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<li><a class="reference internal" href="#">TCP/IP Client and Server</a><ul>
<li><a class="reference internal" href="#echo-server">Echo Server</a></li>
<li><a class="reference internal" href="#echo-client">Echo Client</a></li>
<li><a class="reference internal" href="#client-and-server-together">Client and Server Together</a></li>
<li><a class="reference internal" href="#easy-client-connections">Easy Client Connections</a></li>
<li><a class="reference internal" href="#choosing-an-address-for-listening">Choosing an Address for Listening</a></li>
</ul>
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<div class="section" id="tcp-ip-client-and-server">
<h1>TCP/IP Client and Server<a class="headerlink" href="#tcp-ip-client-and-server" title="Permalink to this headline">¶</a></h1>
<p>Sockets can be configured to act as a <em>server</em> and listen for incoming
messages, or connect to other applications as a <em>client</em>. After both
ends of a TCP/IP socket are connected, communication is
bi-directional.</p>
<div class="section" id="echo-server">
<h2>Echo Server<a class="headerlink" href="#echo-server" title="Permalink to this headline">¶</a></h2>
<p>This sample program, based on the one in the standard library
documentation, receives incoming messages and echos them back to the
sender. It starts by creating a TCP/IP socket.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
</pre></div>
</div>
<p>Then <tt class="xref py py-func docutils literal"><span class="pre">bind()</span></tt> is used to associate the socket with the server
address. In this case, the address is <tt class="docutils literal"><span class="pre">localhost</span></tt>, referring to the
current server, and the port number is 10000.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Bind the socket to the port</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'starting up on </span><span class="si">%s</span><span class="s"> port </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">server_address</span>
<span class="n">sock</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
</pre></div>
</div>
<p>Calling <tt class="xref py py-func docutils literal"><span class="pre">listen()</span></tt> puts the socket into server mode, and
<tt class="xref py py-func docutils literal"><span class="pre">accept()</span></tt> waits for an incoming connection.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Listen for incoming connections</span>
<span class="n">sock</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="c"># Wait for a connection</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'waiting for a connection'</span>
<span class="n">connection</span><span class="p">,</span> <span class="n">client_address</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">accept</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">accept()</span></tt> returns an open connection between the server and
client, along with the address of the client. The connection is
actually a different socket on another port (assigned by the kernel).
Data is read from the connection with <tt class="xref py py-func docutils literal"><span class="pre">recv()</span></tt> and transmitted
with <tt class="xref py py-func docutils literal"><span class="pre">sendall()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre> <span class="k">try</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'connection from'</span><span class="p">,</span> <span class="n">client_address</span>
<span class="c"># Receive the data in small chunks and retransmit it</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">if</span> <span class="n">data</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sending data back to the client'</span>
<span class="n">connection</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'no more data from'</span><span class="p">,</span> <span class="n">client_address</span>
<span class="k">break</span>
<span class="k">finally</span><span class="p">:</span>
<span class="c"># Clean up the connection</span>
<span class="n">connection</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>When communication with a client is finished, the connection needs to
be cleaned up using <tt class="xref py py-func docutils literal"><span class="pre">close()</span></tt>. This example uses a
<tt class="docutils literal"><span class="pre">try:finally</span></tt> block to ensure that <tt class="xref py py-func docutils literal"><span class="pre">close()</span></tt> is always called,
even in the event of an error.</p>
</div>
<div class="section" id="echo-client">
<h2>Echo Client<a class="headerlink" href="#echo-client" title="Permalink to this headline">¶</a></h2>
<p>The client program sets up its <a class="reference internal" href="index.html#module-socket" title="socket: Network communication"><tt class="xref py py-class docutils literal"><span class="pre">socket</span></tt></a> differently from the
way a server does. Instead of binding to a port and listening, it
uses <tt class="xref py py-func docutils literal"><span class="pre">connect()</span></tt> to attach the socket directly to the remote
address.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="c"># Connect the socket to the port where the server is listening</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'connecting to </span><span class="si">%s</span><span class="s"> port </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">server_address</span>
<span class="n">sock</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
</pre></div>
</div>
<p>After the connection is established, data can be sent through the
<a class="reference internal" href="index.html#module-socket" title="socket: Network communication"><tt class="xref py py-class docutils literal"><span class="pre">socket</span></tt></a> with <tt class="xref py py-func docutils literal"><span class="pre">sendall()</span></tt> and received with <tt class="xref py py-func docutils literal"><span class="pre">recv()</span></tt>,
just as in the server.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="c"># Send data</span>
<span class="n">message</span> <span class="o">=</span> <span class="s">'This is the message. It will be repeated.'</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sending "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">message</span>
<span class="n">sock</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="c"># Look for the response</span>
<span class="n">amount_received</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">amount_expected</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="k">while</span> <span class="n">amount_received</span> <span class="o"><</span> <span class="n">amount_expected</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span>
<span class="n">amount_received</span> <span class="o">+=</span> <span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'closing socket'</span>
<span class="n">sock</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>When the entire message is sent and a copy received, the socket is
closed to free up the port.</p>
</div>
<div class="section" id="client-and-server-together">
<h2>Client and Server Together<a class="headerlink" href="#client-and-server-together" title="Permalink to this headline">¶</a></h2>
<p>The client and server should be run in separate terminal windows, so
they can communicate with each other. The server output is:</p>
<div class="highlight-python"><pre>$ python ./socket_echo_server.py
starting up on localhost port 10000
waiting for a connection
connection from ('127.0.0.1', 52186)
received "This is the mess"
sending data back to the client
received "age. It will be"
sending data back to the client
received " repeated."
