[CentOS] Strange TCP ports phenomena

[Scot L. Harris]

On Wed, 2005-08-17 at 14:31, Dominik Składanowski wrote:
> Hello list.
> 
> I have new server on CentOS 4.1 - fresh installation. During security
> tests I've noticed:
> 
> When I scan server ports (nmap) from the outside there is 21 tcp port
> open. But when I check on the server (netstat -tan or lsof -i) there is
> no any open 21 tcp port.
> 
> Any ideas? To be honest I'm confused.
> 
> Regards
> 
> P.S.: of course I don't have started FTP service. Even I don't have
> installed FTP server.

Do you have a router/firewall in front of your server?  If you are using
something like http://www.grc.com to scan from the Internet you are
probably getting a response from the router/firewall in front of your
server not from the server itself.