Tools like "dig", "nslookup", and "host" are useful. I prefer dig. Using dig's options, such as "@" (to specify the DNS server to looup against) are especially helpful. You can do a "whois" query on a domain to see what their authoritative nameservers are, and then query those nameservers about that domain for instance. Dig will show you the query time, and you could run separate bing/mtr checks on those nameservers to get a bigger picture.
For your workstations or other LAN clients, /etc/resolve.conf will have the DNS servers specified by your active DHCP lease if you're using DHCP, or they should be hardcoded there (so you can then find/change them). You can change these on-the-fly too (as root). Beware that other DNS servers might not answer for foreign domains -- I was recently using a UO DNS server [remotely] and then had problems, after which I discovered that UO started ignoring my requests for anything *not* in .uoregon.edu -- so don't be a big dummy like me ;-)
I'd suggest googling for a quickstart-sort of guide to using traceroute, since the output is not that complicated and you should be able to interpret it yourself :)
http://tldp.org/HOWTO/DSL-HOWTO/tuning.html
http://www.yolinux.com/TUTORIALS/LinuxTutorialNetworking.html
http://www.siliconvalleyccie.com/linux-hn/network-trouble.htm
...
best of luck,
Ben
On 3/14/06, horst <
[EMAIL PROTECTED]> wrote:
> Date: Tue, 14 Mar 2006 12:35:22 -0800
> From: Matthew Jarvis < [EMAIL PROTECTED]>
...
>> Larry's Rough Diagnostic Checklist
>> for slow internet. AKA "The Internet is slow, fix it."
>>
>> 1. is it DNS?
>>
>
> Hints or links on diagnosing DNS problems?
>
That's a BIG topic --I'll limit my comment to the slow/fast issue:
w/o looking at the source code(*) of ping (or traceroute), I'd assume it
resolves (DNS) to the destination only once; then it fires the packets (as
you may have defined them with extended options --hint, hint :-)
So the 'ping' times reported don't contain DNS overhead.
(*) You don't need to look at source code -- You can watch above with a
network sniffer, like ethereal --interesting, but doesn't go beyond your
LAN.
In a limited test on DNS influence you would point your application to the
IP quad, not the domain name. (as I said limited)
- Horst
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