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

     divert -- kernel packet diversion mechanism

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


     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 implementa-
     tion 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.

     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 the 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). In the case of an incoming packet the interface name will also
     be placed in the 8 bytes following the address, (assuming it fits).

     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 and minimal error checking is done.
     Packets are written as either incoming or outgoing: if write(2) or send(2) is used to
     deliver the packet, or 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

     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 sup-
     plied 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.

     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.

     To enable divert sockets, your kernel must be compiled with the option IPDIVERT.

     If a packet is diverted but no socket is bound to the port, or if IPDIVERT is not enabled 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 frag-
     ments divert to different ports, then which port ultimately gets chosen is unpredictable.

     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 over-
     written 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

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

     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.

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

     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's too dependent on ipfw(8).

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

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

BSD					  June 18, 1996 				      BSD
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