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+.\" Copyright (c) 1986, 1991, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
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+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
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+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
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+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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+.\" SUCH DAMAGE.
+.\"
+.\"     @(#)icmp.4	8.1 (Berkeley) 6/5/93
+.\" $FreeBSD$
+.\"
+.Dd February 9, 2007
+.Dt ICMP 4
+.Os
+.Sh NAME
+.Nm icmp
+.Nd Internet Control Message Protocol
+.Sh SYNOPSIS
+.In sys/types.h
+.In sys/socket.h
+.In netinet/in.h
+.Ft int
+.Fn socket AF_INET SOCK_RAW proto
+.Sh DESCRIPTION
+.Tn ICMP
+is the error and control message protocol used
+by
+.Tn IP
+and the Internet protocol family.
+It may be accessed
+through a
+.Dq raw socket
+for network monitoring
+and diagnostic functions.
+The
+.Fa proto
+parameter to the socket call to create an
+.Tn ICMP
+socket
+is obtained from
+.Xr getprotobyname 3 .
+.Tn ICMP
+sockets are connectionless,
+and are normally used with the
+.Xr sendto 2
+and
+.Xr recvfrom 2
+calls, though the
+.Xr connect 2
+call may also be used to fix the destination for future
+packets (in which case the
+.Xr read 2
+or
+.Xr recv 2
+and
+.Xr write 2
+or
+.Xr send 2
+system calls may be used).
+.Pp
+Outgoing packets automatically have an
+.Tn IP
+header prepended to
+them (based on the destination address).
+Incoming packets are received with the
+.Tn IP
+header and options intact.
+.Ss 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
+.Xr pf.conf 5 .
+The following types are defined:
+.Bl -column x xxxxxxxxxxxx -offset indent
+.It Sy Num Ta Sy Abbrev. Ta Sy Description
+.It 0 Ta echorep Ta "Echo reply"
+.It 3 Ta unreach Ta "Destination unreachable"
+.It 4 Ta squench Ta "Packet loss, slow down"
+.It 5 Ta redir Ta "Shorter route exists"
+.It 6 Ta althost Ta "Alternate host address"
+.It 8 Ta echoreq Ta "Echo request"
+.It 9 Ta routeradv Ta "Router advertisement"
+.It 10 Ta routersol Ta "Router solicitation"
+.It 11 Ta timex Ta "Time exceeded"
+.It 12 Ta paramprob Ta "Invalid IP header"
+.It 13 Ta timereq Ta "Timestamp request"
+.It 14 Ta timerep Ta "Timestamp reply"
+.It 15 Ta inforeq Ta "Information request"
+.It 16 Ta inforep Ta "Information reply"
+.It 17 Ta maskreq Ta "Address mask request"
+.It 18 Ta maskrep Ta "Address mask reply"
+.It 30 Ta trace Ta Traceroute
+.It 31 Ta dataconv Ta "Data conversion problem"
+.It 32 Ta mobredir Ta "Mobile host redirection"
+.It 33 Ta ipv6-where Ta "IPv6 where-are-you"
+.It 34 Ta ipv6-here Ta "IPv6 i-am-here"
+.It 35 Ta mobregreq Ta "Mobile registration request"
+.It 36 Ta mobregrep Ta "Mobile registration reply"
+.It 39 Ta skip Ta SKIP
+.It 40 Ta photuris Ta Photuris
+.El
+.Pp
+The following codes are defined:
+.Bl -column x xxxxxxxxxxxx xxxxxxxx -offset indent
+.It Sy Num Ta Sy Abbrev. Ta Sy Type Ta Sy Description
+.It 0 Ta net-unr Ta unreach Ta "Network unreachable"
+.It 1 Ta host-unr Ta unreach Ta "Host unreachable"
+.It 2 Ta proto-unr Ta unreach Ta "Protocol unreachable"
+.It 3 Ta port-unr Ta unreach Ta "Port unreachable"
+.It 4 Ta needfrag Ta unreach Ta "Fragmentation needed but DF bit set"
+.It 5 Ta srcfail Ta unreach Ta "Source routing failed"
+.It 6 Ta net-unk Ta unreach Ta "Network unknown"
+.It 7 Ta host-unk Ta unreach Ta "Host unknown"
