Name Resolution Issues

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• if some DNS queries work from client, but others don’t, then there is a more complex problem
  • after:
    • performing successful connectivity test by IP address
    • verified host’s DNS resolvers are working
    • target host or service still does not respond to pinging by name
  • check:
    • for a fault within the name resolution process

Name Resolution Methods

• To trouble shoot name resolution,
  • establish exactly how the process works on that specific host
    • host can use a variety of methods to resolve a name to an IP address
• name resolution methods:
  1. Check local name caches
     • one complication is that there are different types of cache and separate caches for individual applications (i.e. web browsers)
     • on Windows, use:
       • ipconfig /displaydns to monitor DNS cache
       • ipconfig /flushdns to clear system DNS cache
  2. Check HOSTS
     • HOSTS file is a static list of host name to IP address mappings
     • local resolver is likely to try to use any HOSTS file mappings first
       • or mappings might be cached automatically
     • default location for HOSTS file is:
       • Windows: %SystemRoot%\system32\drivers\etc\
       • Linux: usually in /etc
     • in most cases, HOSTS should not contain any entries
       • other than the loopback address
     • any static entries in HOSTS could be the cause of a name resolution issue
     • file can be used for troubleshooting
  3. Verify DNS records using nslookup or dig tools
     • might be some discrepancy between:
       • the records returned by the resolver
       • the records configured on the authoritative DNS server that maintains the zone
     • use these utilities to check what records are returned by the resolver
       • if trying to connect to an Internet resource, compare:
         • these records returned to those by public resolvers
     • consider whether clients have cached a record that has been changed recently
       • reconfiguration of DNS records should be planned and implemented carefully to avoid caching problems

Info

• any text preceded by # in HOSTS file is a comment
• to verify a name resolution problem,
  • edit the HOSTS file and place the correct name and IP address record in the file for the test host
  • when you ping the name, success suggests a name resolution service problem