bsd/man/man4/divert.4

   1 .\" $FreeBSD: src/share/man/man4/divert.4,v 1.15.2.5 2001/08/17 13:08:37 ru Exp $
   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 .Fd #include <sys/types.h>
  11 .Fd #include <sys/socket.h>
  12 .Fd #include <netinet/in.h>
  13 .Ft int
  14 .Fn socket PF_INET SOCK_RAW IPPROTO_DIVERT
  15 .Sh DESCRIPTION
  16 Divert sockets are similar to raw IP sockets, except that they
  17 can be bound to a specific
  18 .Nm
  19 port via the
  20 .Xr bind 2
  21 system call.
  22 The IP address in the bind is ignored; only the port
  23 number is significant.
  24 A divert socket bound to a divert port will receive all packets diverted
  25 to that port by some (here unspecified) kernel mechanism(s).
  26 Packets may also be written to a divert port, in which case they
  27 re-enter kernel IP packet processing.
  28 .Pp
  29 Divert sockets are normally used in conjunction with
  30 .Fx Ns 's
  31 packet filtering implementation and the
  32 .Xr ipfw 8
  33 program.
  34 By reading from and writing to a divert socket, matching packets
  35 can be passed through an arbitrary ``filter'' as they travel through
  36 the host machine, special routing tricks can be done, etc.
  37 .Sh READING PACKETS
  38 Packets are diverted either as they are ``incoming'' or ``outgoing.''
  39 Incoming packets are diverted after reception on an IP interface,
  40 whereas outgoing packets are diverted before next hop forwarding.
  41 .Pp
  42 Diverted packets may be read unaltered via
  43 .Xr read 2 ,
  44 .Xr recv 2 ,
  45 or
  46 .Xr recvfrom 2 .
  47 In the latter case, the address returned will have its port set to
  48 the some tag supplied by the packet diverter, (usually the ipfw rule number)
  49 and the IP address set to the (first) address of
  50 the interface on which the packet was received (if the packet
  51 was incoming) or
  52 .Dv INADDR_ANY
  53 (if the packet was outgoing). In the case of an incoming packet the interface
  54 name will also be placed in the 8 bytes following the address,
  55 (assuming it fits).
  56 .Sh WRITING PACKETS
  57 Writing to a divert socket is similar to writing to a raw IP socket;
  58 the packet is injected ``as is'' into the normal kernel IP packet
  59 processing and minimal error checking is done.
  60 Packets are written as either incoming or outgoing:
  61 if
  62 .Xr write 2
  63 or
  64 .Xr send 2
  65 is used to deliver the packet, or if
  66 .Xr sendto 2
  67 is used with a destination IP address of
  68 .Dv INADDR_ANY ,
  69 then the packet is treated as if it were outgoing, i.e., destined
  70 for a non-local address.  Otherwise, the packet is assumed to be
  71 incoming and full packet routing is done.
  72 .Pp
  73 In the latter case, the
  74 IP address specified must match the address of some local interface,
  75 or an interface name
  76 must be found after the IP address.
  77 If an interface name is found,
  78 that interface will be used and the value of the IP address will be
  79 ignored (other than the fact that it is not
  80 .Dv INADDR_ANY ) .
  81 This is to indicate on which interface the packet ``arrived.''
  82 .Pp
  83 Normally, packets read as incoming should be written as incoming;
  84 similarly for outgoing packets.  When reading and then writing back
  85 packets, passing the same socket address supplied by
  86 .Xr recvfrom 2
  87 unmodified to
  88 .Xr sendto 2
  89 simplifies things (see below).
  90 .Pp
  91 The port part of the socket address passed to the
  92 .Xr sendto 2
  93 contains a tag that should be meaningful to the diversion module.
  94 In the
  95 case of
  96 .Xr ipfw 8
  97 the tag is interpreted as the rule number
  98 .Em after which
  99 rule processing should restart.
 100 .Sh LOOP AVOIDANCE
 101 Packets written into a divert socket
 102 (using
 103 .Xr sendto 2 )
 104 re-enter the packet filter at the rule number
 105 following the tag given in the port part of the socket address, which
 106 is usually already set at the rule number that caused the diversion
 107 (not the next rule if there are several at the same number). If the 'tag'
 108 is altered to indicate an alternative re-entry point, care should be taken
 109 to avoid loops, where the same packet is diverted more than once at the
 110 same rule.
 111 .Sh DETAILS
 112 To enable divert sockets, your kernel must be compiled with the option
 113 .Dv IPDIVERT .
 114 .Pp
 115 If a packet is diverted but no socket is bound to the
 116 port, or if
 117 .Dv IPDIVERT
 118 is not enabled in the kernel, the packet is dropped.
 119 .Pp
 120 Incoming packet fragments which get diverted are fully reassembled
 121 before delivery; the diversion of any one fragment causes the entire
 122 packet to get diverted.
 123 If different fragments divert to different ports,
 124 then which port ultimately gets chosen is unpredictable.
 125 .Pp
 126 Packets are received and sent unchanged, except that
 127 packets read as outgoing have invalid IP header checksums, and
 128 packets written as outgoing have their IP header checksums overwritten
 129 with the correct value.
 130 Packets written as incoming and having incorrect checksums will be dropped.
 131 Otherwise, all header fields are unchanged (and therefore in network order).
 132 .Pp
 133 Binding to port numbers less than 1024 requires super-user access, as does
 134 creating a socket of type SOCK_RAW.
 135 .Sh ERRORS
 136 Writing to a divert socket can return these errors, along with
 137 the usual errors possible when writing raw packets:
 138 .Bl -tag -width Er
 139 .It Bq Er EINVAL
 140 The packet had an invalid header, or the IP options in the packet
 141 and the socket options set were incompatible.
 142 .It Bq Er EADDRNOTAVAIL
 143 The destination address contained an IP address not equal to
 144 .Dv INADDR_ANY
 145 that was not associated with any interface.
 146 .El
 147 .Sh SEE ALSO
 148 .Xr bind 2 ,
 149 .Xr recvfrom 2 ,
 150 .Xr sendto 2 ,
 151 .Xr socket 2 ,
 152 .Xr ipfw 8
 153 .Sh BUGS
 154 This is an attempt to provide a clean way for user mode processes
 155 to implement various IP tricks like address translation, but it
 156 could be cleaner, and it's too dependent on
 157 .Xr ipfw 8 .
 158 .Pp
 159 It's questionable whether incoming fragments should be reassembled
 160 before being diverted.
 161 For example, if only some fragments of a
 162 packet destined for another machine don't get routed through the
 163 local machine, the packet is lost.
 164 This should probably be
 165 a settable socket option in any case.
 166 .Sh AUTHORS
 167 .An Archie Cobbs Aq archie@FreeBSD.org ,
 168 Whistle Communications Corp.