a) the bug where the ban-msg is displayed only once upon a reconnect is still in play
Subnet ban in my system is completely independent on any prior banning code. /reconnect or anything similar doesn't let you in.
That is not my point .)
If one gets banned, he gets a nice popup with the ban reason. If he (tries to) reconnect(s) he will only get the ban msg _one_ time after that the msg wil be "This server is for low ping only".
b) a griefer _will_ come back with a new IP and new GUID and simply re-register his guid.
It takes at least one map reload before new registration requests are loaded. And if there're invites, it takes much longer before they get an invite (if they ever get one).
Sorry, but that will only help you for exactly one game and then?
c) pointing to the backport is simply not enough for 08/15 players that get caught in this baning-mumbo-jumbo.
95 out of 100 players WON'T get caught.
See my point a) you plan on telling those with the ban-msg that they need the backport (and/or the registration) and they get that for exactly 2 seconds before the bug kicks in and they are left clueless as to why they can not connect.
d) do not get hooked up on the existing guid to id a player, even with server-unique-guid it does NOT id a player, unless he wants it.
That's the point. If they don't want to be identified, they won't get in. 
Then why not use the Password-feature?
Players from a banned subnet will not get your msg due to the bug and only your regulars will care enough to actually register other players have incentive to do so.
This WILL solve the griefer problem on one server, yes...
Then why not make it more simple?
Just expand the PWD-feature to accept multiple Passwords which you hand out for every player that registers somewhere. So no need to tinker with guids and such, if one of those does grieve then just revoke his PWD and you are all set.
e) unlimited invites... what's the point of invites then? if you do not keep track of them or limit them, what's the point? (think about an avalanche here)
To keep morons out, not to limit number of registered users.
That will not work. again the avalanche problem. How can you be sure that someone does not invite a moron? Heck you can not even be sure that your closest friend has the same rules to identify someone that _you_'ll call a moron.
f) unless you make the registering mandatory for _all_ servers, servers that require it, will not see many new players, except for the regulars. Where is the incentive for new players to register if there are so many servers that do not require this?
Registration is not required unless you're connecting from a banned subnet. Again, unless you ban 0.0.0.0 /0, 95 out of 100 users won't even notice there's some registration thingie on the server.
And again, the Bug from my point a) hits.
g) if you think about really baning people, do not use the build-in feature, it's faulty as hell and a determined griefer will get around it. Use iptables instead, or build an interface for it into the server-code, but beware of security issues.
Hello? Do you ever read or do you just drool on your keyboard? THAT'S WHAT I HAVE WRITTEN!
Hello? you actually let a user modify the iptables? Or do you mean to tell us that you let the tremulous server runs as root?
How sweet of you to tell us your scripts are useless

h) and please do not use built-in apps that go around the server-console to modify the confs, you will fall hard with that.
You didn't bother to download anything from that link, did you?
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Nope, as all third party apps that are not in the server-code are unusable. And there is just no way for me to let iptables fall under the rule of any user-script, just as there is no way I'd start any game as root.
Like I said "beware of the security issues" .)