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ip - Linux IPv4 protocol implementation




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

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


Linux implements the Internet Protocol, version 4, described in RFC791 and RFC1122. ip contains a level 2 multicasting implementation conforming to RFC1112. It also contains an IP router including a packet filter.

The programmer’s interface is BSD sockets compatible. For more information on sockets, see socket(7).

An IP socket is created by calling the socket(2) function as "socket(PF_INET, socket_type, protocol)". Valid socket types are SOCK_STREAM to open a tcp(7) socket, SOCK_DGRAM to open a udp(7) socket, or 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. The only valid values for protocol are 0 and IPPROTO_TCP for TCP sockets and 0 and IPPROTO_UDP for UDP sockets. For SOCK_RAW you may specify a valid IANA IP protocol defined in RFC1700 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). 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) or connect(2) are called on a unbound socket the socket is automatically bound to a random free 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.


An IP socket address is defined as a combination of an IP interface address and a 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 */
u_int16_t sin_port; /* port in network byte order */
struct in_addr sin_addr; /* internet address */

/* Internet address. */
struct in_addr {
u_int32_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 "reserved ports". Only processes with effective user id 0 or the CAP_NET_BIND_SERVICE capability may bind(2) to these sockets. Note that the raw IPv4 protocol as such has no concept of a port, they are only implemented by higher protocols like tcp(7) and udp(7).

sin_addr is the IP host address. The addr member of struct in_addr contains the host interface address in network order. in_addr should be only accessed 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 only sent or received when the SO_BROADCAST socket flag is set. In the current implementation connection oriented sockets are only allowed to use unicast addresses.

Note that the address and the port are always stored in network 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 order.

There are several special addresses: INADDR_LOOPBACK ( always refers to the local host via the loopback device; INADDR_ANY ( means any address for binding; INADDR_BROADCAST ( means any host and has the same effect on bind as INADDR_ANY for historical reasons.


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 SOL_IP. A boolean integer flag is zero when it is false, otherwise true.

Sets 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 RFC791 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 sysctl. Other options like timestamps are still handled. For datagram sockets, IP options can be only set by the local user. Calling getsockopt(2) with IP_OPTIONS puts the current IP options used for sending into the supplied buffer.

Pass an IP_PKTINFO ancillary message that contains a pktinfo structure that supplies some information about the incoming packet. This only works 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 only be sent/retrieved as 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.
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.

When this flag is set pass a IP_RECVTTL control message with the time to live field of the received packet as a byte. Not supported for SOCK_STREAM sockets.

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.

Identical to IP_RECVOPTS but returns raw unprocessed options with timestamp and route record options not filled in for this hop.

IP_TOS 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 behaviour depends on the configured queueing discipline. Some high priority levels may require an effective user id of 0 or the CAP_NET_ADMIN capability. The priority can also be set in a protocol independent way by the (SOL_SOCKET", "SO_PRIORITY) socket option (see socket(7)).

IP_TTL Set or retrieve the current time to live field that is send in every packet send from this socket.

If enabled the user supplies an ip header in front of the user data. Only valid 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_RECVERR" (defined in <linux/errqueue.h>)"
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 a ancillary message with the type IP_RECVERR and the level SOL_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_ICMP6 3

struct sock_extended_err {
u_int32_t ee_errno; /* error number */
u_int8_t ee_origin; /* where the error originated */
u_int8_t ee_type; /* type */
u_int8_t ee_code; /* code */
u_int8_t ee_pad;
u_int32_t ee_info; /* additional information */
u_int32_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 illegal on SOCK_STREAM sockets. Thus 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 only reported on connected sockets
It sets or retrieves an integer boolean flag. IP_RECVERR defaults to off.

Sets or receives the Path MTU Discovery setting for a socket. When enabled, Linux will perform Path MTU Discovery as defined in RFC1191 on this socket. The don’t fragment flag is set on all outgoing datagrams. The system-wide default is controlled by the ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and disabled on all others. For non SOCK_STREAM sockets 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 packets that are bigger than the known path MTU if this flag is set (with EMSGSIZE ).

"Linux Man Page" IP (7) 2001-06-19

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