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NAME    [Toc]    [Back]

     divert -- kernel packet diversion mechanism

SYNOPSIS    [Toc]    [Back]

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


DESCRIPTION    [Toc]    [Back]

     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    [Toc]    [Back]

     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    [Toc]    [Back]

     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 nonlocal
 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 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    [Toc]    [Back]

     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    [Toc]    [Back]

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

     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 fragments 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 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

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

ERRORS    [Toc]    [Back]

     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

SEE ALSO    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     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

AUTHORS    [Toc]    [Back]

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

FreeBSD 5.2.1			 June 18, 1996			 FreeBSD 5.2.1
[ Back ]
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