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<title>IP Multicasting with Socat</title>
<link rel="stylesheet" type="text/css" href="dest-unreach.css">
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<body>

<h1>IP Multicasting with Socat</h1>

<h2>Introduction</h2>
<p>
Multicasting (and broadcasting which is also discussed in this article)
provides a means to direct a single packet to more than one host. Special
addresses are defined for this purpose and are handled specially by network
adapters, networking hardware, and IP stacks.
</p>
<p>
IPv4 specifications provide broadcasting and multicasting; IPv6 provides
multicasting but replaces broadcasting by special multicast modes. UNIX domain
sockets do not know broadcasting or multicasting.
</p>
<p>
The following examples use UDP/IPv4 only. However, they can easily be
adapted for raw IPv4 sockets. IPv6 multicasting has not yet been successfully
used with socat; please contact the author if you have positive experiences or
ideas that go beyond <tt>IPV6_ADD_MEMBERSHIP</tt>. 
</p>
<p>
All multicast examples presented in this document use multicast address
224.1.0.1; it can be replaced by any valid IPv4 multicast address (except 
<a href="#ALLSYSTEMS">all-systems</a>).
</p>
<p>
We assume a local network with address 192.168.10.0 and mask 255.255.255.0; an
eventual "client" has 192.168.10.1, example "server" and example peer have
192.168.10.2 in all examples. Change these addresses and mask to your own
requirements.
</p>
<p>
All the following examples work bidirectionally except when otherwise noticed. 
For "clients" we just use <tt>STDIO</tt>, and for "servers" we use <tt>EXEC:hostname</tt> which
ingores its input but shows us which host the reply comes from. Replace these
socat addresses with what is appropriate for your needs (e.g. shell script
invocations). Port 6666 can be replaced with any other port (but for ports &lt;
1024 root privilege might be required).
</p>
<p>
Different kinds of broadcast addresses exist: 255.255.255.255 is local network
only; for the IPv4 network 192.168.10.0/24 the "official" broadcast address
is 192.168.10.255; the network address 192.168.10.0 is also interpreted as
broadcast by some hosts. The two latter forms are routed by gateways. In the
following examples we only use broadcast address 192.168.10.255.
</p>

<h2>Example 1: Multicast client and servers</h2>

<p>This example builds something like a "supervisor" or "client" that
communicates with a set of "servers". The supervisor may send packets to the
multicast address, and the servers may send response packets. Note that the
servers would also respond to other clients' requests.</p>

<p>Multicast server:</p>

<span class="frame"><span class="shell">
socat UDP4-RECVFROM:6666,ip-add-membership=224.1.0.1:192.168.10.2,fork EXEC:hostname
</span></span>
<p>
This command receives multicast packets addressed to 224.1.0.1 and forks a
child process for each. The child processes may each send one or more reply
packets back to the particular sender. 192.168.10.2 means the address of the
interface where multicasts should be received.
Run this command on a number of hosts, and they will all respond in
parallel.</p>

<p>Multicast client:</p>

<span class="frame"><span class="shell">
socat STDIO UDP4-DATAGRAM:224.1.0.1:6666,range=192.168.10.0/24
</span></span>
<p>
This process transfers data from stdin to the multicast address, and transfers 
packets received from the local network to stdout. It does not matter in which
direction the first data is passed.
A packet from the network is accepted by the IP stack for our socket if:
<ul>
<li>it is an incoming UDP/IPv4 packet</li>
<li>its target port matches the local port assigned to the socket (random)</li>
<li>its target address matches one of the hosts local addresses or the any-host
multicast address</li>
</ul>
Of these packets, socat handles only those matching the following criteria:
<ul>
<li>the source address is within the given range</li>
<li>the source port is 6666</li>
</ul>
</p>


<h2>Example 2: Broadcast client and servers</h2>

<p>Broadcast server:</p>

<span class="frame"><span class="shell">
socat UDP4-RECVFROM:6666,broadcast,fork EXEC:hostname
</span></span>
<p>
This command receives packets addressed to a local broadcast address and forks
a child process for each. The child processes may each send one or more reply
packets back to the particular sender. 
Run this command on a number of hosts, and they will all respond in
parallel.</p>

<p>Broadcast client:</p>

<span class="frame"><span class="shell">
socat STDIO UDP4-DATAGRAM:192.168.10.255:6666,broadcast,range=192.168.10.0/24
</span></span>
<p>
This process transfers data from stdin to the broadcast address, and transfers 
packets received from the local network to stdout. It does not matter in which
direction the first data is passed.
A packet from the network is accepted by the IP stack for our socket if:
<ul>
<li>it is an incoming UDP/IPv4 packet</li>
<li>its target port matches the local port assigned to the socket (6666)</li>
<li>its target address matches one of the hosts local addresses or the any-host
multicast address, or a local broadcast address</li>
</ul>
Of these packets, socat handles only those matching the following criteria:
<ul>
<li>the source address is within the given range</li>
<li>the source port is 6666</li>
</ul>
</p>
<p>The <tt>broadcast</tt> option is only required for sending or receiving
local broadcasts.</p>

