While getting ready for a trip to DEF CON 22 I wanted to have a VPN set up from my phone and tablet to connect back home. After a little while I had both IPsec and OpenVPN connecting back to the house’s pfSense box and passing IPv4 traffic through the tunnel without issue. But, there was a problem when connecting over the T-Mobile mobile network: the VPN would handle IPv4, but IPv6 was left alone to leak through the carrier.
This can be seen in the screenshot above (link) where IPv4 is passing through my home’s Wide Open West connection, but IPv6 goes through T-Mobile. Such leakage has also been written about here by lxgr, in much greater detail.
By pushing an IPv6 route to the client from OpenVPN I was able to black hole IPv6 on the client and close this leakage. This is done by adding the following to the Advanced configuration section of the OpenVPN server config in pfSense:
push "route-ipv6 ::/128 ::1"
While IPv6 still is configured, all traffic goes to loopback and won’t pass over the mobile network. Connections will then fall back to IPv4, which’ll go via the VPN.
EDIT: I was also reminded that I can turn off IPv6 in the APN settings. This works, but I really do like keeping this at the provider defaults… I like having IPv6 when it is available, I just want data to go via only the connection I prefer.
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