## 6to4 and 6in4 Tunnels

### Eric de Thouars

### Tom Eastep

Copyright © 2003-2004, 2008, 2009, 2012 Eric de Thouars and Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2026/01/29

6to4 tunneling with Shorewall can be used to connect your IPv6 network to another IPv6 network over an IPv4 infrastructure. It can also allow you to experiment with IPv6 even if your ISP doesn't provide IPv6 connectivity.

More information on Linux and IPv6 can be found in the Linux IPv6 HOWTO. Details on how to setup a 6to4 tunnels are described in the section Setup of 6to4 tunnels.

## Getting your Feet Wet with IPv6, by Tom Eastep

6to4 tunnels provide a good way to introduce yourself to IPv6. Shorewall6 was developed on a network whose only IPv6 connectivity was an 6to4 Tunnel; that network is described in the remainder of this section. What is shown here requires Shorewall6 4.2.4 or later.

### Configuring IPv6 using my script

I have created an init script to make the job of configuring your firewall for IPv6 easier.

The script is installed in /etc/init.d and configures ipv6, including a 6to4 tunnel, at boot time. Note that the script is included in the Shorewall6 distribution but is not installed in /etc/init.d by default. The RPMs from shorewall.net, install the file in the package documentation directory.

The script works on OpenSuSE 11.0 and may need modification for other distributions. On OpenSuSE, the script is installed by copying it to `/etc/init.d/` then running the command 'chkconfig --add ipv6'.

At the top of the script, you will see several variables:

- SIT - The name of the tunnel device. Usually 'sit1'

- INTERFACES - local interfaces that you want to configure for IPv6

- ADDRESS4 - A static IPv4 address on your firewall that you want to use for the tunnel.

- SLA - The identity of the first local sub-network that you want to assign to the interfaces listed in INTERFACES. Normally one (0001).

- GATEWAY - The default IPv6 gateway. For 6to4, this is ::192.88.99.1.

Here is the file from my firewall:

>     SIT="sit1"
>     ADDRESS4=206.124.146.180
>     INTERFACES="eth2 eth4"
>     SLA=1
>     GATEWAY=::192.88.99.1

eth2 is the interface to my local network (both wired and wireless). eth4 goes to my DMZ which holds a single server. Here is a diagram of the IPv4 network:

Here is the configuration after IPv6 is configured; the part in bold font is configured by the /etc/init.d/ipv6 script.

>     gateway:~ # ip -6 addr ls
>     1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436
>         inet6 ::1/128 scope host
>            valid_lft forever preferred_lft forever
>     1: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
>         inet6 fe80::202:e3ff:fe08:484c/64 scope link
>            valid_lft forever preferred_lft forever
>     2: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
>         inet6 2002:ce7c:92b4:1::1/64 scope global
>            valid_lft forever preferred_lft forever
>         inet6 fe80::202:e3ff:fe08:55fa/64 scope link
>            valid_lft forever preferred_lft forever
>     3: eth4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
>         inet6 2002:ce7c:92b4:2::1/64 scope global
>            valid_lft forever preferred_lft forever
>         inet6 fe80::2a0:ccff:fed2:353a/64 scope link
>            valid_lft forever preferred_lft forever
>     4: sit1@NONE: <NOARP,UP,LOWER_UP> mtu 1480
>         inet6 ::206.124.146.180/128 scope global
>            valid_lft forever preferred_lft forever
>         inet6 2002:ce7c:92b4::1/128 scope global
>            valid_lft forever preferred_lft forever
>     gateway:~ # ip -6 route ls
>     ::/96 via :: dev sit1  metric 256  expires 21333315sec mtu 1480 advmss 1420 hoplimit 4294967295
>     2002:ce7c:92b4::1 dev sit1  metric 256  expires 21333315sec mtu 1480 advmss 1420 hoplimit 4294967295
>     2002:ce7c:92b4:1::/64 dev eth2  metric 256  expires 21333315sec mtu 1500 advmss 1440 hoplimit 4294967295
>     2002:ce7c:92b4:2::/64 dev eth4  metric 256  expires 21333315sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64 dev eth1  metric 256  expires 20748424sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64 dev eth2  metric 256  expires 20748431sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64 dev eth4  metric 256  expires 20748431sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64 dev sit1  metric 256  expires 21333315sec mtu 1480 advmss 1420 hoplimit 4294967295
>     default via ::192.88.99.1 dev sit1  metric 1  expires 21333315sec mtu 1480 advmss 1420 hoplimit 4294967295
>     gateway:~ #

