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passed to the user process for incoming packets. When the sockaddr_in is passed back to the divert socket later, use thi sas the primary interface lookup and only revert to the IP address when the name fails. This solves a long standing bug with divert sockets: When two interfaces had the same address (P2P for example) the interface "assigned" to the reinjected packet was sometimes incorect. Probably we should define a "sockaddr_div" to officially hold this extended information in teh same manner as sockaddr_dl.
158 lines
5.5 KiB
Groff
158 lines
5.5 KiB
Groff
.\" $Id: divert.4,v 1.10 1998/03/12 07:30:16 charnier Exp $
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.\"
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.Dd June 18, 1996
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.Dt DIVERT 4
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.Os FreeBSD
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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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.Fd #include <sys/types.h>
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.Fd #include <sys/socket.h>
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.Fd #include <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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.Sh DESCRIPTION
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.Pp
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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. 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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FreeBSD's packet filtering implementation and the
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.Xr ipfw 8
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program. 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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the divert port 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). In the case of an incoming packet the interface
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name will also be placed in the 8 bytes following the address,
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(assuming 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 and minimal error checking is done.
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Packets are written as either incoming or outgoing:
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if
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.Xr write 2
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or
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.Xr send 2
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is used to deliver the packet, or 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. 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. 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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).
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This is to indicate on which interface the packet ``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. 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.
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.Sh LOOP AVOIDANCE
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Packets written into a divert socket (using
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.Xr sendto 2 )
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are never rediverted back to the same socket. This means that a
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given packet (either incoming or outgoing) will be diverted to a
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given socket once and once only.
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.Pp
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.Xr Ipfw 8
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rules are executed in order, each time the packet passes through
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the kernel, but only up until a matching
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.Nm
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rule applies. On the second pass, after the packet has been diverted,
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the divert rule is ignored and any subsequent
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.Xr ipfw 8
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rules are applied. For this reason, it is normally best to specify your
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divert rules prior to any others.
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.Sh DETAILS
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To enable divert sockets, your kernel must be compiled with the option
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.Dv IPDIVERT .
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.Pp
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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 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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Packets are received and sent unchanged, except that
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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 8
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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's too dependent on
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.Xr ipfw 8 .
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.Pp
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It's questionable whether incoming fragments should be reassembled
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before being diverted. For example, if only some fragments of a
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packet destined for another machine don't get routed through the
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local machine, the packet is lost. This should probably be
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a settable socket option in any case.
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.Sh AUTHORS
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.An Archie Cobbs Aq archie@whistle.com ,
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Whistle Communications Corp.
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