ICMP(4) FreeBSD Kernel Interfaces Manual ICMP(4)


icmpInternet Control Message Protocol


#include < sys/types.h>
#include < sys/socket.h>
#include < netinet/in.h>

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

The following codes are defined:

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

MIB Variables

The ICMP protocol implements a number of variables in the net.inet.icmp branch of the sysctl(3) MIB.
( boolean) Enable/disable replies to ICMP Address Mask Request packets. Defaults to false.
( 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.
( integer) Bandwidth limit for ICMP replies in packets/second. Used when icmplim_output is non-zero. Defaults to 200.
( boolean) Enable/disable bandwidth limiting of ICMP replies. Defaults to true.
( boolean) Enable/disable dropping of ICMP Redirect packets. Defaults to false.
( boolean) Enable/disable logging of ICMP Redirect packets. Defaults to false.
( boolean) Enable/disable ICMP replies received via broadcast or multicast. Defaults to false.
( 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.
( 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.
( 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 icmp protocol appeared in 4.3BSD.
February 9, 2007 FreeBSD