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.\"     @(#)icmp.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd August 21, 2005
.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 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
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.3 .