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doc/socat-multicast.html
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<!-- $Revision: 1.1 $ $Date: 2007/03/06 20:54:43 $ -->
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<html><head>
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<title>IP Multicasting with Socat</title>
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<link rel="stylesheet" type="text/css" href="dest-unreach.css">
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</head>
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<body>
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<h1>IP Multicasting with Socat</h1>
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<h2>Introduction</h2>
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<p>
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Multicasting (and broadcasting which is also discussed in this article)
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provides a means to direct a single packet to more than one host. Special
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addresses are defined for this purpose and are handled specially by network
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adapters, networking hardware, and IP stacks.
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</p>
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<p>
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IPv4 specifications provide broadcasting and multicasting; IPv6 provides
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multicasting but replaces broadcasting by special multicast modes. UNIX domain
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sockets do not know broadcasting or multicasting.
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</p>
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<p>
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The following examples use UDP/IPv4 only. However, they can easily be
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adapted for raw IPv4 sockets. IPv6 multicasting has not yet been successfully
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used with socat; please contact the author if you have positive experiences or
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ideas that go beyond <tt>IPV6_ADD_MEMBERSHIP</tt>.
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</p>
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<p>
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All multicast examples presented in this document use multicast address
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224.1.0.1; it can be replaced by any valid IPv4 multicast address (except
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<a href="#ALLSYSTEMS">all-systems</a>).
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</p>
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<p>
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We assume a local network with address 192.168.10.0 and mask 255.255.255.0; an
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eventual "client" has 192.168.10.1, example "server" and example peer have
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192.168.10.2 in all examples. Change these addresses and mask to your own
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requirements.
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</p>
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<p>
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All the following examples work bidirectionally except when otherwise noticed.
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For "clients" we just use <tt>STDIO</tt>, and for "servers" we use <tt>EXEC:hostname</tt> which
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ingores its input but shows us which host the reply comes from. Replace these
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addresses with what is appropriate for you (e.g. shell script
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invokations). Port 6666 can be replaced with any other port (but for ports <
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1024 root privilege might be required).
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</p>
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<p>
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Different kinds of broadcast addresses exist: 255.255.255.255 is local network
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only; for the IPv4 network 192.168.10.0/24 the "official" broadcast address
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is 192.168.10.255; the network address 192.168.10.0 is also interpreted as
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broadcast by some hosts. The two latter forms are routed by gateways. In the
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following examples we only use broadcast address 192.168.10.255.
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</p>
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<h2>Example 1: Multicast client and servers</h2>
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<p>This example builds something like a "supervisor" or "client" that
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communicates with a set of "servers". The supervisor may send packets to the
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multicast address, and the servers may send response packets. Note that the
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servers would also respond to other clients' requests.</p>
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<p>Multicast server:</p>
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<span class="frame"><span class="shell">
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socat UDP4-RECVFROM:6666,ip-add-membership=224.1.0.1:192.168.10.2,fork EXEC:hostname
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</span></span>
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<p>
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This command receives multicast packets addressed to 224.1.0.1 and forks a
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child process for each. The child processes may each send one or more reply
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packets back to the particular sender. 192.168.10.2 means the address of the
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interface where multicasts should be received.
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Run this command on a number of hosts, and they will all respond in
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parallel.</p>
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<p>Multicast client:</p>
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<span class="frame"><span class="shell">
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socat STDIO UDP4-DATAGRAM:224.1.0.1:6666,range=192.168.10.0/24
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</span></span>
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<p>
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This process transfers data from stdin to the multicast address, and transfers
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packets received from the local network to stdout. It does not matter in which
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direction the first data is passed.
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A packet from the network is accepted by the IP stack for our socket if:
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<ul>
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<li>it is an incoming UDP/IPv4 packet</li>
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<li>its target port matches the local port assigned to the socket (6666)</li>
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<li>its target address matches one of the hosts local addresses or the any-host
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multicast address</li>
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</ul>
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Of these packets, socat handles only those matching the following criteria:
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<ul>
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<li>the source address is within the given range</li>
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<li>the source port is 6666</li>
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</ul>
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</p>
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<h2>Example 2: Broadcast client and servers</h2>
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<p>Broadcast server:</p>
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<span class="frame"><span class="shell">
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socat UDP4-RECVFROM:6666,broadcast,fork EXEC:hostname
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</span></span>
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<p>
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This command receives packets addressed to a local broadcast address and forks
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a child process for each. The child processes may each send one or more reply
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packets back to the particular sender.