+.It 8 Ta isolate Ta unreach Ta "Host isolated"
+.It 9 Ta net-prohib Ta unreach Ta "Network administratively prohibited"
+.It 10 Ta host-prohib Ta unreach Ta "Host administratively prohibited"
+.It 11 Ta net-tos Ta unreach Ta "Invalid TOS for network"
+.It 12 Ta host-tos Ta unreach Ta "Invalid TOS for host"
+.It 13 Ta filter-prohib Ta unreach Ta "Prohibited access"
+.It 14 Ta host-preced Ta unreach Ta "Precedence violation"
+.It 15 Ta cutoff-preced Ta unreach Ta "Precedence cutoff"
+.It 0 Ta redir-net Ta redir Ta "Shorter route for network"
+.It 1 Ta redir-host Ta redir Ta "Shorter route for host"
+.It 2 Ta redir-tos-net Ta redir Ta "Shorter route for TOS and network"
+.It 3 Ta redir-tos-host Ta redir Ta "Shorter route for TOS and host"
+.It 0 Ta normal-adv Ta routeradv Ta "Normal advertisement"
+.It 16 Ta common-adv Ta routeradv Ta "Selective advertisement"
+.It 0 Ta transit Ta timex Ta "Time exceeded in transit"
+.It 1 Ta reassemb Ta timex Ta "Time exceeded in reassembly"
+.It 0 Ta badhead Ta paramprob Ta "Invalid option pointer"
+.It 1 Ta optmiss Ta paramprob Ta "Missing option"
+.It 2 Ta badlen Ta paramprob Ta "Invalid length"
+.It 1 Ta unknown-ind Ta photuris Ta "Unknown security index"
+.It 2 Ta auth-fail Ta photuris Ta "Authentication failed"
+.It 3 Ta decrypt-fail Ta photuris Ta "Decryption failed"
+.El
+.Ss MIB Variables
+The
+.Tn ICMP
+protocol implements a number of variables in the
+.Va net.inet.icmp
+branch of the
+.Xr sysctl 3
+MIB.
+.Bl -tag -width ".Va icmplim_output"
+.It Va maskrepl
+.Pq Vt boolean
+Enable/disable replies to ICMP Address Mask Request packets.
+Defaults to false.
+.It Va maskfake
+.Pq Vt "unsigned integer"
+When
+.Va 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.
+.It Va icmplim
+.Pq Vt integer
+Bandwidth limit for ICMP replies in packets/second.
+Used when
+.Va icmplim_output
+is non-zero.
+Defaults to 200.
+.It Va icmplim_output
+.Pq Vt boolean
+Enable/disable bandwidth limiting of ICMP replies.
+Defaults to true.
+.It Va drop_redirect
+.Pq Vt boolean
+Enable/disable dropping of ICMP Redirect packets.
+Defaults to false.
+.It Va log_redirect
+.Pq Vt boolean
+Enable/disable logging of ICMP Redirect packets.
+Defaults to false.
+.It Va bmcastecho
+.Pq Vt boolean
+Enable/disable ICMP replies received via broadcast or multicast.
+Defaults to false.
+.It Va reply_src
+.Pq Vt 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.
+.It Va reply_from_interface
+.Pq Vt 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 processed before all others.
+By default, continue with normal source selection.
+Enabling this option is particularly useful on routers because it
+makes external traceroutes show the actual path a packet has taken
+instead of the possibly different return path.
+.It Va quotelen
+.Pq Vt 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.
+.El
+.Sh ERRORS
+A socket operation may fail with one of the following errors returned:
+.Bl -tag -width Er
+.It Bq Er 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;
+.It Bq Er ENOTCONN
+when trying to send a datagram, but
+no destination address is specified, and the socket has not been
+connected;
+.It Bq Er ENOBUFS
+when the system runs out of memory for
+an internal data structure;
+.It Bq Er EADDRNOTAVAIL
+when an attempt is made to create a
+socket with a network address for which no network interface
+exists.
+.El
+.Sh SEE ALSO
+.Xr recv 2 ,
+.Xr send 2 ,
+.Xr inet 4 ,
+.Xr intro 4 ,
+.Xr ip 4 ,
+.Xr pf.conf 5
+.Sh HISTORY
+The
+.Nm
+protocol appeared in
+.Bx 4.3 .