Bridging networks (VPNs)

02 Aug 2021 - tsp
Last update 04 Aug 2021
Reading time 19 mins

Introduction

Another common problem - one has different networks or nodes at different locations and no ability to establish a direct physical wired or wireless link but wants to join the networks. As long as there is any kind of network connection the solution is simple: A virtual private network (VPN). The basic idea is to tunnel the internal traffic over any external or public network like the internet and bridge the gap between hosts and networks that way.

This might be done for different reasons. For example:

There are many different approaches to this problem. Just to mention a few of them:

Note that all settings and services described have to be performed on a router machine that is passed by all traffic or the routing of the default gateway has to be modified (for example by running OLSR or interior BGP to see announcements by the VPN endpoints machine) to allow clients not using routing protocols but just the default route to use routing services. For small networks: Run them on the gateway machine. For large networks: You should already know how to do this without reading this blog post …

Unencrypted solution (GIF, GRE)

The presented unencrypted solutions - the generic routing encapsulation and the generic tunneling interface - are solutions mostly not found for private home deployments or small companies but usually at larger telecommunication systems, internet network operators, internet exchange points, mobile network operators and - for the gif - also as backhaul for IPv6 tunnel broker. In these scenarios they’re also often used to carry protocols such as MPLS that are then used to carry stuff like telephony protocols or IP packets.

Generic routing encapsulation

An unencrypted static tunnel is setup pretty simple - it requires two endpoints with static public IP addresses and the choice between gif or gre. The main difference is in the type of packets that can be encapsulated. The generic tunneling interface is capable of encapsulating all layer 2 packets, the generic routing interface only encapsulates IP traffic - but support for GRE is wider.

Basically a GRE interface only requires some basic configuration. Let’s assume the following:

To establish the tunnel one just has to setup the GRE interface on both ends.

ifconfig gre0 create
ifconfig gre0 inet 128.66.1.1 128.66.2.1
ifconfig gre0 inet tunnel 128.66.0.1 128.66.0.2
ifconfig gre0 up

One can as usual persist the settings in /etc/rc.conf:

cloned_interfaces="gre0"
ifconfig_gre0="inet 128.66.1.1 128.66.2.1 tunnel 128.66.0.1 128.66.0.2 up"

On the other end (router B) the same has to be done with mirrored configuration:

ifconfig gre0 create
ifconfig gre0 inet 128.66.2.1 128.66.1.1
ifconfig gre0 inet tunnel 128.66.0.2 128.66.0.1

Again this can be persisted in /etc/rc.conf:

cloned_interfaces="gre0"
ifconfig_gre0="inet 128.66.2.1 128.66.1.1 tunnel 128.66.0.2 128.66.0.1 up"

In case one wants to route traffic one might use either a routing daemon such as olsrd or even bgpd - which is highly advisable - or configure the routes statically which really only makes sense for really small scale deployments.

To establish the static route on router A:

route add -net 128.66.2.1/24 128.66.0.2

And on router B:

route add -net 128.66.1.1/24 128.66.0.1

Generic interface encapsulation

The gif interface works similar to the gre interface. Assuming the same configuration as above:

The steps are pretty similar on both ends. Again first for router A:

ifconfig gif0 create
ifconfig gif0 inet 128.66.1.1 128.66.2.1
ifconfig gif0 inet tunnel 128.66.0.1 128.66.0.2
ifconfig gif0 up

This also can be persisted in /etc/rc.conf:

cloned_interfaces="gif0"
ifconfig_gif0="inet 128.66.1.1 128.66.2.1 tunnel 128.66.0.1 128.66.0.2 up"

And on router B:

ifconfig gif0 create
ifconfig gif0 inet 128.66.2.1 128.66.1.1
ifconfig gif0 inet tunnel 128.66.0.2 128.66.0.1
ifconfig gif0 up

This also can be persisted in /etc/rc.conf:

cloned_interfaces="gif0"
ifconfig_gif0="inet 128.66.2.1 128.66.1.1 tunnel 128.66.0.2 128.66.0.1 up"

Again in case one wants to route traffic one might use either a routing daemon such as olsrd or even bgpd - which is highly advisable - or configure the routes statically which really only makes sense for really small scale deployments.