You will notice that sit1, eth2 and eth4 each have an IPv6 address beginning with 2002: -- All 6to4 IPv6 addresses have that in their most significant 16 bits. The next 32-bits (ce7c:92b4) encode the IPv4 ADDRESS (206.124.146.180). So once you start the 6to4 tunnel, you are the proud owner of 280 IPv6 addresses! In the case shown here, 2002:ce7c:92b4::/48. The SLA is used to assign each interface in INTERFACES, a subnet of 264 addresses; in the case of eth2, 2002:ce7c:92b4:1::/64.

I run radvd on the firewall to allow hosts conntected to eth2 and eth4 to automatically perform their own IPv6 configuration. Here is my `/etc/radvd.conf` file:

>     interface eth2 {
>             AdvSendAdvert on;
>             MinRtrAdvInterval 3;
>             MaxRtrAdvInterval 10;
>             prefix 2002:ce7c:92b4:1::/64 {
>                     AdvOnLink on;
>                     AdvAutonomous on;
>                     AdvRouterAddr off;
>             };

>             RDNSS 2002:ce7c:92b4:2:2a0:ccff:fedb:31c4 {
>                     AdvRDNSSOpen on;
>                     AdvRDNSSPreference 2;
>             };
>     };

>     interface eth4 {
>             AdvSendAdvert on;
>             MinRtrAdvInterval 3;
>             MaxRtrAdvInterval 10;
>             prefix 2002:ce7c:92b4:2::/64 {
>                     AdvOnLink on;
>                     AdvAutonomous on;
>                     AdvRouterAddr off;
>             };

>             RDNSS 2002:ce7c:92b4:2:2a0:ccff:fedb:31c4 {
>                     AdvRDNSSOpen on;
>                     AdvRDNSSPreference 2;
>             };
>     };

### Note

radvd terminates immediately if IPv6 forwarding is not enabled. So it is a good idea to include this in`/etc/sysctl.conf`:

    net.ipv6.conf.all.forwarding = 1

That way, if radvd starts before Shorewall6, it will continue to run.

An alternative is to modify `/etc/init.d/radvd` so that radvd starts after Shorewall6:

    # Should-Start: shorewall6

Here is the automatic IPv6 configuration on my server attached to eth4:

>     webadmin@lists:~/ftpsite/contrib/IPv6> /sbin/ip -6 addr ls
>     1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436
>         inet6 ::1/128 scope host
>            valid_lft forever preferred_lft forever
>     2: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
>         inet6 2002:ce7c:92b4:2:2a0:ccff:fedb:31c4/64 scope global dynamic
>            valid_lft 2591995sec preferred_lft 604795sec
>         inet6 fe80::2a0:ccff:fedb:31c4/64 scope link
>            valid_lft forever preferred_lft forever
>     webadmin@lists:~/ftpsite/contrib/IPv6> /sbin/ip -6 route ls
>     2002:ce7c:92b4:2::/64 dev eth2  proto kernel  metric 256  expires 2592161sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64 dev eth2  metric 256  expires 20746963sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64 dev ifb0  metric 256  expires 20746985sec mtu 1500 advmss 1440 hoplimit 4294967295
>     default via fe80::2a0:ccff:fed2:353a dev eth2  proto kernel  metric 1024  expires 29sec mtu 1500 advmss 1440 hoplimit 64
>     webadmin@lists:~/ftpsite/contrib/IPv6>

