193 lines
6.1 KiB
Groff
193 lines
6.1 KiB
Groff
.\" $FreeBSD$
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.\"
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.Dd December 17, 2004
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.Dt DIVERT 4
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.Os
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.Sh NAME
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.Nm divert
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.Nd kernel packet diversion mechanism
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/socket.h
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.In netinet/in.h
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.Ft int
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.Fn socket PF_INET SOCK_RAW IPPROTO_DIVERT
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.Pp
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To enable support for divert sockets, place the following lines in the
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kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "options IPFIREWALL"
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.Cd "options IPDIVERT"
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.Ed
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.Pp
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Alternatively, to load
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the driver
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as a module at boot time, add the following lines into the
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.Xr loader.conf 5
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file:
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.Bd -literal -offset indent
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ipfw_load="YES"
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ipdivert_load="YES"
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.Ed
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.Sh DESCRIPTION
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Divert sockets are similar to raw IP sockets, except that they
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can be bound to a specific
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.Nm
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port via the
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.Xr bind 2
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system call.
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The IP address in the bind is ignored; only the port
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number is significant.
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A divert socket bound to a divert port will receive all packets diverted
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to that port by some (here unspecified) kernel mechanism(s).
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Packets may also be written to a divert port, in which case they
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re-enter kernel IP packet processing.
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.Pp
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Divert sockets are normally used in conjunction with
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.Fx Ns 's
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packet filtering implementation and the
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.Xr ipfw 8
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program.
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By reading from and writing to a divert socket, matching packets
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can be passed through an arbitrary ``filter'' as they travel through
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the host machine, special routing tricks can be done, etc.
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.Sh READING PACKETS
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Packets are diverted either as they are ``incoming'' or ``outgoing.''
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Incoming packets are diverted after reception on an IP interface,
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whereas outgoing packets are diverted before next hop forwarding.
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.Pp
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Diverted packets may be read unaltered via
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.Xr read 2 ,
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.Xr recv 2 ,
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or
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.Xr recvfrom 2 .
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In the latter case, the address returned will have its port set to
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some tag supplied by the packet diverter, (usually the ipfw rule number)
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and the IP address set to the (first) address of
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the interface on which the packet was received (if the packet
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was incoming) or
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.Dv INADDR_ANY
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(if the packet was outgoing).
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The interface name (if defined
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for the packet) will be placed in the 8 bytes following the address,
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if it fits.
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.Sh WRITING PACKETS
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Writing to a divert socket is similar to writing to a raw IP socket;
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the packet is injected ``as is'' into the normal kernel IP packet
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processing using
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.Xr sendto 2
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and minimal error checking is done.
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Packets are distinguished as either incoming or outgoing.
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If
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.Xr sendto 2
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is used with a destination IP address of
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.Dv INADDR_ANY ,
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then the packet is treated as if it were outgoing, i.e., destined
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for a non-local address.
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Otherwise, the packet is assumed to be
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incoming and full packet routing is done.
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.Pp
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In the latter case, the
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IP address specified must match the address of some local interface,
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or an interface name
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must be found after the IP address.
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If an interface name is found,
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that interface will be used and the value of the IP address will be
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ignored (other than the fact that it is not
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.Dv INADDR_ANY ) .
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This is to indicate on which interface the packet
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.Dq arrived .
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.Pp
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Normally, packets read as incoming should be written as incoming;
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similarly for outgoing packets.
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When reading and then writing back
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packets, passing the same socket address supplied by
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.Xr recvfrom 2
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unmodified to
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.Xr sendto 2
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simplifies things (see below).
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.Pp
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The port part of the socket address passed to the
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.Xr sendto 2
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contains a tag that should be meaningful to the diversion module.
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In the
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case of
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.Xr ipfw 8
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the tag is interpreted as the rule number
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.Em after which
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rule processing should restart.
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.Sh LOOP AVOIDANCE
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Packets written into a divert socket
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(using
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.Xr sendto 2 )
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re-enter the packet filter at the rule number
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following the tag given in the port part of the socket address, which
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is usually already set at the rule number that caused the diversion
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(not the next rule if there are several at the same number).
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If the 'tag'
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is altered to indicate an alternative re-entry point, care should be taken
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to avoid loops, where the same packet is diverted more than once at the
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same rule.
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.Sh DETAILS
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If a packet is diverted but no socket is bound to the
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port, or if
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.Dv IPDIVERT
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is not enabled or loaded in the kernel, the packet is dropped.
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.Pp
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Incoming packet fragments which get diverted are fully reassembled
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before delivery; the diversion of any one fragment causes the entire
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packet to get diverted.
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If different fragments divert to different ports,
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then which port ultimately gets chosen is unpredictable.
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.Pp
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Note that packets arriving on the divert socket by the
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.Xr ipfw 8
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.Cm tee
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action are delivered as-is and packet fragments do not get reassembled
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in this case.
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.Pp
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Packets are received and sent unchanged, except that
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packets read as outgoing have invalid IP header checksums, and
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packets written as outgoing have their IP header checksums overwritten
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with the correct value.
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Packets written as incoming and having incorrect checksums will be dropped.
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Otherwise, all header fields are unchanged (and therefore in network order).
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.Pp
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Binding to port numbers less than 1024 requires super-user access, as does
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creating a socket of type SOCK_RAW.
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.Sh ERRORS
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Writing to a divert socket can return these errors, along with
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the usual errors possible when writing raw packets:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The packet had an invalid header, or the IP options in the packet
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and the socket options set were incompatible.
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.It Bq Er EADDRNOTAVAIL
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The destination address contained an IP address not equal to
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.Dv INADDR_ANY
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that was not associated with any interface.
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.El
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.Sh SEE ALSO
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.Xr bind 2 ,
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.Xr recvfrom 2 ,
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.Xr sendto 2 ,
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.Xr socket 2 ,
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.Xr ipfw 4 ,
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.Xr ipfw 8
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.Sh AUTHORS
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.An Archie Cobbs Aq Mt archie@FreeBSD.org ,
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Whistle Communications Corp.
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.Sh BUGS
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This is an attempt to provide a clean way for user mode processes
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to implement various IP tricks like address translation, but it
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could be cleaner, and it is too dependent on
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.Xr ipfw 8 .
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.Pp
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It is questionable whether incoming fragments should be reassembled
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before being diverted.
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For example, if only some fragments of a
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packet destined for another machine do not get routed through the
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local machine, the packet is lost.
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This should probably be
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a settable socket option in any case.
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