To establish the static route on router A:

route add -net 128.66.2.1/24 128.66.0.2

And on router B:

route add -net 128.66.1.1/24 128.66.0.1

The main difference between gre and gif that gif is capable of carrying layer 2 frames - and it can carry either IPv4 or IPv6 but not both at the same time - while gre can do that. Generic interface encapsulation is often seen with IPv6 tunnel brokers.

Encrypted solution based on tinc

Since tinc is my favorite VPN client I’ll base my description on this VPN client. Tinc is somewhat special in the kind it works and allows easy bridging of full networks as well as being a backhaul for single clients. The basic design pattern of tinc is a mesh VPN - each node connects to each other node if possible. Of course a VPN might also not be fully meshed in case one doesn’t want to synchronize keysets to all clients, has different administrative domains, etc. - if one configures a routing protocol later on this is no problem as long as at least one known path between each node exists.

Tinc can be installed on many different platforms including:

On FreeBSD installation again is pretty simple. Either from packages:

pkg install security/tinc

or from ports:

cd /usr/ports/security/tinc
make install clean

Basically each VPN mesh that one wants to configure is configured using it’s own configuration file and configuration directory. These are stored inside /usr/local/etc/tinc/ and given a locally unique identification of the VPN network that one’s configuring. For this example I’ll use the examplenet name - thus configuration files will be at /usr/local/etc/tinc/examplenet

The main configuration file is tinc.conf - in this example /usr/local/etc/tinc/examplenet/tinc.conf:

Name = anyexamplenodenamea
Mode = switch
DecrementTTL = yes
Device = /dev/tap1
DeviceType = tap
Forwarding = internal

ConnectTo = anyexamplenodenameb

As one can see the node gets assigned a name - that will also be used as filename for it’s keyfiles and has to be unique in the given mesh. In this case the local node name is anyexamplenodenamea, the name of the only configured reachable remote will be anyexamplenodenameb (more on how to configure that later on).

The mode has been set to switch to allow all ethernet frames to be passed in contrast to router mode. This doesn’t really matter since FreeBSD’s native routing capabilities will be used - one can then imagine the tinc instances just substituting a switch that all routers are attached to. In the example it has also been configured to decrement the TTL which is usually helpful to prevent traffic loops.

More interesting there is a tap device configured and fixed as being /dev/tap1. This device will later be configured using ifconfig inside startup scripts or cloned in rc.conf depending on the setup.

A list of optional ConnectTo statements tells tinc which nodes to connect to - the required IP addresses and keys are contained in host description files that are later kept in /usr/local/etc/tinc/examplenet/hosts

One should create a host configuration for the local node. To do this first create the hosts directory

mkdir -p /usr/local/etc/tinc/examplenet/hosts/

and then edit /usr/local/etc/tinc/examplenet/hosts/anyexamplenodenamea (use the same node name as in tinc.conf). This includes some basic settings:

Address = 128.66.10.20
Compression = 9
Port = 656
Subnet = 128.66.0.2/32

Compression and port specifications are optional - compression just enabled traffic compression which is a trade of between processing power and bandwidth. The port specification selects the TCP port that will be used by this node - it’s a good idea to use the same port on all nodes though.

The Subnet declaration is more crucial. It tells tinc for which subnet this node is responsible. In the example above I’ve set it to a single IP address by specifying the prefix length of 32 bits.

The most important configuration is Address which is the publicly reachable IP address of this node. Other nodes will try to use this IP whenever a ConnectTo statement is found in tincd.conf. Of course only nodes on static IP addresses can supply this information - mobile nodes have to actively dial into the static ones.

