOCK_RAW to open a raw(7) socket to access the IP protocol directly.

protocol is the IP protocol in the IP header to be received or sent. Valid values for protocol include:

•   0 and IPPROTO_TCP for tcp(7) stream sockets;

•   0 and IPPROTO_UDP for udp(7) datagram sockets;

•   IPPROTO_SCTP for sctp(7) stream sockets; and

•   IPPROTO_UDPLITE for udplite(7) datagram sockets.

For SOCK_RAW you may specify a valid IANA IP protocol defined in RFC 1700 assigned numbers.

When a process wants to receive new incoming packets or connections, it should bind a socket to a local interface address using bind(2). In this case, only one IP socket may be bound to any given local (address, port) pair. When INADDR_ANY is specified in the bind call, the socket will be bound to all local interfaces. When listen(2) is called on an unbound socket, the socket is automatically bound to a random free port with the local address set to INADDR_ANY. When connect(2) is called on an unbound socket, the socket is automatically bound to a random free port or to a usable shared port with the local address set to INADDR_ANY.

A TCP local socket address that has been bound is unavailable for some time after closing, unless the SO_REUSEADDR flag has been set. Care should be taken when using this flag as it makes TCP less reliable.

  Address format
An IP socket address is defined as a combination of an IP interface address and a 16-bit port number. The basic IP protocol does not supply port numbers, they are implemented by higher level protocols like udp(7) and tcp(7). On raw sockets sin_port is set to the IP protocol.

    struct sockaddr_in {
        sa_family_t    sin_family; /* address family: AF_INET */
        in_port_t      sin_port;   /* port in network byte order */
        struct in_addr sin_addr;   /* internet address */
    };

    /* Internet address */
    struct in_addr {
        uint32_t       s_addr;     /* address in network byte order */
    };

sin_family is always set to AF_INET. This is required; in Linux 2.2 most networking functions return EINVAL when this setting is missing. sin_port contains the port in network byte order. The port numbers below 1024 are called privileged ports (or sometimes: reserved ports). Only a privileged process (on Linux: a process that has the CAP_NET_BIND_SERVICE capability in the user namespace governing its network namespace) may bind(2) to these sockets. Note that the raw IPv4 protocol as such has no concept of a port, they are implemented only by higher protocols like tcp(7) and udp(7).

sin_addr is the IP host address. The s_addr member of struct in_addr contains the host interface address in network byte order. in_addr should be assigned one of the INADDR_* values (e.g., INADDR_LOOPBACK) using htonl(3) or set using the inet_aton(3), inet_addr(3), inet_makeaddr(3) library functions or directly with the name resolver (see gethostbyname(3)).

IPv4 addresses are divided into unicast, broadcast, and multicast addresses. Unicast addresses specify a single interface of a host, broadcast addresses specify all hosts on a network, and multicast addresses address all hosts in a multicast group. Datagrams to broadcast addresses can be sent or received only when the SO_BROADCAST socket flag is set. In the current implementation, connection-oriented sockets are allowed to use only unicast addresses.

Note that the address and the port are always stored in network byte order. In particular, this means that you need to call htons(3) on the number that is assigned to a port. All address/port manipulation functions in the standard library work in network byte order.

  Special and reserved addresses
There are several special addresses:

INADDR_LOOPBACK (127.0.0.1)
    always refers to the local host via the loopback device;

INADDR_ANY (0.0.0.0)
    means any address for socket binding;

INADDR_BROADCAST (255.255.255.255)
    has the same effect on bind(2) as INADDR_ANY for historical reasons. A packet addressed to INADDR_BROADCAST through a socket which has SO_BROADCAST set will be broadcast to all hosts on the local network segment, as long as the link is broadcast-capable.

