Symptom: The user cannot access the internet or key applications. The network appears to be down.
Cause: Domain Name Services may be to blame. The client PC cannot resolve the name of the server with the IP address of that server, so it will not send a connection request. This is often caused by having the wrong DNS server configured on the client, sending DNS requests that the server does not have in its database, or packet loss on the network. DNS is a UDP-based protocol, so packets that are lost will not be retransmitted, causing DNS to fail.
Resolution: Check the client configuration to see which DNS server it is setup to use. If this is the wrong server, adjust this setting in the client or in the DHCP server which provided it. Test the DNS server from the client connection repeatedly to determine if there is delay in response due to packet loss. If packets are lost, look for Ethernet errors between the client and server. Capture failing DNS requests to determine if there is any response at all from the server. Ideally, setup a tool that will constantly test the DNS server and alert when a problem occurs.
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