<geek mode on>
I changed the port for a number of reasons. 1. 21 is a well-known port and I did not want lots of traffic anymore.
its well known cos thats the port that ftp servers are supposed to use. refer: http://webopedia.internet.com/quick_ref/portnumbers.asp this link isnt comprehensive but its quick to find to answer common port questions :)
2. I am using 21 for something else.
if you're attracting traffic from too many anon users, its because ppl are scanning for open ftp servers. i dont know if you have allow anon logons, but if you do, disable them. if you're using something else on 21, it is probably still answering with enough info to make who ever is doing the scanning investigate it more fully later. to gain a tiny measure of security, go for high ports. dont use 31337. if people are having problems, it could be that application firewalls are preventing things. port 22 is normally reserved for ssh. i'll try and check over the weekend but i'm pretty certain the firewall i'm using will prevent ftp connections on 22, but will allow ssh connections on 22. i can turn it completely on/off pretty easily so i should be able to try both. nearly the weekend, brendan