divert

DIVERT(4)                 BSD Kernel Interfaces Manual                 DIVERT(4)

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);

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
     To enable divert sockets, a kernel must be compiled with options IPDIVERT
     or the ipdivert.ko module can be loaded at run-time:

           kldload ipdivert

     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(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.

BSD                             December 17, 2004                            BSD