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ICMP(4)			 BSD Kernel Interfaces Manual		       ICMP(4)

NAME
     icmp — Internet Control Message Protocol

SYNOPSIS
     #include <sys/types.h>
     #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 net‐
     work 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(2)
     and recvfrom(2) 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.

   Types
     ICMP messages are classified according to the type and code fields
     present in the ICMP header.  The abbreviations for the types and codes
     may be used in rules in pf.conf(5).  The following types are defined:

	   Num	Abbrev.		Description
	   0	echorep		Echo reply
	   3	unreach		Destination unreachable
	   4	squench		Packet loss, slow down
	   5	redir		Shorter route exists
	   6	althost		Alternate host address
	   8	echoreq		Echo request
	   9	routeradv	Router advertisement
	   10	routersol	Router solicitation
	   11	timex		Time exceeded
	   12	paramprob	Invalid IP header
	   13	timereq		Timestamp request
	   14	timerep		Timestamp reply
	   15	inforeq		Information request
	   16	inforep		Information reply
	   17	maskreq		Address mask request
	   18	maskrep		Address mask reply
	   30	trace		Traceroute
	   31	dataconv	Data conversion problem
	   32	mobredir	Mobile host redirection
	   33	ipv6-where	IPv6 where-are-you
	   34	ipv6-here	IPv6 i-am-here
	   35	mobregreq	Mobile registration request
	   36	mobregrep	Mobile registration reply
	   39	skip		SKIP
	   40	photuris	Photuris

     The following codes are defined:

	   Num	Abbrev.		Type	    Description
	   0	net-unr		unreach	    Network unreachable
	   1	host-unr	unreach	    Host unreachable
	   2	proto-unr	unreach	    Protocol unreachable
	   3	port-unr	unreach	    Port unreachable
	   4	needfrag	unreach	    Fragmentation needed but DF bit
					    set
	   5	srcfail		unreach	    Source routing failed
	   6	net-unk		unreach	    Network unknown
	   7	host-unk	unreach	    Host unknown
	   8	isolate		unreach	    Host isolated
	   9	net-prohib	unreach	    Network administratively
					    prohibited
	   10	host-prohib	unreach	    Host administratively prohibited
	   11	net-tos		unreach	    Invalid TOS for network
	   12	host-tos	unreach	    Invalid TOS for host
	   13	filter-prohib	unreach	    Prohibited access
	   14	host-preced	unreach	    Precedence violation
	   15	cutoff-preced	unreach	    Precedence cutoff
	   0	redir-net	redir	    Shorter route for network
	   1	redir-host	redir	    Shorter route for host
	   2	redir-tos-net	redir	    Shorter route for TOS and network
	   3	redir-tos-host	redir	    Shorter route for TOS and host
	   0	normal-adv	routeradv   Normal advertisement
	   16	common-adv	routeradv   Selective advertisement
	   0	transit		timex	    Time exceeded in transit
	   1	reassemb	timex	    Time exceeded in reassembly
	   0	badhead		paramprob   Invalid option pointer
	   1	optmiss		paramprob   Missing option
	   2	badlen		paramprob   Invalid length
	   1	unknown-ind	photuris    Unknown security index
	   2	auth-fail	photuris    Authentication failed
	   3	decrypt-fail	photuris    Decryption failed

   MIB Variables
     The ICMP protocol implements a number of variables in the net.inet.icmp
     branch of the sysctl(3) MIB.

     maskrepl	     (boolean) Enable/disable replies to ICMP Address Mask
		     Request packets.  Defaults to false.

     maskfake	     (unsigned integer) When maskrepl is set and this value is
		     non-zero, it will be used instead of the real address
		     mask when the system replies to an ICMP Address Mask
		     Request packet.  Defaults to 0.

     icmplim	     (integer) Bandwidth limit for ICMP replies in pack‐
		     ets/second.  Used when icmplim_output is non-zero.
		     Defaults to 200.

     icmplim_output  (boolean) Enable/disable bandwidth limiting of ICMP
		     replies.  Defaults to true.

     drop_redirect   (boolean) Enable/disable dropping of ICMP Redirect pack‐
		     ets.  Defaults to false.

     log_redirect    (boolean) Enable/disable logging of ICMP Redirect pack‐
		     ets.  Defaults to false.

     bmcastecho	     (boolean) Enable/disable ICMP replies received via broad‐
		     cast or multicast.	 Defaults to false.

     reply_src	     (str) An interface name used for the ICMP reply source in
		     response to packets which are not directly addressed to
		     us.  By default continue with normal source selection.

     reply_from_interface
		     (boolean) Use the IP address of the interface the packet
		     came through in for responses to packets which are not
		     directly addressed to us.	If enabled, this rule is pro‐
		     cessed before all others.	By default, continue with nor‐
		     mal source selection.  Enabling this option is particu‐
		     larly useful on routers because it makes external tracer‐
		     outes show the actual path a packet has taken instead of
		     the possibly different return path.

     quotelen	     (integer) Number of bytes from original packet to quote
		     in ICMP reply.  This number is internally enforced to be
		     at least 8 bytes (per RFC792) and at most the maximal
		     space left in the ICMP reply mbuf.

ERRORS
     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 data‐
			gram 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 has not 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.

SEE ALSO
     recv(2), send(2), inet(4), intro(4), ip(4), pf.conf(5)

HISTORY
     The icmp protocol appeared in 4.3BSD.

BSD			       February 9, 2007				   BSD
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