DNS Troubleshooting ...
Written by Ken Task
Here's a networking tip ISD's might want to consider … use openDNS servers as secondaries … even in private networks.
Why?
Some Regional ESC private nets use DNS primary servers that are within their own domain (no secondary DNS). When those DNS servers go down, not only does the ISP's domain name go 'dark' but all ISD's for which they provide DNS service are also 'dark' … well, actually, just not reachable by name if the circuits are up for the servers are up and running.
Here's a plug for using Webmin on Linux boxen that's related.
Webmin has a tunnel tool for http. When one cannot reach a site via URL from current location, it's wise to check to see if the issue is related to present connectivity (the network one is on) or if it's also an issue with other networks. The image below shows just a situation/test.
If you note, OpenDNS provides more information past that of a 404. Note the link for 'which nameservers are failing'. It checks DNS from TLD (top level domain) downward to see which DNS servers are failing.
Truth in advertising: ALL things have 'catch 22's' and this tip does also! IF Windows servers/workstations have the 'fall back' position to skip to root hints still active and IN a private network, it's possible they'll cache the temporary protection OpenDNS offers - ie, DNS has recovered, but the workstation has now cached information it got from OpenDNS. Fix is simple though, force another DNS query and/or clear the DNS cache.
Tip for troubleshooting … I keep a text file on my equipment (laptop and in a cell phone) that list the IP addresses of open DNS servers. If I need to, on a Mac and on a Linux server, I can quickly add them to the resolv.conf file to test DNS (adding them does not require reboot - the next DNS query will use the nameservers in the order they are entered in resolv.conf).
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