Tzarium is not correct reguarding the nmap and networking issues. Nmap is 100% accurate; though you might need to use it properly and read its output carefully.
25567/tcp closed means that TCP port 25567 is closed, which is not the same as UDP port 25567. And MC server uses UDP for the "query" feature.
Consider these nmap command lines for reference:
## Probe UDP port 25565 -- default port for the "query" feature
% nmap -sU -pU:25565 minecraft-server
...
PORT STATE SERVICE
25565/udp open|filtered unknown
## Probe TCP ports 25565 and 25575 -- the usual Minecraft client port
## and default RCON port
% nmap -p25565,25575 minecraft-server
...
PORT STATE SERVICE
25565/tcp open unknown
25575/tcp open unknown
7 years later edit
Yea, indeed. "Using nmap properly" includes understanding that different network paths will behave differently in general, and especially likely so when you're debugging an issue. To see with nmap what a client sees, you need to run nmap on the client; or at the very least from its immediate network neighbor (same local subnet), so that the network path is mostly the same except for one host. Notice I wrote minecraft-server as nmap target, not localhost! — implicitly assuming this understanding. A very late thanks to @user56 commenting on this.
1. Login failure
The login is failing because you haven't bought Minecraft for those accounts. When you buy Minecraft it will be listed as one of your games in your account when you log in on Mojang.com, but until then you don't have a Minecraft login, you just have a login to a website that doesn't do anything except tell you that you haven't bought any games to use it with yet. You can still play in offline mode, since Mojang is not so obtuse as to require you be always-online in order to play Minecraft, unlike some other game publishers. However...
2. Server connection failure
When you connect to a server it will check if your account has a Minecraft registration too, and will drop the connection if not. So there are two times your login is checked for Minecraft access: once when you start the game, and once when you start multiplayer.
Fixing the server connection
The server's behaviour can be modified by editing server.properties and setting the line:
online-mode=true
to false instead. The players wanting to connect then pick a name to use consistently (since the server won't enforce identities to match what it has saved in its world files) and "try" to log into the game with that, let it fail, then play offline. Since the server is set to offline mode too, it will allow them to connect under that name.
The problem with this is that anyone can connect – make sure that the server isn't exposed to the internet, and the worst of the complications will be avoided.
The more likely problem though is that you'll have a bit of a headache migrating the kids' work once they have full accounts, since it's likely that the names they pick won't be available to choose once Minecraft is added to the Mojang account (which is when you can pick an official profile name for use in Minecraft). If there are name changes, then you'll have to either walk the child through the idea of losing their inventory and location on the server (blocks they placed will still be there), or you'll have to fiddle with NBT editors to transfer the inventory and location from the old name to the new one in the server's save files. Not hard, either one, but a stumbling block to be aware of.
The real solution here is to register a Mojang account for each child and purchase Minecraft for each account. I understand the desire to test it all out first, but if they already like Minecraft in single player, there aren't really any surprises that come with multiplayer that should change their or your mind. The advantage here is:
You'll immediately get the names that they'll be sticking with (consult and choose wisely, for children are fickle!) and can avoid identity issues in the server's save files.
You'll be able to set custom skins for their avatars, which children inordinately enjoy, in my experience. These are kinda nice in single player, but since they're readily visible in multiplayer they become more relevant when playing with others.
Fewer steps in logging in and connecting to multiplayer where mistakes could happen (a registered account can be remembered with it's password, making logins one-click), which for younger kids can be a bit of a headache-saver for you.
My kid has had an account for a year or so now and loves playing with other people, even if it's only to make random block stacks and randomly-enchanted gear in Creative mode. But then, that's 5-year-olds for you.
Best Answer
I'm assuming you're running minecraft in a screen session, since it's the most convenient way to not force you to keep an ssh connection always open.
You can send a command to screen that it forwards to the application running inside it. For example, I am running it in a screen session called minecraft, and every few hours a script runs, forces a save, toggles autosave off, does a backup of my worlds files, then turns autosave back on.
This is how you start a new screen session called minecraft:
then you get a shell, you may also pass the command you want to run automatically instead of getting a shell.
Then you can send commands to that session:
-p 0sends the message to window 0 in the session-S minecraftis the session name