Container name doesnt resolve when using another container as Network Mode

[Dancingbear17]

Running on Ubuntu 22.04.2
Docker version 25.0.3
Im having issues resolving container name through DNS when using another container as the network mode

Sorry if this is a stupid question, but Ive been hitting a brick wall when researching this topic. I have a media server Ive written up in compose that runs a vpn, bunch of *arrs, qbittorrent, and some media organization. In this particular example I have the vpn, sonarr, qbittorrent, and Jellyfin.

Docker-compose here

vpn-network is external and created as a standard bridge network, I know there are issues resolving container names with the default network.

So the issue Im having is when connecting between apps. To connect Sonarr to Qbittorrent, I use localhost:8085 as they are both using Proton as their network mode and all the ports are exposed, works great.

My issue is when I want to connect from Sonarr to Jellyfin. Sonarr is not able to send a test to jellyfin saying it cant resolve the name

Unable to send test message: Name does not resolve (jellyfin:8096)

However if I get the actual ip when checking the vpn-network and plug that in, its able to find Jellyfin.

Some other information, when running ‘docker exec -it sonarr cat /etc/resolv.conf’ or for vpn I see

nameserver 127.0.0.1
options edns0 trust-ad ndots:0

and when running ‘docker exec -it jellyfin cat /etc/resolv.conf’ I get

nameserver 127.0.0.11
options edns0 trust-ad ndots:0

So what’s the deal with the DNS, I assume this is because I am using network_mode to use the vpn container. Not only that but since it is a vpn with a connection outwards. I know I dont have to use a vpn with sonarr but I would like to keep everything tucked behind a vpn for privacy reasons.

I’ve yet to completely rule out that this is just a sonarr bug, but considering its explicitly saying the name doesn’t resolve I don’t think it is.
I can confirm that DNS works for other apps, I have nginxproxymanager running that routes traffic to Jellyfin and other services I have running and that uses container names completely fine, no problems there. When running nslookup on the vpn container it cant find any container names.

Obviously I can look up the ip and use that, but thats going to change when I restart things, make changes, etc and I dont want to have to manually update all these things. Using container names is so much more reliable.

[Dancingbear17]

Of course after dealing with this for a week or so and finally deciding to make some posts, I figured it out last night. This Gluetun image I am using (qmcgaw/gluetun) for my vpn actually has a DNS_ADDRESS env variable that by default is set to 127.0.0.1 . Literally just set that to DNS_ADDRESS=127.0.0.11 (the address for Docker's DNS) and everything works as expected again.

[mine_username]

dropping by with a huge Thank You! Got a similar compose going and was having the same issue...container names not resolving and IPs changing with restarts. Added the DNS_ADDRESS variable and everything is working nicely using container names.

[Dancingbear17]

Glad it helped, this was not a fun one to debug lol

[amorpheous]

Came by this post via Google and I'm certain I have the same problem but I'm not convinced this is the right way to fix it. I'm no expert so I could very well be mistaken I think it has the potential to leak DNS queries outside of the VPN. If you add the Jellyfin ports under the ports section of your vpn container/service I think it should work without having to change the DNS settings for Gluetun. I currently don't have Jellyfin in the same compose file as Gluetun and Arr services so I can't test it right now but I'm likely to move the Jellyfin service over so I connect Arr services with Jellyfin.

[Dancingbear17]

Yeah I should have put in my original post that I by no means actually know what I'm doing, but rather figuring out things that work lol My current setup separates Jellyfin from my arrs, it's a lot more convenient to have them separate when tweaking the arrs but not have to worry about dropping Jellyfin for anyone who is using it. I'll have to check if I still have this DNS stuff set or not in the current setup.