Network Debugging

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A collection of useful snippets.

  • Node/Client identification
    • Many systems will use dns to make sure the client connecting is indeed who they say they are. Torque (PBS) does this, and so does sshd. To find out what they get from DNS, you cannot rely on host or nslookup. Use this instead: " perl -MSocket -e 'print gethostbyaddr(inet_aton(""),AF_INET)."\n"' , with the replaced with the ip address you are interested in.
  • sshd
    • To debug the passwordless connection of ssh, first try: "ssh -v -v -v server". Better yet, also go to the server, kill sshd and then run it from the terminal as: "sshd -ddd". Tons of debugging info should let you know exactly what it is doing and why it is rejecting you. Now, if only you understood what it was actually saying....