Is Forwarding Faster?

[Payne Freret]

I have wondered whether forwarding DNS queries from my local DNS
nameserver is faster than having my local nameserver resolve them
itself.  I reasoned that my ISP's nameserver would have quicker access
to the Internet than my local nameserver.

I conducted a series of experiments to compare the amount of traffic
generated by my local nameserver (Bind 8.2p1) when it was configured
to forward queries with the amount of traffic it generated when it was
configured not to forward queries. I used tcpdump to monitor IP
domain-port traffic moving between my nameserver host and my ADSL
modem.  I inserted or removed a "forwarders {<ISP DNS1>;<ISP DNS2>;};"
statement in Bind's options block.  I killed and restarted named each
time I inserted or removed the forwarders statment so as to begin each
experiment with the same cache state, and I waited 60 to 90 seconds
for named to initialize its cache before running dig.  I tried
approximately thirty different domain names.  Once I had successfully
looked up a domain name, I did not try it again until I had killed and
restarted named.  For domain names I used domain names contained in
URLs given by Alta Vista in response to an arbitrary search.

I found that when the forwarders statement was present, using dig to
look up a domain name produced a single two-Ethernet-packet exchange
between my local nameserver and the ISP's nameserver.  On the other
hand, when the forwarders statement was absent, anywhere from four to
more than 40 packets were exchanged between my local nameserver and
various remote nameservers. 

I also observed the total query times that dig reported.  I found that
when the forwarders statement was present, dig reported total query
times that were almost always less than half a second and usually less
than a quarter second.  When the forwarders statement was absent, dig
frequently reported total query times of around one second.  In
several extreme cases it reported times of five, six, and eight
seconds, during which my local nameserver awaited responses to its
queries.

The results suggest that even with an ADSL connection, a
non-forwarding local nameserver is not as quick as my ISP's
nameserver.

Payne Freret

P90/RH 4.2 Linux/Bind 8.2p1/PacBell ADSL