As described in earlier articles, I’m using an ephemeral virtual machine to run a privileged container with access to the Docker docker of the virtual machine as a system to build Docker images in a CI pipeline. Occasionally, one of the machines hosting the virtual machines breaks and stops spinning up new virtual machines on demand. The pending docker-machine instances display the error message
Could not find matching IP for MAC address
Here is why and how to solve it until this is fixed upstream.
With the default setup and VirtualBox as the backend, virtual machines managed
by docker-machine receive an IP address from a class C subnet of
192.168.45.123/24. VirtualBox manages a DHCP server responsible
for the assignment. It picks an address in the
100-254 range. Depending on the
number of docker-enabled CI jobs, the range of IP addresses is exhausted after
hours or months. If you hit the limit within hours, there is nothing the DHCP
server could do. It issued IP address leases with a validity of usually 24
hours. So, the address might be reclaimed and assigned to a new ephemeral machine
only after the old lease expired.
If there are not so many jobs, and you hit the limit after, let’s say, a month,
it’s probably safe to assume that one of the earliest leases has expired. The
thing is, the DHCP does not reclaim the IP addresses even after the lease
expired. As a result, the IP address counter never wraps around and does not
192.169.x.100/24. The issue has also been discussed in the
The only way I found to reset the DHCP server is to kill it. This might not be the best solution, and if you have other non-ephemeral virtual machines currently running, be prepared that their IP address will change.
$ ps ax | grep VBoxNetDHCP # Note process number $ kill -9 PID
Have you found a better solution? Let me know.