It is possible to have one node forward all of its network traffic to a host on the VPN,
effectively using this VPN host as the default gateway.
In OpenVPN, there is the
--redirect-gateway option that does this for a client.
With tinc, there is no such option, but the behaviour can be replicated with a host-up and host-down script.
First there is an explaination of the theory behind redirecting the default gateway,
then example scripts will follow.
Normally, there are two entries in the routing table.
One is the route for the local network,
which tells the kernel which IP addresses are directly reachable.
The second is the “default gateway”,
which tells the kernel that in order to reach the rest of the Internet,
traffic should be sent to the gateway of the local network.
Usually the gateway is a router or firewall device,
and its IPv4 address usually ends in
An example output of
route -n on Linux:
Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
Here, the LAN has the IPv4 address range 192.168.1.0/24, and the gateway is 192.168.1.1. Suppose we have a VPN with address range 172.16.0.0/16, on which a server exists with address 172.16.1.1. If we have a VPN connection, and the client wants to replace the standard default route with a default route pointing to 172.16.1.1, then there is a problem: the kernel does not know anymore how to send the encapsulated VPN packets to the server anymore. So we need to add an exception for traffic to the real IP address of the VPN server. Suppose its real address is 188.8.131.52, then the routing table should become:
Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 172.16.1.1 0.0.0.0 255.255.255.255 UH 0 0 0 vpn 184.108.40.206 192.168.1.1 255.255.255.255 UGH 0 0 0 eth0 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 0.0.0.0 172.16.1.1 0.0.0.0 UG 0 0 0 vpn
This will ensure the local LAN is reachable, that the VPN server’s real IP address is reachable via the original gateway, that the VPN server’s VPN IP address is reachable on the vpn interface, and that all other traffic goes via the server on the VPN.
It is better not to remove the original default gateway route,
since someone might kill the
tincd process, such that it doesn’t get a chance to restore the original.
Instead, we use a trick where we add two /1 routes instead of one /0 route:
Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 172.16.1.1 0.0.0.0 255.255.255.255 UH 0 0 0 vpn 220.127.116.11 192.168.1.1 255.255.255.255 UGH 0 0 0 eth0 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 18.104.22.168 172.16.1.1 22.214.171.124 UG 0 0 0 vpn 0.0.0.0 172.16.1.1 126.96.36.199 UG 0 0 0 vpn 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
Since both /1 cover all possible addresses, the real default route will never be used while the two /1 routes are present.
Lets assume we have two nodes called
server, and the netname is
Also, we assume the server has been properly configured to perform routing or masquerading between the VPN and the Internet for the clients that will use it as their default gateway.
Then, only two scripts are necessary on the client.
The following scripts are Linux specific:
#!/bin/sh VPN_GATEWAY=172.16.1.1 ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5` ip route add $REMOTEADDRESS $ORIGINAL_GATEWAY ip route add $VPN_GATEWAY dev $INTERFACE ip route add 0.0.0.0/1 via $VPN_GATEWAY dev $INTERFACE ip route add 188.8.131.52/1 via $VPN_GATEWAY dev $INTERFACE
#!/bin/sh ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5` ip route del $REMOTEADDRESS $ORIGINAL_GATEWAY ip route del $VPN_GATEWAY dev $INTERFACE ip route del 0.0.0.0/1 dev $INTERFACE ip route del 184.108.40.206/1 dev $INTERFACE
These script use the iproute2 commands, because they are easier to work with.
VPN_GATEWAY variable has to be filled in by hand.
ORIGINAL_GATEWAY variable copies the relevant information from the original default route
to create the exception route to the VPN server.
One must also ensure the tinc daemons know which node to send all packets to.
Make sure the following line is in
Subnet = 0.0.0.0/0
Make sure you have masquerading or another form of routing set up on the server. Do not forget to allow forwarding of packets; check your firewall settings, and make sure forwarding is enabled in the kernel:
echo 1 >/proc/sys/net/ipv4/ip_forward
You can also set up portforwarding or proxies to be able to connect to services running on the clients from the rest of the Internet.
The examples given above will work with both router and switch mode. However, in router mode, there is actually no such thing as a gateway route. In that mode, the following four lines in the tinc-up script:
VPN_GATEWAY=... ip route add $VPN_GATEWAY dev $INTERFACE ip route add 0.0.0.0/1 via $VPN_GATEWAY dev $INTERFACE ip route add 220.127.116.11/1 via $VPN_GATEWAY dev $INTERFACE
Can be replaced with the following two lines without any problem:
ip route add 0.0.0.0/1 dev $INTERFACE ip route add 18.104.22.168/1 dev $INTERFACE