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Redirecting traffic using FastD
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FastD is a VPN daemon that has many features of OpenVPN and Tinc and is optimized for small code size and small number of dependencies. Fastd became popular on small devices like routers. In this tutorial we will configure a listening peer (alpha) and a connecting peer (anyremote). On a side note, with fastD you can setup mesh networks (n:n), as opposed to classical clients server networks (1:n). This configuration can be seen as a simple (1:1) setup between the listening *alpha* peer and our connecting client *anyremote*. All traffic from anyremote is redirected to alpha, making alpha the default gateway. This configuration has a lot of similarities with the tinc tutorial (that you can find here: http://www.robinkrens.nl/tutorials/tinc.html). Documentation and manual pages of fastd can be found here http://fastd.readthedocs.io

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

To run the daemon you only need one configuration file. You can place it in fastd's defualt directory _/etc/fastd/fastd.conf_. Here we show a standard configuration of _fastd.conf_ with some minor changes:

        # Log warnings and errors to stderr
        log level warn;

        # Log everything to syslog
        log to syslog level debug;

        # tunnel mode (default is tap). 
        # We use tunneling mode, since we are dealing with routing
        mode tun;

        # Set the interface name
        # you can use any name you like
        # this is the name to configure your interface wit
        interface "vpngateway";

        # encryption method to use
        falls back to null if salsa is not chosen.
        method "salsa2012+umac";
        method "null";

        # Bind to a fixed port, IPv4 only
        # If your remote ip is 1.2.3.4, make sure 1.2.3.4:10000 is accesible
        bind 0.0.0.0:10000;

        # Secret key generated by `fastd --generate-key`
        # --generate-key outputs a file with a secret and public key
        # secret key goes in here. Public keys is distributed amongst other peers
        # read about PKI infrastructures if you don't know about this.
        secret "supersecretkey";

        # (see MTU selection documentation)
        # base MTU is 1500 and you want to use TUN mode over IPv4 with any 
        # crypto method: Choose 1500 - 52 = 1448 bytes.
        mtu 1448;

        # on up: shell script to configure the tun interface on daemon start
        on up "./interface-up";

        # on down: shell script when daemon is terminated
        on down "./interface-down"; 

        # Include peers from the directory 'peers'
        # anyremote is a peer trying to connect to alpha
        include peer "peers/anyremote";

Keys can be generate by running --generate-key (written to stdout):
        
        root@alpha:~$ fastd --generate-key > keys
        root@alpha:~$ cat keys
        2018-04-30 19:25:57 +0800 --- Info: Reading 32 bytes from /dev/random...
        Secret: 5035de5b4ea448b74e9a373765207095057a9485fd9dca5fadb9c1b86347bd75
        Public: 8cb5e8d70d34f52716b6c4de518af2edfd6794e68ef1b3f0608cf05dd6a2ef42

The secret key needs to be added to the above _fastd.conf_ file. The public needs to be spread amongst peers (as we explain later).
on up "./interface-up" will run a simple shell script and configures our network interface vpngateway (make sure this script is executable).
This is our _interface.up_ script: We create a virtual IP: 172.16.16.1.

        #!/bin/bash
        ip link set $INTERFACE up
        ip addr add 172.16.16.1/24 dev $INTERFACE

If we terminate fastd, we run a similar script as defined in interface-down
        
        #!/bin/sh
        ip addr del 172.16.16.1/24 dev $INTERFACE
        ip link set $INTERFACE down

We will create the _peer/anyremote_ file after we finished configuring anyremote and its public key

Anyremote peer
*********

Similar to alpha, we create _/etc/fastd/fastd.conf_. Since we only need to connect to alpha we don't need to bind to a fixed port. 

