xref: /freebsd/share/man/man4/divert.4 (revision 6b3455a7665208c366849f0b2b3bc916fb97516e)
1.\" $FreeBSD$
2.\"
3.Dd June 18, 1996
4.Dt DIVERT 4
5.Os
6.Sh NAME
7.Nm divert
8.Nd kernel packet diversion mechanism
9.Sh SYNOPSIS
10.In sys/types.h
11.In sys/socket.h
12.In netinet/in.h
13.Ft int
14.Fn socket PF_INET SOCK_RAW IPPROTO_DIVERT
15.Sh DESCRIPTION
16Divert sockets are similar to raw IP sockets, except that they
17can be bound to a specific
18.Nm
19port via the
20.Xr bind 2
21system call.
22The IP address in the bind is ignored; only the port
23number is significant.
24A divert socket bound to a divert port will receive all packets diverted
25to that port by some (here unspecified) kernel mechanism(s).
26Packets may also be written to a divert port, in which case they
27re-enter kernel IP packet processing.
28.Pp
29Divert sockets are normally used in conjunction with
30.Fx Ns 's
31packet filtering implementation and the
32.Xr ipfw 8
33program.
34By reading from and writing to a divert socket, matching packets
35can be passed through an arbitrary ``filter'' as they travel through
36the host machine, special routing tricks can be done, etc.
37.Sh READING PACKETS
38Packets are diverted either as they are ``incoming'' or ``outgoing.''
39Incoming packets are diverted after reception on an IP interface,
40whereas outgoing packets are diverted before next hop forwarding.
41.Pp
42Diverted packets may be read unaltered via
43.Xr read 2 ,
44.Xr recv 2 ,
45or
46.Xr recvfrom 2 .
47In the latter case, the address returned will have its port set to
48some tag supplied by the packet diverter, (usually the ipfw rule number)
49and the IP address set to the (first) address of
50the interface on which the packet was received (if the packet
51was incoming) or
52.Dv INADDR_ANY
53(if the packet was outgoing).
54The interface name (if defined
55for the packet) will be placed in the 8 bytes following the address,
56if it fits.
57.Sh WRITING PACKETS
58Writing to a divert socket is similar to writing to a raw IP socket;
59the packet is injected ``as is'' into the normal kernel IP packet
60processing and minimal error checking is done.
61Packets are written as either incoming or outgoing:
62if
63.Xr write 2
64or
65.Xr send 2
66is used to deliver the packet, or if
67.Xr sendto 2
68is used with a destination IP address of
69.Dv INADDR_ANY ,
70then the packet is treated as if it were outgoing, i.e., destined
71for a non-local address.
72Otherwise, the packet is assumed to be
73incoming and full packet routing is done.
74.Pp
75In the latter case, the
76IP address specified must match the address of some local interface,
77or an interface name
78must be found after the IP address.
79If an interface name is found,
80that interface will be used and the value of the IP address will be
81ignored (other than the fact that it is not
82.Dv INADDR_ANY ) .
83This is to indicate on which interface the packet
84.Dq arrived .
85.Pp
86Normally, packets read as incoming should be written as incoming;
87similarly for outgoing packets.
88When reading and then writing back
89packets, passing the same socket address supplied by
90.Xr recvfrom 2
91unmodified to
92.Xr sendto 2
93simplifies things (see below).
94.Pp
95The port part of the socket address passed to the
96.Xr sendto 2
97contains a tag that should be meaningful to the diversion module.
98In the
99case of
100.Xr ipfw 8
101the tag is interpreted as the rule number
102.Em after which
103rule processing should restart.
104.Sh LOOP AVOIDANCE
105Packets written into a divert socket
106(using
107.Xr sendto 2 )
108re-enter the packet filter at the rule number
109following the tag given in the port part of the socket address, which
110is usually already set at the rule number that caused the diversion
111(not the next rule if there are several at the same number).
112If the 'tag'
113is altered to indicate an alternative re-entry point, care should be taken
114to avoid loops, where the same packet is diverted more than once at the
115same rule.
116.Sh DETAILS
117To enable divert sockets, your kernel must be compiled with the option
118.Dv IPDIVERT .
119.Pp
120If a packet is diverted but no socket is bound to the
121port, or if
122.Dv IPDIVERT
123is not enabled in the kernel, the packet is dropped.
124.Pp
125Incoming packet fragments which get diverted are fully reassembled
126before delivery; the diversion of any one fragment causes the entire
127packet to get diverted.
128If different fragments divert to different ports,
129then which port ultimately gets chosen is unpredictable.
130.Pp
131Packets are received and sent unchanged, except that
132packets read as outgoing have invalid IP header checksums, and
133packets written as outgoing have their IP header checksums overwritten
134with the correct value.
135Packets written as incoming and having incorrect checksums will be dropped.
136Otherwise, all header fields are unchanged (and therefore in network order).
137.Pp
138Binding to port numbers less than 1024 requires super-user access, as does
139creating a socket of type SOCK_RAW.
140.Sh ERRORS
141Writing to a divert socket can return these errors, along with
142the usual errors possible when writing raw packets:
143.Bl -tag -width Er
144.It Bq Er EINVAL
145The packet had an invalid header, or the IP options in the packet
146and the socket options set were incompatible.
147.It Bq Er EADDRNOTAVAIL
148The destination address contained an IP address not equal to
149.Dv INADDR_ANY
150that was not associated with any interface.
151.El
152.Sh SEE ALSO
153.Xr bind 2 ,
154.Xr recvfrom 2 ,
155.Xr sendto 2 ,
156.Xr socket 2 ,
157.Xr ipfw 8
158.Sh BUGS
159This is an attempt to provide a clean way for user mode processes
160to implement various IP tricks like address translation, but it
161could be cleaner, and it's too dependent on
162.Xr ipfw 8 .
163.Pp
164It's questionable whether incoming fragments should be reassembled
165before being diverted.
166For example, if only some fragments of a
167packet destined for another machine don't get routed through the
168local machine, the packet is lost.
169This should probably be
170a settable socket option in any case.
171.Sh AUTHORS
172.An Archie Cobbs Aq archie@FreeBSD.org ,
173Whistle Communications Corp.
174