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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.