Smtp port blocked, yet packets delivered in and out

[moimel]

Hey I'm using iptables to block smtp port like this:

[root@ ~]# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
DROP       tcp  --  anywhere             anywhere            tcp dpt:smtp

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
DROP       tcp  --  anywhere             anywhere            tcp dpt:smtp

with this commands:
iptables -A INPUT -p tcp --dport 25 --sport 25 -j DROP

yet when monitoring traffic:

[root@game3 ~]# tcpdump -n tcp port 25
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
22:23:32.339792 IP 121.35.170.181.can-ferret > MY-IP.smtp: S 3636350420:3636350420(0) win 65535 <mss 1440,nop,nop,sackOK>
22:23:32.638640 IP 121.34.200.183.slush > MY-IP.smtp: S 2084693075:2084693075(0) win 65535 <mss 1440,nop,nop,sackOK>
22:23:33.256984 IP 121.35.168.87.vpntpp > MY-IP.smtp: S 3096707673:3096707673(0) win 65535 <mss 1440,nop,nop,sackOK>
22:23:34.430466 IP 121.34.200.189.a16-an-an > MY-IP.smtp: S 2972377123:2972377123(0) win 65535 <mss 1440,nop,nop,sackOK>

4 packets captured
30 packets received by filter
0 packets dropped by kernel

I can see a lot of packets inside and outside in port 25.

However, when checking netstat I CAN'T see even one connection on smtp port!

How come?

p.s. I cannot access with telnet to port 25. (from my own pc)

Thanks

[Irithori]

The problem is here:

iptables -A INPUT -p tcp --dport 25 --sport 25 -j DROP

This rule will only block packets, that have *both* destination port 25 and source port 25

Break it up in two rules

iptables -A INPUT -p tcp --sport 25 -j DROP
iptables -A OUTPUT -p tcp --dport 25 -j DROP

[zombykillah]

if you wanna check the port you may wanna use this command:
nmap localhost -p 25
Beginner's Guide to Nmap | Linux.com

Or to find the program receiving at smtp:
netstat -anp | less

telnet usually runs at port 23 and not at 25.