You will note that the public IPv6 address of eth2 (2002:ce7c:92b4:2:2a0:ccff:fedb:31c4) was formed by concatenating the prefix for eth2 shown in radvd.conf (2002:ce7c:92b4:2) and the lower 64 bits of the link level address of eth2 (2a0:ccff:fedb:31c4). You will also notice that the address 2002:ce7c:92b4:2:2a0:ccff:fedb:31c4 appears in the RDNSS clauses in radvd.conf; that causes my server to be automatically configured as a DNS server.

The default route is described using the link level address of eth2 on the firewall (fe80::2a0:ccff:fed2:353a).

On my laptop, ursa:

>     ursa:~ # ip -6 addr ls dev eth0
>     3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
>         inet6 2002:ce7c:92b4:1:21a:24ff:fecb:2bcc/64 scope global dynamic
>            valid_lft 2591996sec preferred_lft 604796sec
>         inet6 fe80::21a:73ff:fedb:8c35/64 scope link
>            valid_lft forever preferred_lft forever
>     ursa:~ # ip -6 route ls dev eth0
>     2002:ce7c:92b4:1::/64  proto kernel  metric 256  expires 2592160sec mtu 1500 advmss 1440 hoplimit 4294967295
>     fe80::/64  metric 256  expires 21314573sec mtu 1500 advmss 1440 hoplimit 4294967295
>     default via fe80::202:e3ff:fe08:55fa  proto kernel  metric 1024  expires 28sec mtu 1500 advmss 1440 hoplimit 64
>     ursa:~ #

Here is the resulting simple IPv6 Network:

### Configuring IPv6 the Debian Way

In May 2009, I rebuilt the above firewall using Debian GNU/Linux and decided to configure IPv6 using the `/etc/network/interfaces` file.

When I installed Debian Lenny on the system, the network interfaces were reunmbered as follows:

Table 1. Interface Renaming

Old Configuration | New Configuration
eth0 (Avvanta Interface) | eth3
eth3 (Comcast Interface) | eth0
eth2 (Local Interface) | eth1
eth4 (DMZ Interface) | eth2

So the IPv4 network was transformed to this:

To implement the same IPv6 network as described above, I used this /etc/shorewall/interfaces file:

>     auto lo
>     iface lo inet loopback

>     auto eth0
>     iface eth0 inet dhcp
>             hwaddress ether 00:11:85:89:da:9b

>     auto eth1
>     iface eth1 inet static
>             address 172.20.1.254
>             netmask 255.255.255.0
>             network 172.20.1.0
>             broadcast 172.20.1.255

>     iface eth1 inet6 static
>             address 2002:ce7c:92b4:1::1
>             netmask 64

>     auto eth2
>     iface eth2 inet static
>             address 206.124.146.176
>             netmask 255.255.255.255
>             up ip route add 206.124.146.177/32 dev eth2

>     iface eth2 inet6 static
>             address 2002:ce7c:92b4:2::1
>             netmask 64

>     auto eth3 eth3:0 eth3:1 eth3:2
>     iface eth3 inet static
>             address 206.124.146.176
>             netmask 255.255.255.0
>             network 206.124.146.0
>             broadcast 206.124.146.255

>     iface eth3:0 inet static
>             address 206.124.146.178
>             netmask 255.255.255.0
>             broadcast 206.124.146.255

>     iface eth3:1 inet static
>             address 206.124.146.179
>             netmask 255.255.255.0
>             broadcast 206.124.146.255

>     iface eth3:2 inet static
>             address 206.124.146.180
>             netmask 255.255.255.0
>             broadcast 206.124.146.255

>     auto sit1
>     iface sit1 inet6 v4tunnel
>           address 2002:ce7c:92b4::1
>           netmask 64
>           endpoint 192.88.99.1
>           local 206.124.146.180
>           gateway ::192.88.99.1
>           ttl 64
>           post-up echo 1 > /proc/sys/net/ipv6/conf/all/forwarding

That file produces the following IPv6 network.

