PSA: uBlock Origin will interfere with your DNS prefetching setting

[jobbautista9]

Just updated to 33.3.0, and I noticed that my DNS prefetching was disabled, which surprised me. I enabled it and then restarted only to see it got flipped back to false again. I had to go through all my add-ons, and turns out I had the Disable pre-fetching checkbox ticked in my uBlock's settings, which explains why the DNS prefetcher pref gets flipped every restart...

[Moonchild]

When prefetching is enabled, the browser can establish connections to remote servers even if the resources from these remote servers were supposed to be blocked by uBO.

This is incorrect for DNS prefetching. This only applies to prefetching of content/predictor/prefetch-next -- uBO should not block DNS prefetching if what is described in the quote is the intent.

[jobbautista9]

Indeed. It shouldn't bypass the user's filters, which seems what uBlock intends to avoid. This might also be why the checkbox is ticked by default; everyone who has uBlock Origin installed will most likely have DNS prefetching forcibly disabled too instead of being flipped by the browser update. Hopefully UCyborg will fix this in their fork.

[back2themoon]

Thanks for the warning, jobbautista9.

[gepus]

DNS-prefetch is off
DNS-prefetch is on

[Moonchild]

DNS-prefetch is off
DNS-prefetch is on

Working exactly as-intended. Standard DNS queries don't connect to the addresses looked up.

[gepus]

Working exactly as-intended. Standard DNS queries don't connect to the addresses looked up.

Of course they don't.
Bombing the DNS-server for milliseconds of speed up?

[Moonchild]

Bombing the DNS-server for milliseconds of speed up?

Wow, so dramatic!

Remember that lookups are cached. Whether they are looked up in a burst right away or piecemeal doesn't matter for that. But if you don't want to "bomb" the DNS server with work it's designed to be doing (it's all UDP so it's not like you have a connection overhead...) then feel free to switch it off.

[back2themoon]

Just updated to 33.3.0, and I noticed that my DNS prefetching was disabled, which surprised me.

Wait, doesn't disablePrefetch=True (the default setting) mean that prefetching is in fact, disabled?

That's what I get from the wording anyway.

[jobbautista9]

Oh yeah you're right, my bad

[LuftWafflePilot]

So is this actually a problem or not? I can't figure out whether the addon is doing something it shouldn't or what's going on.

[Moonchild]

To clarify...

The default for network.dns.disablePrefetch in 33.3.0 is false. disable=false means it's prefetching (i.e. the feature is enabled).

[back2themoon]

Right, this setting's default state changed in 33.0.0. Was probably looking at 33.2.1, sorry for the confusion.

[jobbautista9]

So is this actually a problem or not? I can't figure out whether the addon is doing something it shouldn't or what's going on.

As MC just said earlier network.dns.disablePrefetch should be false by default in 33.3.0. But uBlock Origin's default settings (which has the offending "Disable pre-fetching" checkbox ticked) is forcing that pref to be true even with browser restarts (as long as that uBlock checkbox is ticked). This can confuse users who upgrade to 33.3.0 while having uBlock Origin installed expecting for DNS prefetching to be enabled.

[moonbat]

To add - using the FoxyProxy addon will also result in disabling DNS prefetches.

[back2themoon]

Am I correct to assume that this uBO setting does not make a direct change in how uBO operates? It only toggles this particular browser setting?

[jobbautista9]

It seems to only toggle the browser's own prefs, AFAICS.

[jobbautista9]

FWIW, this is the add-on author's justification in 2016 on why DNS prefetching is forcibly disabled: https://github.com/gorhill/uBlock/issues/1716

[Moonchild]

Misinterpretation of wireshark results, I think. There's obvious lookups missing in Chromium's dump that should occur for a page load, i.e. googleapis.com for the fonts used in the page, etc., so he was probably seeing the caching in effect, and deciding to block DNS based on "I see fewer queries in Chromium so Firefox needs more aggressive settings" while that's not really the case.
Regardless, those lookups do not mean connections to the looked-up servers are made, which has been my point here.

In case people still don't understand I'll try to break down the sequence, AFAIU without combing through the code in question (some steps may be combined or out of order):

A. Original page request

1. Request is made for example.com
2. DNS lookup for example.com -> IP + caching of result
3. Connection to example.com's IP
4. Page load from example.com
5. Parsing of page, scanning for resource URLs, e.g. images
6. DNS lookup of resource URLs of display elements -> IP + caching of result
7. Connection to resource URL locations, and loading page elements from them.
8. Scan of page content for links (external.com)
9. DNS lookup for link URLs -> IP + caching of result

Done.

B. Navigation to linked URL:

1. Request is made for external.com
2. DNS cache lookup of external.com. Found? -> IP and skip next step, otherwise do next step
3. if not cached: DNS lookup for external.com -> IP + caching of result
4. Connection to external.com's IP
5. Page load from external.com

followed by the rest of the steps in the sequence above.

Steps A8 and A9 happen when prefetching is enabled, resulting in B2 always having a cached IP, skipping B3. As you can see there are no connecting steps after A8 and A9, the page load is done. A9 happens when the original page is being displayed and viewed by the user (i.e. in the background when the browser is otherwise idle), while without prefetching it would be done when actively navigating, incurring a wait.

[UCyborg]

So this checkbox hasn't practically altered the behavior of Pale Moon as set out-of-the-box in any way until now, as the 3 prefs' default values were already set like the checked box sets them.

Code: Select all

if ( settingVal ) {
    this.clear('network', 'prefetch-next');
    this.clear('network.http', 'speculative-parallel-limit');
    this.clear('network.dns', 'disablePrefetch');
} else {
    this.setValue('network', 'prefetch-next', false);
    this.setValue('network.http', 'speculative-parallel-limit', 0);
    this.setValue('network.dns', 'disablePrefetch', true);
}

Until now that network.dns.disablePrefetch's default value has changed, which would be better handled by separate checkbox, if at all, considering the option is exposed in Pale Moon's own settings window?

To conclude, clearing the checkbox for now will allow DNS prefetching to remain enabled while other prefetching will still be disabled, unless manually changing settings in about:config.