In this tip, I want to let you know about a couple of my favorite troubleshooting sites. I think these are occasionally very useful for network administrators.
The first site is telnet://route-server.ip.att.net. You can get there via the telnet application or by typing that into your browser, which will most likely cause it to open up a HyperTerm session. This site is basically just a Cisco router collecting all the routes from AT&T's peers, so administrators can keep track of them, mostly for diagnostic purposes. In fact, if you are interested in this sort of thing and have the resources, you might consider setting up your own route server. But for most readers, this tool is useful for occasional internet troubleshooting. If for instance, you use multiple providers and want to make sure that your networks are being advertised the way you want, you can use this tool from AT&T. If you want to see if your AS path has changed, you could also use this tool. If you operate routers that get internet routes via BGP, this service is also useful for a simple sanity check. For example, each time a BGP routing table changes, it gets a new version. If your table changes about once a minute, is that a high or low number? The ability to look inside a few other internet routers can help answer questions like that.
(Note that server is quite slow, so be patient.)
Another very useful site is www.dnsstuff.com. There are lots of ways to get WHOIS searches and nslookup equivalents out there, but this site has a few extra goodies worth checking out. The best one is a spam database lookup. Type your mail domain in and it will search about 150 blacklists and tell you which ones you're on.
Another tool at this site lets you check the DNS cache at some major ISPs. If you're making DNS changes, and your management wants hourly reports about the DNS propagation across the internet, just show them how to use this tool.
The nice thing about both the tools at dslstuff is that they list all the places they check, not just the ones where you're cached or blacklisted.
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.