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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] 3 points 3 weeks ago* (last edited 3 weeks ago) (1 children)

Private browsing has no effect. Cannot connect, no packets observed in Wireshark. What are these logs that you might suggest viewing? There's a console, but I didn't observe any relevant long messages.

I downloaded an older version from three months ago and it cannot load the page on the test machine. A arbitrarily-tested older version wasn't compatible with my Mac.

I tried Firefox on another machine (Linux) and it can load the page no problems. Sadly, I don't have a second Mac to test if it's a Mac thing, but then why this IP? Seems strange to me.

If I use an SSH tunnel to direct localhost:8000 to 192.168.0.2:80 using a third box as a go-between, it connects just fine. Enter it directly in the address bar and no dice. Cannot connect.

Thank you for the suggestions! I'm stumped. I can work around it, but it's really weird and it would be nice to know why it doesn't like this IP.

[–] NegativeLookBehind 1 points 3 weeks ago (1 children)

You’re welcome! Can you change the IP of the target host?

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

Hmm, that's a reasonable thing to test. Sadly, this Linksys router doesn't allow changing the IP in bridge mode. It will be the subnet mask ending in .2. It's really lame to be limited in such a way, but nearly all settings are disabled in bridge mode.

This browser cannot access the router settings. Other browsers and devices can. It's very odd that it seems unable to communicate to this IP.

I can live with this, but it really makes me wonder why.