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How does Firefox DNS cache work with multi A records?

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  • Last reply by cor-el

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Hi Everyone,

I'm trying to understand how Firefox works internally when querying its DNS cache regarding domains with multi A records, this means a domain with more than one IP associated, like

Facts: - The SO has priority when resolving DNS and sends his cache content to the browser when asked. It sends ALL IPs associated to a requested domain. - Firefox's cache then has all IPs for every domain and calls the shots. - DNS Timeouts change per browser, in Firefox I've seen some domains have 4 minutes of TTL, others even less (1 minute TTL) - If I open Firebug plugin I can see the "IP Address" field in each request and see which IP was resolved per domain

Scenario 1: - I hit (which has 12 defined IPs, found it in chrome's dns cache: chrome://net-internals/#dns, or querying the OS: nslookup in a Mac Console ) - Then one IP is resolved and the SAME will be used during the 4 minutes the cache lasts (as validated with trial and error) - After 4 minutes the dice is thrown again and any other of the 12 IPs may be returned when hitting the URL again. (in one test I got IP finishing in 58, second hit IP finishing in 56, they are not ordered either)

Scenario 2: - I hit and then filter for hits to to see which IPs are used through AJAX in all the multiple hits - Then one IP is resolved and the SAME is used for all google-analytics hits during the 4 minutes the cache lasts.

Scenario 3 (Exactly SAME as 2): - I hit and then filter for hits to to see which IPs are used through AJAX in all the multiple hits - Then different IPs are defined for different AJAX hits of, seamlessly random. In every request each AJAX hit will be resolved to a different IP (or repeat some, with no pattern). Behavior will keep going until 4 minutes come by and the DICE of DNS is rolled again.

Conclusions and Questions: - Scenario 2 and Scenario 3 are easy to reproduce but it's not deterministic, every 4 minutes Firefox seems to choose whether to use ONE or ALL IPs during the period and then if it's ALL, it throws a new decision dice on EVERY hit. What is Firefox's internal behavior regarding reading its own DNS cache? - Does this mean that if my site has multi-record DNS then there is no level of IP stickiness? - Is there any pattern on how it happens? The context path is used?

NOTE: I've seen this happen both in Firefox and Chrome and both in MAC and Windows. I've validated this behavior in many multi-record domains, including, google-analytics,,, etc.

Any help or insight on this is MOST appreciated. Don't hesitate to ask if you need more details on this. Thanks! Federico

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