Docker / Networking, Ports & Service Connectivity
Inspect a container's actual DNS configuration without assuming host equivalence
Docker can generate resolver settings and use an embedded DNS service on certain networks. Host lookup success does not prove the container's path.
Environment and inputs: Docker Engine · Compose v2 and Buildx where specified; Linux containers unless noted. Verify daemon context, installed feature support and authorized host or Swarm manager access first.
Start with this check
Confirm the environment and replace sample objects, namespaces, images, paths and hosts with approved targets. Share only relevant, sanitized output. Follow the workload owner’s process for changes.
docker container exec app cat /etc/resolv.confRead the result
This runs a read command where cat exists in the intended running Linux container. The file contains resolver configuration, not evidence that every upstream query succeeds.
Resolve and verify
Compare daemon overrides, network type and permitted upstream paths with the failing name. Repair the specific resolver or policy dependency and test the real application.
Before changing production
Do not hard-code public DNS for private names or modify runtime-generated files as a durable fix.