Legacy support on IPv6-only infra

This post details the technical implementation of supporting IPv4 external requests on an IPv6-only data center infrastructure. It describes modifications to Layer 4 (L4LB/shiv) and Layer 7 (L7LB/proxygen) load balancers, including the use of IPv4 link-local addresses as BGP next-hops for IPv6-only servers, tunneling IPv4 traffic inside IPv6 using a custom kernel module for decapsulation, and the use of gnlpy to configure IPVS forwarding. It also explains how Direct Server Return (DSR) is maintained for IPv4 responses.