<h2>Example 3: Multicast peers</h2>

<p>It is possible to combine multicast sender and receiver in one socat
address. This allows to start processes on different hosts on the local network
that will communicate symmetrically, so each process can send messages that are
received by all the other ones.</p>

<span class="frame"><span class="shell">
socat STDIO UDP4-DATAGRAM:224.1.0.1:6666,bind=:6666,range=192.168.10.0/24,ip-add-membership=224.1.0.1:192.168.10.2
</span></span>
<p>
This command is valid for host 192.168.10.2; adapt this address to the
particular interface addresses of the hosts.
</p>
<p>
Starting this process opens a socket on port 6666 that will receive packets
directed to multicast address 224.1.0.1. Only packets with matching source
address and source port 6666 will be handled though. When this process sends
data to the network the packets will be addressed to 224.1.0.1:6666 and have a
source address of 192.168.10.2:6666, matching the accept criteria of the peers
on the local network.
</p>

<p>Note: this command receives the packets it just has sent; add option
<tt>ip-multicast-loop=0</tt> if this in undesired.</p>

<h2>Example 4: Broadcast peers</h2>

<p>Just as with multicast, it is possible to combine broadcast sender and
receiver in one socat address.</p>

<span class="frame"><span class="shell">
socat STDIO UDP4-DATAGRAM:255.255.255.255:6666,bind=:6666,range=192.168.10.0/24,broadcast
</span></span>
<p>
Starting this process opens a socket on port 6666 that will receive packets
directed to a local broadcast addresses. Only packets with matching source
address and source port 6666 will be handled though. When this process sends
data to the network the packets will be addressed to 255.255.255.255:6666 and
have a source address of 192.168.10.2:6666, matching the accept criteria of
the peers on the local network.
</p>

<p>Note: this command receives the packets it just has sent; there does not
seem to exist a simple way to prevent this.</p>


<h2>Troubleshooting</h2>

<p>
If you do not get an error message during operation, but the packets do not
reach the target processes, use <tt>tcpdump</tt> to see if the packets have the
correct source and destination addresses and ports, and if they leave and enter
the hosts as expected.
</p>
<p>
The following subsections discuss some typical sources of trouble.
</p>

<h3>IP filters</h3>
<p>
If you do not succeed in receiving multicast or broadcast packets, check if
iptables are activated on the receiving or sending host. They might be
configured to disallow this traffic. 
</p>

<h3>Do not bind()</h3>
<p>
When using multicast communications, you should not bind the sockets to a
specific IP address. It seems that the (Linux) IP stack compares the
destination address with the bind address, not taking care of the multicast
property of the incoming packet.
</p>

<h3>Routing</h3>
<p>
When you receive an error like:</p>
<table border="1" bgcolor="#e08080"><tr><td><tt>... E sendto(3, 0x80c2e44, 4,
0, AF=2 224.1.0.1:6666, 16): Network is unreachable</tt></td></tr></table>
<p>you have a routing problem. The (Linux) IP stack seems to handle multicast
addresses just like unicast addresses when determining their route (interface
  and gateway), i.e. the routing table needs an entry that somehow matches the
  target address. </p>
<p>
For the same reason, multicast packets will probably leave your host on the
interface with the default route if it is specified.</p>
<p>
Set a multicast/broadcast route with the following command (Linux):</p>
<span class="frame"><span class="shell">
route add -net 224.0.0.0/3 gw 192.168.10.2
</span></span>

<a name="ALLSYSTEMS"></a>
<h3>ALL-SYSTEMS multicast address</h3>
<p>
<tt>224.0.0.1</tt> is the all-systems multicast address: all
datagram sockets appear to be automatically member of this group on all
interfaces. This membership cannot be dropped on Linux (you need iptables to
filter packets).
</p>


<h2>(In)Security</h2>

<p>When you use the above examples you should understand that all datagram
sockets without exception accept all packets that are directly addressed to
them;
the multi- and broadcast receiving features are just extensions to this
functionality. socat currently has no means to handle incoming packets
differently whether they are addressed to unicast, multicast, or broadcast
addresses. However, for EXEC'd scripts socat can provide this info in
environment variables.
</p>

<p>Authentication or encryption are not available.</p>

<p>It is very easy to fake the source address of UDP (or raw IP) packets. You
should understand whether your network is protected from address spoofing
attacks.</p>

<p>Broadcast and multicast traffic can trivially be received by <em>any</em>
host on the local network.</p>


<h2>History</h2>

Starting with version 1.5.0, socat provides a set of address types that
allow various operations on datagram oriented sockets:
<dl>
<dt>SENDTO</dt><dd>send packets to a remote socket and receive packet from this
remote socket only</dd>
<dt>RECV</dt><dd>receive all packets that arrive on the local socket, but do
not reply</dd>
<dt>RECVFROM</dt><dd>receive all packets that arrive on the local socket, and
reply using child processes</dd>
</dl>