Highest-numbered address
Lowest-numbered address
    On any locally-attached non-point-to-point IP subnet with a link type that supports broadcasts, the highest-numbered address (e.g., the .255 address on a subnet with netmask 255.255.255.0) is designated as a broadcast address. It cannot usefully be assigned to an individual interface, and can only be addressed with a socket on which the SO_BROADCAST option has been set. Internet standards have historically also reserved the lowest-numbered address (e.g., the .0 address on a subnet with netmask 255.255.255.0) for broadcast, though they call it "obsolete" for this purpose. (Some sources also refer to this as the "network address.") Since Linux 5.14, it is treated as an ordinary unicast address and can be assigned to an interface.

Internet standards have traditionally also reserved various addresses for particular uses, though Linux no longer treats some of these specially.

[0.0.0.1, 0.255.255.255]
[240.0.0.0, 255.255.255.254]
    Addresses in these ranges (0/8 and 240/4) are reserved globally. Since Linux 5.3 and Linux 2.6.25, respectively, the 0/8 and 240/4 addresses, other than INADDR_ANY and INADDR_BROADCAST, are treated as ordinary unicast addresses. Systems that follow the traditional behaviors may not interoperate with these historically reserved addresses.

[127.0.0.1, 127.255.255.254]
    Addresses in this range (127/8) are treated as loopback addresses akin to the standardized local loopback address INADDR_LOOPBACK (127.0.0.1);

[224.0.0.0, 239.255.255.255]
    Addresses in this range (224/4) are dedicated to multicast use.

  Socket options
IP supports some protocol-specific socket options that can be set with setsockopt(2) and read with getsockopt(2). The socket option level for IP is IPPROTO_IP. A boolean integer flag is zero when it is false, otherwise true.

When an invalid socket option is specified, getsockopt(2) and setsockopt(2) fail with the error ENOPROTOOPT.

IP_ADD_MEMBERSHIP (since Linux 1.2)
    Join a multicast group. Argument is an ip_mreqn structure.

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

    imr_multiaddr contains the address of the multicast group the application wants to join or leave. It must be a valid multicast address (or setsockopt(2) fails with the error EINVAL). imr_address is the address of the local interface with which the system should join the multicast group; if it is equal to INADDR_ANY, an appropriate interface is chosen by the system. imr_ifindex is the interface index of the interface that should join/leave the imr_multiaddr group, or 0 to indicate any interface.

    The ip_mreqn structure is available only since Linux 2.2. For compatibility, the old ip_mreq structure (present since Linux 1.2) is still supported; it differs from ip_mreqn only by not including the imr_ifindex field. (The kernel determines which structure is being passed based on the size passed in optlen.)

    IP_ADD_MEMBERSHIP is valid only for setsockopt(2).

IP_ADD_SOURCE_MEMBERSHIP (since Linux 2.4.22 / Linux 2.5.68)
    Join a multicast group and allow receiving data only from a specified source. Argument is an ip_mreq_source structure.

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

    The ip_mreq_source structure is similar to ip_mreqn described under IP_ADD_MEMBERSHIP. The imr_multiaddr field contains the address of the multicast group the application wants to join or leave. The imr_interface field is the address of the local interface with which the system should join the multicast group. Finally, the imr_sourceaddr field contains the address of the source the application wants to receive data from.

    This option can be used multiple times to allow receiving data from more than one source.

IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)
    Inform the kernel to not reserve an ephemeral port when using bind(2) with a port number of 0. The port will later be automatically chosen at connect(2) time, in a way that allows sharing a source port as long as the 4-tuple is unique.

IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
    Stop receiving multicast data from a specific source in a given group. This is valid only after the application has subscribed to the multicast group using either IP_ADD_MEMBERSHIP or IP_ADD_SOURCE_MEMBERSHIP.

    Argument is an ip_mreq_source structure as described under IP_ADD_SOURCE_MEMBERSHIP.

IP_DROP_MEMBERSHIP (since Linux 1.2)
    Leave a multicast group. Argument is an ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.

IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)
    Leave a source-specific group—that is, stop receiving data from a given multicast group that come from a given source. If the application has subscribed to multiple sources within the same group, data from the remaining sources will still be delivered. To stop receiving data from all sources at once, use IP_DROP_MEMBERSHIP.

    Argument is an ip_mreq_source structure as described under IP_ADD_SOURCE_MEMBERSHIP.

