Use netcat to check udp port status

Damn you, Windows.  You’ve conditioned me to use telnet for network connectivity checks instead of superior tools like netcat (which supports UDP ports). 

I nearly forgot this one so I thought I’d write it down:

nc -vzu <ip address> <udp port or port range>

  • -v enables verbose mode for easier feedback.
  • -z scans for listening daemons without sending any data to them.
  • -u uses UDP instead of TCP.

‘Nuff said.

Shh. You had me at CONNECTED(00000004)…

I’ve been absorbed into the wonderful world of SSL recently (particularly certificate chain validation) and thought I’d pass along a few helpful commands:

openssl verify -purpose sslserver -CAfile <ca bundle> <cert file>
This helps check if a signed cert matches with a cert bundle (including all intermediates and root). It’s useful since some certificate authorities can require different intermediates and have somewhat misleading instructions on which to use (I’m looking at you, Network Solutions).

openssl s_client -CApath <root truststore directory> -connect <recipient web server>:443 -showcerts
This command string is useful for examining standard SSL connection information and validating the provided certificates against a trustStore directory (containing root certificates).  

openssl s_client -cert tls-cert.pem -starttls smtp -connect <recipient mail server>:25
This command string is a slight variation of the one above it.  It’s useful for troubleshooting TLS-related issues as they pertain to SMTP. In addition to providing information on the certificates being exchanged, it will submit the STARTTLS command as part of the connection attempt and allow admins to continue a manual SMTP session through a TLS encrypted tunnel.