DIVERT(4) BSD Kernel Interfaces Manual DIVERT(4)
divert -- kernel packet diversion mechanism
socket(PF_INET, SOCK_RAW, IPPROTO_DIVERT);
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
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.
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 inter-
face 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 incom-
ing 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; simi-
larly 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 pro-
cessing 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
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 out-
going have invalid IP header checksums, and packets written as outgoing
have their IP header checksums overwritten with the correct value. Pack-
ets 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 incompati-
[EADDRNOTAVAIL] The destination address contained an IP address not
equal to INADDR_ANY that was not associated with any
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
Archie Cobbs <archieATFreeBSD.org>, Whistle Communications Corp.
BSD June 18, 1996 BSD