[thelist] DNS caching? Tips to override, refresh DNS?

[Daniel J. Cody]

Hey Joe -

Welcome to one of the most frustrating and common problems on the 
internet today :)

The best way to check where the problem lies is to do an nslookup 
against the nameserver that your client uses. That is, if your clients 
ISP or provider is myISP.com, you want to do an nslookup *against* the 
nameservers for myISP.com to see if they have the old or new info. To do 
this, find out what the hostname of msISP.com's primary DNS server is 
using the whois command or by looking on the networksolutions website. 
For exmaple, the DNS servers for myISP.com are
AUTH1.DNS.GXN.NET
AUTH2.DNS.GXN.NET

So, we can assume(although this always isn't the case) that the customer 
of myISP.com is using the nameservers of myISP.com. Now we want to 
lookup the domain in question that doesn't seem to be propogating 
correctly *against* the DNS server auth1.dns.gxn.net and NOT our own DNS 
server. To do this, type 'nslookup' and hit return(this is unix, but it 
should work the same on NT) at which point you should see something like 
this(my example follows):

[djc at leo djc]$ nslookup
Default Server:  ns1.starkmedia.com
Address:  63.237.54.2
 >

now we're at the > prompt and can type in a domain name or IP address 
for nslookup to query against(in this caes) ns1.starkmedia.com. Or we 
can pass it some different options, like which nameserver to use. So, at 
the > prompt type 'server auth1.dns.gxn.net' and hit return, at which 
point you should see something like so:

 > server auth1.dns.gxn.net
Default Server:  auth1.dns.gxn.net
Address:  195.224.255.2
 >

We're back at the > prompt again, but now you can see the default DNS 
server which we're looking domain names against is now 
auth1.dns.gxn.net. If you type the domain in question and hit return, 
and auth1.dns.gxn.net returns the old IP address of the domain, then the 
DNS server hasn't recieved the new host information(the domain name 
hasn't 'propogated' accross the 'net). At this point, you can try a 
couple things the best of which is to email the administrator at the 
gxn.net domain name(example :) and let them know that the TTL(time to 
live) for their DNS server is set to high and they should refresh thier 
DNS servers. Once they do that, the DNS server will report the correct 
IP address, and you're client will be happy :)

*whew*

Now, if you do an nslookup against their DNS server and it returns the 
correct IP address, then the 3 situations below can be explained as 
sitting behind a firewall/NAT box/proxy server that has its *own* DNS 
cache and *that* needs to be refreashed. Most every proxy server has its 
own DNS cache to speed up its response time, and it often takes a hard 
refresh of the proxy server before it will *ever* go to the DNS server 
to get a domain name it doesn't have in its cache. There are some tricks 
you can try to get it to refresh, but the best is to usually email the 
admin(or  have your clients talk to them) and let them know the 
situation. Hopefully they'll understand the problem and fix it for ya :)

Hope that long assed explanation helps a bit, if not, lemme know :)

.djc.



Joe Crawford wrote:

> A little hard to explain - but we have a client site which changed hosts
> and DNS recently, but the DNS has not propogated to everywhere on the
> net.
> 
> How does one go about getting local DNS administrators to get a fresh
> copy?
> 
> Examples of people with the old version:
>         - people on cable modems
>         - people behind firewalls
>         - people behind proxys
> 
> Any tricks for these people to query their local DNS controlling server
> so that it gets a fresh copy?
> 
> I only half understand how DNS works - so simple explanations would be
> best.