Docker container DNS fails: 'Temporary failure in name resolution' or 'Could not resolve host' inside a container while the host works - how do I fix it?

Cause: containers get their DNS settings from Docker, not directly from your applications. On the default bridge network a container gets a copy of the host's /etc/resolv.conf; on user-defined networks it uses Docker's embedded DNS server at 127.0.0.11, which forwards to the host's upstream resolvers. Resolution breaks when those upstream servers aren't reachable from the container (a VPN or corporate resolver only available on the host, a host resolver bound only to a loopback address, split DNS), when firewall rules stop container traffic, or when Docker's firewall management has been turned off. Fix (best first): 1. Test in a throwaway container: docker run --rm busybox nslookup example.com, and docker run --rm busybox cat /etc/resolv.conf. If a bare IP works (ping 192.0.2.1 style test to a reachable address) but names don't, it is DNS rather than routing. 2. Set reachable DNS servers for one container with --dns <ip>, or for all containers in /etc/docker/daemon.json: {"dns": ["<your-resolver-ip>"]} and then sudo systemctl restart docker. Use your organization's resolver if you are behind a VPN or internal DNS. Note that a --dns value of 127.0.0.1 points at the container's own loopback, not the host. 3. For internal names add --dns-search, and for individual host entries use --add-host; the host's /etc/hosts is not inherited. 4. Check the firewall: if you disabled Docker's iptables management (iptables set to false in the daemon config) without equivalent rules, bridge containers can't reach the internet through masquerading. If firewalld is running, verify the docker zone exists, and remember IP forwarding must be enabled. 5. Restart Docker after changing VPN or network settings so the daemon picks up the new resolvers. Verify: nslookup example.com inside a new container returns an address, and the failing build or app now resolves hostnames. Sources: https://docs.docker.com/engine/network/ https://docs.docker.com/reference/cli/dockerd/ https://docs.docker.com/engine/network/packet-filtering-firewalls/