I tried to run a Pcap4J container with runC.

What is Pcap4J?

Pcap4J is a Java library for capturing, crafting, and sending packets. It’s actually a Java wrapper for libpcap/WinPcap plus packet analyzer. We can see the details in its README.

What is runC?

runC is a container runtime developed by Docker and released on June 22, 2015. With runC, we can start a container from a docker image without the docker service or the docker command.

That said, as of now, runC cannot directory use docker images. We need to create a container form a docker image and export its filesystem before executing runC.

It seems currently it supports only Linux but Windows support is in the roadmap.

What I did

• Environment
  • OS: CentOS 7 (on VMware Player 7.1.0 on Windows 7)
  • user: root
  • runC version: 0.2
  • Pcap4J version: 1.5.1-SNAPSHOT
  • Docker version: 1.6.2

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

• Step by step

  1. Install runC

     # mkdir -p $GOPATH/src/github.com/opencontainers
     # cd $GOPATH/src/github.com/opencontainers
     # git clone https://github.com/opencontainers/runc
     # cd runc
     # make && make install

  2. Pull the Pcap4J docker image.

     # docker pull kaitoy/pcap4j

  3. Create a container from the image.

     # docker run -d --name pcap4j-tmp kaitoy/pcap4j:latest /bin/bash

  4. Export the container’s file system.

     # mkdir /tmp/pcap4j-test
     # cd /tmp/pcap4j-test
     # docker export pcap4j-tmp > pcap4j.tar
     # tar xf pcap4j.tar

     We are now free from Docker. We don’t need Docker service, Docker command, Docker images, nor Docker containers anymore.

  5. Generate a container config file.

     # runc spec | sed -e 's/rootfs/\/root\/Desktop\/pcap4j-container/' -e 's/"readonly": true/"readonly": false/' -e 's/"NET_BIND_SERVICE"/"NET_BIND_SERVICE","NET_ADMIN","NET_RAW"/' > config.json

     In the above command, runc spec generates a standard container config file and sed modifies it for Pcap4J.

  6. Run a container.

     # runc

  7. In the container, enable lo.

     As far as I saw, lo is the only interface we can use in a container. So, I used it to capture packets.

     In container:

     # ifconfig lo up

  8. Generate a script to ping localhost and run it background.

     In container:

     # cd /usr/local/src/pcap4j/bin
     # echo ping 127.0.0.1 \> /dev/null > pinger.sh
     # chmod +x pinger.sh
     # ./pinger.sh &

     In the next step, ICMP packets from this pinger.sh will be captured.

  9. Generate a script to start capturing packets with Pcap4J and run it.

     In container:

     # cat runGetNextPacket.sh | sed -e 's/eth0/lo/' > foo.sh
     # chmod +x foo.sh
     # ./foo.sh

     We will see the ICMP packets are dumped on the terminal. That’s it!