1416675 - When disable mDNSResponder, Firefox Quantum loses internet connection permanently.

Status: RESOLVED INVALID

(Core :: Networking, defect, P2)

Description

[Qingxiang Jia]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:57.0) Gecko/20100101 Firefox/57.0
Build ID: 20171109183137

Steps to reproduce:

I disabled mDNSResponder and Firefox Quantum immediately lost internet access. This holds true as long as mDNSResponder is kept disabled.

To reproduce the problem, in terminal, run the following command to disable mDNSResponder:
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

To enable mDNSResponder, run the following command in terminal:
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

You will find Firefox Quantum immediately regains internet access.


Actual results:

When mDNSResponder is disabled, Firefox Quantum has no internet access.
When mDNSResponder is enabled, internet access is regained.


Expected results:

No matter mDNSResponder is disabled or not, Firefox Quantum should have internet access. The two things should not be associated. For example, google chrome works fine no matter the status of mDNSResponder.

Comment 1

[Gary Chen [:xeonchen]]

I can reproduce this on my mac (w/ System Integrity Protection disabled).
When mDNSResponder is disabled, it also disables the DNS functionality.

- Safari has the same issue.
- Google Chrome works because it seems to have its own internal DNS resolver (open [1] in Google Chrome).

[1] chrome://net-internals/#dns

Comment 2

[--]

(In reply to Qingxiang Jia from comment #0)

> I disabled mDNSResponder and Firefox Quantum immediately lost internet
> access. This holds true as long as mDNSResponder is kept disabled.

How do you determine that it lost "internet access" ? Doesn't this "only" affect name resolving?

Why do you disable it in the first place?

Comment 3

[Qingxiang Jia]

(In reply to Daniel Stenberg [:bagder] from comment #2)
> (In reply to Qingxiang Jia from comment #0)
> 
> > I disabled mDNSResponder and Firefox Quantum immediately lost internet
> > access. This holds true as long as mDNSResponder is kept disabled.
> 
> How do you determine that it lost "internet access" ? Doesn't this "only"
> affect name resolving?
> 
> Why do you disable it in the first place?

Here is what I did: run nslookup to find the DNS of a website A, say IP address 172.217.9.14 (Google).
Make sure mDNSResponder is on, visit 172.217.9.14 using Firefox and it loads the page without problems.

Turn off mDNSresponder, from the running Firefox instance, using 172.217.9.14, the page still loads due to cache (I guess). Open private browsing, using 172.217.9.14, I expect the page to load since I am using the IP address and we don't need DNS. However, the page doesn't load. In the same private browsing instance, using www.google.com, the page also doesn't load (which is expected since it needs DNS and private browsing doesn't access the old cached DNS results).

By the time I am writing this reply, I am using Firefox 59. But the above behavior didn't change. For completeness, I should also try it with Firefox 57.

Comment 4

[--]

According to a superuser.com comment[1] I found:

"In recent versions of macOS, mDNSResponder is also the primary unicast DNS resolver agent, so shutting down mDNSResponder is terrible advice; you're disabling the OS's ability to do DNS resolution"

So I'll ask again: why did you disable mDNSResponder? What do you want to gain by this and why did you presume that Firefox and other applications that need name resolving should work fine without it?

> visit 172.217.9.14 using Firefox and it loads the page without problems

If you go to "http://172.217.9.14", it will immediately redirect you to another host with a name and if the name resolving has effectively been switched off it won't work. That it doesn't work with Safari either is also a good clue I think.

As Gary mentioned in comment #1, it probably works in Chrome because it has its own name resolver functionality so it isn't affected by mDNSResponder getting switched off.

I'm struggling to see how this is a Firefox bug and not "working exactly as intended"...

[1] = https://superuser.com/questions/1150494/mdnsresponder-on-macos-sierra-malware#comment1656550_1151569

Comment 5

[Qingxiang Jia]

> So I'll ask again: why did you disable mDNSResponder? What do you want to gain by this and why did you presume that Firefox and other applications that need name resolving should work fine without it?

I no longer do that. I did that because I was living in a school apartment where the internet is configured in a way that mDNSResponder kept generating traffic that was around 200KB/s. We encountered internet outages several times a day. (Most of the residents are mac users) We thought it was traffic generated by it that led to the frequent outages.

> I'm struggling to see how this is a Firefox bug and not "working exactly as intended"...

Now I see http://172.217.9.14/ will not work due to a redirect which then requires DNS. Yes, I agree this should not be a bug and we can probably close this one.

Thanks for the help.

Comment 6

[--]

Thanks!