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Run this command on a number of hosts, and they will all respond in
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parallel.</p>
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<p>Broadcast client:</p>
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<span class="frame"><span class="shell">
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socat STDIO UDP4-DATAGRAM:192.168.10.255:6666,broadcast,range=192.168.10.0/24
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</span></span>
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<p>
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This process transfers data from stdin to the broadcast address, and transfers
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packets received from the local network to stdout. It does not matter in which
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direction the first data is passed.
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A packet from the network is accepted by the IP stack for our socket if:
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<ul>
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<li>it is an incoming UDP/IPv4 packet</li>
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<li>its target port matches the local port assigned to the socket (6666)</li>
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<li>its target address matches one of the hosts local addresses or the any-host
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multicast address, or a local broadcast address</li>
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</ul>
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Of these packets, socat handles only those matching the following criteria:
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<ul>
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<li>the source address is within the given range</li>
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<li>the source port is 6666</li>
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</ul>
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</p>
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<p>The <tt>broadcast</tt> option is only required for sending or receiving
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local broadcasts.</p>
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<h2>Example 3: Multicast peers</h2>
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<p>It is possible to combine multicast sender and receiver in one socat
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address. This allows to start processes on different hosts on the local network
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that will communicate symmetrically, so each process can send messages that are
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received by all the other ones.</p>
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<span class="frame"><span class="shell">
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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
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</span></span>
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<p>
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This command is valid for host 192.168.10.2; adapt this address to the
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particular interface addresses of the hosts.
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</p>
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<p>
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Starting this process opens a socket on port 6666 that will receive packets
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directed to multicast address 224.1.0.1. Only packets with matching source
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address and source port 6666 will be handled though. When this process sends
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data to the network the packets will be addressed to 224.1.0.1:6666 and have a
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source address of 192.168.10.2:6666, matching the accept criteria of the peers
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on the local network.
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</p>
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<p>Note: this command receives the packets it just has sent; add option
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<tt>ip-multicast-loop=0</tt> if this in undesired.</p>
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<h2>Example 4: Broadcast peers</h2>
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<p>Just as with multicast, it is possible to combine broadcast sender and
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receiver in one socat address.</p>
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<span class="frame"><span class="shell">
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socat STDIO UDP4-DATAGRAM:255.255.255.255:6666,bind=:6666,range=192.168.10.0/24,broadcast
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</span></span>
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<p>
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Starting this process opens a socket on port 6666 that will receive packets
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directed to a local broadcast addresses. Only packets with matching source
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address and source port 6666 will be handled though. When this process sends
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data to the network the packets will be addressed to 255.255.255.255:6666 and
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have a source address of 192.168.10.2:6666, matching the accept criteria of
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the peers on the local network.
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</p>
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<p>Note: this command receives the packets it just has sent; there does not
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seem to exist a simple way to prevent this.</p>
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<h2>Troubleshooting</h2>
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<p>
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If you do not get an error message during operation, but the packets do not
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reach the target processes, use <tt>tcpdump</tt> to see if the packets have the
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correct source and destination addresses and ports, and if they leave and enter
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the hosts as expected.
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</p>
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<p>
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The following subsections discuss some typical sources of trouble.
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</p>
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<h3>IP filters</h3>
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<p>
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If you do not succeed in receiving multicast or broadcast packets, check if
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iptables are activated on the receiving or sending host. They might be
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configured to disallow this traffic.
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</p>
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<h3>Do not bind()</h3>
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<p>
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When using multicast communications, you should not bind the sockets to a
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specific IP address. It seems that the (Linux) IP stack compares the
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destination address with the bind address, not taking care of the multicast
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property of the incoming packet.
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</p>
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<h3>Routing</h3>
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<p>
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When you receive an error like:</p>
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<table border="1" bgcolor="#e08080"><tr><td><tt>... E sendto(3, 0x80c2e44, 4,
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0, AF=2 224.1.0.1:6666, 16): Network is unreachable</tt></td></tr></table>
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<p>you have a routing problem. The (Linux) IP stack seems to handle multicast
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addresses just like unicast addresses when determining their route (interface and gateway).</p>
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<p>
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For the same reason, multicast packets will probably leave your host on the
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interface with the default route.</p>
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<p>
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Set a multicast/broadcast route with the following command:</p>
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<span class="frame"><span class="shell">
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route add -net 224.0.0.0/3 gw 192.168.10.2
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</span></span>
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<h3>ALL-SYSTEMS multicast address</h3>
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<p>
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<a name="ALLSYSTEMS"><tt>224.0.0.1</tt></a> is the all-systems multicast address: all
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datagram sockets appear to be automatically member of this group on all
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interfaces. This membership cannot be dropped on Linux.
