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ICMP(4) Kernel Interfaces Manual ICMP(4) NAME icmp -- Internet Control Message Protocol SYNOPSIS #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> int socket(AF_INET, SOCK_RAW, proto); DESCRIPTION ICMP is the error and control message protocol used by IP and the In- ternet 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. Types 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 (sysctl) Variables The ICMP protocol implements a number of variables in the net.inet.icmp branch of the sysctl(3) MIB, which can also be read or modified with sysctl(8). bmcastecho (boolean) Enable/disable ICMP replies received via broadcast or multicast. Defaults to false. drop_redirect (boolean) Enable/disable dropping of ICMP Redirect packets. Defaults to false. icmplim (unsigned integer) Mean rate limit for replies in pack- ets/second. The actual limit is icmplim plus a random jitter limited by icmplim_jitter. If set to zero, no limiting will occur. Defaults to 200. icmplim_jitter (unsigned integer) A random jitter between the negative of icmplim_jitter and icmplim_jitter is applied to icmplim for limiting the sending rate of replies. icmplim_jitter must be smaller than icmplim, if icmplim is not zero. If set to zero, no jitter will be ap- plied. Defaults to 16. icmplim_output (boolean) Enable/disable logging of ICMP replies band- width limiting. Defaults to true. log_redirect (boolean) Enable/disable logging of ICMP Redirect pack- ets. Defaults to false. maskfake (unsigned integer) When maskrepl is set and this value is non-zero, it will be used instead of the real ad- dress mask when the system replies to an ICMP Address Mask Request packet. Defaults to 0. maskrepl (boolean) Enable/disable replies to ICMP Address Mask Request packets. Defaults to false. quotelen (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 maxi- mal space left in the ICMP reply mbuf. redirtimeout (integer) Delay in seconds before expiring route cre- ated by ICMP redirect. reply_from_interface (boolean) Use the IP address of the interface the packet came in through for responses to packets which are not directly addressed to us. If enabled, this rule is processed before all others. By default, con- tinue with normal source selection. Enabling this op- tion 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. reply_src (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 selec- tion. tstamprepl (boolean) Enable/disable replies to ICMP Timestamp packets. Defaults to true. ERRORS A socket operation may fail with one of the following errors returned: [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; [ENOTCONN] when trying to send a datagram, but no destination address is specified, and the socket has not been connected; [ENOBUFS] when the system runs out of memory for an internal data structure; [EADDRNOTAVAIL] when an attempt is made to create a socket with a network address for which no network interface ex- ists. SEE ALSO recv(2), send(2), sysctl(3), inet(4), intro(4), ip(4), pf.conf(5), sysctl(8) HISTORY The icmp protocol implementation appeared in 4.2BSD. FreeBSD 14.3 December 11, 2024 ICMP(4)
NAME | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | HISTORY
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