Created attachment 261075 [details] Crash dump summary I often use USB tethering via my Google Pixel 7a phone. My carrier is T-Mobile. For a long time I was simply using IPv4 with DHCP. This has worked seamlessly with FreeBSD for as long as I can recall. When set to tethering mode the phone is detected by FreeBSD as a cdce(4) USB network device. I can simply use dhclient(8) to obtain an IP address. A while back I found out that Android can do IPv6 tethering if your carrier supports it, which T-Mobile does, so I decided to try that with FreeBSD too. The strategy I use is to simply add this line to my /etc/rc.conf file: ifconfig_ue0_ipv6="inet6 accept_rtadv" Now, when I plug my phone and enable tethering, FreeBSD receives an IPv6 router advertisement and is automatically assigned an IPv6 address. From here, things Just Work (tm) as expected. The problem is, since the phone is a USB device it can be surprise-removed. This leads to the ue0 network interface being destroyed. I have never had a problem with this with IPv4: the interface goes a way, dhclient(8) exits, and the system keeps going. This works *mostly* with IPv6, but not always. Sometimes it causes the kernel to panic shortly after I unplug the phone. I first noticed this with FreeBSD 14.1 and it still happens in FreeBSD 14.2. Based on the latest crash dump info (which I will include as an attachment), it looks like there is a TCP timeout that triggers for a stream socket that was opened while the interface was active which is now resulting in the TCP stack trying to send a reply, even though the interface is now no longer there to send it. Eventually in6_selecthlim() is called, but it explodes, likely because it's being provided an invalid interface handle. The crash dump summary shows the dmesg output with my complete system info, but briefly my configuration is: OS: FreeBSD/amd64 14.2-RELEASE System: Dell OptiPlex 9010 (all-in-one desktop) CPU: Intel(R) Core(TM) i7-3770S CPU @ 3.10GHz Cores: 4 physical, 8 logical (hyperthreading is enabled) RAM: 16GB Chipset: Intel Panther Point Steps to reproduce: 1) Get a phone with tethering support with a carrier that supports IPv6 (which hopefully should be most of them), or a USB ethernet adapter. If using an ethernet adapter, you will need to set up a test machine that supports IPv6 with router advertisements. 2) Install FreeBSD/amd64 14.2-RELEASE on an Intel system, with your favorite graphical desktop environment and web browser 3) Edit /etc/rc.conf and add the line: ifconfig_ue0_ipv6="inet6 accept_rtadv" (If necessary, change ue0 to whatever the interface name is for your device.) 4) Plug the phone/adapter into a free USB port. If using a phone, enable tethering over USB. For Android this would be under Settings->Network & Internet->Hotspot & tethering. 5) Use ifconfig -a to verify that the device has been detected and that it has an IPv6 address assigned 6) Generate some network traffic over the port: browse the web for a while. 7) At a randomly selected time, unplug the phone/adapter from its USB port If you do this often enough, you should eventually encounter a panic. -Bill