Troubleshooting Tools

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When it comes to troubleshooting, I have three tools that I go back to time and time again. They're pretty basic, but I've found them to be useful in diagnosing about 90% of the issues out there. They can provide insight into what's wrong, and more importantly, they can provide proof as to what isn't wrong.

Telnet

Telnet allows me to test through Layer 4. If I try to telnet to a IP address and port, such as port 80 for a web server, and a connection is established, then network connectivity through Layer 4 is working. Routing, switching, and firewall configuration are all properly working in that case. Ping doesn't allow me to do that, especially since most load balancers won't (by default) respond to ICMP on a virtual IP.

Telnet also allows me to test through Layer 7, if I know a bit of the protocol. For instance, I telnet to port 80 of a web server, and then I type in GET / and hit return twice. Technically, that's not a properly formed request, but virtually ever web server (if functioning) will respond to, even if it's some type of error.

OpenSSL

I can use OpenSSL much like telnet, if I'm trying to test a network service that operates with SSL/TLS. SSL/TLS is a transparent protocol, in that once it's properly in place, it's just like using a non-encrypted connection. As such, once connected with OpenSSL, you can do a "GET /" just like you would with Telnet.

You can check out it's usage on the Telnet for SSL documentation page.

HTTP Analyzer

The third tool I use is some sort of HTTP analyzer. There are two basic types of HTTP analyzers: An HTTP header monitor, and an actual full-blown HTTP analyzer. In particular, an HTTP analyzer allows me to see:

  • The request header sent to the server, including
    • Host header
    • Full request
    • The cookies I have set
  • The response header sent by the server, including
    • HTTP return code (200, 404, 302, etc.)
    • Any redirects from the server (HTTP 302)
    • Any cookies the server sets

Some HTTP analyzers have the ability to specify what cookies and their values you send to the server, as well as manipulate other request headers to do specific testing.

Available Header Monitors

There are two very popular and very useful HTTP header monitors available, one for IE and one for Firefox. They are both free.

Available HTTP Analyzers

HttpFox is free, while the others have licensing costs.




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