HOWTO Setup iptables

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Kernel Configuration

NOTE This configuration is for basic firewalling only; we don't do NAT/packet-forwarding... so, if you're reading this, and wish to use NAT/forwarding, you will be missing a few key configuration items :-O 

   Typical kernel 2.6.22 and higher kernel options for our very-basic firewalling:

Networking  ---->
 Networking options  ---->
  Network packet filtering framework (Netfilter)--->
   Core Netfilter Configuration ---->
    ["enable"] Netfilter connection tracking support--->Layer 3 Independent Connection tracking
    ["enable"] Netfilter Xtables support (required for ip_tables)
    ["enable"] "state" match support 
   IP: Netfilter Configuration --->
    ["enable"] IPv4 connection tracking support (required for NAT) required by "Layer 3 Independent Connection tracking" above (caused many headaches)
    ["enable"] IP tables support (required for filtering/masq/NAT)
    ["enable"]   Packet Filtering
    ["enable"]     REJECT target support

Iptables Installation

emerge iptables
rc-update add iptables default

Usually, when you try to start a new installation of iptables, you get an error, sometimes like this: 

 hostname ~ # /etc/init.d/iptables start
* Not starting iptables.  First create some rules then run:
* /etc/init.d/iptables save

Or, you may see this kind of error: 

FATAL: Module ip_tables not found.
iptables v1.3.8: can't initialize iptables table `filter': iptables who? (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.

Occasionally, there are complaints about "mangle"... it may be that there is some previous ruleset which included NAT/forwarding. For our application, we do not need mangling-capability!
In any of the above startup cases, we will manually start iptables (that is, not using the init-script), and give it a very-simple command-line rule, just to get iptables going: 

hostname ~ # /sbin/iptables -A INPUT -i lo -j ACCEPT

This usually (should!) result in very simple, default "all-pass" ruleset, which doesn't actually do anything except keep the init-scripts happy: 

hostname ~ # iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

We then use the init-script to save our ruleset, which will at least then allow iptables to start properly the next time with the init-scripts: 

hostname ~ # /etc/init.d/iptables save
* Saving iptables state ...

Scripting the Rules

Once iptables is up-and-running, simply execute the script below, to implement the policies: 

sh /etc/iptables.bak

Example 1 - /etc/iptables.bak for a web-server with vsftpd upload, also SSH connectivity, and being monitored by nagios: 

#! /bin/sh
# /etc/iptables.bak

# Let's save typing & confusion with variables
IPTABLES=/sbin/iptables

# Flush active rules and custom tables
$IPTABLES --flush
$IPTABLES --delete-chain

# set the defaults so that by-default incoming packets are dropped, unless explicitly allowed;
# for a desktop workstation, we'll let lots of (unpredictable) outgoing packets go freely.
$IPTABLES -P INPUT DROP
$IPTABLES -P FORWARD DROP
$IPTABLES -P OUTPUT ACCEPT

# INBOUND POLICY
# ==============
# of course, accepting loopback is a good idea
$IPTABLES -A INPUT -i lo -j ACCEPT 

#   (Applies to packets entering our network interface from the network, 
#   and addressed to this host.)

$IPTABLES -A INPUT -m state --state INVALID -j DROP 
$IPTABLES -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 

# ftp incoming
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 20 -j ACCEPT
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 21 -j ACCEPT

# ssh incoming, including non-standard port (if needed)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT 
#$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 222 -j ACCEPT

# web serving, let's allow it!
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 80 -j ACCEPT

# nagios (5666); monitor time (123), allow snmp (161)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 5666 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 123 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 161 -j ACCEPT


# OUTBOUND POLICY
# ===============
# of course, accepting loopback is a good idea
$IPTABLES -A OUTPUT -o lo -j ACCEPT

#   (Applies to packets sent to the network interface from local processes)

$IPTABLES -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Example 2 - /etc/iptables.bak for a web-server with both http and https, including an nfs-mounted directory (this machine client-only). Also, our amanda-tape-server reaches out to back up this example, and we have SSH connectivity. To complicate matters, we run a Sassafrass keyserver, and a flexlm license-server. Again, monitoring by nagios: 

#! /bin/sh
# /etc/iptables.bak

# Let's save typing & confusion with variables
IPTABLES=/sbin/iptables

# Flush active rules and custom tables
$IPTABLES --flush
$IPTABLES --delete-chain

# set the defaults so that by-default incoming packets are dropped, unless explicitly allowed;
# for a desktop workstation, we'll let lots of (unpredictable) outgoing packets go freely.
$IPTABLES -P INPUT DROP
$IPTABLES -P FORWARD DROP
$IPTABLES -P OUTPUT ACCEPT

# INBOUND POLICY
# ==============
# of course, accepting loopback is a good idea
$IPTABLES -A INPUT -i lo -j ACCEPT 

#   (Applies to packets entering our network interface from the network, 
#   and addressed to this host.)

$IPTABLES -A INPUT -m state --state INVALID -j DROP 
$IPTABLES -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 

# ftp incoming
#$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 20 -j ACCEPT
#$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 21 -j ACCEPT

# ssh incoming, including non-standard port (if needed)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT 
#$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 222 -j ACCEPT

# this machine is a mail-server, aggregating logs + hosting mailing-lists
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 25 -j ACCEPT

# web serving, let's allow it!  Both http and https ports
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 80 -j ACCEPT
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 443 -j ACCEPT

# portmapper, in support of NFS-client
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 111 -j ACCEPT

# nagios (5666); monitor time (123), allow snmp (161)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 5666 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 123 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 161 -j ACCEPT

# amanda tape-backups; we reach out and tape things from this machine
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 10080 -j ACCEPT
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 10082 -j ACCEPT
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 10083 -j ACCEPT

# flexlm (lmgrd) license-server listens here (set in license.dat file)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 7111 -j ACCEPT

# Sassafrass keyserver listens here on both udp and tcp
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 19283 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 19283 -j ACCEPT


# OUTBOUND POLICY
# ===============
# of course, accepting loopback is a good idea
$IPTABLES -A OUTPUT -o lo -j ACCEPT

#   (Applies to packets sent to the network interface from local processes)

$IPTABLES -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Save the configuration: 

etc/init.d/iptables save

And then back up your working configuration in case you break something later you can quickly revert: 

cp /var/lib/iptables/rules-save /var/lib/iptables/rules.working

Viewing/checking the active ruleset: 

iptables -L
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