Linux – Sendmail doesn’t work with iptables, even though smtp and dns are allowed

iptableslinuxsendmail

I have sendmail installed on Ubuntu 10.04 solely for the use of the php mail() function. This works fine unless iptables is running (I've been using sendmail myemailaddress@domain.com to test this).

I think that I have allowed SMTP and DNS (the script I am using to test iptables rules is below, in my version are the actual IPs of my hosts nameservers), but to no avail!

iptables --flush

iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Postgres
iptables -A INPUT -p tcp --dport 5432 -j ACCEPT

# Webmin
iptables -A INPUT -p tcp --dport 10000 -j ACCEPT

# Ping
iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
iptables -A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT

# sendmail
iptables -A INPUT -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT

# DNS
iptables -A INPUT -p udp --sport 53 -s <nameserver1> -j ACCEPT
iptables -A INPUT -p udp --sport 53 -s <nameserver2> -j ACCEPT
iptables -A INPUT -p tcp --sport 53 -s <nameserver1> -j ACCEPT
iptables -A INPUT -p tcp --sport 53 -s <nameserver2> -j ACCEPT

iptables -A OUTPUT -p udp --dport 53 -d <nameserver1> -j ACCEPT
iptables -A OUTPUT -p udp --dport 53 -d <nameserver2> -j ACCEPT
iptables -A OUTPUT -p tcp --dport 53 -d <nameserver1> -j ACCEPT
iptables -A OUTPUT -p tcp --dport 53 -d <nameserver2> -j ACCEPT


iptables -A INPUT -j DROP

# Add loopback
iptables -I INPUT 1 -i lo -j ACCEPT

Best Answer

Currently you have:

iptables -A INPUT -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT

Which allows:

  • packets to your sendmail in,
  • packets from your port 25 out, but only for already established connections (so only connections from the outside).

For outgoing E-mails, you need your sendmail to be able to connect to the outside world.

So you'll need something like this as well:

iptables -A OUTPUT -p tcp --dport 25 -j ACCEPT
iptables -A INPUT -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT

Please make sure you don't become an open relay.

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