sending data back to the client
received ""
no more data from ('127.0.0.1', 52186)
waiting for a connection</pre>
</div>
<p>The client output is:</p>
<div class="highlight-python"><pre>$ python socket_echo_client.py
connecting to localhost port 10000
sending "This is the message. It will be repeated."
received "This is the mess"
received "age. It will be"
received " repeated."
closing socket
$</pre>
</div>
</div>
<div class="section" id="easy-client-connections">
<h2>Easy Client Connections<a class="headerlink" href="#easy-client-connections" title="Permalink to this headline">¶</a></h2>
<p>TCP/IP clients can save a few steps by using the convenience function
<tt class="xref py py-func docutils literal"><span class="pre">create_connection()</span></tt> to connect to a server. The function takes
one argument, a two-value tuple containing the address of the server,
and derives the best address to use for the connection.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="k">def</span> <span class="nf">get_constants</span><span class="p">(</span><span class="n">prefix</span><span class="p">):</span>
<span class="sd">"""Create a dictionary mapping socket module constants to their names."""</span>
<span class="k">return</span> <span class="nb">dict</span><span class="p">(</span> <span class="p">(</span><span class="nb">getattr</span><span class="p">(</span><span class="n">socket</span><span class="p">,</span> <span class="n">n</span><span class="p">),</span> <span class="n">n</span><span class="p">)</span>
<span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">dir</span><span class="p">(</span><span class="n">socket</span><span class="p">)</span>
<span class="k">if</span> <span class="n">n</span><span class="o">.</span><span class="n">startswith</span><span class="p">(</span><span class="n">prefix</span><span class="p">)</span>
<span class="p">)</span>
<span class="n">families</span> <span class="o">=</span> <span class="n">get_constants</span><span class="p">(</span><span class="s">'AF_'</span><span class="p">)</span>
<span class="n">types</span> <span class="o">=</span> <span class="n">get_constants</span><span class="p">(</span><span class="s">'SOCK_'</span><span class="p">)</span>
<span class="n">protocols</span> <span class="o">=</span> <span class="n">get_constants</span><span class="p">(</span><span class="s">'IPPROTO_'</span><span class="p">)</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">create_connection</span><span class="p">((</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">))</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'Family :'</span><span class="p">,</span> <span class="n">families</span><span class="p">[</span><span class="n">sock</span><span class="o">.</span><span class="n">family</span><span class="p">]</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'Type :'</span><span class="p">,</span> <span class="n">types</span><span class="p">[</span><span class="n">sock</span><span class="o">.</span><span class="n">type</span><span class="p">]</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'Protocol:'</span><span class="p">,</span> <span class="n">protocols</span><span class="p">[</span><span class="n">sock</span><span class="o">.</span><span class="n">proto</span><span class="p">]</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span>
<span class="k">try</span><span class="p">:</span>
<span class="c"># Send data</span>
<span class="n">message</span> <span class="o">=</span> <span class="s">'This is the message. It will be repeated.'</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sending "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">message</span>
<span class="n">sock</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="n">amount_received</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">amount_expected</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="k">while</span> <span class="n">amount_received</span> <span class="o"><</span> <span class="n">amount_expected</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span>
<span class="n">amount_received</span> <span class="o">+=</span> <span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'closing socket'</span>
<span class="n">sock</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="xref py py-func docutils literal"><span class="pre">create_connection()</span></tt> uses <tt class="xref py py-func docutils literal"><span class="pre">getaddrinfo()</span></tt> to find candidate
connection parameters, and returns a <a class="reference internal" href="index.html#module-socket" title="socket: Network communication"><tt class="xref py py-class docutils literal"><span class="pre">socket</span></tt></a> opened with the
first configuration that creates a successful connection. The
<tt class="xref py py-attr docutils literal"><span class="pre">family</span></tt>, <tt class="xref py py-attr docutils literal"><span class="pre">type</span></tt>, and <tt class="xref py py-attr docutils literal"><span class="pre">proto</span></tt> attributes can be
examined to determine the type of <a class="reference internal" href="index.html#module-socket" title="socket: Network communication"><tt class="xref py py-class docutils literal"><span class="pre">socket</span></tt></a> being returned.</p>
<div class="highlight-python"><pre>$ python socket_echo_client_easy.py
Family : AF_INET
Type : SOCK_STREAM
Protocol: IPPROTO_TCP
sending "This is the message. It will be repeated."
received "This is the mess"
received "age. It will be"
received " repeated."
