Configured Tunnels

Tunneling interfaces have the following format:

ip.tun ppa

ppa is the physical point of attachment.

At system startup, the tunneling module (tun) is pushed, by the ifconfig command, on top of IP to create a virtual interface. The push is accomplished by creating the appropriate hostname6.* file.

For example, to create a tunnel to encapsulate IPv6 packets over an IPv4 network, IPv6 over IPv4, you would create the following file name:

/etc/hostname6.ip.tun0

The content of this file is passed to ifconfig after the interfaces have been plumbed. The content becomes the parameters that are necessary to configure a point-to-point tunnel.

Example 10–11 hostname6.ip.tun0 File for an IPv6 Over IPv4 Tunnel

The following is an example of entries in the hostname6.ip.tun0 file:

tsrc 10.10.10.23 tdst 172.16.7.19 up
addif 2001:db8:3b4c:1:5678:5678::2 up

In this example, the IPv4 source and destination addresses are used as tokens to autoconfigure IPv6 link-local addresses. These addresses are the source and destination for the ip.tun0 interface. Two interfaces are configured. The ip.tun0 interface is configured. A logical interface, ip.tun0:1, is also configured. The logical interface has the source and destination IPv6 addresses specified by the addif command.

The contents of these configuration files are passed to ifconfig without change when the system is started in multiuser mode. The entries in Example 10–11 are equivalent to the following:

# ifconfig ip.tun0 inet6 plumb
# ifconfig ip.tun0 inet6 tsrc 10.0.0.23 tdst 172.16.7.19 up
# ifconfig ip.tun0 inet6 addif 2001:db8:3b4c:1:5678:5678::2 up

The following shows the output of ifconfig -a for this tunnel.

ip.tun0: flags=2200850<UP,POINTOPOINT,RUNNING,MULTICAST,
  NONUD,IPv6> mtu 1480 index 6
        inet tunnel src 10.0.0.23  tunnel dst 172.16.7.19
        inet6 fe80::c0a8:6417/10 --> fe80::c0a8:713
ip.tun0:1: flags=2200850<UP,POINTOPOINT,RUNNING,MULTICAST,NONUD,IPv6> mtu 1480 
  index 5
        inet6 2001:db8:3b4c:1:5678:5678::2 

You can configure more logical interfaces by adding lines to the configuration file by using the following syntax:

addif IPv6-source IPv6-destination up

When either end of the tunnel is an IPv6 router that advertises one or more prefixes over the tunnel, you do not need addif commands in the tunnel configuration files. Only tsrc and tdst might be required because all other addresses are autoconfigured.

In some situations, specific source and destination link-local addresses need to be manually configured for a particular tunnel. Change the first line of the configuration file to include these link-local addresses. The following line is an example:

tsrc 10.0.0.23 tdst 172.16.7.19 fe80::1/10 fe80::2 up

Notice that the source link-local address has a prefix length of 10. In this example, the ip.tun0 interface resembles the following:

ip.tun0: flags=2200850<UP,POINTOPOINT,RUNNING,MULTICAST,NONUD,IPv6> mtu 1480 
index 6
        inet tunnel src 10.0.0.23  tunnel dst 172.16.7.19
        inet6 fe80::1/10 --> fe80::2

To create a tunnel to encapsulate IPv6 packets over an IPv6 network, IPv6 over IPv6, you create the following file name:

/etc/hostname6.ip6.tun0

Example 10–12 hostname6.ip6.tun0 File for an IPv6 over IPv6 Tunnel

The following is an example of entries in the hostname6.ip6.tun0 file for IPv6 encapsulation over an IPv6 network:

tsrc 2001:db8:3b4c:114:a00:20ff:fe72:668c 
        tdst 2001:db8:15fa:25:a00:20ff:fe9b:a1c3
fe80::4 fe80::61 up

To create a tunnel to encapsulate IPv4 packets over an IPv6 network, IPv4 over IPv6, you would create the following file name:

/etc/hostname.ip6.tun0

Example 10–13 hostname.ip6.tun0 File for an IPv4 Over IPv6 Tunnel

The following is an example of entries in the hostname.ip6.tun0 file for IPv4 encapsulation over an IPv6 network:

tsrc 2001:db8:3b4c:114:a00:20ff:fe72:668c 
         tdst 2001:db8:15fa:25:a00:20ff:fe9b:a1c3
10.0.0.4 10.0.0.61 up

To create a tunnel to encapsulate IPv4 packets over an IPv4 network, IPv4 over IPv4, you would create the following file name:

/etc/hostname.ip.tun0

Example 10–14 hostname.ip.tun0 for an IPv4 Over IPv4 Tunnel

The following is an example of entries in the hostname.ip.tun0 file for IPv4 encapsulation over an IPv4 network:

tsrc 172.16.86.158 tdst 192.168.86.122
10.0.0.4 10.0.0.61 up

For specific information about tun, see the tun(7M) man page. .For a general description of tunneling concepts during the transition to IPv6, see Overview of IPv6 Tunnels. For a description of procedures for configuring tunnels, see Tasks for Configuring Tunnels for IPv6 Support (Task Map).