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 the driver 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.