Allow for some kind of ignored gateway

[jannic]

Just an idea:
How about some way to mark a gateway as invalid, so that a connection to that gateway won't be sufficient to enable the online-SSID?

The use case I have in mind is the possible deprecation of old gateways. One would want to keep those gateway online as long as possible, to allow old nodes re-appearing after an extended period of time to reach the update servers. But at the same time, one wouldn't want those gateways to be active part of the Freifunk network - after all, they are deprecated and will be disabled some day.

If we could mark those gateways as inactive, we could let them run with minimal resources, perhaps even without full internet access, knowing that normal users will never use them to access the internet (because of the offline-ssid), while still providing connectivity to capture ancient nodes and connect them to an update server.

One possibility to reach this goal would be to just reserve a range of mac addresses as illegal, and filter them out from the output of batctl gwl in the ssid changer script.