title: How to route IPv4 via IPv6 --- pub_date: 2019-12-10 --- author: ungleich network --- twitter_handle: ungleich --- _hidden: yes --- _discoverable: no --- abstract: Bringing IPv4 into the IPv6 world --- body: Imagine the following: you are running an IPv6 only network. And now someone asks you to pass IPv4 traffic through it, without tunneling it. Was sounds crazy at first, is actually quite feasible. ## A short routing recap Routers have routing tables. The routing tables basically say "if you receive a packet for this host, send it to that router". ![IP routing](/u/image/ip-routing.png) The important thing about this process is that the information on where to send it to, is **not in the packet**. ## How to send IPv4 packets via IPv6 Because the next hop is not written into the IPv4 packet, the router is free to forward the packet via any method it thinks is the best. And if that happens to be IPv6 - well, it will forward the IPv4 packet via an IPv6 neighbour. ## A practical example! ``` [root@diamond ~]# ip -6 r ::1 dev lo proto kernel metric 256 pref medium 2a0a:e5c1:137::/48 dev wgungleich proto kernel metric 256 pref medium fe80::/64 dev wlp0s20f3 proto kernel metric 256 pref medium [root@diamond ~]# ip r default via 192.168.84.1 dev wlp0s20f3 proto dhcp src 192.168.84.7 metric 302 192.168.84.0/22 dev wlp0s20f3 proto dhcp scope link src 192.168.84.7 metric 302 [root@diamond ~]# ip route add 10.0.0.0/8 via inet6 2a0a:e5c1:137::22 [root@diamond ~]# ip -6 route ::1 dev lo proto kernel metric 256 pref medium 2a0a:e5c1:137::/48 dev wgungleich proto kernel metric 256 pref medium fe80::/64 dev wlp0s20f3 proto kernel metric 256 pref medium [root@diamond ~]# ip r default via 192.168.84.1 dev wlp0s20f3 proto dhcp src 192.168.84.7 metric 302 10.0.0.0/8 via inet6 2a0a:e5c1:137::22 dev wgungleich 192.168.84.0/22 dev wlp0s20f3 proto dhcp scope link src 192.168.84.7 metric 302 [root@diamond ~]# ``` ## More in the cloud Actually, what happens behind the scenes is that the VM is running VNC (we are also experimenting with XRDP), so I have actually full access to a remote Linux desktop via browser and can even run applications like libreoffice, blender or gimp remotely. Because I think it's a cool thing to have, our team at ungleich added it as an offer to our [Black IPv6 Friday Crowdfunding](https://swiss-crowdfunder.com/campaigns/black-ipv6-friday?locale=en). Below you can actually see how it looks like: root@beebox ~ # route add 192.168.0.0/16 2a0a:e5c1:100::1 add net 192.168.0.0/16: gateway 2a0a:e5c1:100::1 root@beebox ~ # route -n get 192.168.1.2 route to: 192.168.1.2 destination: 192.168.0.0 mask: 255.255.0.0 gateway: 2a0a:e5c1:100::1 interface: tun3 if address: 2a0a:e5c1:11e::1 priority: 8 (static) flags: use mtu expire 2 0 0