We’ve seen yesterday that an easy way to significantly decrease the time spent doing DNS resolution was to put a small (1024 items!) cache on each of our fetchers.
This technique allowed us to save all network traffic on about 50% of our DNS requests: pretty significant, but still, for the rest (50%) we would be relying on remote servers which are both far (latency going thru the roof) and crowded (public servers are used by a lot of people!).
A shared cache
For that reason, we decided to host our very own recursive DNS cache: we picked unbound, but we could have gone with the older and venerable Bind9 or Powerdns’s recursor. The setup is pretty simple, even though the defaults are quite conservative.
The goal of this central DNS server (we actually have 2 for redundancy and load balancing) is to cache results accross all of our fetching servers while still provide a reliable resolution latency.
When adding our server’s IP as the second line to our
/etc/resolv.conf (right after
127.0.0.1 for dnsmasq), we get the following performance: Min:0 Max:24565 Median:1 Average:5.572257, to be compared to Min:0 Max:3028 Median:8 Average:18.41814* (with Google’s 220.127.116.11 server).
This is again a great gain compared to using only a local cache and public DNS servers, because the latency is about 1ms compared to 8ms with the ‘best’ public server in our tests.
The following graph illustrates the efficiency of our cache:
You’ll notice that the hit rate is still rather small, with only about 60% hit rate.
Publishers, fix your TTLs!
After investigating further we quickly found out that a lot of domains had no TTLs or very low TTLs, which means that caching is impossible. This has a performance impact on our side, but it also has a strong impact on the performance of their website too.
Here is a quick script to check what your TTL is.
300 seconds is a rather common default.
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