Domain Name Services, or DNS, is a critical service on almost all Local and Wide Area Networks. DNS is used for host name to IP address resolution of all Internet hosts, many WAN hosts, and may be used for address resolution of LAN hosts too. DNS resolution is so important that Windows supports configuration of 2 DNS servers in basic IP configuration; with more work, you can define even 3 or more DNS servers. Many NAT routers will let you define up to 3 DNS servers.
Any time you try to access a server on the Internet, and get "server not found" or "unknown host", check your DNS server settings. Run "ipconfig /all", and look for the DNS servers entry, such as:
DNS Servers . . . . . . . . . . . : 192.168.1.11
The DNS server sequence is important. When DNS resolution is needed, server #1 is queried first. If server #1 is busy or otherwise unavailable, server #2 is used in that query, and all subsequent queries. If server #2 is needed to provide a backup to server #1, server #1 may not be used again, until you reset the computer or router. This behaviour is not consistent, though, some DNS clients may always try DNS Server #1 first, then #2, and finally (if defined), #3.
If you're researching a problem where the symptoms indicate a DNS issue, and the problem isn't consistent between computers, compare the DNS server settings on each computer.
If all DNS servers in the sequence don't have balanced ability (availability, capacity, connection to higher level DNS server), you can get to a situation where the next server in the sequence is used, and won't provide consistent service. Resetting the DNS client, generally by restarting the computer or router, after DNS server #1 is returned to service, is the normal recovery from this problem.
Recognising a DNS problem may not be easy, though. Without some minimal diagnosis, a DNS problem can be confused with a physical connectivity problem, a security problem, or even a simple CKI fault.
The long term solution, for a DNS server sequence problem, is to have a properly balanced DNS server sequence. Many networks plan their primary DNS server very carefully, and throw a surplus (generally old and underpowered) computer in as the secondary. Some networks may even have 2 primary servers (with the clients split between the two), and an single, surplus, secondary.
What happens when the primary DNS server goes down? If your clients are using the secondary server suddenly, and it doesn't have the same capacity as the primary server, you're going to have performance problems. Make sure that your backup server is equal to the task of replacing, even temporarily, the primary server. Remember that the clients will be using the backup server, after the primary server comes back online. And if there's a chance that a secondary DNS server will be in use during an outage of other equipment, don't compound the stress. The stress that your clients experience will be passed on to you, generally doubled.
If you relay DNS requests to external DNS servers, and ones that you don't control, again try to specify servers of equal ability. Also, make sure that both external servers have good servers feeding them, and that they are secured against exploits that would permit pharming. If, for any reason, some of your clients are using the backup external server, and others the primary, both servers need to be able to resolve your DNS queries properly. If either server filters addresses differently, for instance, you'll have some clients able to access websites that other clients can't. Again, more stress for you.
If you're using DNS for address resolution on your LAN, make sure that both the server and all clients are setup properly.
If your Internet service goes thru a NAT router, you may be using the NAT router as a DNS relay.
If you think that you have a DNS problem, but aren't quite sure, read Identifying A DNS Problem In Your Internet Service.