Mac can’t get IPv4 address but can get IPv6

Question

I’ve been struggling for two days now. My 2015 MacBook Pro with Catalina can connect to the Xfinity router by either WiFi or Ethernet. Both of them can obtain IPv6 address and thus access IPv6 websites, such as Google. However, neither methods can obtain IPv4 address, thus I can not even ping any IPv4 addresses. My iPhone however, can connect to the same router and obtain both IPv4 and IPv6 with DHCP.

An interesting observation: on the router page, I can see my iPhone with a human readable device name. For my MacBook I only see it’s MAC address, not the human readable host name like “xxx’s MacBook”. Not sure if this indicates anything.

I’ve tried resetting the router, resetting some network configurations under /Library/Preferences/SystemConfigurations, restarting my MacBook, renewing DHCP lease, killing mDNSResponders, none of them works.

Please help ... If you know how to reset DNS related configurations, restarting any DNS related system services, or reloading any kernel modules, or resetting all networking interfaces ... please let me know how to do it.

Btw, I recently updated to Catalina and it was working fine. I then updated LittleSnitch which involves some kernel stuff I don’t understand. Anyway it failed halfway after a restart, saying something like a “version mismatch”. At that point, IPv4 is already gone although I don’t know since when. I uninstalled it completely did not solve the problem. I also turned off Mac firewall.

Answer 1

Turns out LittleSnitch "version mismatch" is indeed related. The actual fix is described here at https://www.obdev.at/support/index.html?product=LS&topic=faq&entry=245694426255130

I think the important part is to rebuild kernel extensions by

1. Let the Mac enter recovery mode.
2. touch "/Volumes/Macintosh HD/System/Library/Extensions" which I think triggers the rebuilding of kernel extensions.

Anyway, it works now.