I was taking a look at the network configuration of one of my FreeNAS machines, and I noticed a few network interfaces that I did not immediately recognize, something I am sure I had not created manually myself. These interfaces are named bridge0, epair0a and epair1a.

What looked especially funny about these is that none of them had an IP address assigned to them, so I decided to investigate a little. After having no luck finding some useful information in the forum, I found a documentation page in the old doc website that had a section specifically about this:

The IPv4 and IPv6 bridge interface is used to bridge the epair(4) device, which is automatically created for each started jail, to a physical network device. The default network device is the one that is configured with a default gateway. So, if em0 is the FreeBSD name of the physical interface and three jails are running, the following virtual interfaces will be automatically created: bridge0, epair0a, epair1a, and epair2a. The physical interface em0 will be added to the bridge, as well as each epair device. The other half of the epair will be placed inside the jail and will be assigned the IP address specified for that jail. The bridge interface will be assigned an alias of the default gateway for that jail, if configured, or the bridge IP, if configured; either is correct.

Therefore, these are simply network interfaces that are related to your jails and are required for you to run them successfully, so you shouldn’t worry about this should you see them on your machine as well.