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</p>
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<h2>(In)Security</h2>
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<p>When you use the above examples you should understand that all datagram
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sockets without exception accept packets that are directly addressed to them;
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the multi- and broadcast receiving features are just extensions to the normal
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functionality. socat has no way to find out if an incoming packet is addressed
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to a unicast, multicast or broadcast address. Please contact the author if you
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know how the target address can be determined.</p>
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<p>Authentication or encryption are not available.</p>
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<p>It is very easy to fake the source address of UDP (or raw IP) packets. You
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should understand whether your network is protected from address spoofing
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attacks.</p>
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<p>Broadcast and multicast traffic can trivially be received by <em>any</em>
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host on the local network.</p>
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<h2>History</h2>
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Starting with version 1.5.0, socat provides a set of address types that
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allow various operations on datagram oriented sockets:
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<dl>
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<dt>SENDTO</dt><dd>send packets to a remote socket and receive packet from this
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remote socket only</dd>
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<dt>RECV</dt><dd>receive all packets that arrive on the local socket, but do
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not reply</dd>
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<dt>RECVFROM</dt><dd>receive all packets that arrive on the local socket, and
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reply using child processes</dd>
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</dl>
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<p>
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These modes already enable several different client/server oriented operations.
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Moreover, the SENDTO addresses can send to multicast and broadcast addresses
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(the latter requires the <tt>broadcast</tt> option though). RECV and RECVFROM
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also would accept packets addressed to a local broadcast address (with option
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<tt>broadcast</tt>) or the all-systems multicast address.
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</p>
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<p>
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These address types had, however, two major caveats:
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<ul>
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<li>Missing control of multicast group membership in the RECV and RECVFROM
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addresses</li>
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<li>The SENDTO address would never accept a reply to a broadcast or multicast
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addressed packet because the source address of incoming replies would not match
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the target address of the sent packet.
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</ul>
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</p>
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<h3>New Features in socat 1.6.0</h3>
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<p>
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socat version 1.6.0 addresses these problems and provides a new more generic
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datagram address type (*-DATAGRAM) and the new address option IP-ADD-MEMBERSHIP.
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</p>
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<p>
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Please note that the new features could not be successfully tested on IPv6;
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these sections thus apply to IPv4 only.
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</p>
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<p>This document was last modified in March 2007.</p>
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<h2>More info about socat datagrams</h2>
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<h3>Links regarding this tutorial</h3>
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<a href="socat.html#ADDRESS_UDP4_DATAGRAM">address udp4-datagram</a><br>
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<a href="socat.html#ADDRESS_UDP4_RECVFROM">address udp4-recvfrom</a><br>
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<a href="socat.html#OPTION_RANGE">option range</a><br>
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<a href="socat.html#OPTION_SO_BROADCAST">option broadcast</a><br>
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<a href="socat.html#OPTION_IP_ADD_MEMBERSHIP">option ip-add-membership</a><br>
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<a href="socat.html#OPTION_FORK">option fork</a><br>
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<a href="socat.html#OPTION_BIND">option bind</a><br>
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<h3>Other datagram addresses</h3>
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<a href="socat.html#ADDRESS_UDP4_RECV">address udp4-recv</a>: pure datagram receiver<br>
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<a href="socat.html#ADDRESS_UDP4_SENDTO">address udp4-sendto</a>: communicate
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with one peer address<br>
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<a href="socat.html#ADDRESS_UDP4_LISTEN">address udp4-listen</a>: pseudo stream server<br>
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<a href="socat.html#ADDRESS_UDP4_CONNECT">address udp4-connect</a>: pseudo stream client<br>
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<h3>Related socat option groups</h3>
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<a href="socat.html#GROUP_IP">IP options</a><br>
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<a href="socat.html#GROUP_SOCKET">socket options</a><br>
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<a href="socat.html#GROUP_FD">file descriptor options</a><br>
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<a href="socat.html#GROUP_RANGE">range options</a><br>
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<a href="socat.html#GROUP_CHILD">child process options</a><br>
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<h2>References</h2>
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<a href="http://www.dest-unreach.org/socat">socat home page</a><br>
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<a href="socat.html">socat man page</a><br>
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<a href="http://en.wikipedia.org/wiki/Multicast">multicasting on Wikipedia</a><br>
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<a href="http://en.wikipedia.org/wiki/Broadcast_address">broadcasting on Wikipedia</a><br>
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<p>
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<small>Copyright: Gerhard Rieger 2007</small><br>
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<small>License: <a href="http://www.fsf.org/licensing/licenses/fdl.html">GNU Free Documentation License (FDL)</a></small>
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</p>
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</body>
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</html>
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