Have you ever found yourself needing to simulate network latency or packet loss? I recently wanted to test a replication scenario but needed to have at least 15ms or so of latency between the source and destination.
Years ago I used a WANem virtual appliance as a router to do something similar. This project can be found online (https://wanem.sourceforge.net), however it appears that development has stopped, with the latest releases being nearly 10 years old. WANem provides a PHP web interface allowing the user to configure latency and packet loss, among other things. When you apply settings, the web interface issues commands to Linux Traffic Control (tc) Network Emulator (netem) to enforce those settings.
Instead of using the WANem appliance, which ships as a very old and unpatched Knoppix Linux distribution, I decided to take the command it ran and try it on an Ubuntu template available in my lab.
I created an Ubuntu VM with 2 network interfaces. One interface connected to my routable lab network using a static IP (ens192: 192.168.40.11) and the other connected to non-routable VLAN 19 that was available and was assigned another static IP (ens224: 192.168.19.1) — which would become the default gateway for this new network.
I then added the following two lines to the /etc/rc.local file on this system:
echo "1" > /proc/sys/net/ipv4/ip_forward
tc qdisc add dev ens192 root handle 1: netem delay 19ms loss 2%The first line makes the system act as a router, the second adds 19ms of latency and 2% packet loss to traffic which passes the network interface. More configuration options for this second command can be found here: https://man7.org/linux/man-pages/man8/tc-netem.8.html, including how to send corrupt/duplicate/reordered packets — really make your WAN experience terrible. Since it is in /etc/rc.local these commands will run automatically when the system starts.
I also wanted this system to act as a DHCP server for the clients at the remote site. I did this by installing a DHCP server (apt install isc-dhcp-server), making environment specific changes to /etc/dhcp/dhcpd.conf such as DNS servers and lease-time, and also adding my new subnet with:
subnet 192.168.19.0 netmask 255.255.255.0 {
range 192.168.19.50 192.168.19.99;
option subnet-mask 255.255.255.0;
option routers 192.168.19.1;
}To activate these DHCP changes, I ran /etc/init.d/isc-dhcp-server restart.
Finally I added a static route to the primary router in my lab, such that any requests for 192.168.19.0/24 go to the gateway at 192.168.40.11. Now any request in or out of my WAN site has latency injected at some occasional lost packets:
I’m now able to deploy virtual machines at a “remote location” about 19-20ms away, but actually run on the same vSphere cluster.