[Techtalk] Slow DNS

David Sumbler david at aeolia.co.uk
Tue Oct 12 01:44:05 EST 2004

Since I upgraded from Fedora Core 1 (with 2.4 kernel) to FC2 (with
2.6), I have noticed that often it takes a long time to resolve host
names.  I haven't changed my /etc/resolv.conf file (which is correct)
or anything like that.

It isn't associated with any particular program: for instance, Mozilla
Firefox and Links both have the problem.

I just tried an experiment: I entered 'dig newswww.bbc.net.uk +trace'

and got the following output:

; <<>> DiG 9.2.3 <<>> newswww.bbc.net.uk +trace
;; global options:  printcmd
.                       165188  IN      NS      d.root-servers.net.
.                       165188  IN      NS      e.root-servers.net.
.                       165188  IN      NS      f.root-servers.net.
.                       165188  IN      NS      g.root-servers.net.
.                       165188  IN      NS      h.root-servers.net.
.                       165188  IN      NS      i.root-servers.net.
.                       165188  IN      NS      j.root-servers.net.
.                       165188  IN      NS      k.root-servers.net.
.                       165188  IN      NS      l.root-servers.net.
.                       165188  IN      NS      m.root-servers.net.
.                       165188  IN      NS      a.root-servers.net.
.                       165188  IN      NS      b.root-servers.net.
.                       165188  IN      NS      c.root-servers.net.
;; Received 436 bytes from in 36 ms

uk.                     172800  IN      NS      NS1.NIC.uk.
uk.                     172800  IN      NS      NS2.NIC.uk.
uk.                     172800  IN      NS      NS4.NIC.uk.
uk.                     172800  IN      NS      NS3.NIC.uk.
uk.                     172800  IN      NS      NS5.NIC.uk.
uk.                     172800  IN      NS      NSA.NIC.uk.
uk.                     172800  IN      NS      NSB.NIC.uk.
uk.                     172800  IN      NS      NS6.NIC.uk.
uk.                     172800  IN      NS      NS7.NIC.uk.
;; Received 346 bytes from in 30144 ms

bbc.net.uk.             172800  IN      NS      ns0.thny.bbc.co.uk.
bbc.net.uk.             172800  IN      NS      ns0.thdo.bbc.co.uk.
;; Received 89 bytes from in 30144 ms

newswww.bbc.net.uk.     300     IN      A
;; Received 52 bytes from in 1 ms

I did this 4 times, within a few minutes, and got almost identical
results each time.

Despite the times reported, only the first step took perhaps the time
it says.  The 2nd an 3rd steps say that they took about 30 seconds
each, but in fact it was about 45 seconds for each step, and the final
step took not 1 millisecond, but about 30 seconds!

So where and why are these delays occurring?  Any ideas anybody?  My
ISP assures me that they have no known problems relating to this.



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