title: pptpd in docker stop working after container is restarted
body: <p>I build a docker image via this <a href="https://registry.hub.docker.com/u/vimagick/pptpd/" rel="nofollow">Dockerfile</a>:</p>
  
  <pre><code>#
  # Dockerfile for pptpd
  #
  
  FROM debian:jessie
  MAINTAINER kev&lt;noreply@datageek.info&gt;
  
  RUN apt-get update \
      &amp;&amp; apt-get install -y iptables pptpd \
      &amp;&amp; rm -rf /var/lib/apt/lists/*
  
  COPY pptpd.conf    /etc/
  COPY chap-secrets  /etc/ppp/
  COPY pptpd-options /etc/ppp/
  
  EXPOSE 1723
  
  CMD iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE \
      &amp;&amp; pptpd --fg
  </code></pre>
  
  <h3>before restart</h3>
  
  <pre><code>$ docker pull vimagick/pptpd
  $ docker run -d --name pptpd_pptpd_1 -p 1723:1723 --privileged vimagick/pptpd
  $ tcpdump -ni eth0 proto gre
  
  13:21:16.877858 IP 1.2.3.4 &gt; 5.6.7.8: GREv1, call 16501, seq 0, length 40: LCP, Conf-Request (0x01), id 1, length 26
  13:21:16.944894 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 512, seq 0, length 40: LCP, Conf-Request (0x01), id 1, length 26
  13:21:16.945002 IP 1.2.3.4 &gt; 5.6.7.8: GREv1, call 16501, seq 1, ack 0, length 44: LCP, Conf-Ack (0x02), id 1, length 26
  13:21:16.945932 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 512, seq 1, length 25: LCP, Conf-Nack (0x03), id 1, length 11
  13:21:16.946006 IP 1.2.3.4 &gt; 5.6.7.8: GREv1, call 16501, seq 2, ack 1, length 45: LCP, Conf-Request (0x01), id 2, length 27
  13:21:16.984018 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 512, seq 2, length 41: LCP, Conf-Ack (0x02), id 2, length 27
  13:21:16.984224 IP 1.2.3.4 &gt; 5.6.7.8: GREv1, call 16501, seq 3, ack 2, length 26: LCP, Echo-Request (0x09), id 0, length 10
  </code></pre>
  
  <h3>after restart</h3>
  
  <pre><code>$ docker restart pptpd_pptpd_1
  $ tcpdump -ni eth0 proto gre
  
  13:31:32.071308 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 256, seq 0, length 40: LCP, Conf-Request (0x01), id 1, length 26
  13:31:35.123217 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 256, seq 1, length 40: LCP, Conf-Request (0x01), id 1, length 26
  13:31:40.112179 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 256, seq 2, length 40: LCP, Conf-Request (0x01), id 1, length 26
  13:31:41.111172 IP 5.6.7.8 &gt; 1.2.3.4: GREv1, call 256, seq 3, length 40: LCP, Conf-Request (0x01), id 1, length 26
  </code></pre>
  
  <ul>
  <li>server:
  
  <ul>
  <li>eth0: 1.2.3.4</li>
  <li>docker0: 192.168.42.1</li>
  </ul></li>
  <li>client: 5.6.7.8</li>
  </ul>
  
  <p>I notice that container's ip changed (192.168.42.2 -> 192.168.42.3) after restart.<br>
  I enable/disable firewall, the result is the same.<br>
  Do I need a iptables rule to make it work again? Thanks!</p>
  
  <hr>
  
  <p>UPDATE: I can append <code>--net host</code> option to walk around this problem.  </p>