pptpd in docker stop working after container is restarted

Question

I build a docker image via this Dockerfile:

#
# Dockerfile for pptpd
#

FROM debian:jessie
MAINTAINER kev<[email protected]>

RUN apt-get update \
    && apt-get install -y iptables pptpd \
    && 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 \
    && pptpd --fg

before restart

$ 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 > 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 > 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 > 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 > 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 > 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 > 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 > 5.6.7.8: GREv1, call 16501, seq 3, ack 2, length 26: LCP, Echo-Request (0x09), id 0, length 10

after restart

$ docker restart pptpd_pptpd_1
$ tcpdump -ni eth0 proto gre

13:31:32.071308 IP 5.6.7.8 > 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 > 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 > 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 > 1.2.3.4: GREv1, call 256, seq 3, length 40: LCP, Conf-Request (0x01), id 1, length 26

• server:
  • eth0: 1.2.3.4
  • docker0: 192.168.42.1
• client: 5.6.7.8

I notice that container's ip changed (192.168.42.2 -> 192.168.42.3) after restart.
I enable/disable firewall, the result is the same.
Do I need a iptables rule to make it work again? Thanks!

---

UPDATE: I can append --net host option to walk around this problem.

Totalizator

Answer 1

When I edit /etc/default/ufw, it gives me some hint:

# Extra connection tracking modules to load. Complete list can be found in
# net/netfilter/Kconfig of your kernel source. Some common modules:
# nf_conntrack_irc, nf_nat_irc: DCC (Direct Client to Client) support
# nf_conntrack_netbios_ns: NetBIOS (samba) client support
# nf_conntrack_pptp, nf_nat_pptp: PPTP over stateful firewall/NAT
# nf_conntrack_ftp, nf_nat_ftp: active FTP support
# nf_conntrack_tftp, nf_nat_tftp: TFTP support (server side)
IPT_MODULES="nf_conntrack_ftp nf_nat_ftp nf_conntrack_netbios_ns"

After I run the command below, every thing back to normal.

modprobe nf_conntrack_pptp nf_nat_pptp

Totalizator