.\"	$OpenBSD: icmp.4,v 1.10 2004/12/23 20:33:03 jaredy Exp $
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.\"     @(#)icmp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 5, 1993
.Dt ICMP 4
.Os
.Sh NAME
.Nm icmp
.Nd Internet Control Message Protocol
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Fd #include <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 xxxxxxx -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 unreac 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
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.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 hasn't 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 ip 4 ,
.Xr netintro 4
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.3 .