OpenBGPD not listening on IPv6

[routermonkey]

When "Listen on IP" is left blank on the Web interface, OpenBGPD should bind to all IPs, but "listen on 0.0.0.0" is written to config file, thus it's only listening on IPv4 addresses.  :(

Correct config should be no "listen" directive at all.

That way, OpenBGPD can have IPv4 and IPv6 neighbors and exchange routes for both protocols.