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[SOLVED] Firefox Can't Connect to Specific LAN IP (infosec.pub)

submitted 3 weeks ago* (last edited 3 weeks ago) by [email protected] to c/techsupport

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This is a really weird problem that I can't seem to track down further. Perhaps a creative person could suggest some test ideas. Here are the facts:

Firefox "Unable to connect" to my LAN server (a router) at 192.168.0.2 port 80.
Network error is specifically "NS_CONNECTION_REFUSED".
Wireshark on a Raspberry Pi placed between the laptop and server shows no packets exchanged trying to connect. Any packet containing 192.168.0.2, any port.
Chrome and Safari work just fine on the same machine. I can see the packets in Wireshark. This validates my test setup works.
Curl works, loads the web page. I can see the packets.
I have reinstalled, refreshed, removed all extensions, cleared all history and cookies in Firefox and still cannot load the page.
Firefox in Safe Mode cannot load the page.
Disabled DNS over HTTPS, made sure No Proxy is selected in network settings. Still cannot load the page.
Disabled IPv6 in Firefox with about:config setting. Still fails.
I have no security software installed of any kind on this Mac. No antivirus or firewall except the default OS one.
Turned off Mac built-in Firewall. Still unable to connect.

Why is Firefox apparently refusing to connect to my server? Other LAN IP addresses work fine, even local ones. It specifically hates this one.

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[–] [email protected] 2 points 3 weeks ago* (last edited 3 weeks ago) (1 children)

The fact you're not seeing any exit packets, along with the ability to connect using anything other than Firefox means it must be an issue with Firefox itself.

Not to insult your intelligence, but do you have any extensions installed on Firefox such as an ad-blocker? Ones that are allowed to operate in private mode as well? I've had random issues with blacklists in my ad-blocker having bad entries in the past.

[–] [email protected] 2 points 3 weeks ago (1 children)

No insult taken! I reset Firefox and I’m using a new profile. There are no extensions installed.

I’m not sure how I would go about debugging Firefox further to understand why it doesn’t want to attempt connecting to that IP. Currently I agree with you. It seems like there’s some bug with Firefox itself or perhaps an unexpected configuration hidden elsewhere on the system on which it depends but nobody else.

[–] [email protected] 2 points 3 weeks ago

You might try a quick shell script to set NSPR_LOG_MODULES and NSPR_LOG_FILE to "all" debug mode to fish for some clues. https://firefox-source-docs.mozilla.org/nspr/reference/nspr_log_modules.html#nspr-log-modules