+++ rss = "Teredo tunnel simulation in virtual machines" date = Date(2020, 7, 3) tags = ["fun", "ipv6", "recipe"] +++
Internet Protocol version 6 (IPv6), the most recent version of the Internet Protocol, was developed by the IETF to deal with the long-anticipated problem of IPv4 address exhaustion. Despite being superior to IPv4 in multiple aspect (e.g. larger address space, extension headers), IPv6 has not been widely adopted, although it has been semi-standardized in 1998 and fully-standardized in 2017.[^rfc8200]
During the transition period, teredo tunneling has been used to give IPv6 connectivity for IPv6-capable hosts that are on the IPv4 Internet but have no native connection to an IPv6 network.[^rfc4380] In this article, I will demontrate a way to set up such tunnel up on virtual machines, then examine the packets being sent by IPv6 nodes connected by the tunnel.
<figure> <a href=https://video.hardlimit.com/videos/watch/61a0e40c-a74d-4dc6-bb10-9be67b2c1ac5> <img src=/assets/teredo.png alt=Screenshot> </a> <figcaption>Teredo Tunnel Simulation Speedrun</figcaption> </figure>
In order to simulate Teredo tunneling, one needs two IPv6 nodes and two routers
with both IPv4 and IPv6 access. In total, there needs to be four
virtual machines to be set up, thus I went for Void Linux, which is known
for its low memory foot print thanks to using
runit instead of
To minimize resource usage and speed up the setup process, I chose the barebone
live image which uses
musl instead of
glibc. At boot, the image uses only
40 MB of memory.
For virtualization, I used
vert-manager, simply because it is available
in Debian's repository (my host OS). For some reason, on
amd64, the kernel
refuses to boot until I give it over 200 MB, but apparently that is still
a really modest number. Networking is provided to the guest OSes via NAT
with default configurations.
It is worth mentioning that through
virtio, one may use SSH to log into
the guests systems from the host OS. I find this especially convenient as
it enables me to copy and paste not only commands but also IP addresses between
host and guests as well as between guests.
For convenience, from now on, the outside nodes will be referred to as PC A and
PC B, on the other hand the routers are named Router A and Router B. Upon boot,
they were given an Ethernet interface
eth0 with the following addresses.
|Node||MAC address||IPv4 address|
Local IPv6 addresses were also given but we are not going to need them.
First, I set up a IPv4 tunnel between the two routers:
# On Router A ip tunnel add tunn mode sit remote 192.168.122.134 ttl 255 ip link set tunn up # On Router B ip tunnel add tunn mode sit remote 192.168.122.127 ttl 255 ip link set tunn up
For this tunnel to be able to act as a Teredo one, the two routers needs
to have IPv6 addresses prefixed by
# On Router A ip -6 addr add 2001:2::1/64 dev eth0 # On Router B ip -6 addr add 2001:3::1/64 dev eth0
Finally, I fellback all IPv6 lookups to the tunnel and enabled IPv6 forwarding:
ip -6 route add default dev tunn sysctl -w net.ipv6.conf.all.forwarding=1
The IPv6 addresses of the PCs were set up as follows
PC in ASCII).
# On PC A ip -6 address add 2001:2::8067/64 dev eth0 # On PC B ip -6 address add 2001:3::8067/64 dev eth0
By giving both Router A and PC A addresses prefixed by
(similarly for Router B and PC B), I implied that they can find each other
through the local IPv6 network, for example on PC B:
$ ip -6 route | head -n1 2001:3::/64 dev eth0 proto kernel metric 256 pref medium
To use the newly created tunnel, the PCs simple had to be routed directly to the routers:
# On PC A ip -6 route add default via 2001:2::1 # On PC B ip -6 route add default via 2001:3::1
The connection could then be verified by running on PC A:
$ traceroute 2001:3::8067 traceroute to 2001:3::8067 (2001:3::8067), 30 hops max, 80 byte packets 1 2001:2::1 (2001:2::1) 0.572 ms 0.441 ms 0.328 ms 2 2001:3::1 (2001:3::1) 0.906 ms 0.888 ms 1.049 ms 3 2001:3::8067 (2001:3::8067) 1.325 ms 1.174 ms 1.091 ms
To gain further understanding on how packets are transferred over the Teredo tunnel, I captured and took a closer look at some of them.
Fortunately for me[^ipfs], all traffic of guests OSes were wired
to an separate interface named
virbr0. To capture going through
the tunnel, I simply had to tell Wireshark to listen to the interface,
while letting PC A ping PC B though IPv6:
ping -c1 2001:3::8067.
I then skimmed through the packets sent between the two nodes and looked for
the IPv6-in-IPv4 ones.
Catured IPv6-in-IPv4 looks exactly like how I would imagined it to be. The content of the ping request can be partially decoded as follows.
52 54 00 2b 01 cc: MAC address of Router B (destination)
52 54 00 f0 85 c7: MAC address of Router A (source)
08 00: EtherType of IPv4
45 00 00 7c 9b 43 40 00 ff: Some flags
29: Protocol of IPv6
69 be: Checksum
c0 a8 7a 86: IPv4 address of Router B (destination)
c0 a8 7a 7f: IPv4 address of Router A (source)
60 00 07 e7 00 40: Some flags
3a: Next header (ICMPv6)
3f: Hop limit of 63
20 01 00 02 00 00 00 00 00 00 00 00 00 00 80 67: PC A's IPv6 address
20 01 00 03 00 00 00 00 00 00 00 00 00 00 80 67: PC B's IPv6 address
80: Type of ping request
00 cf be 03 d9 00 01: Some flags
e3 0d fe 5e 00 00 00 00 bc d6 0e 00 00 00 00 00 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37: Binary data to be echoed
Via the activities elaborated above, the procedure to set up a Teredo tunnel and the content of the packets travelling through it could be well understood. These understanding may help facilite the adoption of IPv6, even for IPv6 nodes having no native connection to an IPv6 network. I hope that the IPv6 will grow fast enough that I can see the day measures like this tunnel can soon be deprecated.
[^rfc8200]: RFC 8200 [^rfc4380]: RFC 4380 [^ipfs]: Aside from web browsing, I also run an IPFS node and a bunch of local servers. I probably need to retire some of them soon since they really clutter the traffic.