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Debugging connection problems

With today's connectivity stretching over the internet via VPN and firewalls it is often necessary to put sniffers on Lans to diagnose issues with filtering, ports etc. There are a number of tools such as PING, CWBPING NBTSTAT etc and we generally use these tools in certain areas depending on the problem. One of the techniques I use to determine if base connectivity on a port level is functioning is to use TELNET SYSTENAME PORTNUMBER from a DOS prompt on a PC. Normally we just use TELNET SERVERNAME and expect to get a signon screen. By specifying a port number we can determine if a connection at a socketlevel can be made. Things like firewalls can restrict ports. TELNET SERVERNAME 449 will bring up a blank telnet screen with the cursor in the top left. This tells us that we have established a connection to the servermapper. On the AS/400 NETSTAT option 3 will show the connection established. TELNET SERVERNAME 139 creates a socket connection to as400 NETSERVER netbios over TCP. If the connection cannot be established we get "could not open a connection to servername. By specifying servername we can also determine if name resolution is taking place via HOSTS file or DNS server.

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