[SOLVED] BIND 9.7: does it optimise amongst forwarders like dnsmasq?

catkin · 04-21-2011, 03:36 AM

Hello

Doe BIND 9.7 optimise the use of forwarders as dnsmasq does?

The logs show that when dnsmasq initialises it contacts all its upstream nameservers and selects one to use. After that it can be seen switching to another upstream nameserver from time to time. Presumably this shows that it is dynamically choosing the best performing upstream DNS server.

Does BIND 9.7 do something similar? Here's from O'Reilly's "DNS and BIND 5th Edition": "On BIND 8 nameservers from 8.2.3 on, and BIND 9 nameservers from 9.3.0 on, you don't need to list forwarders more than once. These nameservers don't necessarily query the forwarders in the order listed; they interpret the nameservers in the list as "candidate" forwarders and choose which one to query first based on roundtrip time, the time it took to respond to previous queries". That sounds promising and could be similar to what dnsmasq seems to do.

The question is important because our ISPs' nameservers are at best the fastest but are inconsistent and are sometimes unavailable. dnsmasq-like behaviour allows us to get optimal name resolution by listing all our ISPs' nameservers and some public ones.

Best

Charles

catkin · 12-15-2011, 04:27 AM

Solved.

The answer is "yes". From docstore.mik.ua:

BIND name servers use a metric called roundtrip time, or RTT,to choose between name servers authoritative for the same zone. Roundtrip time is a measurement of how long a remote name server takes to respond to queries. Each time a BIND name server sends a query to a remote name server, it starts an internal stopwatch. When it receives a response, it stops the stopwatch and makes a note of how long that remote name server took to respond. When the name server must choose which of a group of authoritative name servers to query, it simply chooses the one with the lowest RTT.

Before a BIND name server has queried a name server, it gives it a random RTT value, but lower than any real-world RTT. This ensures that the BIND name server queries all of the name servers authoritative for a given zone in a random order before playing favorites.

On the whole, this simple but elegant algorithm allows BIND name servers to "lock on" to the closest name servers quickly and without the overhead of an out-of-band mechanism to measure performance.