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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 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.

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

     maskrepl        (boolean) Enable/disable replies to ICMP Address Mask Request packets.
                     Defaults to false.

     maskfake        (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.

     icmplim         (integer) Bandwidth limit for ICMP replies in packets/second.  Used when
                     icmplim_output is non-zero.  Defaults to 200.

     icmplim_output  (boolean) Enable/disable bandwidth limiting of ICMP replies.  Defaults to
                     true.

     drop_redirect   (boolean) Enable/disable dropping of ICMP Redirect packets.  Defaults to
                     false.

     log_redirect    (boolean) Enable/disable logging of ICMP Redirect packets.  Defaults to
                     false.

     bmcastecho      (boolean) Enable/disable ICMP replies received via broadcast or multicast.
                     Defaults to false.

     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 selection.

     reply_from_interface
                     (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.

     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 maximal space left in the ICMP reply mbuf.

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 exists.

SEE ALSO

     recv(2), send(2), inet(4), intro(4), ip(4), pf.conf(5)

HISTORY

     The icmp protocol appeared in 4.3BSD.