ICMP(4P)

NAME
icmp - Internet Control Message Protocol

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

s = socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION
ICMP is the error and control message protocol used by the Internet protocol family. It is used by the kernel to handle and report errors in protocol processing. It may also be accessed through a “raw socket” for network monitoring and diagnostic functions. The protocol number for ICMP, used in the proto parameter to the socket call, can be obtained from getprotobyname (see getprotoent(3N)). ICMP sockets are connectionless, and are normally used with the sendto and recvfrom calls, though the connect(2) call may also be used to fix the destination for future packets (in which case the read(2V) or recv(2) and write(2V) or send(2) system calls may be used).

Outgoing packets automatically have an Internet Protocol (IP) header prepended to them. Incoming packets are provided to the holder of a raw socket with the IP header and options intact.

ICMP is an unreliable datagram protocol layered above IP. It is used internally by the protcol code for various purposes including routing, fault isolation, and congestion control. Receipt of an ICMP “redirect” message will add a new entry in the routing table, or modify an existing one. ICMP messages are routinely sent by the protocol code. Received ICMP messages may be reflected back to users of higher-level protocols such as TCP or UDP as error returns from system calls. A copy of all ICMP message received by the system is provided using the ICMP raw socket.

ERRORS
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.

SEE ALSO
connect(2), read(2V), recv(2), send(2), write(2V), getprotoent(3N), inet(4F), ip(4P), routing(4N)

Postel, Jon, Internet Control Message Protocol — DARPA Internet Program Protocol Specification, RFC 792, Network Information Center, SRI International, Menlo Park, Calif., September 1981. (Sun 800-1064-01)

BUGS

Replies to ICMP “echo” messages which are source routed are not sent back using inverted source routes, but rather go back through the normal routing mechanisms.