Tools for simulating network unreliability

On OSX Yosemite and above, in a terminal window:

1. Switch to the root user.
  
  $ sudo su -
  
2. Create a Dummynet pipe that represents a slow, unreliable network:

  # dnctl pipe 1 config bw 10Kbit/s delay 300 plr 0.1 noerror

3. Add the pipe to the packet filter, and enable it. Change 'local.wordpress.dev' to the hostname or IP of your local test VM.

  # echo "dummynet in proto {tcp,icmp} from local.wordpress.dev to any pipe 1" | pfctl -f -
  # pfctl -e

4. Ping the VM to confirm that traffic shaping is working as expected.

  # ping local.wordpress.dev

  PING local.wordpress.dev (192.168.50.4): 56 data bytes
  64 bytes from 192.168.50.4: icmp_seq=0 ttl=64 time=368.255 ms
  64 bytes from 192.168.50.4: icmp_seq=1 ttl=64 time=368.513 ms
  64 bytes from 192.168.50.4: icmp_seq=2 ttl=64 time=369.118 ms
  64 bytes from 192.168.50.4: icmp_seq=3 ttl=64 time=368.516 ms
  Request timeout for icmp_seq 4
  64 bytes from 192.168.50.4: icmp_seq=5 ttl=64 time=368.274 ms
  ^C
  --- local.wordpress.dev ping statistics ---
  7 packets transmitted, 5 packets received, 28.6% packet loss
  round-trip min/avg/max/stddev = 368.255/368.535/369.118/0.312 ms

5. To disable filtering and restore normal traffic to the VM:

  # pfctl -f /etc/pf.conf
  # dnctl -q flush
  
The rule given in step 3 affects traffic outbound from the VM to your browser. To apply it to inbound traffic, in order to test file uploads:

  # echo "dummynet out proto {tcp,icmp} from any to local.wordpress.dev pipe 1" | pfctl -f -
  
To simulate dropped connections, instead of dropped packets, dnctl is not needed:

  # echo "block return proto {tcp,icmp} from any to local.wordpress.dev probability 1%" | pfctl -f -
  # pfctl -e

Note that the probability is per packet, not per request, so you'll want a much lower value than for packets. I recommend around 1%.

Disabling dropped connections is the same as step 5.


Some links to more details and other tools that do a similar job:

https://spin.atomicobject.com/2016/01/05/simulating-poor-network-connectivity-mac-osx/
http://www.thomashanning.com/network-link-conditioner/
https://www.npmjs.com/package/crapify
https://github.com/tylertreat/Comcast
http://toxiproxy.io/
https://jagt.github.io/clumsy/

And downloadable PDFs of the retro BASIC programming books I mentioned in my talk:

http://www.usborne.com/catalogue/feature-page/computer-and-coding-books.aspx