Simple iptables syntax

Just a couple of examples of simple iptables rules for fun and profit.

This was tested on RHEL 6.

See the chapter on iptables in the Red Hat Enterprise Linux (RHEL) Security Guide here. If using a different version of RHEL or Fedora, be sure to check any doc specific to the distribution and version.

The iptables utility is used to manage the packet filter rules for the Linux kernel. By default RHEL is only configured to allow access to port 22 (ssh). Everything else is blocked.

Note that the default, at least on RHEL, is that anything not explicitly permitted is excluded.

To restrict access to port 636 to all but two specific IP addresses (say, an application server and your workstation):

iptables -I INPUT -p tcp -m tcp -s 192.168.4.20 --dport 636 -j ACCEPT
iptables -I INPUT -p tcp -m tcp -s 10.2.11.67 --dport 636 -j ACCEPT

The above assumes you don’t have any “DROP” or “REJECT” rules otherwise allowing or rejecting access to port 636. Note that adding the subnet mask for an address (e.g. 192.168.4.0/16), will be interpreted as a direction to allow access to a subnet.

A note on syntax: when issuing an iptables command, take “-I” to mean “Insert at top”, while “-A” means “Append”. Because most tables will have their exclusionary rules at the bottom you’ll want to use the “-I” switch when creating new exceptions. Some manual adjustment may be needed later on for clarity or to tune firewall behavior.

To save the configuration so it survives the next reboot:

iptables-save >/etc/sysconfig/iptables

The actual file would look something like this:

# Generated by iptables-save v1.4.7 on Mon Feb 25 15:17:41 2013
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [38:11824]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A INPUT -p icmp -j ACCEPT 
-A INPUT -i lo -j ACCEPT
-A INPUT -s 192.168.4.20 -p tcp -m tcp --dport 636 -j ACCEPT 
-A INPUT -s 10.2.11.67 -p tcp -m tcp --dport 636 -j ACCEPT
-A INPUT -s 172.16.0.0/16 -p tcp -m tcp --dport 636 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT 
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT 
-A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT 
-A INPUT -j REJECT --reject-with icmp-host-prohibited 
-A FORWARD -j REJECT --reject-with icmp-host-prohibited 
COMMIT
# Completed on Mon Feb 25 15:17:41 2013

In the table above those lines beginning with “-A” are “Added” to the table at their location (recall that the command used to created them employed the “-I”, or “Insert”, switch). I did manually edit this table a bit for clarity.

That third “-s” directive above allows access to any host on the 172.16.0.0/16 network.

Note that the iptables-save command can be run without piping to a file to see what would be written.

The above is tidied up a bit from the raw output of iptables-save. In particular I moved the ACCEPT statements for those two specific hosts to put them just after the ACCEPT for the lo interface.

To effect changes made directly to the file simply bounce the iptables service. On RHEL 6:

service iptables restart

Notice that new rules are added to the top of the file by default. Remember this when you’re creating DROP or REJECT rules. If you want to avoid trouble move those to the bottom of the rule list so your ACCEPT rules will take precedence (you could also add your blocking rules first, if you plan that far ahead — my own preference is to edit the tables file directly and then apply). For example, take a look at this:

-A INPUT -s 192.168.4.20 -p tcp -m tcp --dport 636 -j ACCEPT 
-A INPUT -s 10.2.11.67 -p tcp -m tcp --dport 636 -j ACCEPT
-A INPUT -s 172.16.0.0/16 -p tcp -m tcp --dport 636 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 636 -j DROP 

Everything will work fine so long as you remember to allow first, deny after.

Further Reading
Take a look at these two tutorials, one from the CentOS Wiki and the other from Nixcraft:

http://wiki.centos.org/HowTos/Network/IPTables

http://www.cyberciti.biz/faq/rhel-fedorta-linux-iptables-firewall-configuration-tutorial/

Here’s more:

25 Most Frequently Used Linux IPTables Rules Examples

Linux: 20 IPTables Examples for New SysAdmins

iptables: Small manual and tutorial with some examples and tips

Access Control with Netfilter/iptables

Linux Firewalls Using iptables

iptables debugging

Google around a bit and you’ll find even more.

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About phil

My name is Phil Lembo. In my day job I’m an enterprise IT architect for a leading distribution and services company. The rest of my time I try to maintain a semi-normal family life in the suburbs of Raleigh, NC. E-mail me at philipATlembobrothersDOTcom. The opinions expressed here are entirely my own and not those of my employers, past, present or future (except where I quote others, who will need to accept responsibility for their own rants).