Manage automatically SOCKS5 tunnel created with tun2socks from package badvpn. This allows for setting up a cheap VPN on top of for instance an SSH tunnel.

Install dnsmasq-base:

sudo apt install dnsmasq-base
sudo vi /etc/NetworkManager/NetworkManager.conf
# Edit as follows:
#      [main]
#      plugins=ifupdown,keyfile
#     +dns=dnsmasq
sudo vi /etc/dhcp/dhclient.conf
# Edit as follows:
#      #supersede domain-name "fugue.com home.vix.com";
#     -#prepend domain-name-servers 127.0.0.1;
#     +prepend domain-name-servers 127.0.0.1;

Build badvpn-tun2socks from source:

sudo apt install cmake
git clone https://github.com/ambrop72/badvpn.git
cd badvpn
mkdir badvpn-build
cd badvpn-build
cmake .. -DBUILD_NOTHING_BY_DEFAULT=1 -DBUILD_TUN2SOCKS=1
make
sudo cp tun2socks/badvpn-tun2socks /usr/local/bin

Install tun2socks-manager:

cd tun2socks-manager
sudo make install

By default tun2socks is configured to use UDP forwarding (parameter TUN2SOCKS_UDPGW_REMOTE) for better performance. This requires running badvpn-udpgw on the server side (as standard user, no root access mandatory). For instance:

git clone https://github.com/ambrop72/badvpn.git
cd badvpn
mkdir badvpn-build
cd badvpn-build
cmake .. -DBUILD_NOTHING_BY_DEFAULT=1 -DBUILD_UDPGW=1
make
sudo cp badvpn-udpgw /usr/local/bin      # if no root: install in ~/bin

Then we need to run it automatically. Using systemd, we first create a service file /etc/systemd/system/udpgw.service containing:

[Unit]
Description=UDP forwarding for badvpn-tun2socks
After=nss-lookup.target

[Service]
ExecStart=/usr/local/bin/badvpn-udpgw --loglevel none --listen-addr 127.0.0.1:7300
User=udpgw

[Install]
WantedBy=multi-user.target

Then we create the user, and enable / start the service:

sudo useradd -m udpgw
sudo systemctl enable udpgw
sudo systemctl start udpgw

An non-root alternative is to edit the crontab (crontab -e) and add a user @reboot event:

@reboot           sleep 15 && ~/bin/badvpn-udpgw --loglevel none --listen-addr 127.0.0.1:7300

See file /etc/tun2socks-manager.conf and per-connection sample file in /etc/tun2socks-manager.d.

Error org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.NetworkManager.dnsmasq
was not provided by any .service files

This is likely because dnsmasq is not running. Check that NetworkManager started it in background. Otherwise follows the install instructions above.

ps faux | grep dnsmasq
# nobody    128496  0.0  0.0  12976  4548 ?        S    10:06   0:00 /usr/sbin/dnsmasq --no-resolv
# --keep-in-foreground --no-hosts --bind-interfaces --pid-file=/run/NetworkManager/dnsmasq.pid
# --listen-address=127.0.0.1 --cache-size=400 --clear-on-reload --conf-file=/dev/null --proxy-dnssec
# --enable-dbus=org.freedesktop.NetworkManager.dnsmasq --conf-dir=/etc/NetworkManager/dnsmasq.d

Also check that the parameter given to --enable-dbus matches the expected string.

The script heavily interferes with the routing table, and may interfere with the routing of IP packets from the WiFi interface. In particular, if the computer is also used as hotspot, the routing may be broken.

$ ip route
default via 10.137.3.254 dev enp0s31f6 proto dhcp metric 100
10.0.10.0/24 dev tun0 proto kernel scope link src 10.0.10.1
10.42.0.0/24 dev wlp1s0 proto kernel scope link src 10.42.0.1 metric 600
10.137.2.0/23 dev enp0s31f6 proto kernel scope link src 10.137.2.174 metric 100

We see we have a dedicated route for the hotspot on itf wlsp1s0. However when looking in the tun table, we don't find such routing, but instead the routing is caught in a general routing rule.

$ ip route show table tun
default via 10.0.10.2 dev tun0
10.0.0.0/8 via 10.137.3.254 dev enp0s31f6
10.0.10.0/24 dev tun0 proto kernel scope link src 10.0.10.1
10.137.2.0/23 dev enp0s31f6 proto kernel scope link src 10.137.2.174
...

To fix this, we must duplicate the hotspot routing rule in the tun table:

sudo ip route add 10.42.0.0/24 dev wlp1s0 proto kernel scope link src 10.42.0.1 metric 600 table tun

Messages are sent to /var/log/messages:

grep tun2socks-manager /var/log/messages

When troubleshooting, it helps to follow these messages in a separate window. For instance:

tail -f /var/log/messages

tun2socks-manager is installed as a Network Manager (NM) dispatcher script. When NM starts, it launches the tun2socks-manager with:

/usr/local/bin/tun2socks-manager start

Then, NM sends up or down event to the script, which updates the routing table based on configuration files in /etc/tun2socks-manager.d/ . This can be triggered manually with:

/usr/local/bin/tun2socks-manager update eth0 up       # Simulate up event on itf eth0