There are four servers in the environment. One was recently forcibly demoted after it couldn't reach the PDC. From that server, the other controllers respond to ping by IP, but their names don't resolve. Its DNS settings point to two of the remaining controllers.
Where should I start looking?
Start with DNS queries rather than ping. From the demoted server, use nslookup to query each controller's fully qualified name, first normally and then directly against each configured DNS server. For example, use nslookup A.localdomain followed by nslookup A.localdomain IP-of-DNS-server.
Compare the responses. A timeout points you toward reachability or a DNS service problem. An answer saying the name doesn't exist points you toward the zone or its records. Also check that the client has the expected DNS suffix and that the names being tested match the actual records.
Being able to ping an IP doesn't prove DNS traffic can get through. Check routing and firewall rules for DNS between the demoted server and both DNS servers, including UDP and TCP port 53. Also verify the DNS service is running and listening on the expected addresses.
If queries time out, capture traffic at both ends while making a query. That should show whether it leaves the client, reaches the DNS server, and gets a response back.
Since D was forcibly demoted, check the domain metadata too. Run netdom query fsmo from a remaining controller and verify the role holders are active. If D still owns a role, assess whether it can be recovered before transferring or seizing it.
Check for stale D records and references in DNS and Active Directory, and follow the appropriate metadata cleanup procedure. That may be separate from the current name resolution failure, but a forced demotion can leave things behind.
Take a look at the full client configuration with ipconfig /all, including IPv6 DNS servers. A DNS address supplied over IPv6 could change which server gets queried, even if the IPv4 settings look right. Test the actual configured addresses rather than assuming the primary IPv4 server handles every lookup.
Answers are reviewed before publication.
Tavren.8p:
Even if the firewall and routing haven't knowingly changed, a capture can help narrow down where the query stops.