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.\"     @(#)icmp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd June 19, 2002
.Dt ICMP 4
.Os BSD 4.3
.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
and
.Xr recvfrom
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.
Note that the ip_off and ip_len fields are in host byte order.
For more information about the IP header structure, see 
.Xr ip 4 .
.Ss "Non-privileged ICMP"
.Pp
.Tn ICMP
sockets can be opened with the 
.Dv SOCK_DGRAM
socket type without requiring root privileges. The synopsis is the following:
.Pp
.Fn socket AF_INET SOCK_DGRAM IPPROTO_ICMP
.Pp
Datagram oriented
.Tn ICMP
sockets offer a subset of the functionality available to raw 
.Tn ICMP
sockets. Only
.Tn 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 
.Tn ICMP 
message request is eight (8) octets.
.Pp
The advantage of using datagram oriented
.Tn ICMP
sockets is that even a non-privileged process can use 
.Tn ICMP
echo requests to gauge the quality of the connectivity to a host, 
or to receive 
.Tn ICMP
destination unreachable message for path MTU discovery, or to receive 
time exceeded messages for traceroute. 
.Pp
The following 
.Tn IP
level option can be used with datagram oriented 
.Tn ICMP
sockets:
.Bl -column ip_opts -offset 4n
.It IP_OPTIONS
.It IP_HDRINCL
.It IP_TOS
.It IP_TTL
.It IP_RECVOPTS
.It IP_RECVRETOPTS
.It IP_RECVDSTADDR
.It IP_RETOPTS
.It IP_MULTICAST_IF
.It IP_MULTICAST_TTL
.It IP_MULTICAST_LOOP
.It IP_ADD_MEMBERSHIP
.It IP_DROP_MEMBERSHIP
.It IP_MULTICAST_VIF
.It IP_PORTRANGE
.It IP_RECVIF
.It IP_IPSEC_POLICY
.It IP_STRIPHDR
.El
.Pp
When the  
.Tn IP
option IP_HDRINCL is used, the provided 
.Tn IP 
header must obey the following rules:
.Bl -column ip_opts -offset 4n
.It ip_v Ta Must be IPVERSION (4);
.It ip_hl Ta Between 5 and 10 (inclusive);
.It ip_tos Ta Any value;
.It ip_len Ta Must be the total length of IP datagram (IP header + ICMP message);
.It ip_id Ta Must be zero, will be automatically set;
.It ip_off Ta Must be zero, will be automatically set;
.It ip_ttl Ta Any value;
.It ip_p Ta Must be IPPROTO_IP;
.It ip_sum Ta Value ignored, will be automatically set;
.It ip_src Ta Must be an 
.Tn IP
address currently assigned to one of the local interface or INADDR_ANY;
.It ip_dst Ta Any address;
.It ip_opts Ta Any option.
.El
.Pp
The maximum length of a IMCP message that can be sent is controlled 
by the sysctl 
variable net.inet.raw.maxdgram.
.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;
.It Bq Er EINVAL
when an invalid value is used with
.Tn IMCP
datagram socket for a field of the  
.Tn IP
or
.Tn ICMP
header.
.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 .