### Configuring Shorewall

We need to add an entry in /etc/shorewall/tunnels and restart Shorewall:

>     #TYPE           ZONE            GATEWAY                 GATEWAY_ZONE
>     6to4            net

### Configuring Shorewall6

STOP -- If you have followed the instructions above, you should have a completely functional IPv6 network. Try:

    ping6 www.kame.net
    ping6 ipv6.chat.eu.freenode.net

If neither of those work from your firewall and from any local IPv6 systems that you have behind your firewall, do not go any further until one of them does work. If you ask for help from the Shorewall team, the first question we will ask is 'With Shorewall6 cleared, can you ping6 kame or freenode?'.

The Shorewall6 configuration on my firewall is a very basic three-interface one.

Key entry in `/etc/shorewall6/shorewall6.conf`:

>     IP_FORWARDING=On

`/etc/shorewall6/zones`:

>     #ZONE           TYPE            OPTIONS         IN_OPTIONS      OUT_OPTIONS

>     fw	        firewall
>     net	        ipv6
>     loc	        ipv6
>     dmz	        ipv6

`/etc/shorewall6/interfaces`:

>     ?FORMAT 2
>     #ZONE           INTERFACE               OPTIONS

>     net             sit1                    tcpflags,forward=1,nosmurfs
>     loc             eth0                    tcpflags,forward=1
>     dmz             eth2                    tcpflags,forward=1

`/etc/shorewall6/policy`:

>     #SOURCE    DEST    POLICY    LOGLEVEL    LIMIT
>     net        all     DROP      info
>     loc        net     ACCEPT
>     dmz        net     ACCEPT
>     all        all     REJECT    info

`/etc/shorewall6/rules`:

>     #ACTION         SOURCE          DEST            PROTO   DPORT   SPORT   ORIGDEST        RATE            USER    MARK    CONNLIMIT       TIME    HEADERS SWITCH  HELPER

>     ?SECTION ALL
>     ?SECTION ESTABLISHED
>     ?SECTION RELATED
>     ?SECTION INVALID
>     ?SECTION UNTRACKED
>     ?SECTION NEW
>     #
>     #       Accept DNS connections from the firewall to the network
>     #
>     DNS(ACCEPT)      $FW             net
>     #
>     #       Accept SSH connections from the local network for administration
>     #
>     SSH(ACCEPT)      loc             $FW
>     #
>     #       Allow Ping everywhere
>     #
>     Ping(ACCEPT)     all             all

## 6in4 Tunnel

6in4 is very similar to 6to4:

- Both Tunnel IPv6 traffic over IPv4 using Protocol 41

- Both allow you access to the IPv6 network even though your ISP doesn't offer native IPv6 connectivity.

The differences are:

- 6in4 gives you a /64 prefix outside of the 2002::0/16 network

- You have a dedicated fixed endpoint for the tunnel rather than the nebulous anycast endpoint 192.88.99.1. This is:

  - Much more reliable

  - Much easier to troubleshoot (there is ONE host and one company to call on the other end of the tunnel rather than an indefinite cloud with noone in charge)

I converted to a 6in4 Tunnel from Hurricane Electric in April of 2010. Converting from the 6to4 tunnel configuration above to a 6in4 tunnel from HE took less than an hour.

