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.

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)

This can be used by non root privileged processes to send ICMP echo requests to gauge the quality of the connectivity to a host, to receive ICMP destination unreachable message for path MTU discovery, or to receveive time exceeded message for traceroute.

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 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

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

History

The icmp protocol appeared in 4.3BSD.

Table of Contents

• Name
• Synopsis
• Description
• Diagnostics
• See Also
• History