<p>
These modes already enable several different client/server oriented operations.
Moreover, the SENDTO addresses can send to multicast and broadcast addresses
(the latter requires the <tt>broadcast</tt> option though). RECV and RECVFROM
also would accept packets addressed to a local broadcast address (with option
<tt>broadcast</tt>) or the all-systems multicast address.
</p>

<p>
These address types had, however, two major caveats: 
<ul>
<li>Missing control of multicast group membership in the RECV and RECVFROM
addresses</li>
<li>The SENDTO address would never accept a reply to a broadcast or multicast
addressed packet because the source address of incoming replies would not match
the target address of the sent packet.
</ul>
</p>

<h3>New Features in socat 1.6.0</h3>

<p>
socat version 1.6.0 addresses these problems and provides a new more generic
datagram address type (*-DATAGRAM) and the new address option IP-ADD-MEMBERSHIP.
</p>

<p>
Please note that the new features could not be successfully tested on IPv6;
these sections thus apply to IPv4 only.
</p>

<h3>New Features in socat 1.7.0</h3>

<p>
socat version 1.7.0 helps to find more information about incoming packets in
environment variables that can be used in scripts or programs invoked by
socat. The option <tt>ip-pktinfo</tt> (on non-BSD systems) 
or <tt>ip-recvdstaddr</tt> (on BSD systems) is required to get basic
information about incoming packets.
</p>

<p>
Example: Start a receiver of the following form (tried on Linux):
</p>

<span class="frame"><span class="shell">
socat -u UDP-RECVFROM:8888,reuseaddr,ip-add-membership=224.1.0.1:192.168.10.2,ip-pktinfo,fork SYSTEM:export
</span></span>

<p>
Then send a multicast packet from the client:
</p>

<span class="frame"><span class="shell">
echo |socat -u STDIO UDP-DATAGRAM:224.1.0.1:8888
</span></span>

<p>
On the server the following text should appear (only interesting lines shown):
</p>

<pre>
export SOCAT_IP_DSTADDR="224.1.0.1"
export SOCAT_IP_IF="eth0"
export SOCAT_IP_LOCADDR="192.168.10.2"
export SOCAT_PEERADDR="192.168.10.1"
export SOCAT_PEERPORT="41159"
</pre>

<p>
<tt>SOCAT_IP_IF</tt> shows the interface where the packet entered the server;
<tt>SOCAT_IP_LOCADDR</tt> shows the IP address of this interface;
<tt>SOCAT_IP_DSTADDR</tt> shows the target address of the packet;
<tt>SOCAT_PEERADDR</tt> and <tt>SOCAT_PEERPORT</tt> are the client socket
values.
</p>

<h2>More info about socat datagrams</h2>

<h3>Links regarding this tutorial</h3>
<a href="socat.html#ADDRESS_UDP4_DATAGRAM">address UDP4-DATAGRAM</a><br>
<a href="socat.html#ADDRESS_UDP4_RECVFROM">address UDP4-RECVFROM</a><br>
<a href="socat.html#OPTION_RANGE">option range</a><br>
<a href="socat.html#OPTION_SO_BROADCAST">option broadcast</a><br>
<a href="socat.html#OPTION_IP_ADD_MEMBERSHIP">option ip-add-membership</a><br>
<a href="socat.html#OPTION_FORK">option fork</a><br>
<a href="socat.html#OPTION_BIND">option bind</a><br>

<h3>Other datagram addresses</h3>
<a href="socat.html#ADDRESS_UDP4_RECV">address UDP4-RECV</a>: pure datagram receiver<br>
<a href="socat.html#ADDRESS_UDP4_SENDTO">address UDP4-SENDTO</a>: communicate
with one peer address<br>
<a href="socat.html#ADDRESS_UDP4_LISTEN">address UDP4-LISTEN</a>: pseudo stream server<br>
<a href="socat.html#ADDRESS_UDP4_CONNECT">address UDP4-CONNECT</a>: pseudo stream client<br>

<h3>Related socat option groups</h3>
<a href="socat.html#GROUP_IP">IP options</a><br>
<a href="socat.html#GROUP_SOCKET">socket options</a><br>
<a href="socat.html#GROUP_FD">file descriptor options</a><br>
<a href="socat.html#GROUP_RANGE">range options</a><br>
<a href="socat.html#GROUP_CHILD">child process options</a><br>


<h2>References</h2>
<a href="http://www.dest-unreach.org/socat">socat home page</a><br>
<a href="socat.html">socat man page</a><br>
<a href="http://en.wikipedia.org/wiki/Multicast">multicasting on Wikipedia</a><br>
<a href="http://en.wikipedia.org/wiki/Broadcast_address">broadcasting on Wikipedia</a><br>

<p>
<small>This document was last modified in May 2009.</small><br>
<small>Copyright: Gerhard Rieger 2007-2009</small><br>
<small>License: <a href="http://www.fsf.org/licensing/licenses/fdl.html">GNU Free Documentation License (FDL)</a></small>
</p>

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