IP_FREEBIND (since Linux 2.4)
    If enabled, this boolean option allows binding to an IP address that is nonlocal or does not (yet) exist. This permits listening on a socket, without requiring the underlying network interface or the specified dynamic IP address to be up at the time that the application is trying to bind to it. This option is the per-socket equivalent of the ip_nonlocal_bind /proc interface described below.

IP_HDRINCL (since Linux 2.0)
    If enabled, the user supplies an IP header in front of the user data. Valid only for SOCK_RAW sockets; see raw(7) for more information. When this flag is enabled, the values set by IP_OPTIONS, IP_TTL, and IP_TOS are ignored.

IP_LOCAL_PORT_RANGE (since Linux 6.3)
    Set or get the per-socket default local port range. This option can be used to clamp down the global local port range, defined by the ip_local_port_range /proc interface described below, for a given socket.

    The option takes an uint32_t value with the high 16 bits set to the upper range bound, and the low 16 bits set to the lower range bound. Range bounds are inclusive. The 16-bit values should be in host byte order.

    The lower bound has to be less than the upper bound when both bounds are not zero. Otherwise, setting the option fails with EINVAL.

    If either bound is outside of the global local port range, or is zero, then that bound has no effect.

    To reset the setting, pass zero as both the upper and the lower bound.

IP_MSFILTER (since Linux 2.4.22 / 2.5.68)
    This option provides access to the advanced full-state filtering API. Argument is an ip_msfilter structure.

        struct ip_msfilter {
            struct in_addr imsf_multiaddr; /* IP multicast group
                                              address */
            struct in_addr imsf_interface; /* IP address of local
                                              interface */
            uint32_t       imsf_fmode;     /* Filter-mode */

            uint32_t       imsf_numsrc;    /* Number of sources in
                                              the following array */
            struct in_addr imsf_slist[1];  /* Array of source
                                              addresses */
        };

    There are two macros, MCAST_INCLUDE and MCAST_EXCLUDE, which can be used to specify the filtering mode. Additionally, the IP_MSFILTER_SIZE(n) macro exists to determine how much memory is needed to store ip_msfilter structure with n sources in the source list.

    For the full description of multicast source filtering refer to RFC 3376.

IP_MTU (since Linux 2.2)
    Retrieve the current known path MTU of the current socket. Returns an integer.

    IP_MTU is valid only for getsockopt(2) and can be employed only when the socket has been connected.

IP_MTU_DISCOVER (since Linux 2.2)
    Set or receive the Path MTU Discovery setting for a socket. When enabled, Linux will perform Path MTU Discovery as defined in RFC 1191 on SOCK_STREAM sockets. For non-SOCK_STREAM sockets, IP_PMTUDISC_DO forces the don't-fragment flag to be set on all outgoing packets. It is the user's responsibility to packetize the data in MTU-sized chunks and to do the retransmits if necessary. The kernel will reject (with EMSGSIZE) datagrams that are bigger than the known path MTU. IP_PMTUDISC_WANT will fragment a datagram if needed according to the path MTU, or will set the don't-fragment flag otherwise.

    The system-wide default can be toggled between IP_PMTUDISC_WANT and IP_PMTUDISC_DONT by writing (respectively, zero and nonzero values) to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.
    Path MTU discovery value | Meaning
    IP_PMTUDISC_WANT | Use per-route settings.
    IP_PMTUDISC_DONT | Never do Path MTU Discovery.
    IP_PMTUDISC_DO | Always do Path MTU Discovery.
    IP_PMTUDISC_PROBE | Set DF but ignore Path MTU.

    When PMTU discovery is enabled, the kernel automatically keeps track of the path MTU per destination host. When it is connected to a specific peer with connect(2), the currently known path MTU can be retrieved conveniently using the IP_MTU socket option (e.g., after an EMSGSIZE error occurred). The path MTU may change over time. For connectionless sockets with many destinations, the new MTU for a given destination can also be accessed using the error queue (see IP_RECVERR). A new error will be queued for every incoming MTU update.

