Some of the networking code tries to support dual socket support. However, it doesn't work with IPv6 only systems.
With our custom kernel that disables IPv4, setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, 0) fails, and then the networking code bails.

If IPv4 is detected to not be working, the IPV6_V6ONLY option should not be set.