If a Linux host is used as an ISATAP router this router becomes a kind of access router for an ISATAP subnet. This subnet has to be provided with a prefix just like any other subnet and this prefix has to be routed.
During a test setup, the host lemy.uni-muenster.de has been configured as an ISATAP router. At the same time this host was already a normal access router to another IPv6 subnet with the prefix 2001:638:500:200::/64 and connected to the rest of the IPv6 internet by an IPv6-in-IPv4 tunnel. For simplicity the /64 prefix was shortened to /63, so 2001:638:500:201::/64 could be used for the ISATAP subnet.
After compiling the kernel and rebooting the host the first thing to do is to configure an ISATAP interface (is0). In the test setup the IPv4 address of the interface where the packets come through (eth0 on host lemy.uni-muenster.de) was 128.176.184.113. So the interface was configured by typing:
#/sbin/ip tunnel add is0 mode isatap local 128.176.184.113 ttl 64
To enable the interface:
#/sbin/ip link set is0 up
Now the interface has to be configured with an IPv6 ISATAP address. An ISATAP address includes
the IPv4 address of the host and has the format:
<prefix/64>:0:5efe:<IPv4 address in hex format>
In this case host lemy.uni-muenster.de has the IPv4 IP 128.176.184.113 and is configured with an
address for the prefix 2001:638:500:201::/64. So the ISATAP address for lemy is:
2001:638:500:201:0:5efe:80b0:b871
To add this address to the interface configuration type:
#/sbin/ip addr add 2001:638:500:201:0:5efe:80b0:bf71/64 dev is0
Now the host has to actually be configured to become a router and send out router advertisements.
Usually ‘radvd’ is used for router advertisements. It can be found at
http://v6web.litech.org/radvd/
In this case the configuration file for radvd (usually /etc/radvd.conf) has the following entries:
Now the router configuration is done. To have this setup come up at boot one can add the “/sbin/ip” lines from above to /etc/rc.local.
Configuring a Linux Host as an ISATAP client
To configure a Linux host as an ISATAP client one has to know the IPv4 address of the host acting as the ISATAP gateway (in this case lemy.uni-muenster.de = 128.176.184.113) and the IPv4 address of the client in question (here 128.176.245.58). With this information configuration is performed by two commands:
#/sbin/ip tunnel add is0 mode isatap local 128.176.245.58 v4any \
128.176.184.113 ttl 64
# /sbin/ip link set is0 up
Now the client should be connected to the rest of the IPv6 world, as long as the ISATAP server has global IPv6 connectivity.