2020-02-10 16:09:49 +01:00
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title: How to route IPv4 via IPv6
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---
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pub_date: 2020-02-10
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---
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author: ungleich network
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---
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twitter_handle: ungleich
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---
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_hidden: no
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---
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_discoverable: yes
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---
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abstract:
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Bringing IPv4 into the IPv6 world
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---
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body:
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Imagine the following: you are running an IPv6 only network. And now
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someone asks you to pass IPv4 traffic through it, without tunneling
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it. Was sounds crazy at first, is actually quite feasible.
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## A short routing recap
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Routers have routing tables. The routing tables basically say
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"if you receive a packet for this host, send it to that router".
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The important thing about this process is that the information on
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where to send it to, is **not in the packet**.
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## How to send IPv4 packets via IPv6
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Because the next hop is not written into the IPv4 packet, the router
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is free to forward the packet via any method it thinks is the
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best. And if that happens to be IPv6 - well, it will forward the IPv4
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packet via an IPv6 neighbour.
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## A practical example!
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In the IPv6 only coworking network in the [Digital
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Chalet](/u/projects/digital-chalet/), I can add an IPv4 default route
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via the IPv6 router:
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```
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[root@diamond ~]# ip route add 0/0 nexthop via inet6 fe80::21b:21ff:febb:6934 dev wlp0s20f3
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[root@diamond ~]# ip r
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default via inet6 fe80::21b:21ff:febb:6934 dev wlp0s20f3
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[root@diamond ~]#
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```
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Now to be able to actually transmit IPv4 packets, I do need a source
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IPv4 address. In the current network I can use an address in the
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2020-02-10 19:45:02 +01:00
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unused 10.0.8.0/22 network, however I'll add it with a /32 mask to
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make it clear that there is no interface local route applied:
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2020-02-10 16:09:49 +01:00
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```
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2020-02-10 19:45:02 +01:00
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[root@diamond ~]# ip addr add 10.0.8.42/32 dev wlp0s20f3
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2020-02-10 16:09:49 +01:00
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[root@diamond ~]# ip r
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default via inet6 fe80::21b:21ff:febb:6934 dev wlp0s20f3
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[root@diamond ~]# ip a sh dev wlp0s20f3
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2: wlp0s20f3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
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link/ether 24:ee:9a:54:c3:bf brd ff:ff:ff:ff:ff:ff
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inet 10.0.8.42/32 scope global wlp0s20f3
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valid_lft forever preferred_lft forever
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inet6 2a0a:e5c0:0:4:c6ea:b1a8:ec14:6f35/64 scope global dynamic mngtmpaddr noprefixroute
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valid_lft 86400sec preferred_lft 14400sec
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inet6 fe80::3b98:cb58:ed02:c25/64 scope link
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valid_lft forever preferred_lft forever
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[root@diamond ~]#
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```
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And I can indeed ping another IPv4 address, routed via IPv6!
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```
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[root@diamond ~]# ping -4 10.0.8.3
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PING 10.0.8.3 (10.0.8.3) 56(84) bytes of data.
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64 bytes from 10.0.8.3: icmp_seq=1 ttl=64 time=2.37 ms
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^C
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--- 10.0.8.3 ping statistics ---
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1 packets transmitted, 1 received, 0% packet loss, time 0ms
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rtt min/avg/max/mdev = 2.365/2.365/2.365/0.000 ms
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[root@diamond ~]#
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```
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2020-02-10 16:22:55 +01:00
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## Why?
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Why would anyone want to do this? It's quite easy: with this you can
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route an IPv4 address to an IPv6 only host. This enables IPv6 only
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resources to create and send IPv4 packets, even if they don't have
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IPv4 routes.
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2020-02-10 16:09:49 +01:00
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## Do it yourself
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If you don't believe us that it is possible, you can test it yourself
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on IPv6 only VMs on [IPv6OnlyHosting.com](https://ipv6onlyhosting.com).
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