ICMP(4)                  BSD Kernel Interfaces Manual                  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 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.  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 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 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 mes-
     sage 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 con-
                      nected;

     [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 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.

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

HISTORY
     The icmp protocol appeared in 4.3BSD.

4.3 Berkeley Distribution        June 19, 2002       4.3 Berkeley Distribution