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[robinkrens.nl] / tinc.txt

robinkrens.nl - TINC as a gateway
=====

Tinc is a VPN daemon which tunnels IP packets and Ethernet frames over UDP. More on Tinc can be found on: http://tinc-vpn.org
Here I will show a tinc setup with an *alpha* (as a listening peer) and a *beta* (a peer connecting to alpha).   After setting up the VPN, alpha will be the gateway for beta. All traffic from beta will be routed through alpha and back. I will basically retell the man page documentation: https://tinc-vpn.org/documentation-1.1/tinc.conf.5 but in a more tutorial kind of way. 

-------------

Alpha peer
********** 

Create config files
+++++++

_/etc/tinc/gatewayvpn/tinc.conf_
        
        Name: alpha 
        Device: /dev/net/tun
        
The name will be used by other tinc daemons for identification. Device in here means the virtual network to bind to. Because we are going to use routing we use a tunneling device. For alpha we don't fill out a ConnectTo option, so alpha will passively listen for incoming connections.

_/etc/tinc/gatewayvpn/tinc-up_
        
        #!/bin/sh
        ip link set $INTERFACE up
        ip addr add  172.16.16.1/24 dev $INTERFACE      

This is a shell script executed right after the tinc daemon has been started and has connected to the virtual network device.  It should be used to set up the corresponding network interface, but can also be used to start other things (as we show later for routing). $INTERFACE contains the name of our virtual network interface that the tinc daemon uses (in our case gatewayvpn). So later on, if you run tinc, it will show something like:

        $ ifconfig gatewayvpn
        gatewayvpn   Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00  
        inet addr:172.16.16.1  P-t-P:172.16.16.1  Mask:255.255.255.0
        UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1

We use an IP address in the private range: 172.16.16.0/24, but you can use any address that you like (i.e. 10.0.0.0). The /24 means the subnet in which the daemon is going to serve. Here, we assing 172.16.16.1 to alpha. 

_/etc/tinc/gatewayvpn/tinc-down_

        #!/bin/sh
        ip addr del 172.16.16.1/24 dev $INTERFACE
        ip link set $INTERFACE down
        
Shell script that will be executed right before the tinc daemon is going to close its connection to the virtual network device. Similar to tinc-up, but in reverse.


_/etc/tinc/gatewayvpn/hosts/alpha_

        Address = 1.2.3.4
        Port = 7999
        Subnet = 0.0.0.0/0 

This file should be send to all the other peers (only beta in this example). We _create_ this file here so we can automatically add a public key to this file. Since we want beta to connect to alpha we should assign the external IP and port number to it. Make sure this connection is accesible! We set the subnet to 0.0.0.0/0, you can read this as alpha serve on the *whole* internet (as opposed to a limiting it in a local range. If no Port option is specified, the socket will be bound to the standard port 655 of tinc.

Create private keys
+++++++++


        $ root@alpha tincd -n gatewayvpn --generate-keys
        Generating 2048 bits keys: ...........
        Done
        Please enter a file to save private RSA key to [/etc/tinc/gatewayvpn/rsa-key.priv]: 
        Please enter a file to save public RSA key to [/etc/tinc/gatewayvpn/hosts/alpha]: 

Generate public/private RSA keypair. Private key will be written to _/etc/tinc/gatewayvpn/rsa-key.priv_. Public key will be written to all the files in /etc/tinc/gatewayvpn/hosts/*. So after running this command. _/etc/tinc/gatewayvpn/hosts/alpha_ will look like this

        Address = 1.2.3.4
        Port = 7999
        Subnet = 0.0.0.0/0 

        -----BEGIN RSA PUBLIC KEY-----
        Blabla
        -----END RSA PUBLIC KEY-----

Test your configuration
+++++++++

Now alpha is setup. We can test our configuration as follows:

        root@alpha:~$ tincd -n gatewayvpn --logfile /root/log
        root@alpha:~$

Logfile should show the following output:

        root@alpha:~# cat /root/log
        2018-04-28 04:43:21 tinc.gatewayvpn[9589]: tincd 1.0.26 (Jul  5 2015 23:17:54) starting, debug level 0
        2018-04-28 04:43:21 tinc.gatewayvpn[9589]: /dev/net/tun is a Linux tun/tap device (tun mode)
        2018-04-28 04:43:21 tinc.gatewayvpn[9589]: Ready

