HOWTO Setup iptables
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 2.6.25 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
["enable"] Amanda backup protocol support
["enable"] Netfilter Xtables support (required for ip_tables)
["enable"] "limit" match support really only needed if you want to rate-limit some connection, like ICMP-echo
["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
hostname ~ # emerge iptables hostname ~ # 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 ...
Creating iptables Rulesets
It's a great starting-point for ruleset development to see what ports are currently in-use, and consider that these may need to be opened through our firewall. To see what ports are in-use, and by what programs/services:
hostname ~ # netstat -alnp
On a complex server, there can be many unfamiliar ports open, and some may vary with each invocation of the associated program :-( Google around, and first see if you can identify all the services. Secondly, see if you can nail down these services to always use a known/specific port.
In a worst-case scenario, you may decide it's too scary to actually begin blocking ports and breaking services to users - this can work OK with fail2ban! You will gain some protection against dictionary-attacks, which is better than you had a moment ago (see Example 1).
Example 1 - very-basic /etc/iptables.bak to prevent breaking extensive/weird/complex services
Scripting the Rules
Once iptables is up-and-running, simply execute one of the scripts below, to implement the policies, Or, use these scripts as a basis for your own similar-but-custom rulesets, which Google can help you with. Our standard is to save the ruleset-script as /etc/iptables.bak. So, once iptables is running (in very-basic-form), invoking the ruleset-script is very simple:
hostname ~ # sh /etc/iptables.bak
Save the configuration:
hostname ~ # etc/init.d/iptables save
And then back up your working configuration in case you break something later you can quickly revert:
hostname ~ # cp /var/lib/iptables/rules-save /var/lib/iptables/rules.working
Viewing/checking the active ruleset:
hostname ~ # iptables -L
Manually Working With Rules
You may wish to manually block some bad guy who is trying to access your SSH service (in this example). You can add lines like these to /etc/iptables.bak (of course, substitute the bad-guy's IP-address):
# Manually added blocking-rules, often from watching log-output:
# =============================================================
#awk '($(NF-7) = /invalid user/){print $(NF-3)}' /var/log/messages | sort | uniq -c | sort
iptables -I INPUT -p tcp -s 83.103.96.33 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 41.206.41.90 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 87.139.25.251 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 200.171.229.109 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 217.91.80.206 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 61.74.75.56 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 80.169.105.159 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 62.96.29.34 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 190.146.246.36 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 195.202.52.155 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 80.153.59.28 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 82.106.226.77 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 194.51.12.238 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 61.74.75.43 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 113.105.82.13 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 211.115.234.143 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 92.79.128.167 --dport ssh -j REJECT --reject-with tcp-reset
iptables -I INPUT -p tcp -s 58.247.222.163 --dport ssh -j REJECT --reject-with tcp-reset