closing socket</pre>
</div>
</div>
<div class="section" id="choosing-an-address-for-listening">
<h2>Choosing an Address for Listening<a class="headerlink" href="#choosing-an-address-for-listening" title="Permalink to this headline">¶</a></h2>
<p>It is important to bind a server to the correct address, so that
clients can communicate with it. The previous examples all used
<tt class="docutils literal"><span class="pre">'localhost'</span></tt> as the IP address, which limits connections to clients
running on the same server. Use a public address of the server, such
as the value returned by <tt class="xref py py-func docutils literal"><span class="pre">gethostname()</span></tt>, to allow other hosts to
connect. This example modifies the echo server to listen on an
address specified via a command line argument.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="c"># Bind the socket to the address given on the command line</span>
<span class="n">server_name</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="n">server_name</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'starting up on </span><span class="si">%s</span><span class="s"> port </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">server_address</span>
<span class="n">sock</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'waiting for a connection'</span>
<span class="n">connection</span><span class="p">,</span> <span class="n">client_address</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">accept</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'client connected:'</span><span class="p">,</span> <span class="n">client_address</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">if</span> <span class="n">data</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">break</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>A similar modification to the client program is needed before the
server can be tested.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="c"># Connect the socket to the port on the server given by the caller</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="mi">10000</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'connecting to </span><span class="si">%s</span><span class="s"> port </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">server_address</span>
<span class="n">sock</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">message</span> <span class="o">=</span> <span class="s">'This is the message. It will be repeated.'</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sending "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">message</span>
<span class="n">sock</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="n">amount_received</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">amount_expected</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="k">while</span> <span class="n">amount_received</span> <span class="o"><</span> <span class="n">amount_expected</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span>
<span class="n">amount_received</span> <span class="o">+=</span> <span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">sock</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>After starting the server with the argument
<tt class="docutils literal"><span class="pre">farnsworth.hellfly.net</span></tt>, the <strong class="command">netstat</strong> command shows it
listening on the address for the named host.</p>
<div class="highlight-python"><pre>$ host farnsworth.hellfly.net
farnsworth.hellfly.net has address 192.168.1.17
$ netstat -an
Active Internet connections (including servers)
Proto Recv-Q Send-Q Local Address Foreign Address (state)
...
tcp4 0 0 192.168.1.17.10000 *.* LISTEN
...</pre>
</div>
<p>Running the the client on another host, passing
<tt class="docutils literal"><span class="pre">farnsworth.hellfly.net</span></tt> as the host where the server is running,
produces:</p>
<div class="highlight-python"><pre>$ hostname
homer
$ python socket_echo_client_explicit.py farnsworth.hellfly.net
connecting to farnsworth.hellfly.net port 10000
sending "This is the message. It will be repeated."
received "This is the mess"
received "age. It will be"
received " repeated."</pre>
</div>
<p>And the server output is:</p>
<div class="highlight-python"><pre>$ python ./socket_echo_server_explicit.py farnsworth.hellfly.net
starting up on farnsworth.hellfly.net port 10000
waiting for a connection
client connected: ('192.168.1.8', 57471)
received "This is the mess"
received "age. It will be"
received " repeated."
received ""
waiting for a connection</pre>
</div>
<p>Many servers have more than one network interface, and therefore more
than one IP address. Rather than running separate copies of a service
bound to each IP address, use the special address <tt class="xref py py-const docutils literal"><span class="pre">INADDR_ANY</span></tt>
to listen on all addresses at the same time. Although <a class="reference internal" href="index.html#module-socket" title="socket: Network communication"><tt class="xref py py-mod docutils literal"><span class="pre">socket</span></tt></a>
defines a constant for <tt class="xref py py-const docutils literal"><span class="pre">INADDR_ANY</span></tt>, it is an integer value and
must be converted to a dotted-notation string address before it can be
passed to <tt class="xref py py-func docutils literal"><span class="pre">bind()</span></tt>. As a shortcut, use the empty string <tt class="docutils literal"><span class="pre">''</span></tt>
instead of doing the conversion.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="c"># Bind the socket to the address given on the command line</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">''</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'starting up on </span><span class="si">%s</span><span class="s"> port </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">sock</span><span class="o">.</span><span class="n">getsockname</span><span class="p">()</span>
<span class="n">sock</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'waiting for a connection'</span>
<span class="n">connection</span><span class="p">,</span> <span class="n">client_address</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">accept</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'client connected:'</span><span class="p">,</span> <span class="n">client_address</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">data</span>
<span class="k">if</span> <span class="n">data</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">break</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>To see the actual address being used by a socket, call its
<tt class="xref py py-func docutils literal"><span class="pre">getsockname()</span></tt> method. After starting the service, running
<strong class="command">netstat</strong> again shows it listening for incoming connections
on any address.</p>
<div class="highlight-python"><pre>$ netstat -an
Active Internet connections (including servers)
Proto Recv-Q Send-Q Local Address Foreign Address (state)
...
tcp4 0 0 *.10000 *.* LISTEN
...</pre>
</div>
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