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DNS caching

Yeah, some sites use some bad ju-ju in bypassing DNS's safeguards. They hold onto the numbers and only release on scheduled reboots, etc., meaning that their customers (as I assume this is largely ISP behavior) get worse service. My favorite problem with DNS involves poorly aged ARP caches on my local network! OOps...restart the Ethernet switch...ah...

Glenn Fleishman
Thursday, November 1, 2001

I was wondering about that. It's driving me crazy as I try to move services from machine to machine ... In order to provide a "seamless" experience I think I will have to set up the new machine to answer to the old machine's IP address for a while.

Joel Spolsky
Thursday, November 1, 2001

Some ISPs here in India refresh their DNS caches only every 24 hours to once a week. When I made a nameserver switch for one of my domains, I had to wait 4 days before my ISP's DNS server recognised the change :(

Madhu Menon
Friday, November 2, 2001


It looks like your DNS is configured wrongly. Even though your SOA is configured with a minimum/ttl of 900s, individual records have a TTL of 259200s (72h):

% dig
; <<>> DiG 2.0 <<>>
;; ANSWERS:      259200  CNAME  259200  A

Maybe you should have a closer look at your DNS server's documentation (bind allows configuration of TTL's on a per-file or per-record basis for example).


Tim Kleingeld
Sunday, November 4, 2001

Of course, none of this helps when crapware or deliberately misconfigured servers refuse to behave.  One .nz ISP, for example, forcibly caches DNS records (and, no doubt many other things) for days, overriding the TTLs supplied for the record.

Sadly, this seems to be becoming more common as organisations try to save a penny, even at the expense of their customers getting the wrong infomation.

Rodger Donaldson
Sunday, November 4, 2001

When moving from an old servers to new, set up a web server answering on the old IP address which redirects to the new one.  I don't know about IIS, but it's a one-liner in Apache:

Redirect / http://1.2.newipaddress.4/

Of course, this might be on the same physical server as the new web server.

Until the DNS change propogates, people that have the old address will access the new server using the IP address instead.  You can set up an additional DNS name for the new server ( if that is problematic.

Kyle Cordes
Monday, November 5, 2001

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