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 the divert port 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).

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 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. 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.

LOOP AVOIDANCE
To avoid having a packet sent from a divert socket rediverted back to the same socket, use the sendto(2) system call supplying any non-zero destination port number. This indicates to ipfw(8) and other diverting mechanisms to not divert the packet back to the same socket it was written from.

Since ipfw checks incoming as well as outgoing packets, a packet written as incoming may get checked twice. Loop avoidance will be enabled for both checks.

DETAILS
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 fragments divert to different ports, then which port ultimately gets chosen is unpredictable.

Packets are received and sent unchanged, except that 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)

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'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.

AUTHOR
Archie Cobbs <archie@whistle.com>, Whistle Communications Corp.