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Port checking


I remember that there's a standard utility to verify that a port is available on a host computer. If memory serves, I can telnet to the port if it's open and available. For example, from a command prompt, I can type "telnet xxx.xxx.xxx.xxx: (port number)." If the port is available, the telnet session will connect. There is no functionality via telnet, but the connection is established. Can you verify this and let me know what utility or tool will accomplish this?

Yes, telnet can be used to verify that a server port is available. The syntax for the command is usually "telnet host name port number" with a space between the host name (or address) and the port number. Depending on the service you're checking there may be some functionality you can use. Telnetting to the mail port (25) on a host running Sendmail will let you converse with the mail server. You can also telnet to port 80 of a Web server and issue HTTP commands by hand. My favorite freeware tool for such things is Netcat. You can find a Windows 9X/NT version and a pointer to the original Unix source code at www.l0pht.com/~weld/netcat. (note that l0pht is spelled L-ZERO-P-H-T). You can find a copy of the cygwin32-based PC Netcat kit I use, containing Netcat plus a set of basic Unix commands for the PC, including Perl, Tar, Gzip, Vi, Grep and others, at www.home.earthlink.net/~swblass/pc-netcat.zip.

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As a network architect at Change at Work in Houston, Blass understands the strain of developing and managing networks. Send your problems to dr.internet@changeatwork.com

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