        # log arnings and errors to stderr
        log level warn;

        # Log everything to syslog
        log to syslog level debug;

        # tunnel mode (default is tap)
        mode tun;

        # Set the interface name
        interface "vpngateway";

        # Support salsa2012+umac and null methods, prefer salsa2012+umac
        method "salsa2012+umac";
        method "null";

        # Secret key generated by `fastd --generate-key`
        secret "supersecretkey";

        # (see MTU selection documentation)
        mtu 1448;

        # daemon start
        on up "./interface-up";

        # daemon terminated
        on down "./interface-down";

        # if a connection is established set up the gateway
        on establish "./set-gateway";

        # if the connection is lost restore the default gateway
        on disestablish "./restore-gateway";

        # Include peers from the directory 'peers'
        include peer "peers/alpha";

For anyremote we also need to generate a key pair and replace the "supersecretkey" with the secret key value. The public key will be given to alpha (explained in a little while)

        root@anyremote~ $ fastd --generate-keys > anyremote-keypair
        root@anyremote~ $ cat anyremote-keypair
        2018-05-01 19:48:49 +0800 --- Info: Reading 32 bytes from /dev/random...
        Secret: c0a611e0d4f3075b45cf172d3221c8427008e2c6f541b5b6adda0368cb79f271
        Public: 2598c5d7e72f171731658ce35734ff7599e1840367422e1a9c5943c327ab5ea9

        
*on up* and *on down* are similar to alpha (except the ip address). interface-up:
        
        #!/bin/bash
        ip link set $INTERFACE up
        ip addr add 172.16.16.2/24 dev $INTERFACE
        
interface-down:

        #!/bin/sh
        ip addr del 172.16.16.1/24 dev $INTERFACE
        ip link set $INTERFACE down

We need to include some information about how to connect to alpha. We define in a file (_/etc/fastd/peers/alpha_):

        root@anyremote:/etc/fastd/peers/ $ cat alpha
        # alpha 
        key "8cb5e8d70d34f52716b6c4de518af2edfd6794e68ef1b3f0608cf05dd6a2ef42";
        remote 1.2.3.4:10000;

*key* means the public key we just created with --generate-keys the alpha section. Here we add a remote ip to which anyremote tries to connect to. Make sure port numbers are the same. 
Don't forget to also add our our just created public key to our alpha server:

        root@alpha:/etc/fastd/peers/ $ cat anyremote
        # anyremote
        key "2598c5d7e72f171731658ce35734ff7599e1840367422e1a9c5943c327ab5ea9";

This will allow alpha to accept connections from anyremote. Note: you don't need to specify a remote address, this will make it more dynamic and you can connect with anyremote from anywhere as long as you have the private key.

After these steps you should be able to run both alpha and anyremote. You can run the daemon as follows:
        
        root@alpha:~ $ fastd -c /etc/fastd/fastd.conf &
        root@anyremote:~ $ fastd -c /etc/fastd/fastd.conf &

The interface *vpngateway* should show up and you should be able to ping to both hosts us.
        
Now, in our config file of anyremote we see two additionals values: *on establish* and *on disestablish*. Once the connection is (dis)established, fastd will execute these scripts. This brings us two the last step: setting the default gateway of anyremote to point to alpha

Alpha as gateway for anyremote
**********

Have a look at the tinc tutorial (gateway section) about the theory of routing and gateways.  
We add the following scripts in _/etc/fastd_ of anyremote if a connection with alpha is established: (set-gateway)
        
        #!/bin/bash
        #ip link set $INTERFACE up
        #ip addr add 172.16.16.2/24 dev $INTERFACE

        VPN_GATEWAY=172.16.16.1
        ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5`
        REMOTEADDRESS=1.2.3.4

        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
        
And, similar, if the connecting is lost: (restore-gateway):

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

        ORIGINAL_GATEWAY=`ip route show | grep ^default | cut -d ' ' -f 2-5`
        REMOTEADDRESS=45.76.159.1

        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


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 anyremote) 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

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


Test the gateway
********

Restart the daemon on alpha and anyremote. Use route -n to see check your routing tables. Ping both 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@anyremote:~$ 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
        
* Fastd's log to _/var/log/syslog_ You can define these locations in your fast.conf file. You can also change the log level, in case you need more information:

        --log-level error|warn|info|verbose|debug|debug2
        Sets the stderr log level; default is info if no alternative log
        destination is configured.

* Use tcpdump or wireshark to analyze your network devices