freebsd/share/man/man4/divert.4

.Dd June 18, 1996
.Dt DIVERT 4
.Os FreeBSD
.Sh NAME
.Nm divert 
.Nd kernel packet diversion mechanism
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Fd #include <netinet/in.h>
.Ft int
.Fn socket PF_INET SOCK_RAW IPPROTO_DIVERT
.Sh DESCRIPTION
.Pp
Divert sockets are similar to raw IP sockets, except that they
can be bound to a specific
.Nm
port via the
.Xr 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.
.Pp
Divert sockets are normally used in conjunction with 
FreeBSD's packet filtering implementation and the
.Xr 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.
.Sh 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.
.Pp
Diverted packets may be read unaltered via
.Xr read 2 ,
.Xr recv 2 ,
or
.Xr 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
.Dv INADDR_ANY
(if the packet was outgoing).
.Sh 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
.Xr write 2
or
.Xr send 2
is used to deliver the packet, or if
.Xr sendto 2
is used with a destination IP address of
.Dv 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.
.Pp
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.''
.Pp
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
.Xr recvfrom 2
unmodified to
.Xr sendto 2
simplifies things.
.Sh LOOP AVOIDANCE
To avoid having a packet sent from a divert socket rediverted back
to the same socket, use the
.Xr sendto 2
system call supplying any non-zero destination port number.
This indicates to
.Xr ipfw 8
and other diverting mechanisms to not divert the packet back
to the same socket it was written from.
.Pp
Since
.Xr 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.
.Sh DETAILS
To enable divert sockets, your kernel must be compiled with the option
.Dv IPDIVERT .
.Pp
If a packet is diverted but no socket is bound to the
port, or if
.Dv IPDIVERT
is not enabled in the kernel, the packet is dropped.
.Pp
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.
.Pp
Packets are received and sent unchanged, with two exceptions:
read as incoming will have their IP header checksum zeroed,
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).
.Pp
Binding to port numbers less than 1024 requires super-user access.
.Sh ERRORS
Writing to a divert socket can return these errors, along with
the usual errors possible when writing raw packets:
.Bl -tag -width Er
.It Bq Er EINVAL
The packet had an invalid header, or the IP options in the packet
and the socket options set were incompatible.
.It Bq Er EADDRNOTAVAIL
The destination address contained an IP address not equal to
.Dv INADDR_ANY
that was not associated with any interface.
.El
.Sh SEE ALSO
.Xr ipfw 8 ,
.Xr socket 2 ,
.Xr bind 2 ,
.Xr sendto 2 .
.Xr recvfrom 2 ,
.Sh 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
.Xr ipfw 8 .
.Pp
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.
.Sh AUTHOR
Archie Cobbs <archie@whistle.com>, Whistle Communications Corp.