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<table class="head">
<tr>
<td class="head-ltitle">DIVERT(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">DIVERT(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">divert</code> — <span class="Nd">kernel
packet diversion mechanism</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="In">#include
<<a class="In">sys/types.h</a>></code>
<br/>
<code class="In">#include <<a class="In">sys/socket.h</a>></code>
<br/>
<code class="In">#include <<a class="In">netinet/in.h</a>></code></p>
<p class="Pp"><var class="Ft">int</var>
<br/>
<code class="Fn">socket</code>(<var class="Fa" style="white-space: nowrap;">PF_DIVERT</var>,
<var class="Fa" style="white-space: nowrap;">SOCK_RAW</var>,
<var class="Fa" style="white-space: nowrap;">0</var>);</p>
<p class="Pp">To enable support for divert sockets, place the following lines in
the kernel configuration file:</p>
<div class="Bd Pp Bd-indent"><code class="Cd">options IPDIVERT</code></div>
<p class="Pp">Alternatively, to load the driver as a module at boot time, add
the following lines into the <a class="Xr">loader.conf(5)</a> file:</p>
<div class="Bd Pp Bd-indent Li">
<pre>ipdivert_load="YES"</pre>
</div>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">Divert sockets allow to intercept and re-inject packets flowing
through the <a class="Xr">ipfw(4)</a> and <a class="Xr">pf(4)</a> firewalls.
A divert socket can be bound to a specific <code class="Nm">divert</code>
port via the <a class="Xr">bind(2)</a> system call. The sockaddr argument
shall be sockaddr_in with sin_port set to the desired value. Note that the
<code class="Nm">divert</code> port has nothing to do with TCP/UDP ports. It
is just a cookie whose value depends on the firewall in use. For
<a class="Xr">ipfw(4)</a> this is the number of the rule which diverted the
packet; for <a class="Xr">pf(4)</a> this is a value which indicates the
original direction through the firewall of the diverted packet. A divert
socket bound to a divert port will receive all packets diverted to that port
by the firewall. Packets may also be written to a divert port, in which case
they re-enter firewall processing at the next rule.</p>
<p class="Pp">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.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="READING_PACKETS"><a class="permalink" href="#READING_PACKETS">READING
PACKETS</a></h1>
<p class="Pp">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.</p>
<p class="Pp">Diverted packets may be read unaltered via
<a class="Xr">read(2)</a>, <a class="Xr">recv(2)</a>, or
<a class="Xr">recvfrom(2)</a>. In the latter case, the address returned will
have its port set to some tag supplied by the packet diverter, (usually the
cookie described above) and the IP address set to the (first) address of the
interface on which the packet was received (if the packet was incoming) or
<code class="Dv">INADDR_ANY</code> (if the packet was outgoing). The
interface name (if defined for the packet) will be placed in the 8 bytes
following the address, if it fits.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="WRITING_PACKETS"><a class="permalink" href="#WRITING_PACKETS">WRITING
PACKETS</a></h1>
<p class="Pp">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 using <a class="Xr">sendto(2)</a> and minimal error checking is
done. Packets are distinguished as either incoming or outgoing. If
<a class="Xr">sendto(2)</a> is used with a destination IP address of
<code class="Dv">INADDR_ANY</code>, 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.</p>
<p class="Pp">In the latter case, the IP address specified must match the
address of some local interface, or an interface name must be found after
the IP address. If an interface name is found, that interface will be used
and the value of the IP address will be ignored (other than the fact that it
is not <code class="Dv">INADDR_ANY</code>). This is to indicate on which
interface the packet “arrived”.</p>
<p class="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 <a class="Xr">recvfrom(2)</a>
unmodified to <a class="Xr">sendto(2)</a> simplifies things (see below).</p>
<p class="Pp" id="after">The port part of the socket address passed to the
<a class="Xr">sendto(2)</a> contains a tag that should be meaningful to the
diversion module. In the case of <a class="Xr">ipfw(8)</a> the tag is
interpreted as the rule number
<a class="permalink" href="#after"><i class="Em">after which</i></a> rule
processing should restart.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="LOOP_AVOIDANCE"><a class="permalink" href="#LOOP_AVOIDANCE">LOOP
AVOIDANCE</a></h1>
<p class="Pp">Packets written into a divert socket (using
<a class="Xr">sendto(2)</a>) re-enter the packet filter at the rule number
following the tag given in the port part of the socket address, which is
usually already set at the rule number that caused the diversion (not the
next rule if there are several at the same number). If the 'tag' is altered
to indicate an alternative re-entry point, care should be taken to avoid
loops, where the same packet is diverted more than once at the same
rule.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="DETAILS"><a class="permalink" href="#DETAILS">DETAILS</a></h1>
<p class="Pp">If a packet is diverted but no socket is bound to the port, or if
<code class="Dv">IPDIVERT</code> is not enabled or loaded in the kernel, the
packet is dropped.</p>
<p class="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.</p>
<p class="Pp">Note that packets arriving on the divert socket by the
<a class="Xr">ipfw(8)</a> <code class="Cm">tee</code> action are delivered
as-is and packet fragments do not get reassembled in this case.</p>
<p class="Pp">Packets are received and sent unchanged, except that packets read
as outgoing have invalid IP header checksums, 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).</p>
<p class="Pp">Creating a <code class="Nm">divert</code> socket requires
super-user access.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="ERRORS"><a class="permalink" href="#ERRORS">ERRORS</a></h1>
<p class="Pp">Writing to a divert socket can return these errors, along with the
usual errors possible when writing raw packets:</p>
<dl class="Bl-tag">
<dt id="EINVAL">[<a class="permalink" href="#EINVAL"><code class="Er">EINVAL</code></a>]</dt>
<dd>The packet had an invalid header, or the IP options in the packet and the
socket options set were incompatible.</dd>
<dt id="EADDRNOTAVAIL">[<a class="permalink" href="#EADDRNOTAVAIL"><code class="Er">EADDRNOTAVAIL</code></a>]</dt>
<dd>The destination address contained an IP address not equal to
<code class="Dv">INADDR_ANY</code> that was not associated with any
interface.</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">bind(2)</a>, <a class="Xr">recvfrom(2)</a>,
<a class="Xr">sendto(2)</a>, <a class="Xr">socket(2)</a>,
<a class="Xr">ipfw(4)</a>, <a class="Xr">pf(4)</a>,
<a class="Xr">ipfw(8)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp"><span class="An">Archie Cobbs</span>
<<a class="Mt" href="mailto:archie@FreeBSD.org">archie@FreeBSD.org</a>>,
Whistle Communications Corp.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="BUGS"><a class="permalink" href="#BUGS">BUGS</a></h1>
<p class="Pp">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.</p>
<p class="Pp">It is questionable whether incoming fragments should be
reassembled before being diverted. For example, if only some fragments of a
packet destined for another machine do not get routed through the local
machine, the packet is lost. This should probably be a settable socket
option in any case.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">January 23, 2026</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
