Use `slirp4netns` to add a tap device with working networking to an unprivileged/rootless namespace

main.sh

# Also see: https://passt.top/passt/about/ for an alternative setup

# In first terminal (namespace 1)
unshare --user --map-root-user --net --mount
echo $$ > /tmp/pid

# In second terminal
# For port-forwarding: http://web.archive.org/web/20240112152726/https://rootlesscontaine.rs/how-it-works/netns/incoming/ for port-forwarding
# For communicating between different slirp4netns instances: https://github.com/rootless-containers/slirp4netns/blob/master/slirp4netns.1.md#inter-namespace-communication
slirp4netns --configure --mtu=65520 --disable-host-loopback --macaddress 26:87:71:8f:6f:57 --outbound-addr 100.111.168.3 --outbound-addr6 fd7a:115c:a1e0::1def:a803 --enable-ipv6 $(cat /tmp/pid) tap0

# In third terminal (namespace 2)
unshare --user --map-root-user --net --mount
echo $$ > /tmp/pid2

# In fourth terminal
slirp4netns --configure --mtu=65520 --disable-host-loopback --macaddress 26:87:71:8f:6f:57 --outbound-addr 100.111.168.3 --outbound-addr6 fd7a:115c:a1e0::1def:a803 --enable-ipv6 $(cat /tmp/pid2) tap0

# Now both network namespaces have the same internal view of the network (same IP/MAC address) but can still talk to the oustside (and optionaly with each other!)