    While MTU discovery is in progress, initial packets from datagram sockets may be dropped. Applications using UDP should be aware of this and not take it into account for their packet retransmit strategy.

    To bootstrap the path MTU discovery process on unconnected sockets, it is possible to start with a big datagram size (headers up to 64 kilobytes long) and let it shrink by updates of the path MTU.

    To get an initial estimate of the path MTU, connect a datagram socket to the destination address using connect(2) and retrieve the MTU by calling getsockopt(2) with the IP_MTU option.

    It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM or SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE (available since Linux 2.6.22). This is also particularly useful for diagnostic tools such as tracepath(8) that wish to deliberately send probe packets larger than the observed Path MTU.

IP_MULTICAST_ALL (since Linux 2.6.31)
    This option can be used to modify the delivery policy of multicast messages. The argument is a boolean integer (defaults to 1). If set to 1, the socket will receive messages from all the groups that have been joined globally on the whole system. Otherwise, it will deliver messages only from the groups that have been explicitly joined (for example via the IP_ADD_MEMBERSHIP option) on this particular socket.

IP_MULTICAST_IF (since Linux 1.2)
    Set the local device for a multicast socket. The argument for setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq structure similar to IP_ADD_MEMBERSHIP, or an in_addr structure. (The kernel determines which structure is being passed based on the size passed in optlen.) For getsockopt(2), the argument is an in_addr structure.

IP_MULTICAST_LOOP (since Linux 1.2)
    Set or read a boolean integer argument that determines whether sent multicast packets should be looped back to the local sockets.

IP_MULTICAST_TTL (since Linux 1.2)
    Set or read the time-to-live value of outgoing multicast packets for this socket. It is very important for multicast packets to set the smallest TTL possible. The default is 1 which means that multicast packets don't leave the local network unless the user program explicitly requests it. Argument is an integer.

IP_NODEFRAG (since Linux 2.6.36)
    If enabled (argument is nonzero), the reassembly of outgoing packets is disabled in the netfilter layer. The argument is an integer.

    This option is valid only for SOCK_RAW sockets.

IP_OPTIONS (since Linux 2.0)
    Set or get the IP options to be sent with every packet from this socket. The arguments are a pointer to a memory buffer containing the options and the option length. The setsockopt(2) call sets the IP options associated with a socket. The maximum option size for IPv4 is 40 bytes. See RFC 791 for the allowed options. When the initial connection request packet for a SOCK_STREAM socket contains IP options, the IP options will be set automatically to the options from the initial packet with routing headers reversed. Incoming packets are not allowed to change options after the connection is established. The processing of all incoming source routing options is disabled by default and can be enabled by using the accept_source_route /proc interface. Other options like timestamps are still handled. For datagram sockets, IP options can be set only by the local user. Calling getsockopt(2) with IP_OPTIONS puts the current IP options used for sending into the supplied buffer.

IP_PASSSEC (since Linux 2.6.17)
    If labeled IPSEC or NetLabel is configured on the sending and receiving hosts, this option enables receiving of the security context of the peer socket in an ancillary message of type SCM_SECURITY retrieved using recvmsg(2). This option is supported only for UDP sockets; for TCP or SCTP sockets, see the description of the SO_PEERSEC option below.

    The value given as an argument to setsockopt(2) and returned as the result of getsockopt(2) is an integer boolean flag.

    The security context returned in the SCM_SECURITY ancillary message is of the same format as the one described under the SO_PEERSEC option below.

    Note: the reuse of the SCM_SECURITY message type for the IP_PASSSEC socket option was likely a mistake, since other IP control messages use their own numbering scheme in the IP namespace and often use the socket option value as the message type. There is no conflict currently since the IP option with the same value as SCM_SECURITY is IP_HDRINCL and this is never used for a control message type.

