如果我使用nmap扫描我的服务器,则表明已打开21端口.但是当我登录到这个服务器并运行netstat时,我什么也看不见.
$nmap -sT server
Starting Nmap 4.76 ( http://nmap.org ) at 2009-06-24 11:54 MSD
Interesting ports on server (x.x.x.x):
Not shown: 994 filtered ports
PORT STATE SERVICE
21/tcp open ftp
22/tcp open ssh
...
Nmap done: 1 IP address (1 host up) scanned in 7.97 seconds
$netstat --listen --tcp --numeric
怎么会这样？这可能是安全问题吗？
UPD：与nmap连接时输出tcpdump
13:13:02.982805 IP 10.19.10.2.51983 > server.ftp: S 767068541:767068541(0) win 5840
13:13:04.096705 IP 10.19.10.2.52000 > server.ftp: S 792080356:792080356(0) win 5840
13:13:04.131169 IP server.ftp > 10.19.10.2.52000: S 3312178661:3312178661(0) ack 792080357 win 32768
13:13:04.131346 IP 10.19.10.2.52000 > server.ftp: . ack 1 win 46
13:13:04.131737 IP 10.19.10.2.52000 > server.ftp: R 1:1(0) ack 1 win 46
