Containers can ping each other, but hostname resolution fails

Hello Community,

I have an interesting network behavior in my Docker setup and hope for help from experienced users.

Current situation:

• All containers are running in a shared Docker Bridge network (frontend)
• Containers can ping each other
• Hostname resolution between containers does not work
• unRAID Version: 7.0.0

Network details:

• Docker network: frontend (bridge)
• Subnet: 172.18.0.0/16
• Affected containers: SWAG, Authelia, Paperless

Debugging steps already performed:

• docker network ls
• docker network inspect frontend
• docker inspect swag | grep -A 10 “Networks”
• docker inspect authelia | grep -A 10 “Networks”
• docker exec swag cat /etc/resolv.conf
• docker exec swag nslookup authelia
• docker exec swag ping authelia

Observations:

• ping authelia works
• nslookup authelia works
• NGINX cannot resolve “authelia”

nginx error log:

2025/02/18 22:50:26 [error] 1042#1042: *83 authelia could not be resolved (3: Host not found), client: 100.x.x.x, server: paperless.*, request: "GET /favicon.ico HTTP/2.0", subrequest: "/authelia/api/authz/auth-request", host: "paperless.home.xxx.xx", referrer: "https://paperless.home.xxx.xx/"

What could be the reason for this? Which configuration steps could help? Are there known problems with Docker networking in unRAID?

I look forward to your expertise!