When I signed up for a tunnel with HE, I received these assignments:

> Server IPv4 address: 216.218.226.238

> Server IPv6 address: 2001:470:a:227::1/64

> Client IPv4 address: 206.124.146.180 (Same as the 6to4 tunnel)

> Client IPv6 address: 2001:470:a:227::2/64

I also took advantage of their offer for a /48 prefix routed via 2001:470:a:227::2. The prefix I was assigned is

> 2001:470:e857::/48

Here are the key changes:

`/etc/network/interfaces:`

    iface eth1 inet6 static
            address 2001:470:e857:1::1
            netmask 64

    auto eth2
    ...
    iface eth2 inet6 static
          address 2001:470:e857:2::1
          netmask 64

    auto sit1
    iface sit1 inet6 v4tunnel
          address 2001:470:a:227::2
          netmask 64
          endpoint 216.218.226.238
          local 206.124.146.180
          gateway 2001:470:a:227::1
          ttl 64
          post-up echo 1 > /proc/sys/net/ipv6/conf/all/forwarding

`/etc/radvd.conf (I'm currently not using RDNSS so I've simply commented out the existing entries)`:

    interface eth1 {
            AdvSendAdvert on;
            MinRtrAdvInterval 60;
            MaxRtrAdvInterval 600;
            AdvDefaultLifetime 9000;
            prefix 2001:470:e857:1::/64 {
                    AdvOnLink on;
                    AdvAutonomous on;
                    AdvRouterAddr off;
            };

            route ::/0 {
                    AdvRouteLifetime infinity;
            };

    #       RDNSS 2002:ce7c:92b4:2:221:5aff:fe22:ace0 {
    #                AdvRDNSSOpen on;
    #                AdvRDNSSPreference 2;
    #        };
    };

    interface eth2 {
            AdvSendAdvert on;
            MinRtrAdvInterval 60;
            MaxRtrAdvInterval 600;
            prefix 2001:470:e857:2::/64 {
                    AdvOnLink on;
                    AdvAutonomous on;
                    AdvRouterAddr off;
            };

    #       RDNSS 2002:ce7c:92b4:2:221:5aff:fe22:ace0 {
    #              AdvRDNSSOpen on;
    #               AdvRDNSSPreference 2;
    #        };
    };

## Connecting two IPv6 Networks, by Eric de Thouars

Suppose that we have the following situation:

We want systems in the 2002:100:333::/64 subnetwork to be able to communicate with the systems in the 2002:488:999::/64 network. This is accomplished through use of the `/etc/shorewall/tunnels` file and the “ip” utility for network interface and routing configuration.

Unlike GRE and IPIP tunneling, the `/etc/shorewall/policy`, `/etc/shorewall/interfaces` and `/etc/shorewall/zones` files are not used. There is no need to declare a zone to represent the remote IPv6 network. This remote network is not visible on IPv4 interfaces and to iptables. All that is visible on the IPv4 level is an IPv4 stream which contains IPv6 traffic. Separate IPv6 interfaces and ip6tables rules need to be defined to handle this traffic.

In `/etc/shorewall/tunnels` on system A, we need the following:

    #TYPE           ZONE            GATEWAY                 GATEWAY_ZONE
    6to4            net             134.28.54.2

This entry in `/etc/shorewall/tunnels` opens the firewall so that the IPv6 encapsulation protocol (41) will be accepted to/from the remote gateway.

Use the following commands to setup system A:

    >ip tunnel add tun6to4 mode sit ttl 254 remote 134.28.54.2
    >ip link set dev tun6to4 up
    >ip addr add 3ffe:8280:0:2001::1/64 dev tun6to4
    >ip route add 2002:488:999::/64 via 3ffe:8280:0:2001::2

Similarly, in `/etc/shorewall/tunnels` on system B we have:

    #TYPE           ZONE            GATEWAY                 GATEWAY_ZONE
    6to4            net             206.191.148.9

And use the following commands to setup system B:

    >ip tunnel add tun6to4 mode sit ttl 254 remote 206.191.148.9
    >ip link set dev tun6to4 up
    >ip addr add 3ffe:8280:0:2001::2/64 dev tun6to4
    >ip route add 2002:100:333::/64 via 3ffe:8280:0:2001::1

On both systems, restart Shorewall and issue the configuration commands as listed above. The systems in both IPv6 subnetworks can now talk to each other using IPv6.