Eu finalmente resolvo o problema. Eu uso systemd (com networkd) e novo recurso sobre o encaminhamento de ip foi introduzido no systemd-221: "IPForwarding=" - consulte link
De man systemd.network
:
[NETWORK] SECTION OPTIONS
...
IPForward=
Configures IP forwarding for the network interface. If enabled incoming packets on the network interface will be forwarded to other interfaces according to the routing table. Takes either a boolean argument, or the values "ipv4" or "ipv6", which only enables IP forwarding for the specified address family, or "kernel", which preserves existing sysctl settings. This controls the net.ipv4.conf..forwarding and net.ipv6.conf..forwarding sysctl options of the network interface (see ip-sysctl.txt[1] for details about sysctl options). Defaults to "no".
Note: unless this option is turned on, or set to "kernel", no IP forwarding is done on this interface, even if this is globally turned on in the kernel, with the net.ipv4.ip_forward, net.ipv4.conf.all.forwarding, and net.ipv6.conf.all.forwarding sysctl options.
Então, agora eu uso o arquivo de rede como segue para ativar o encaminhamento de ip (por interface):
# cat /etc/systemd/network/tun0.network
[Match]
Name=tun0
[Network]
IPForward=ipv4