1 [[!meta title="redirect-gateway"]]
3 ## Example: redirecting the default gateway to a host on the VPN
5 It is possible to have one node forward all of its network traffic to a host on the VPN,
6 effectively using this VPN host as the default gateway.
7 In OpenVPN, there is the `--redirect-gateway` option that does this for a client.
8 With tinc, there is no such option, but the behaviour can be replicated with a host-up and host-down script.
9 First there is an explaination of the theory behind redirecting the default gateway,
10 then example scripts will follow.
14 Normally, there are two entries in the routing table.
15 One is the route for the local network,
16 which tells the kernel which IP addresses are directly reachable.
17 The second is the "default gateway",
18 which tells the kernel that in order to reach the rest of the Internet,
19 traffic should be sent to the gateway of the local network.
20 Usually the gateway is a router or firewall device,
21 and its IPv4 address usually ends in `.1`.
22 An example output of `route -n` on Linux:
24 Kernel IP routing table
25 Destination Gateway Genmask Flags Metric Ref Use Iface
26 192.168.1.42 0.0.0.0 255.255.255.0 U 0 0 0 eth0
27 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
29 Here, the LAN has the IPv4 address range 192.168.1.0/24, and the gateway is 192.168.1.1.
30 Suppose we have a VPN with address range 172.16.0.0/16, on which a server exists with address 172.16.1.1.
31 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,
32 then there is a problem: the kernel does not know anymore how to send the encapsulated VPN packets to the server anymore.
33 So we need to add an exception for traffic to the real IP address of the VPN server.
34 Suppose its real address is 192.0.32.10, then the routing table should become:
36 Kernel IP routing table
37 Destination Gateway Genmask Flags Metric Ref Use Iface
38 192.0.32.10 192.168.1.1 255.255.255.255 UGH 0 0 0 eth0
39 192.168.1.42 0.0.0.0 255.255.255.0 U 0 0 0 eth0
40 0.0.0.0 192.0.32.10 0.0.0.0 UG 0 0 0 vpn
42 This will ensure the local LAN is reachable, that the VPN server's real IP address is reachable via the original gateway,
43 and that all other traffic goes via the server on the VPN.
45 It is better not to remove the original default gateway route,
46 since someone might kill the `tincd` process, such that it doesn't get a chance to restore the original.
47 Instead, we use a trick where we add two /1 routes instead of one /0 route:
49 Kernel IP routing table
50 Destination Gateway Genmask Flags Metric Ref Use Iface
51 192.0.32.10 192.168.1.1 255.255.255.255 UGH 0 0 0 eth0
52 192.168.1.42 0.0.0.0 255.255.255.0 U 0 0 0 eth0
53 128.0.0.0 192.0.32.10 128.0.0.0 UG 0 0 0 vpn
54 0.0.0.0 192.0.32.10 128.0.0.0 UG 0 0 0 vpn
55 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
57 Since both /1 cover all possible addresses, the real default route will never be used while the two /1 routes are present.
61 Lets assume we have two nodes called `client` and `server`, and the netname is `myvpn`.
62 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.
63 Then, only two scripts are necessary on the client.
64 The following scripts are Linux specific:
66 `/etc/tinc/myvpn/hosts/server-up`:
69 VPN_GATEWAY=192.168.1.1
70 ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5`
72 ip route add $REMOTEADDRESS $ORIGINAL_GATEWAY
73 ip route add 0.0.0.0/1 via $VPN_GATEWAY dev $INTERFACE
74 ip route add 128.0.0.0/1 via $VPN_GATEWAY dev $INTERFACE
76 `/etc/tinc/myvpn/hosts/server-down`:
79 ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5`
81 ip route del $REMOTEADDRESS $ORIGINAL_GATEWAY
82 ip route del 0.0.0.0/1 dev $INTERFACE
83 ip route del 128.0.0.0/1 dev $INTERFACE
85 These script use the iproute2 commands, because they are easier to work with.
86 The `VPN_GATEWAY` variable has to be filled in by hand.
87 The `ORIGINAL_GATEWAY` variable copies the relevant information from the original default route
88 to create the exception route to the VPN server.
90 ### Further configuration
92 The only thing left is to ensure the tinc daemons know which node to send all packets to.
93 Make sure the following line is in `/etc/tinc/myvpn/hosts/server`: