containernetworking/cni - "Container Network Interface - networking for Linux containers."
containernetworking/plugins - "Some reference and example networking plugins, maintained by the CNI team."
go-cni - "A generic CNI library to provide APIs for CNI plugin interactions."
The library can load CNI network configs, setup networks for a container namespace, remove networks from a container namespace, query status of CNI network plugin initialization, etc.
slirp4netns - "User-mode networking for unprivileged network namespaces."
netavark - "A container network stack."
pipework - "Software-Defined Networking tools for LXC (LinuX Containers)."
Pipework lets you connect together LXC and/or Docker containers in arbitrarily complex scenarios using cgroups and (network?) namespaces.