You should also be able to ping to the tunneling device:
        
        root@alpha:~$ ping 172.16.16.1
        PING 172.16.16.1 (172.16.16.1) 56(84) bytes of data.
        64 bytes from 172.16.16.1: icmp_seq=1 ttl=64 time=0.065 ms
        64 bytes from 172.16.16.1: icmp_seq=2 ttl=64 time=0.060 ms
        ^C
        --- 172.16.16.1 ping statistics ---
        2 packets transmitted, 2 received, 0% packet loss, time 999ms
        rtt min/avg/max/mdev = 0.060/0.062/0.065/0.008 ms

The tinc daemon is listening on our configured port 7999:

        root@alpha:~$ netstat -pnaut
        tcp        0      0 0.0.0.0:7999            0.0.0.0:               LISTEN      9828 tincd         
        udp        0      0 0.0.0.0:7999            0.0.0.0:                           9828 tincd


Beta peer
********


Create config files
+++++++

_/etc/tinc/gatewayvpn/tinc.conf_
        
        Name: beta 
        Device: /dev/net/tun
        ConnectTo: alpha

Beta will connect to alpha, for this connection beta will look in _/etc/tinc/gatewayvpn/hosts/alpha_ and connect to this IP:PORT
        
_/etc/tinc/gatewayvpn/tinc-up_
        
        #!/bin/sh
        ip link set $INTERFACE up
        ip addr add  172.16.16.2/24 dev $INTERFACE      

Beta will get assigned 172.16.16.2

_/etc/tinc/gatewayvpn/tinc-down_

        #!/bin/sh
        ip addr del 172.16.16.2/24 dev $INTERFACE
        ip link set $INTERFACE down
        

_/etc/tinc/gatewayvpn/hosts/beta_

        Subnet = 172.16.16.2/32
        Port = 7999

We don't need to set a Address. No peer will actively connect to this peer. The subnet will be limited to just the the peer itself, since it is not serving in any local network. 

Create private keys
+++++++++

        tincd -n gatewayvpn --generate-keys
        Generating 2048 bits keys: ...........
        Done.
        Please enter a file to save private RSA key to [/etc/tinc/gatewayvpn/rsa_key.priv]: 
        Please enter a file to save public RSA key to [/etc/tinc/gatewayvpn/hosts/beta]:

So now you will also have created the private key file for beta. Public keys are written to files in the host directory.
*Note: don't forget to put _/etc/tinc/gatewayvpn/hosts/beta_ on the alpha side and alpha on the beta side.

        root@beta:/etc/tinc/gatewayvpn/hosts$ sudo scp root@alpha:/etc/tinc/gatewayvpn/hosts/alpha .
        alpha                                               100%  481     0.5KB/s   00:00    
        root@beta:/etc/tinc/gatewayvpn/hosts$ sudo scp beta root@alpha:/etc/tinc/gatewayvpn/hosts/
        beta                                                100%  463     0.5KB/s   00:00    

Test your configuration
+++++++++++

See alpha.


Initial run
*********

Now let's see if the configuration is correct and both peers' connections are accepted. 
We, in a sense, have used a very verbose way to make a tunnel between two computers over the internet.

First start alpha:
        
        root@alpha:~$
        root@alpha:~$ tincd -n gatewayvpn --log-file /root/log

Then start beta: 

        root@beta:~$
        root@beta:~$ tincd -n gatewayvpn --log-file /root/log


Now test if you can ping!

        root@alpha:~# ping 172.16.16.2
        PING 172.16.16.2 (172.16.16.2) 56(84) bytes of data.
        64 bytes from 172.16.16.2: icmp_seq=1 ttl=64 time=118 ms
        64 bytes from 172.16.16.2: icmp_seq=2 ttl=64 time=118 ms
        64 bytes from 172.16.16.2: icmp_seq=3 ttl=64 time=118 ms

        root@beta:~# ping 172.16.16.1
        ping 172.16.16.1
        PING 172.16.16.1 (172.16.16.1) 56(84) bytes of data.
        64 bytes from 172.16.16.1: icmp_seq=1 ttl=64 time=118 ms
        64 bytes from 172.16.16.1: icmp_seq=2 ttl=64 time=118 ms
        64 bytes from 172.16.16.1: icmp_seq=3 ttl=64 time=117 ms


Routing gateway 
**********

(Note: mostly copied from the tinc manual)
It is possible to have one peer forward all of its network traffic to another peer on the VPN, effectively using this peer as the default gateway. This behaviour can configured in the tinc-up or tinc-down scripts. First, we explain some theory about redirecting, then the example scripts will follow.


