Linux Iptables: How to block or open mail server / SMTP protocol

Posted on in Categories CentOS, Debian Linux, Iptables, Linux, Networking, RedHat/Fedora Linux, Security last updated July 22, 2005

SMTP is used to send mail. Sendmail, Qmail, Postfix, Exim etc all are used on Linux as mail server. Mail server uses the TCP port 25. Following two iptable rule allows incoming SMTP request on port 25 for server IP address 202.54.1.20 (open port 25):
iptables -A INPUT -p tcp -s 0/0 –sport 1024:65535 -d 202.54.1.20 –dport 25 -m state –state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -p tcp -s 202.54.1.20 –sport 25 -d 0/0 –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

In order to block port 25 simply use target REJECT instead of ACCEPT in above rules.

And following two iptables rules allows outgoing SMTP server request for server IP address 202.54.1.20:
iptables -A OUTPUT -p tcp -s 202.54.1.20 –sport 1024:65535 -d 0/0 –dport 25 -m state –state NEW,ESTABLISHED -j ACCEPT

iptables -A INPUT -p tcp -s 0/0 –sport 25 -d 202.54.1.20 –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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  1. Im running Debian etch with sendmail for sending email from web pages and normal account gui and this just DOES NOT WORK!!!!! what does work is:

    (yes it could be altered and improved but i spent ages trying different things and this is the first that worked)

    ###########################################################
    #ALLOW SMTP
    iptables -A INPUT -p tcp -m state –state NEW,ESTABLISHED –dport 25 -j ACCEPT
    iptables -A OUTPUT -p tcp -m state –state NEW,ESTABLISHED,RELATED –sport 25 -j ACCEPT
    iptables -A FORWARD -p tcp -m state –state ESTABLISHED,RELATED –sport 25 -j ACCEPT

    ##########################################################
    #ALLOW DNS

    iptables -A INPUT -p tcp –dport 53:50956 -j ACCEPT
    iptables -A OUTPUT -p tcp –sport 53:50956 -j ACCEPT
    iptables -A INPUT -p udp –dport 53:50956 -j ACCEPT
    iptables -A OUTPUT -p udp –sport 53:50956 -j ACCEPT

    YOU MUST ALLOW DNS NOT ONLY FOR UPDATES BUT TO GET THE CONTACT SERVER ADDRESS OF COURSE OR IT WONT KNOW WHERE TO SEND THE MAIL.

    most of the other stuff on this site will work on debian so stick with it.

  2. Hi,

    I have run those two rules for allowing SMTP port 25, but its still not working,
    I did nmap for the server, and it still says
    .
    25/tcp filtered smtp
    .

    Is there anything like i need to restart some network service to take affect.
    Excuse me for my childish question, i new to linux n network system.

  3. If it doesn’t work for you and your connection is hanging on outgoing mail then it probably means that your firewall is blocking DNS queries. This should always be allowed since many software (web server, mail server, etc.) perform DNS lookups to get IPs for domains.

    Note: whether you need to have DNS software installed is irrelative, you just need a DNS resolver to perform DNS queries to.

    Just add these two records:

    iptables -A INPUT -p udp --sport 53 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
    iptables -A INPUT -p tcp --sport 53 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

    DNS queries can both be either UDP or TCP based, depending on the size of the request, therefore both types must be allowed.

    Hope this helps

    1. Are you needing iptables – chains for only ftp and email?

      Note: for ftp you will need to load the ip_conntrack_ftp otherwise it will not work.

      in – /etc/sysconfig/iptables-config
      IPTABLES_MODULES=”ip_conntrack_ftp”

      port 21 you will need this rule:

      -A INPUT -s 192.168.1.0/24 -d 192.168.1.5/32 -i eth0 -p tcp -m tcp –sport 1024:65535 –dport 21 -m state –state NEW -j ACCEPT

      example above place below:

      -A INPUT -m state –state RELATED,ESTABLISHED -j ACCEPT

      then for port 25

      -A fw -s 192.168.0.1/24 -d 192.168.0.8/24 -i eth0 -p tcp -m state –state NEW -m tcp –dport 25 -j ACCEPT

      another example just plug in your ip range

      see if this help you out any

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