April 27, 2015

Linux - Simulating Degraded Network Conditions

When testing an application or service, it can be very useful to simulate degraded network conditions. This allows you to see how they might perform for real users. Testing on a controlled LAN is not always realistic, since the LAN has much different network properties than a real user would experience over a WAN.

I will give examples using 2 tools that alter a network interface's properties. The tools are: NetEm and Wondershaper.

NetEm is an enhancement of the Linux traffic control facilities that allows you to add delay, packet loss, duplication, corruption, reordering, and more to outgoing packets from a specified network interface. NetEm is controlled by the command line tool tc, which is part of the iproute2 package and included in most Linux distros.

Wondershaper is a traffic shaping script that allows you to throttle network transfers on a specified network interface. This is useful for testing latency when bandwidth is limited.


add fixed 250ms delay to all outgoing packets on Ethernet interface eth1:
  $ sudo tc qdisc add dev eth1 root netem delay 250ms
turn it off:
  $ sudo tc qdisc del dev eth1 root netem

limit bandwidth of eth1 interface to 256kbps upload and 128kbps download:
  $ sudo wondershaper eth1 256 128
turn it off:
  $ sudo wondershaper clear eth1