1493619 - DNS cache: pad very short TTLs with a grace period

Status: RESOLVED FIXED

(Core :: Networking: DNS, enhancement, P2)

Description

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(split out from bug 1463374)

With proper support for DNS TTLs, we occasionally get responses with single-digit seconds life-times. This makes them expire from the cache ridiculously fast, and for TTL-support we don't set a grace period.

Since the non-TTL version of the DNS cache still works fine with 60 second default TTL + 60 second grace timeout, there's a strong indicator that just adding a little grace will not break Firefox for users with TTL supported DNS resolves.

I propose we set a grace for all TTLs less than a minute to pad out the cache life time to a full minute (at least). Like this:

 if (ttl < 60) {
   grace = 60 - ttl;
 }

Comment 1

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Attachment: bug 1493619 - pad DNS entry grace period for low TTL cases

MozReview-Commit-ID: 8AsmyaSYVJr

Comment 2

[Dragana Damjanovic [:dragana]]

Is there a way to notice that we are breaking something? Do we have some telemetry probe? If we do not have any, we should add some, e.g. maybe black-listed address rate when a record is used in grace period.

Comment 3

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We don't have any decent probe to measure if this breaks anything. But again: we already set a 60 second grace period to *all* DNS entries when using the native resolver on Linux and mac so if adding (up to 60 seconds) grace would be a problem, that problem would already exist in Firefox on these platforms since a few years back.

But adding some sort of counter for how many blacklisted addresses that are added while in grace period compared to non-grace could certainly be an interesting probe.

However; isn't the blacklisting basically the only current method we have to disable further IPv6 attempts for specific hosts for the cases where you resolve IPv6 addresses but IPv6 isn't working (correctly) ? If so, then I presume those blocks will drown all other blocks.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/ddcd7cc2f3cd
pad DNS entry grace period for low TTL cases r=dragana

Comment 5

[Arthur Iakab [arthur_iakab]]

https://hg.mozilla.org/mozilla-central/rev/ddcd7cc2f3cd