There are two additional really useful files tinc-up and tinc-down. These are simple shell scripts that are executed whenever the tinc daemon starts up or is stopped. In case one wants to configure the interface in tinc-up one could create a simple /usr/local/etc/tinc/examplenet/tinc-up with executable bit set (chmod 755 /usr/local/etc/tinc/examplenet/tinc-up):

#!/bin/sh
ifconfig tap1 create
ifconfig tap1 inet 128.66.0.1

# One might even configure some static routes here:
# route add -net 128.66.2.1/24 128.66.0.2

The counterpart is the tinc-down script that should then perform cleanup:

#!/bin/sh
route del -net 128.66.2.1/24 128.66.0.2
ifconfig tap1 destroy

Before one can really launch the VPN daemon one has to create the keyset by launching tinc in key creation mode:

tincd -n examplenet -K

This will create the /usr/local/etc/tinc/examplenet/rsa_key.priv private keyfile that contains the nodes private key - this should never ever be shared or leave the host except for backup purposes.

It will also add the key to mkdir -p /usr/local/etc/tinc/examplenet/hosts/anyexamplenodenamea (the filename is generated from the node name set in tinc.conf)

The last step that has to be taken on the node is to enable tinc in /etc/rc.conf and tell the startup script which network to launch:

tincd_enable="YES"
tincd_cfg="examplenet"

The same steps have to be taken on all other nodes that should join the VPN - and their respective hosts files have to be copied to all other nodes that they should connect to. Then one can list external nodes that the daemon should connect to as a ConnectTo statement on the given machines. Personally I’ve built a small shell script to automate that process that periodically fetches a set of hostfiles from a central location, verifies an GPG signature of the file, extracts the hosts files, scans them for Address lines and if they’re present lists them with ConenctTo in the tincd.conf files.

Metadata hiding using TOR

Of course sometimes it might be interesting to hide metadata of nodes. This is of course entirely possible for TCP based tinc by using the TOR daemons SOCKS5 proxy server and setting up hidden services. This can be done pointing to the proxy using proxy = socks5 127.0.0.1 9050 - but beware that DNS resolution might leak hostnames. Since tincd uses the systems DNS resolver one has to use the DNSPort option of tor inside torrc, set the port to DNSPort 53 and redirect all local DNS resolutions towards the TOR daemon in /etc/resolv.conf. Since machines that support TOR hidden services should be usually isolated from any other public internet access to prevent data leakage this should not be a real problem anyways.

Of course using tinc on top of TOR can be considered safe usually since the VPNs traffic is already connected to travel via an unencrypted network.

Setting up olsrd for automatic route configuration

Since it’s nice to have when bridging networks - especially when extending them later on - here a short introduction on how to setup olsrd for this simple scenario. The optimized link state routing protocol basically is a proactive route advisory protocol that allows all other routers on the same mesh to discover new routes and automatically configure their routing tables. One often sees OLSR being employed in wireless LAN mesh networks such as FunkFeuer. One could of course also use some exterior protocol such as BGP - they’re somewhat equivalent for the given problem with BGP being currently the only routing protocol that can handle networks of the size of the Internet.

Installing olsrd is pretty easy - either from packages or ports:

pkg install net/olsrd

or from ports:

cd /usr/ports/net/olsrd
make install clean

After that one just has to configure olsrd by editing /usr/local/etc/olsrd/olsrd.conf. First there is a set of configurations specific to this daemons instance and operation:

DebugLevel              0
AllowNoInt              yes
FIBMetric               "flat"
TcRedundancy            2
MprCoverage             1

LinkQualityAlgorithm    "etx_ff"
LinkQualityFishEye      1

UseHysteresis           no

This is just an example of my usual configuration when bridging networks via VPN links:

The next blocks configure host network associations. There is one block for IPv4 and one for IPv6. This basically includes the routes or subnets that are available through this node and that it will initially advertise (in addition to learned routes). In case this is a border router one should include 0.0.0.0 0.0.0.0 or 0:: 0 to allow routing towards the public internet (i.e. a default route).

