I've just installed pureftpd and pureadmin. I'm behind a dlink firewall/router and it gives my computer a private ip. I want a few select people to connect to this box through the firewall, and I've opened port 21 which pureftpd uses. What ip can they connect on? I know the router has an internet ip, should I tell them them to try to connect to that ip or what?

Yes, you'll want to point them at your internet IP address, not the computer's local address. That is correct. If you've opened up port 21 on the router, and made it forward to the computer running pureftpd, then as far as the router is concerned, everything should be all set. It can be tried at this point, and will probably have success, so long as you have no additional firewalling happening on the ftp server machine.

My brother is trying to get into my ftpserver, but he can't. I've opened port 21 in the firewall and pointed all connections to that port to the machine the server is on, 192.168.0.102, but it doesn't work.

I told him to connect to the ip 213.214.xxx.xxx:21 (which is supposed to be my itnernet ip, blurted out the last six digits here on lq, don't want skiddies to know my ip ^_^) and I've given him a user account with password. I can see that he is trying to get in, but the server kicks him out every time he tries.
How do I check the internet ip of the router in the shell btw?

Masking is just fine, and encouraged, I'd say...no worries there.

Checking the internet IP of the router...I think for a D-Link, I would just go to the web interface it has, and go check in there (I think it's displayed where you would normally release/renew).

As far as the server rejecting him... I guess I would make sure that the ftp server is actually running on 21 (is 21 the default FTP or is 23? I forget...). If you have an ssh service running as well, you might try to sftp to your machine also. I don't know why yet (haven't had time to look), but on my own machine, using wu-ftpd at the moment, everything runs peachy if you connect via sftp (ssh is port 22, by the way), but it just lags like no tomorrow if I try just a standard ftp connection. Lucky for me, I don't allow a standard, and only the sftp, so it's not high priority either...

If sftp works, its probably going to be a configuration issue with the ftp server. If it doesn't, then we're kinda back at square one. By the time you reply with results, I should hopefully think of something else...a little distracted at the moment.

Hope it helps though.

Nah, can't be the port, I've specically started it on port 21, besides, it's the default
I simply can't figure out what the problem is. Can it have something to do with him being on wind0z3?

Let's go through this from the beginning...

I installed pureftpd from source with the options --with-ftpwho --with-puredb, just like the guys who made pureadmin said I should in their readme. Then I made a standrard installation of pureadmin.
Then I opened port 21 and routed the connections to it to 192.168.0.102.
Then I added login/passwd for my brother and told him to connect to xxx.xxx.xxx.xxx:21, which is the internet ip. I could see that he tried to connect in pureadmin, but he got thrown out.
Can it have anything to do with filepermissions?

I've added the groups and users the readme for pureadmin said I should:

groupadd ftpgroup
useradd -g ftpgroup -d /dev/null -s /etc ftpuser

I'm trying to share the files in /export with him, and it's owned by the group users.
Could it be that? I feel like a n00b...

I think now that it is pureadmin who's making a mess out of things... I've doodled around a bit and I've realized that the only thing wrong is the password not being accepted.

You need to check your cable or DSL modem for any firewall functions. Most of the ones Ive used not only have a dhcp server but also have firewall functions and that can complicate things. For this reason, it is my opinion that people shouldnt use routers on top of their modem but get switches instead. But in your case just make sure the ports is forwarded to your server. BTW I needed port 21(FTP control) and port 20(FTP data) for my ftp to work. Here's a link explaining: http://slacksite.com/other/ftp.html