IP_PKTINFO (since Linux 2.2)
    Pass an IP_PKTINFO ancillary message that contains a pktinfo structure that supplies some information about the incoming packet. This works only for datagram oriented sockets. The argument is a flag that tells the socket whether the IP_PKTINFO message should be passed or not. The message itself can be sent/retrieved only as a control message with a packet using recvmsg(2) or sendmsg(2).

        struct in_pktinfo {
            unsigned int   ipi_ifindex;  /* Interface index */
            struct in_addr ipi_spec_dst; /* Local address */
            struct in_addr ipi_addr;     /* Header Destination
                                            address */
        };

    ipi_ifindex is the unique index of the interface the packet was received on. ipi_spec_dst is the local address of the packet and ipi_addr is the destination address in the packet header. If IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst is not zero, then it is used as the local source address for the routing table lookup and for setting up IP source route options. When ipi_ifindex is not zero, the primary local address of the interface specified by the index overwrites ipi_spec_dst for the routing table lookup.

IP_RECVERR (since Linux 2.2)
    Enable extended reliable error message passing. When enabled on a datagram socket, all generated errors will be queued in a per-socket error queue. When the user receives an error from a socket operation, the errors can be received by calling recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_extended_err structure describing the error will be passed in an ancillary message with the type IP_RECVERR and the level IPPROTO_IP. This is useful for reliable error handling on unconnected sockets. The received data portion of the error queue contains the error packet.

    The IP_RECVERR control message contains a sock_extended_err structure:

        #define SO_EE_ORIGIN_NONE    0
        #define SO_EE_ORIGIN_LOCAL   1
        #define SO_EE_ORIGIN_ICMP    2
        #define SO_EE_ORIGIN_ICMP6   3

        struct sock_extended_err {
            uint32_t ee_errno;   /* error number */
            uint8_t  ee_origin;  /* where the error originated */
            uint8_t  ee_type;    /* type */
            uint8_t  ee_code;    /* code */
            uint8_t  ee_pad;
            uint32_t ee_info;    /* additional information */
            uint32_t ee_data;    /* other data */
            /* More data may follow */
        };

        struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

    ee_errno contains the errno number of the queued error. ee_origin is the origin code of where the error originated. The other fields are protocol-specific. The macro SO_EE_OFFENDER returns a pointer to the address of the network object where the error originated from given a pointer to the ancillary message. If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the sockaddr are undefined.

    IP uses the sock_extended_err structure as follows: ee_origin is set to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet, or SO_EE_ORIGIN_LOCAL for locally generated errors. Unknown values should be ignored. ee_type and ee_code are set from the type and code fields of the ICMP header. ee_info contains the discovered MTU for EMSGSIZE errors. The message also contains the sockaddr_in of the node caused the error, which can be accessed with the SO_EE_OFFENDER macro. The sin_family field of the SO_EE_OFFENDER address is AF_UNSPEC when the source was unknown. When the error originated from the network, all IP options (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and contained in the error packet are passed as control messages. The payload of the packet causing the error is returned as normal payload. Note that TCP has no error queue; MSG_ERRQUEUE is not permitted on SOCK_STREAM sockets. IP_RECVERR is valid for TCP, but all errors are returned by socket function return or SO_ERROR only.

    For raw sockets, IP_RECVERR enables passing of all received ICMP errors to the application, otherwise errors are reported only on connected sockets

    It sets or retrieves an integer boolean flag. IP_RECVERR defaults to off.

IP_RECVOPTS (since Linux 2.2)
    Pass all incoming IP options to the user in a IP_OPTIONS control message. The routing header and other options are already filled in for the local host. Not supported for SOCK_STREAM sockets.

IP_RECVORIGDSTADDR (since Linux 2.6.29)
    This boolean option enables the IP_ORIGDSTADDR ancillary message in recvmsg(2), in which the kernel returns the original destination address of the datagram being received. The ancillary message contains a struct sockaddr_in.

IP_RECVTOS (since Linux 2.2)
    If enabled, the IP_TOS ancillary message is passed with incoming packets. It contains a byte which specifies the Type of Service/Precedence field of the packet header. Expects a boolean integer flag.

IP_RECVTTL (since Linux 2.2)
    When this flag is set, pass a IP_TTL control message with the time-to-live field of the received packet as a 32 bit integer. Not supported for SOCK_STREAM sockets.

