ICMP(4) BSD Kernel Interfaces Manual ICMP(4)
icmp -- Internet Control Message Protocol
socket(AF_INET, SOCK_RAW, proto);
ICMP is the error and control message protocol used by IP and the Inter-
net 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.
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 avail-
able 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
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
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.
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
[ENOTCONN] when trying to send a datagram, but no destination
address is specified, and the socket hasn't been con-
[ENOBUFS] when the system runs out of memory for an internal data
[EADDRNOTAVAIL] when an attempt is made to create a socket with a net-
work 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.
send(2), recv(2), intro(4), inet(4), ip(4)
The icmp protocol appeared in 4.3BSD.
4.3 Berkeley Distribution June 19, 2002 4.3 Berkeley Distribution