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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).
p 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"
p .Tn ICMP sockets can be opened with the .Dv SOCK_DGRAM socket type without requiring root privileges. The synopsis is the following:
p .Fn socket AF_INET SOCK_DGRAM IPPROTO_ICMP
p 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.
p 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.
p The following .Tn IP level option can be used with datagram oriented .Tn ICMP sockets: l -column ip_opts -offset 4n t IP_OPTIONS t IP_HDRINCL t IP_TOS t IP_TTL t IP_RECVOPTS t IP_RECVRETOPTS t IP_RECVDSTADDR t IP_RETOPTS t IP_MULTICAST_IF t IP_MULTICAST_TTL t IP_MULTICAST_LOOP t IP_ADD_MEMBERSHIP t IP_DROP_MEMBERSHIP t IP_MULTICAST_VIF t IP_PORTRANGE t IP_RECVIF t IP_IPSEC_POLICY t IP_STRIPHDR .El
p When the .Tn IP option IP_HDRINCL is used, the provided .Tn IP header must obey the following rules: l -column ip_opts -offset 4n t ip_v Ta Must be IPVERSION (4); t ip_hl Ta Between 5 and 10 (inclusive); t ip_tos Ta Any value; t ip_len Ta Must be the total length of IP datagram (IP header + ICMP message); t ip_id Ta Must be zero, will be automatically set; t ip_off Ta Must be zero, will be automatically set; t ip_ttl Ta Any value; t ip_p Ta Must be IPPROTO_IP; t ip_sum Ta Value ignored, will be automatically set; t ip_src Ta Must be an .Tn IP address currently assigned to one of the local interface or INADDR_ANY; t ip_dst Ta Any address; t ip_opts Ta Any option. .El
p 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: l -tag -width [EADDRNOTAVAIL] t 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; t Bq Er ENOTCONN when trying to send a datagram, but no destination address is specified, and the socket hasn't been connected; t Bq Er ENOBUFS when the system runs out of memory for an internal data structure; t Bq Er EADDRNOTAVAIL when an attempt is made to create a socket with a network address for which no network interface exists; t 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 x 4.3 .