icmp(4) Internet Control Message Protocol


In sys/types.h In sys/socket.h In netinet/in.h Ft int Fn socket AF_INET SOCK_RAW proto


ICMP is the error and control message protocol used by IP and the Internet protocol family. It may be accessed through a ``raw socket'' for network monitoring and diagnostic functions. The Fa proto parameter to the socket call to create an ICMP socket is obtained from getprotobyname(3). ICMP sockets are connectionless, and are normally used with the sendto(2) and recvfrom(2) calls, though the connect(2) call may also be used to fix the destination for future packets (in which case the read(2) or recv(2) and write(2) or send(2) system calls may be used).

Outgoing packets automatically have an IP header prepended to them (based on the destination address). Incoming packets are received with the IP header and options intact.


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 pf.conf5. The following types are defined:

Num Ta Abbrev. Ta Description
0 Ta echorep Ta Echo reply
3 Ta unreach Ta Destination unreachable
4 Ta squench Ta Packet loss, slow down
5 Ta redir Ta Shorter route exists
6 Ta althost Ta Alternate host address
8 Ta echoreq Ta Echo request
9 Ta routeradv Ta Router advertisement
10 Ta routersol Ta Router solicitation
11 Ta timex Ta Time exceeded
12 Ta paramprob Ta Invalid IP header
13 Ta timereq Ta Timestamp request
14 Ta timerep Ta Timestamp reply
15 Ta inforeq Ta Information request
16 Ta inforep Ta Information reply
17 Ta maskreq Ta Address mask request
18 Ta maskrep Ta Address mask reply
30 Ta trace Ta Traceroute
31 Ta dataconv Ta Data conversion problem
32 Ta mobredir Ta Mobile host redirection
33 Ta ipv6-where Ta IPv6 where-are-you
34 Ta ipv6-here Ta IPv6 i-am-here
35 Ta mobregreq Ta Mobile registration request
36 Ta mobregrep Ta Mobile registration reply
39 Ta skip Ta SKIP
40 Ta photuris Ta Photuris

The following codes are defined:

Num Ta Abbrev. Ta Type Ta Description
0 Ta net-unr Ta unreach Ta Network unreachable
1 Ta host-unr Ta unreach Ta Host unreachable
2 Ta proto-unr Ta unreach Ta Protocol unreachable
3 Ta port-unr Ta unreach Ta Port unreachable
4 Ta needfrag Ta unreach Ta Fragmentation needed but DF bit set
5 Ta srcfail Ta unreach Ta Source routing failed
6 Ta net-unk Ta unreach Ta Network unknown
7 Ta host-unk Ta unreach Ta Host unknown
8 Ta isolate Ta unreach Ta Host isolated
9 Ta net-prohib Ta unreach Ta Network administratively prohibited
10 Ta host-prohib Ta unreach Ta Host administratively prohibited
11 Ta net-tos Ta unreach Ta Invalid TOS for network
12 Ta host-tos Ta unreach Ta Invalid TOS for host
13 Ta filter-prohib Ta unreach Ta Prohibited access
14 Ta host-preced Ta unreach Ta Precedence violation
15 Ta cutoff-preced Ta unreach Ta Precedence cutoff
0 Ta redir-net Ta redir Ta Shorter route for network
1 Ta redir-host Ta redir Ta Shorter route for host
2 Ta redir-tos-net Ta redir Ta Shorter route for TOS and network
3 Ta redir-tos-host Ta redir Ta Shorter route for TOS and host
0 Ta normal-adv Ta routeradv Ta Normal advertisement
16 Ta common-adv Ta routeradv Ta Selective advertisement
0 Ta transit Ta timex Ta Time exceeded in transit
1 Ta reassemb Ta timex Ta Time exceeded in reassembly
0 Ta badhead Ta paramprob Ta Invalid option pointer
1 Ta optmiss Ta paramprob Ta Missing option
2 Ta badlen Ta paramprob Ta Invalid length
1 Ta unknown-ind Ta photuris Ta Unknown security index
2 Ta auth-fail Ta photuris Ta Authentication failed
3 Ta decrypt-fail Ta photuris Ta Decryption failed

MIB Variables

The ICMP protocol implements a number of variables in the net.inet.icmp branch of the sysctl(3) MIB.

(Vt boolean ) Enable/disable replies to ICMP Address Mask Request packets. Defaults to false.
(Vt unsigned integer ) When 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.
(Vt integer ) Bandwidth limit for ICMP replies in packets/second. Used when icmplim_output is non-zero. Defaults to 200.
(Vt boolean ) Enable/disable bandwidth limiting of ICMP replies. Defaults to true.
(Vt boolean ) Enable/disable dropping of ICMP Redirect packets. Defaults to false.
(Vt boolean ) Enable/disable logging of ICMP Redirect packets. Defaults to false.
(Vt boolean ) Enable/disable ICMP replies received via broadcast or multicast. Defaults to false.
(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.
(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.
(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.


A socket operation may fail with one of the following errors returned:

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;
when trying to send a datagram, but no destination address is specified, and the socket has not been connected;
when the system runs out of memory for an internal data structure;
when an attempt is made to create a socket with a network address for which no network interface exists.


The protocol appeared in BSD 4.3