An intro to MiniUPNP

2020-04-14T00:00:00+00:00
If you have a software that requires port forwarding, you may want to implement UPnP to make things easy for your end users.</p>
When I wanted to implement it, the first library I found was MiniUPnP</a>, sadly it doesn't have much documentation but a quick look at the header files and some examples on the internet I managed to make it work, here is how:</p>
First the required include directives we need:</p>
#include </span><miniupnpc/miniupnpc.h>
</span>#include </span><miniupnpc/upnpcommands.h>
</span>#include </span><miniupnpc/upnperrors.h>
</span>#include </span><stdio.h>
</span></code></pre>
The first thing we need to do is discover the UPnP devices on the network, this is done using upnpDiscover</code>:</p>
int </span>main</span>() {
</span>  </span>struct</span> UPNPDev </span>*</span>upnp_dev </span>= </span>0</span>;
</span>
</span>  </span>int</span> error </span>= </span>0</span>;
</span>  upnp_dev </span>= </span>upnpDiscover</span>(</span>2000</span>, </span>NULL</span>, </span>NULL</span>, </span>0</span>, </span>0</span>, </span>2</span>, </span>&</span>error)</span>;
</span>
</span>  </span>if</span>(error </span>!= </span>0</span>) {
</span>    </span>printf</span>(</span>"error discovering upnp devices: </span>%s</span>\n</span>"</span>, </span>strupnperror</span>(error))</span>;
</span>    </span>return </span>1</span>;
</span>  }
</span>}
</span></code></pre>
Then we need to retrieve the Internet Gateway Device we discovered:</p>
struct</span> UPNPUrls upnp_urls</span>;
</span>struct</span> IGDdatas upnp_data</span>;
</span>char</span> aLanAddr[</span>64</span>]</span>;
</span>const char </span>*</span>pPort </span>= </span>"8000"</span>;
</span>
</span>// Retrieve a valid Internet Gateway Device
</span>int</span> status </span>= </span>UPNP_GetValidIGD</span>(
</span>    upnp_dev</span>,
</span>    </span>&</span>upnp_urls</span>,
</span>    </span>&</span>upnp_data</span>,
</span>    aLanAddr</span>,
</span>    </span>sizeof</span>(aLanAddr)
</span>    )</span>;
</span>printf</span>(</span>"status=</span>%d</span>, lan_addr=</span>%s</span>\n</span>"</span>,</span> status</span>,</span> aLanAddr)</span>;
</span></code></pre>
We check if we got the correct status and then map the port.
We also declare the port we want to use, in this case I use the same number for the internal and external ports:</p>
if </span>(status </span>== </span>1</span>)
</span>{
</span>    </span>printf</span>(</span>"found valid IGD: </span>%s</span>\n</span>"</span>,</span> upnp_urls</span>.</span>controlURL)</span>;
</span>    error </span>=
</span>        </span>UPNP_AddPortMapping</span>(
</span>            upnp_urls</span>.</span>controlURL</span>,
</span>            upnp_data</span>.</span>first</span>.</span>servicetype</span>,
</span>            pPort</span>, </span>// external port
</span>            pPort</span>, </span>// internal port
</span>            aLanAddr</span>, </span>"My Application Name"</span>, </span>"UDP"</span>,
</span>            </span>0</span>,  </span>// remote host
</span>            </span>"0" </span>// lease duration, recommended 0 as some NAT
</span>            </span>// implementations may not support another value
</span>        )</span>;
</span>
</span>    </span>if </span>(error)
</span>    {
</span>        </span>printf</span>(</span>"failed to map port</span>\n</span>"</span>)</span>;
</span>        </span>printf</span>(</span>"error: </span>%s</span>\n</span>"</span>, </span>strupnperror</span>(error))</span>;
</span>    }
</span>    </span>else
</span>        </span>printf</span>(</span>"successfully mapped port</span>\n</span>"</span>)</span>;
</span>}
</span>else
</span>    </span>printf</span>(</span>"no valid IGD found</span>\n</span>"</span>)</span>;
</span></code></pre>
Now the port is mapped, at this point you can open a socket bound to the internal port and external clients will be able to connect using the external port.</p>
Then we do some cleanup:</p>
Note: The port here is the external one.</em></p>
error </span>= </span>UPNP_DeletePortMapping</span>(upnp_urls</span>.</span>controlURL</span>,
</span>                               upnp_data</span>.</span>first</span>.</span>servicetype</span>,
</span>                               pPort</span>,
</span>                               </span>"UDP"</span>,
</span>                               </span>0</span>)</span>;
</span>
</span>if </span>(error </span>!= </span>0</span>) {
</span>    </span>printf</span>(</span>"port map deletion error: </span>%s</span>\n</span>"</span>, </span>strupnperror</span>(error))</span>;
</span>}
</span>
</span>FreeUPNPUrls</span>(</span>&</span>upnp_urls)</span>;
</span>freeUPNPDevlist</span>(upnp_dev)</span>;
</span>return </span>0</span>;
</span></code></pre>
Complete source code: gist</a></p>
Compiled using: cc -lminiupnpc upnp.c</code></p>
Edgar Luque - c

An intro to MiniUPNP
