I’ll show how to capture packets on a local network with Pcap4J container.

Docker network

By default, Docker containers are not connected to a local network. They are connected only to a virtual network Docker creates as like below:

Docker network

Refer to the Docker doc for more details.

What’s a challenge

In order to let a Pcap4J container capture packets in a local (real) network, we need to directly connect the container to the local network, because docker0 forwards only packets the destinations of which are in the virtual network.

How to do it is explained in some articles. I referred to one of them, Four ways to connect a docker container to a local network in Odd Bits blog, and succeeded in local network capturing using the 4th way.

What I actually did is as follows.

What I did

  • Environment

    • OS: CentOS 7.0 (on VMware Player 7.1.0 on Windows 7)

      # uname -a
      Linux localhost.localdomain 3.10.0-229.el7.x86_64 #1 SMP Fri Mar 6 11:36:42 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
    • user: root

    • Pcap4J version: 1.5.1-SNAPSHOT

    • Docker version: 1.6.2

    • Network interfaces:

        # ip addr show
        1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
            link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
            inet scope host lo
               valid_lft forever preferred_lft forever
            inet6 ::1/128 scope host
               valid_lft forever preferred_lft forever
        2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
            link/ether 00:0c:29:8e:95:27 brd ff:ff:ff:ff:ff:ff
            inet brd scope global dynamic eth0
               valid_lft 85975sec preferred_lft 85975sec
            inet6 2601:282:8102:2623:20c:29ff:fe8e:9527/64 scope global dynamic
               valid_lft 221469sec preferred_lft 221469sec
            inet6 fe80::20c:29ff:fe8e:9527/64 scope link
               valid_lft forever preferred_lft forever
        3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN
            link/ether 56:84:7a:fe:97:99 brd ff:ff:ff:ff:ff:ff
            inet scope global docker0
               valid_lft forever preferred_lft forever

  • Prerequisites:
    • Docker is installed and Docker service is started
    • nsenter

  • Step by step

    1. Preparing

      Create a utility script docker-pid with the following content and place it somewhere in the PATH.

        exec docker inspect --format '{{ .State.Pid }}' "$@"

      This script show the PID of a docker container by name or ID.

    2. Pull the latest Pcap4J image

      # docker pull kaitoy/pcap4j
    3. Start a Pcap4J container with wait mode

      # docker run --name pcap4j-br kaitoy/pcap4j:latest eth1 true

      This container (pcap4j-br) waits for a ping to eth0 on the container before staring capturing packets with eth1 on the container. After the container starts, you will see messages like below:

      17:49:21.196 [main] INFO  org.pcap4j.core.Pcaps - 3 NIF(s) found.
      eth0 (null)
      IP address: /
      IP address: /fe80:0:0:0:42:acff:fe11:3

      The messages say IP address of eth0 is We will use it later.

    4. Configure a bridge interface

      Open another terminal and do the following:

      # ip link add eth1 link eth0 type macvlan mode bridge
      # ip link set netns $(docker-pid pcap4j-br) eth1
      # nsenter -t $(docker-pid pcap4j-br) -n ip link set eth1 up
      # nsenter -t $(docker-pid pcap4j-br) -n ip route del default
      # nsenter -t $(docker-pid pcap4j-br) -n ip addr add dev eth1
      # nsenter -t $(docker-pid pcap4j-br) -n ip route add default via dev eth1

      The above commands 1) add an interface eth1 bridged to eth0 to the Docker host machine, 2) move the eth1 to the name space of pcap4j-br, 3) start eth1, 4) delete the default route in pcap4j-br, 5) add an IP address to eth1, 6) and set the default route in pcap4j-br to

      Too much hassle? I agree. Let’s use an awesome tool, pipework. This tool accomplishes the above 6 steps in easier way as shown below:

      # git clone https://github.com/jpetazzo/pipework.git
      # cd pipework
      # ./pipework eth0 pcap4j-br[email protected]

      pipework uses ip netns exec command instead of nsenter to manipulate a container. Incidentally, docker exec didn’t work for the step 3 due to an error “RTNETLINK answers: Operation not permitted”.

      In addition, in my case, because I was doing it on a VMware VM, I needed to enable the promiscuous mode of eth0 (on the docker host machine) as follows:

      # ip link set dev eth0 promisc on
    5. Try to poke the container

      You can now communicate with pcap4j-br using eth1 from another host. I tried some pings from the VM’s host to pcap4j-br and saw replies.

      Note that you can NOT communicate with pcap4j-br via eth1 from the docker host. See the Odd Bits blog for the details.

    6. Start packet capturing

      Ping to eth0 of pcap4j-br form the docker host to start packet capturing.

      # ping -c 1