Hna4
{
	0.0.0.0                0.0.0.0
	128.66.1.0             255.255.255.0
}

Hna6
{
	0::                    0
	fec0:2200:1:0:0:0:0:0  48
}

Then one just has to specify the interfaces that olsrd should listen on, setting the operation mode as well as optionally the broadcast addresses:

Interface "tap1"
{
        Mode            "mesh"
        Ip4Broadcast    128.66.0.255
}

Just in case one wants to monitor the node via some basic mechanism one might also want to enable the txtinfo plugin - one has to check the current version number of the plugin though:

LoadPlugin "olsrd_txtinfo.so.1.1"
{
        PlParam "Accept" "127.0.0.1"
}

This plugin exposes neighbor tables, HNAs and learned routes as well as the discovered topology.

After that one can simply enable the service in /etc/rc.conf

olsrd_enable="YES"

and launch it using the rc script:

/usr/local/etc/rc.d/olsrd start

In case one has loaded the txtinfo plugin successfully one might query the status of the node using netcat:

echo "/all" | nc 127.0.0.1 2006

This produces output comparable to the following (for a small mesh consisting of 7 OLSR capable endpoints):

Table: Neighbors
IP address	SYM	MPR	MPRS	Will.	2-hop count
10.0.3.15	YES	NO	NO	3	5
10.0.3.12	YES	NO	NO	3	5
10.0.3.17	YES	NO	NO	3	5
10.0.3.21	YES	NO	NO	3	5
10.0.3.9	YES	NO	NO	3	5
10.0.3.6	YES	NO	NO	3	5

Table: Links
Local IP	Remote IP	Hyst.	LQ	NLQ	Cost
10.0.3.2	10.0.3.12	0.000	1.000	1.000	1.000
10.0.3.2	10.0.3.21	0.000	1.000	1.000	1.000
10.0.3.2	10.0.3.17	0.000	1.000	1.000	1.000
10.0.3.2	10.0.3.6	0.000	1.000	1.000	1.000
10.0.3.2	10.0.3.15	0.000	1.000	1.000	1.000
10.0.3.2	10.0.3.9	0.000	1.000	1.000	1.000

Table: Routes
Destination	Gateway IP	Metric	ETX	Interface
10.0.3.6/32	10.0.3.6	1	1.000	tap1
10.0.3.9/32	10.0.3.9	1	1.000	tap1
10.0.3.12/32	10.0.3.12	1	1.000	tap1
10.0.3.15/32	10.0.3.15	1	1.000	tap1
10.0.3.17/32	10.0.3.17	1	1.000	tap1
10.0.3.21/32	10.0.3.21	1	1.000	tap1
10.2.1.0/24	10.0.3.15	1	1.000	tap1
10.2.2.0/24	10.0.3.15	1	1.000	tap1
10.2.4.0/24	10.0.3.17	1	1.000	tap1
10.2.5.0/24	10.0.3.17	1	1.000	tap1
10.2.6.0/24	10.0.3.12	1	1.000	tap1
10.3.0.0/16	10.0.3.9	1	1.000	tap1

Table: HNA
Destination	Gateway
10.0.10.0/24	10.0.3.2
10.2.1.0/24	10.0.3.15
10.2.2.0/24	10.0.3.15
10.0.3.12/32	10.0.3.12
10.2.6.0/24	10.0.3.12
10.0.3.17/32	10.0.3.17
10.2.4.0/24	10.0.3.17
10.2.5.0/24	10.0.3.17
10.0.3.21/32	10.0.3.21
10.0.3.9/32	10.0.3.9
10.3.0.0/16	10.0.3.9
10.0.3.6/32	10.0.3.6

