I have several services running on my machine (web, pop, smtp) running on my server and I'm having a hell of a hard time with IPTABLES and Sendmail in particular.

I've been testing a simple IPTABLES configuration, and even though I open port 25 I can still not telnet to my server on port 25. All the other ports/services seem to work without any problems. I can remove the rule for port 80 and my web site stops working, I add it back in, and it works again.

This is my simple config

iptables -A INPUT -p tcp -s 0/0 -d 0/0 --destination-port 22 --syn -j ACCEPT
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --destination-port 25 --syn -j ACCEPT
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --destination-port 110 --syn -j ACCEPT
iiptables -A INPUT -p tcp -s 0/0 -d 0/0 --destination-port 80 --syn -j ACCEPT
iptables -A INPUT -p tcp -s 0/0 -d 0/0 --destination-port 443 --syn -j ACCEPT
iptables -A INPUT -s 0/0 -d 0/0 -p udp -j DROP
iptables -A INPUT -s 0/0 -d 0/0 -p tcp --syn -j DROP

So when I do this everything works except my SMTP on port 25, when I try to telnet to port 25 it just hangs. Then I do a IPTABLES -F and I can then telnet to port 25 again.

This doesn't make any sense. Is there some other port that I need to open for Sendmail SMTP? With IPTABLES flushed, I telnet to port 25 and do a NETSTAT at the console and all I see it my IP connected on SMTP.

I've tried several other iptable configs, and all of them always dropped my SSH connection and I had to reboot the server (this is a virtual host on the internet). This one has gotten me the closest to have a primative firewall, but only SMTP is not working???

Any ideas?

Thanks!

Brad

The best way to find out what other traffic is happening is to flush the firewall, start tcpdump in a second window, and connect. The output of tcpdump will show you what's happening.

My first guess would be ident (port 113) If that's the case then it can be turned off in the sendmail.conf file by setting Timeout.ident = 0.

Mike

That makes sense, because I just discovered that if I let that telnet sit for a about 90sec it will connect.

Yeah that ident setting was already in my SENDMAIL config, so that didn't help. I know I'm on the right track, but??

I did the flush and the TCPDUMP. The only thing I see happening immediately after I see my connection on port 25 the server seems to do a reverse DNS lookup on my ip address. It looks like somehow I'm blocking this DNS lookup from happening although I'm not blocking any outgoing packets with iptables. My statements are all incoming???

Thanks!

Brad

DNS would be something else that sendmail needs. Try putting your IP address and hostname in /etc/hosts.

Also adding this to your script would probably help....

iptables -A INPUT -i lo -j ACCEPT

I assume that you have a policy of ACCEPT on your OUTPUT table.

Mike

This VPS that I'm running doesn't have the stateful module installed for iptables. I think that is the main problem.

I went to a simpler config that seems to be working OK for me now.

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
iptables -N valid-tcp-flags
iptables -A INPUT -p tcp -j valid-tcp-flags
iptables -A OUTPUT -p tcp -j valid-tcp-flags
iptables -A FORWARD -p tcp -j valid-tcp-flags
iptables -N valid-source-address
iptables -A INPUT -p ! tcp -j valid-source-address
iptables -A INPUT -p tcp --syn -j valid-source-address
iptables -A FORWARD -p ! tcp -j valid-source-address
iptables -A FORWARD -p tcp --syn -j valid-source-address
iptables -N valid-destination-address
iptables -A OUTPUT -j valid-destination-address
iptables -A FORWARD -j valid-destination-address
iptables -A valid-tcp-flags -p tcp --tcp-flags ALL NONE -j DROP
iptables -A valid-tcp-flags -p tcp --tcp-flags ACK,FIN FIN -j DROP
iptables -A valid-tcp-flags -p tcp --tcp-flags ACK,PSH PSH -j DROP
iptables -A valid-tcp-flags -p tcp --tcp-flags ACK,URG URG -j DROP
iptables -A valid-tcp-flags -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
iptables -A valid-tcp-flags -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
iptables -A valid-tcp-flags -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
iptables -A valid-source-address -s 10.0.0.0/8 -j DROP
iptables -A valid-source-address -s 172.16.0.0/12 -j DROP
iptables -A valid-source-address -s 192.168.0.0/16 -j DROP
iptables -A valid-source-address -s 224.0.0.0/4 -j DROP
iptables -A valid-source-address -s 240.0.0.0/5 -j DROP
iptables -A valid-source-address -s 127.0.0.0/8 -j DROP
iptables -A valid-source-address -s 0.0.0.0/8 -j DROP
iptables -A valid-source-address -d 255.255.255.255 -j DROP
iptables -A valid-source-address -s 169.254.0.0/16 -j DROP
iptables -A valid-source-address -s 192.0.2.0/24 -j DROP
iptables -A OUTPUT -p udp --dport 53 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p udp --sport 53 --dport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 110 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 995 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 --sport 1024:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 8443 --sport 1024:65535 -j DROP
iptables -A INPUT -p tcp --dport 3306 --sport 1024:65535 -j DROP
iptables -A INPUT -p tcp --dport 1:1023 -j DROP

Thanks!

Brad