what's the use of "dev null"

phdm

Hello,

I recently made what I thought would be a closed openvpn network as a meeting point for a set of devices that get dynamic IP adresses (and sometimes even in a 10.x.y.z
network) to communicate with each other. In my config file I had written :
dev null
dev-type tun
client-to-client
That works: my devices can communicate with each other.

I have then tried to make a second instance of such a closed openvpn network, for another set, but that fails with :
Note: Cannot ioctl TUNSETIFF null: Device or resource busy (errno=16)

Now I am surprised. What's the meaning of 'dev null' ? The documentation mentions it, but does not explain it. I thus thought the meaning was 'no dev', but now I see that I have an interface called 'null', with an ip address. So, did 'dev null' work with an older openvpn version, or is that something that has been added after the openvpn version that I use (openvpn-2.2.1)

Thanks in advance

phdm

dev null in an openvpn config file is certainly not /dev/null. With openvpn-2.2.1, I get an IP interface called 'null'. That's not what I expected.

ovs:/etc/openvpn # ifconfig
eth0 Link encap:Ethernet HWaddr 00:xx:yy:zz:uu:vv
...

lo Link encap:Local Loopback
...

null Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:192.168.42.1 P-t-P:192.168.42.1 Mask:255.255.255.0
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)

ovs:/etc/openvpn #

And the second problem, of course, is that the interface called null cannot be used again for a second openvpn server on the same machine 🙁

Note: Cannot ioctl TUNSETIFF null: Device or resource busy (errno=16)

So, what was the intended use ?
Or did that work differently/better before ?

TIA