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 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 mes-
     sages 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 destina-
		      tion 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.

4.3 Berkeley Distribution					   June 19, 2002					 4.3 Berkeley Distribution