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NAME

     divert -- kernel packet diversion mechanism

SYNOPSIS

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

     int
     socket(PF_INET, SOCK_RAW, IPPROTO_DIVERT);

     To enable support for divert sockets, place the following lines in the
     kernel configuration file:

           options IPFIREWALL
           options IPDIVERT

     Alternatively, to load divert as a module at boot time, add the following
     lines into the loader.conf(5) file:

           ipfw_load="YES"
           ipdivert_load="YES"

DESCRIPTION

     Divert sockets are similar to raw IP sockets, except that they can be
     bound to a specific divert port via the bind(2) system call.  The IP
     address in the bind is ignored; only the port number is significant.  A
     divert socket bound to a divert port will receive all packets diverted to
     that port by some (here unspecified) kernel mechanism(s).  Packets may
     also be written to a divert port, in which case they re-enter kernel IP
     packet processing.

     Divert sockets are normally used in conjunction with FreeBSD's packet
     filtering implementation and the ipfw(8) program.  By reading from and
     writing to a divert socket, matching packets can be passed through an
     arbitrary ``filter'' as they travel through the host machine, special
     routing tricks can be done, etc.

READING PACKETS

     Packets are diverted either as they are ``incoming'' or ``outgoing.''
     Incoming packets are diverted after reception on an IP interface, whereas
     outgoing packets are diverted before next hop forwarding.

     Diverted packets may be read unaltered via read(2), recv(2), or
     recvfrom(2).  In the latter case, the address returned will have its port
     set to some tag supplied by the packet diverter, (usually the ipfw rule
     number) and the IP address set to the (first) address of the interface on
     which the packet was received (if the packet was incoming) or INADDR_ANY
     (if the packet was outgoing).  The interface name (if defined for the
     packet) will be placed in the 8 bytes following the address, if it fits.

WRITING PACKETS

     Writing to a divert socket is similar to writing to a raw IP socket; the
     packet is injected ``as is'' into the normal kernel IP packet processing
     using sendto(2) and minimal error checking is done.  Packets are
     distinguished as either incoming or outgoing.  If sendto(2) is used with
     a destination IP address of INADDR_ANY, then the packet is treated as if
     it were outgoing, i.e., destined for a non-local address.  Otherwise, the
     packet is assumed to be incoming and full packet routing is done.

     In the latter case, the IP address specified must match the address of
     some local interface, or an interface name must be found after the IP
     address.  If an interface name is found, that interface will be used and
     the value of the IP address will be ignored (other than the fact that it
     is not INADDR_ANY).  This is to indicate on which interface the packet
     ``arrived''.

     Normally, packets read as incoming should be written as incoming;
     similarly for outgoing packets.  When reading and then writing back
     packets, passing the same socket address supplied by recvfrom(2)
     unmodified to sendto(2) simplifies things (see below).

     The port part of the socket address passed to the sendto(2) contains a
     tag that should be meaningful to the diversion module.  In the case of
     ipfw(8) the tag is interpreted as the rule number after which rule
     processing should restart.

LOOP AVOIDANCE

     Packets written into a divert socket (using sendto(2)) re-enter the
     packet filter at the rule number following the tag given in the port part
     of the socket address, which is usually already set at the rule number
     that caused the diversion (not the next rule if there are several at the
     same number).  If the 'tag' is altered to indicate an alternative re-
     entry point, care should be taken to avoid loops, where the same packet
     is diverted more than once at the same rule.

DETAILS

     If a packet is diverted but no socket is bound to the port, or if
     IPDIVERT is not enabled or loaded in the kernel, the packet is dropped.

     Incoming packet fragments which get diverted are fully reassembled before
     delivery; the diversion of any one fragment causes the entire packet to
     get diverted.  If different fragments divert to different ports, then
     which port ultimately gets chosen is unpredictable.

     Note that packets arriving on the divert socket by the ipfw(8) tee action
     are delivered as-is and packet fragments do not get reassembled in this
     case.

     Packets are received and sent unchanged, except that packets read as
     outgoing have invalid IP header checksums, and packets written as
     outgoing have their IP header checksums overwritten with the correct
     value.  Packets written as incoming and having incorrect checksums will
     be dropped.  Otherwise, all header fields are unchanged (and therefore in
     network order).

     Binding to port numbers less than 1024 requires super-user access, as
     does creating a socket of type SOCK_RAW.

ERRORS

     Writing to a divert socket can return these errors, along with the usual
     errors possible when writing raw packets:

     [EINVAL]           The packet had an invalid header, or the IP options in
                        the packet and the socket options set were
                        incompatible.

     [EADDRNOTAVAIL]    The destination address contained an IP address not
                        equal to INADDR_ANY that was not associated with any
                        interface.

SEE ALSO

     bind(2), recvfrom(2), sendto(2), socket(2), ipfw(4), ipfw(8)

AUTHORS

     Archie Cobbs <archie@FreeBSD.org>, Whistle Communications Corp.

BUGS

     This is an attempt to provide a clean way for user mode processes to
     implement various IP tricks like address translation, but it could be
     cleaner, and it is too dependent on ipfw(8).

     It is questionable whether incoming fragments should be reassembled
     before being diverted.  For example, if only some fragments of a packet
     destined for another machine do not get routed through the local machine,
     the packet is lost.  This should probably be a settable socket option in
     any case.