IP_RETOPTS (since Linux 2.2)
    Identical to IP_RECVOPTS, but returns raw unprocessed options with timestamp and route record options not filled in for this hop.

IP_ROUTER_ALERT (since Linux 2.2)
    Pass all to-be forwarded packets with the IP Router Alert option set to this socket. Valid only for raw sockets. This is useful, for instance, for user-space RSVP daemons. The tapped packets are not forwarded by the kernel; it is the user's responsibility to send them out again. Socket binding is ignored, such packets are filtered only by protocol. Expects an integer flag.

IP_TOS (since Linux 1.0)
    Set or receive the Type-Of-Service (TOS) field that is sent with every IP packet originating from this socket. It is used to prioritize packets on the network. TOS is a byte. There are some standard TOS flags defined: IPTOS_LOWDELAY to minimize delays for interactive traffic, IPTOS_THROUGHPUT to optimize throughput, IPTOS_RELIABILITY to optimize for reliability, IPTOS_MINCOST should be used for "filler data" where slow transmission doesn't matter. At most one of these TOS values can be specified. Other bits are invalid and shall be cleared. Linux sends IPTOS_LOWDELAY datagrams first by default, but the exact behavior depends on the configured queueing discipline. Some high-priority levels may require superuser privileges (the CAP_NET_ADMIN capability).

IP_TRANSPARENT (since Linux 2.6.24)
    Setting this boolean option enables transparent proxying on this socket. This socket option allows the calling application to bind to a nonlocal IP address and operate both as a client and a server with the foreign address as the local endpoint. NOTE: this requires that routing be set up in a way that packets going to the foreign address are routed through the TProxy box (i.e., the system hosting the application that employs the IP_TRANSPARENT socket option). Enabling this socket option requires superuser privileges (the CAP_NET_ADMIN capability).

    TProxy redirection with the iptables TPROXY target also requires that this option be set on the redirected socket.

IP_TTL (since Linux 1.0)
    Set or retrieve the current time-to-live field that is used in every packet sent from this socket.

IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)
    Unblock previously blocked multicast source. Returns EADDRNOTAVAIL when given source is not being blocked.

    Argument is an ip_mreq_source structure as described under IP_ADD_SOURCE_MEMBERSHIP.

SO_PEERSEC (since Linux 2.6.17)
    If labeled IPSEC or NetLabel is configured on both the sending and receiving hosts, this read-only socket option returns the security context of the peer socket connected to this socket. By default, this will be the same as the security context of the process that created the peer socket unless overridden by the policy or by a process with the required permissions.

    The argument to getsockopt(2) is a pointer to a buffer of the specified length in bytes into which the security context string will be copied. If the buffer length is less than the length of the security context string, then getsockopt(2) returns -1, sets errno to ERANGE, and returns the required length via optlen. The caller should allocate at least NAME_MAX bytes for the buffer initially, although this is not guaranteed to be sufficient. Resizing the buffer to the returned length and retrying may be necessary.

    The security context string may include a terminating null character in the returned length, but is not guaranteed to do so: a security context "foo" might be represented as either {'f','o','o'} of length 3 or {'f','o','o','\0'} of length 4, which are considered to be interchangeable. The string is printable, does not contain non-terminating null characters, and is in an unspecified encoding (in particular, it is not guaranteed to be ASCII or UTF-8).

    The use of this option for sockets in the AF_INET address family is supported since Linux 2.6.17 for TCP sockets, and since Linux 4.17 for SCTP sockets.

    For SELinux, NetLabel conveys only the MLS portion of the security context of the peer across the wire, defaulting the rest of the security context to the values defined in the policy for the netmsg initial security identifier (SID). However, NetLabel can be configured to pass full security contexts over loopback. Labeled IPSEC always passes full security contexts as part of establishing the security association (SA) and looks them up based on the association for each packet.