Theory
+++++++
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 .1. An example output of route -n on Linux:

        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 172.16.16.0/24 (as in our case) on which a server (alpha in our setup) exists with address 172.16.16.1. If we have a VPN connection, and a peer wants to replace the standard default route with a default route pointing to 172.16.16.1, 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 (remote) IP address of the VPN server. Suppose its real address is 1.2.3.4, then the routing table should become:

        Kernel IP routing table
        Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
        172.16.16.1     0.0.0.0         255.255.255.255 UH    0      0        0 gatewayvpn
        1.2.3.4         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         172.16.16.1     0.0.0.0         UG    0      0        0 gatewayvpn

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
        172.16.16.1     0.0.0.0         255.255.255.255 UH    0      0        0 gatewayvpn
        1.2.3.4         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
        128.0.0.0       172.16.16.1     128.0.0.0       UG    0      0        0 gatewayvpn
        0.0.0.0         172.16.16.1     128.0.0.0       UG    0      0        0 gatewayvpn
        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.

Scripts
+++++++++

To achieve this, two scripts are needed on beta. We can add the following code to the the already existing tinc-up and tinc-down files.

_/etc/tinc/gatewayvpn/tinc-up_

        #!/bin/sh
        VPN_GATEWAY=172.16.16.1
        REMOTEADDRESS=1.2.3.4
        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 128.0.0.0/1 via $VPN_GATEWAY dev $INTERFACE

_/etc/tinc/gatewayvpn/tinc-down_

        #!/bin/sh       
        ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5`
        REMOTEADDRESS=1.2.3.4

        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 128.0.0.0/1 dev $INTERFACE

These script use the iproute2 commands, because they are easier to work with. The VPN_GATEWAY and REMOTEADDRESS variables have to be filled in by hand. The ORIGINAL_GATEWAY variable copies the relevant information from the original default route to create the exception route to the VPN server.


Setup firewall
+++++++++

Make sure forwarding is enabled on alpha. Make sure you have masquerading or another form of routing set up on alpha. If you don't masquerade outgoing (forwarded beta) packets, the source address in in the TCP/UDP package will still remain 172.16.16.2. Please have a look here: http://www.tldp.org/LDP/nag2/x-087-2-ipmasq.html if you don't know about NAT and masquerading.

        #!/bin/sh
        # iptables config line to masquerade
        
        echo "Enabling IPv4 forwarding"
        echo 1 >/proc/sys/net/ipv4/ip_forward
        
        echo "Appending Masquerade rule to iptables"
        iptables -t nat -A POSTROUTING -s 172.16.16.0/255.255.255.0 -o eth0 -j MASQUERADE

Here I use iptables to masquerade the (-s) source address on the (-o) interface eth0. 


Test the gateway
+++++++++++

Restart the daemon on alpha and beta. Use route -n to see check your routing table on beta. It should look similar to the one that is displayed above. Ping both the 172.16.16.1 and 1.2.3.4 (external address). In case of problems, trace the connections or analyze the data with tools like wireshark.


Troubleshooting help
*******

* DNS request are not forwarded through the gateway. Check your resolver config files (/etc/resolv.conf). Debian-based systems might have the following configuration

        root@beta:~$ cat /etc/resolv.conf       
        # resolv.conf file
        nameserver 127.0.1.0

* and in your routing table you might have the following entry. A local / caching DNS server might still send packages to your router. Use wireshark to see if there are any DNS queries, not going to the VPN gateway
        
        IP ROUTING TABLE
        link-local      *               255.255.0.0     U     1000   0        0 wlp7s0

* A simple fix would to change your resolv.conf and point it to nameserver 8.8.8.8
        
* Check your logfile while running tinc (i.e. you might forgot to create a key pair):

        2018-04-28 04:49:53 tinc.gatewayvpn[9684]: Error reading RSA private key file
         `/etc/tinc/gatewayvpn/rsa_key.priv': No such file or directory

* Overview of created files
        
        root@alpha:~$ ls -R /etc/tinc/gatewayvpn
        /etc/tinc/gatewayvpn:
        hosts/  rsa-key.priv  tinc.conf  tinc-down  tinc-up
        /etc/tinc/gatewayvpn/hosts:
        alpha beta
        
        root@beta:~$ ls -R /etc/tinc/gatewayvpn
        /etc/tinc/gatewayvpn:
        hosts/ rsa-key.priv tinc.conf tinc-down tinc-up
        /etc/tinc/gatewayvpn/hosts:
        alpha beta

* Use tcpdump or wireshark to analyze your network devices