How does the Docker assign MAC addresses to containers?

Question

When I start new containers, Docker automatically assigns some MAC address to them. I am curious if there is a pattern to this assignment. Can the MAC address be changed?

$ docker network inspect bridge

"Containers": {             "3386a527aa08b37ea9232cbcace2d2458d49f44bb05a6b775fba7ddd40d8f92c": {                 "EndpointID": "647c12443e91faf0fd508b6edfe59c30b642abb60dfab890b4bdccee38750bc1",                 "MacAddress": "02:42:ac:11:00:02",                 "IPv4Address": "172.17.0.2/16",                 "IPv6Address": ""             },             "94447ca479852d29aeddca75c28f7104df3c3196d7b6d83061879e339946805c": {                 "EndpointID": "b047d090f446ac49747d3c37d63e4307be745876db7f0ceef7b311cbba615f48",                 "MacAddress": "02:42:ac:11:00:03",                 "IPv4Address": "172.17.0.3/16",                 "IPv6Address": ""             } 

Answer 1

Docker start assigning always the same mac 02:42:ac:11:00:02 for the first container and then is increasing by one each mac for each different container.

Not sure why they are using that mac address. It seems 02:42:ac doesn't match any real vendor in oui databases. Look at the official documentation about this. They say:

The MAC address is generated using the IP address allocated to the container to avoid ARP collisions, using a range from 02:42:ac:11:00:00 to 02:42:ac:11:ff:ff

Anyway, you can set any mac address on container generation using --mac-address parameter on the docker run command. For example doing a command like this docker run -ti --mac-address 00:00:00:00:00:11 ubuntu:trusty

Hope it helps.

Answer 2

If you look at the MAC address of the container interface(s), you can see that the last 4 octets are the hex representation of the IPv4 address. This part is prefixed by 02:42: For example:-

The docker generated MAC address of the container interface with IPv4 address 172.19.0.6 would be 02:42:ac:13:00:06