  /proc interfaces
The IP protocol supports a set of /proc interfaces to configure some global parameters. The parameters can be accessed by reading or writing files in the directory /proc/sys/net/ipv4/. Interfaces described as Boolean take an integer value, with a nonzero value ("true") meaning that the corresponding option is enabled, and a zero value ("false") meaning that the option is disabled.

ip_always_defrag (Boolean; since Linux 2.2.13)
    [New with Linux 2.2.13; in earlier kernel versions this feature was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG option; this option is not present in Linux 2.4.x and later]

    When this boolean flag is enabled (not equal 0), incoming fragments (parts of IP packets that arose when some host between origin and destination decided that the packets were too large and cut them into pieces) will be reassembled (defragmented) before being processed, even if they are about to be forwarded.

    Enable only if running either a firewall that is the sole link to your network or a transparent proxy; never ever use it for a normal router or host. Otherwise, fragmented communication can be disturbed if the fragments travel over different links. Defragmentation also has a large memory and CPU time cost.

    This is automagically turned on when masquerading or transparent proxying are configured.

ip_autoconfig (since Linux 2.2 to Linux 2.6.17)
    Not documented.

ip_default_ttl (integer; default: 64; since Linux 2.2)
    Set the default time-to-live value of outgoing packets. This can be changed per socket with the IP_TTL option.

ip_dynaddr (Boolean; default: disabled; since Linux 2.0.31)
    Enable dynamic socket address and masquerading entry rewriting on interface address change. This is useful for dialup interface with changing IP addresses. 0 means no rewriting, 1 turns it on and 2 enables verbose mode.

ip_forward (Boolean; default: disabled; since Linux 1.2)
    Enable IP forwarding with a boolean flag. IP forwarding can be also set on a per-interface basis.

ip_local_port_range (since Linux 2.2)
    This file contains two integers that define the default local port range allocated to sockets that are not explicitly bound to a port number—that is, the range used for ephemeral ports. An ephemeral port is allocated to a socket in the following circumstances:

    •   the port number in a socket address is specified as 0 when calling bind(2);

    •   listen(2) is called on a stream socket that was not previously bound;

    •   connect(2) was called on a socket that was not previously bound;

    •   sendto(2) is called on a datagram socket that was not previously bound.

    Allocation of ephemeral ports starts with the first number in ip_local_port_range and ends with the second number. If the range of ephemeral ports is exhausted, then the relevant system call returns an error (but see BUGS).

    Note that the port range in ip_local_port_range should not conflict with the ports used by masquerading (although the case is handled). Also, arbitrary choices may cause problems with some firewall packet filters that make assumptions about the local ports in use. The first number should be at least greater than 1024, or better, greater than 4096, to avoid clashes with well known ports and to minimize firewall problems.

ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)
    If enabled, don't do Path MTU Discovery for TCP sockets by default. Path MTU discovery may fail if misconfigured firewalls (that drop all ICMP packets) or misconfigured interfaces (e.g., a point-to-point link where the both ends don't agree on the MTU) are on the path. It is better to fix the broken routers on the path than to turn off Path MTU Discovery globally, because not doing it incurs a high cost to the network.

ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)
    If set, allows processes to bind(2) to nonlocal IP addresses, which can be quite useful, but may break some applications.

ip6frag_time (integer; default: 30)
    Time in seconds to keep an IPv6 fragment in memory.

ip6frag_secret_interval (integer; default: 600)
    Regeneration interval (in seconds) of the hash secret (or lifetime for the hash secret) for IPv6 fragments.

ipfrag_high_thresh (integer), ipfrag_low_thresh (integer)
    If the amount of queued IP fragments reaches ipfrag_high_thresh, the queue is pruned down to ipfrag_low_thresh. Contains an integer with the number of bytes.

neigh/*
    See arp(7).

  Ioctls
All ioctls described in socket(7) apply to ip.

Ioctls to configure generic device parameters are described in netdevice(7).

ERRORS

EACCES
    The user tried to execute an operation without the necessary permissions. These include: sending a packet to a broadcast address without having the SO_BROADCAST flag set; sending a packet via a prohibit route; modifying firewall settings without superuser privileges (the CAP_NET_ADMIN capability); binding to a privileged port without superuser privileges (the CAP_NET_BIND_SERVICE capability).

