ICMP(4)

NAME
icmp - Internet Control Message Protocol

SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION
ICMP is the error and control message protocol used by IP and the Internet protocol family. It may be accessed through a “raw socket” for network monitoring and diagnostic functions. The proto parameter to the socket call to create an ICMP socket is obtained from getprotobyname(3). 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(2) or recv(2) and write(2) or send(2) system calls may be used).

Outgoing packets automatically have an IP header prepended to them (based on the destination address). Incoming packets are received with the IP header and options intact. Note that the ip_off and ip_len fields are in host byte order. For more information about the IP header structure, see ip(4).

  Non-privileged ICMP

ICMP sockets can be opened with the SOCK_DGRAM socket type without requiring root privileges. The synopsis is the following:

socket(AF_INET, SOCK_DGRAM, IPPROTO_ICMP)

Datagram oriented ICMP sockets offer a subset of the functionality available to raw ICMP sockets. Only IMCP request messages of the following types can be sent: ICMP_ECHO, ICMP_TSTAMP or ICMP_MASKREQ. The code field must be the value zero (0). The minimal length of an ICMP message request is eight (8) octets.

The advantage of using datagram oriented ICMP sockets is that even a non-privileged process can use ICMP echo requests to gauge the quality of the connectivity to a host, or to receive ICMP destination unreachable message for path MTU discovery, or to receive time exceeded messages for traceroute.

The following IP level option can be used with datagram oriented ICMP sockets:
IP_OPTIONS
IP_HDRINCL
IP_TOS
IP_TTL
IP_RECVOPTS
IP_RECVRETOPTS
IP_RECVDSTADDR
IP_RETOPTS
IP_MULTICAST_IF
IP_MULTICAST_TTL
IP_MULTICAST_LOOP
IP_ADD_MEMBERSHIP
IP_DROP_MEMBERSHIP
IP_MULTICAST_VIF
IP_PORTRANGE
IP_RECVIF
IP_IPSEC_POLICY
IP_STRIPHDR

When the IP option IP_HDRINCL is used, the provided IP header must obey the following rules:
ip_v | Must be IPVERSION (4);
ip_hl | Between 5 and 10 (inclusive);
ip_tos | Any value;
ip_len | Must be the total length of IP datagram (IP header + ICMP message);
ip_id | Must be zero, will be automatically set;
ip_off | Must be zero, will be automatically set;
ip_ttl | Any value;
ip_p | Must be IPPROTO_IP;
ip_sum | Value ignored, will be automatically set;
ip_src | Must be an
IP address currently assigned to one of the local interface or INADDR_ANY;
ip_dst | Any address;
ip_opts | Any option.

The maximum length of a IMCP message that can be sent is controlled by the sysctl variable net.inet.raw.maxdgram.

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;

[EINVAL]
    when an invalid value is used with IMCP datagram socket for a field of the IP or ICMP header.

SEE ALSO
recv(2), send(2), inet(4), intro(4), ip(4)

HISTORY
The icmp protocol appeared in 4.3BSD.