Legacy support on IPv6-only infra

What this post added

This post details the technical implementation of supporting IPv4 traffic 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 BGP with ExaBGP, IPv4 link-local addresses as BGP next-hops for IPv6-only servers, IP tunneling of IPv4 traffic within IPv6, the development of the gnlpy library for IPVS forwarding configuration via Netlink, and a custom kernel module for mixed-version IP tunnel decapsulation. It also covers the use of direct server return (DSR) for responses and the configuration of IPv4 default gateways and link-local addresses on servers.