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 signifi-cant. significant.
cant. 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 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 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, 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) unmodi-fied unmodified
fied 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, 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 ker-nel, kernel,
nel, 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 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.
AUTHORS
Archie Cobbs <archie@FreeBSD.org>, Whistle Communications Corp.
BSD June 18, 1996 BSD
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