carl at byington.org
Tue Sep 11 18:42:21 UTC 2018
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On Tue, 2018-09-11 at 14:19 -0400, Alex wrote:
> This is when our 20mbs cable upstream link was saturated and resulted
> in DNS query timeout errors. resulting in these SERVFAIL messages.
Not specific to dns, but this looks like a bufferbloat problem, which is
common with cable modems. When the upstream link is saturated, the
buffers in the interface device (cable modem or possibly a standalone
router) become full. If there is a lot of buffer space, the latency
becomes very large, and that will cause many problems, including issues
with dns. A partial fix is to prioritize small packets like dns queries
and tcp acks, so they don't wait behind a large queue of full size
packets. A more complete fix is switching to fq-codel queue discipline.
google for bufferbloat for more details.
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