EADDRINUSE
    Tried to bind to an address already in use.

EADDRNOTAVAIL
    A nonexistent interface was requested or the requested source address was not local.

EAGAIN
    Operation on a nonblocking socket would block.

EALREADY
    A connection operation on a nonblocking socket is already in progress.

ECONNABORTED
    A connection was closed during an accept(2).

EHOSTUNREACH
    No valid routing table entry matches the destination address. This error can be caused by an ICMP message from a remote router or for the local routing table.

EINVAL
    Invalid argument passed. For send operations this can be caused by sending to a blackhole route.

EISCONN
    connect(2) was called on an already connected socket.

EMSGSIZE
    Datagram is bigger than an MTU on the path and it cannot be fragmented.

ENOBUFS, ENOMEM
    Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory, but this is not 100% consistent.

ENOENT
    SIOCGSTAMP was called on a socket where no packet arrived.

ENOPKG
    A kernel subsystem was not configured.

ENOPROTOOPT and EOPNOTSUPP
    Invalid socket option passed.

ENOTCONN
    The operation is defined only on a connected socket, but the socket wasn't connected.

EPERM
    User doesn't have permission to set high priority, change configuration, or send signals to the requested process or group.

EPIPE
    The connection was unexpectedly closed or shut down by the other end.

ESOCKTNOSUPPORT
    The socket is not configured or an unknown socket type was requested.

Other errors may be generated by the overlaying protocols; see tcp(7), raw(7), udp(7), and socket(7).

NOTES
IP_FREEBIND, IP_MSFILTER, IP_MTU, IP_MTU_DISCOVER, IP_RECVORIGDSTADDR, IP_PASSSEC, IP_PKTINFO, IP_RECVERR, IP_ROUTER_ALERT, and IP_TRANSPARENT are Linux-specific.

Be very careful with the SO_BROADCAST option - it is not privileged in Linux. It is easy to overload the network with careless broadcasts. For new application protocols it is better to use a multicast group instead of broadcasting. Broadcasting is discouraged. See RFC 6762 for an example of a protocol (mDNS) using the more modern multicast approach to communicating with an open-ended group of hosts on the local network.

Some other BSD sockets implementations provide IP_RCVDSTADDR and IP_RECVIF socket options to get the destination address and the interface of received datagrams. Linux has the more general IP_PKTINFO for the same task.

Some BSD sockets implementations also provide an IP_RECVTTL option, but an ancillary message with type IP_RECVTTL is passed with the incoming packet. This is different from the IP_TTL option used in Linux.

Using the SOL_IP socket options level isn't portable; BSD-based stacks use the IPPROTO_IP level.

INADDR_ANY (0.0.0.0) and INADDR_BROADCAST (255.255.255.255) are byte-order-neutral. This means htonl(3) has no effect on them.

  Compatibility
For compatibility with Linux 2.0, the obsolete socket(AF_INET, SOCK_PACKET, protocol) syntax is still supported to open a packet(7) socket. This is deprecated and should be replaced by socket(AF_PACKET, SOCK_RAW, protocol) instead. The main difference is the new sockaddr_ll address structure for generic link layer information instead of the old sockaddr_pkt.

BUGS
There are too many inconsistent error values.

The error used to diagnose exhaustion of the ephemeral port range differs across the various system calls (connect(2), bind(2), listen(2), sendto(2)) that can assign ephemeral ports.

The ioctls to configure IP-specific interface options and ARP tables are not described.

Receiving the original destination address with MSG_ERRQUEUE in msg_name by recvmsg(2) does not work in some Linux 2.2 kernels.

SEE ALSO
recvmsg(2), sendmsg(2), byteorder(3), capabilities(7), icmp(7), ipv6(7), netdevice(7), netlink(7), raw(7), socket(7), tcp(7), udp(7), ip(8)

The kernel source file Documentation/networking/ip-sysctl.txt.

RFC 791 for the original IP specification. RFC 1122 for the IPv4 host requirements. RFC 1812 for the IPv4 router requirements.