Table: MID
IP address	(Alias)+

Table: Topology
Dest. IP	Last hop IP	LQ	NLQ	Cost
10.0.3.6	10.0.3.2	1.000	1.000	1.000
10.0.3.9	10.0.3.2	1.000	1.000	1.000
10.0.3.12	10.0.3.2	1.000	1.000	1.000
10.0.3.15	10.0.3.2	1.000	1.000	1.000
10.0.3.17	10.0.3.2	1.000	1.000	1.000
10.0.3.21	10.0.3.2	1.000	1.000	1.000
10.0.3.2	10.0.3.6	1.000	1.000	1.000
10.0.3.9	10.0.3.6	1.000	1.000	1.000
10.0.3.12	10.0.3.6	1.000	1.000	1.000
10.0.3.15	10.0.3.6	1.000	1.000	1.000
10.0.3.17	10.0.3.6	1.000	1.000	1.000
10.0.3.21	10.0.3.6	1.000	1.000	1.000
10.0.3.2	10.0.3.9	1.000	1.000	1.000
10.0.3.6	10.0.3.9	1.000	1.000	1.000
10.0.3.12	10.0.3.9	1.000	1.000	1.000
10.0.3.15	10.0.3.9	1.000	1.000	1.000
10.0.3.17	10.0.3.9	1.000	1.000	1.000
10.0.3.21	10.0.3.9	1.000	1.000	1.000
10.0.3.2	10.0.3.12	1.000	1.000	1.000
10.0.3.6	10.0.3.12	1.000	1.000	1.000
10.0.3.9	10.0.3.12	1.000	1.000	1.000
10.0.3.15	10.0.3.12	1.000	1.000	1.000
10.0.3.17	10.0.3.12	1.000	1.000	1.000
10.0.3.21	10.0.3.12	1.000	1.000	1.000
10.0.3.2	10.0.3.15	1.000	1.000	1.000
10.0.3.6	10.0.3.15	1.000	1.000	1.000
10.0.3.9	10.0.3.15	1.000	1.000	1.000
10.0.3.12	10.0.3.15	1.000	1.000	1.000
10.0.3.17	10.0.3.15	1.000	1.000	1.000
10.0.3.21	10.0.3.15	1.000	1.000	1.000
10.0.3.2	10.0.3.17	1.000	1.000	1.000
10.0.3.6	10.0.3.17	1.000	1.000	1.000
10.0.3.9	10.0.3.17	1.000	1.000	1.000
10.0.3.12	10.0.3.17	1.000	1.000	1.000
10.0.3.15	10.0.3.17	1.000	1.000	1.000
10.0.3.21	10.0.3.17	1.000	1.000	1.000
10.0.3.2	10.0.3.21	1.000	1.000	1.000
10.0.3.6	10.0.3.21	1.000	1.000	1.000
10.0.3.9	10.0.3.21	1.000	1.000	1.000
10.0.3.12	10.0.3.21	1.000	1.000	1.000
10.0.3.15	10.0.3.21	1.000	1.000	1.000
10.0.3.17	10.0.3.21	1.000	1.000	1.000

Table: Interfaces
Name	State	MTU	WLAN	Src-Adress	Mask	Dst-Adress
tap1	UP	1472	No	10.0.3.2	255.255.255.0	10.0.3.255

Table: Neighbors
IP address	SYM	MPR	MPRS	Will.	(2-hop address)+
10.0.3.15	YES	NO	NO	3	10.0.3.9	10.0.3.21	10.0.3.12	10.0.3.6	10.0.3.17
10.0.3.12	YES	NO	NO	3	10.0.3.9	10.0.3.21	10.0.3.6	10.0.3.15	10.0.3.17
10.0.3.17	YES	NO	NO	3	10.0.3.9	10.0.3.21	10.0.3.15	10.0.3.12	10.0.3.6
10.0.3.21	YES	NO	NO	3	10.0.3.9	10.0.3.15	10.0.3.12	10.0.3.17	10.0.3.6
10.0.3.9	YES	NO	NO	3	10.0.3.12	10.0.3.17	10.0.3.6	10.0.3.15	10.0.3.21
10.0.3.6	YES	NO	NO	3	10.0.3.9	10.0.3.21	10.0.3.15	10.0.3.17	10.0.3.12

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Dipl.-Ing. Thomas Spielauer, Wien (webcomplains389t48957@tspi.at)

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