Fixing internet access failures after upgrading to MacOS 10.14 Mojave

I innocently upgraded my 2012 Macbook Pro to Mojave (10.14) from High Sierra (10.13) purely because HomeBrew was refusing to let me install any new goodies on such an old OS. The upgrade itself was painless and everything seemed to be in good shape until I tried to access the wider Internet (access to my own local network was completely fine).

Now Internet access problems are extra-infuriating, because "Googling for the solution" instantly becomes far more of a palaver. An interesting quirk of this particular issue was that using my iPhone's "Personal Hotspot" worked perfectly with the Mac. Only accessing the internet directly through the home WiFi was problematic, and other devices were of course completely fine.

So to recap the scenario (and hopefully give this post some Google-ju to help anyone else in this situation), here's what we had:

• Macbook Pro was running OSX High Sierra (10.13) with no prior network problems
• Network has been carefully set up and tuned for "meshed" operation
• Local DNS services provided by DNSMasq running on a Raspberry Pi
• DHCP services provided by DNSMasq, handing out fixed addresses to "known" devices like the Macbook Pro
• All local network services working fine - e.g. accessing router admin webpages
• Internet access when tethered to an iPhone works fine (Mac gets assigned 172.20.10.2, iPhone is 172.20.10.1)
• Internet access through wired Ethernet (via a Thunderbolt adapter) works fine
• Internet access on home Wifi completely fails; no DNS, no ping, no browsing

After a frustrating day, I decided to blow through the Mojave upgrade in case it had been fixed in Catalina (10.15) ... it hadn't.

I went through probably a dozen different iterations/versions of the "recreate your network stack"/reboot/reset PRAM and/or SMC cycle; nothing was helping, and I was still mystified by how the wired Ethernet was working immaculately while the WiFi failed to get "out" of the local network.

Comparing settings with my wife's Macbook that had perfect Wifi yielded nothing, so just for the hell of it, I decided to check the gateway router's settings one last time to see if for some reason I had a special route or firewall rule set for the Macbook Pro's Wifi IP address (10.240.0.72) only, which would explain why the wired connection (10.240.0.77) had no problems. It wouldn't normally be something I'd do, but given the output of traceroute was showing that the hop to the gateway was working in all cases, I had to start suspecting that the MacOS internal routing of packets was not faulty ...

And lo and behold, WHAT IS THIS? On my (ISP-provided) TP-Link router's Security/DoS prevention page, a place I have rarely if ever visited ... My laptop's Wifi IP address has been automatically added to a blocklist for attempted DoSing of my network ... from the inside ?!?!

I removed the blocklist entry and instantly everything worked perfectly. Oh dear. I have no idea when my laptop started absolutely hammering pings/UDP/TCP SYNs but it must have been at some point during or immediately after the upgrade to Mojave, and the router did what it was configured (completely by default) to do when it saw more than 3600 packets/sec coming from my laptop. Wow.

Reading the rather scant documentation for this router feature indicates that it uses up (extremely precious) router CPU so I've turned this feature totally off - internal DoS prevention seems like a waste of time to me.

Addendum: Disabling SPI and DoS detection/prevention has massively sped up using the router's web UI so I can only imagine it's working wonders for the overall performance. So I guess it's been a win ... overall 😕