Let's modify rc.conf and enable IPFW.
# ee /etc/rc.conf
Add this to rc.conf:
firewall_enable="YES"
firewall_type="simple"
firewall_script="/etc/ipfw.rules"
firewall_logging="YES"
Additional changes may be required with sysctl (in regards to maximum firewall rules allowed). If you have a very busy server, you will get the following error message:
ipfw: install_state: Too many dynamic rules
The follow lines in /etc/sysctl.conf are highly recommended
# IPFW enhancements:
net.inet.ip.fw.verbose=1
net.inet.ip.fw.verbose_limit=5
net.inet.ip.fw.dyn_max=65536
net.inet.ip.fw.dyn_keepalive=1
# *NEW* Recycle finwait2 connections faster:
net.inet.tcp.fast_finwait2_recycle=1
# *NEW* Faster finwait2 timeouts:
net.inet.tcp.finwait2_timeout=15000
Add some basic rules to your firewall.
Time to modify /etc/ipfw.rules:
# ee /etc/ipfw.rules
Paste the following in /etc/ipfw.rules:
#!/bin/sh
#################################################
# ipfw Firewall Commands
#################################################
cmd="ipfw -q add"
ipfw -q -f flush
pif="em0"
#################################################
# Allow Loopback and Deny Loopback Spoofing
#################################################
$cmd allow all from any to any via lo0
$cmd deny all from any to 127.0.0.0/8
$cmd deny all from 127.0.0.0/8 to any
$cmd deny tcp from any to any frag
#################################################
# Stateful rules
#################################################
$cmd check-state
$cmd deny tcp from any to any established
$cmd allow all from any to any out keep-state
$cmd allow icmp from any to any
#################################################
# Table 10 for IP blocks
#################################################
ipfw -q table 10 add 127.0.0.2
ipfw -q add 900 deny ip from 'table(10)' to any
#################################################
# Incoming/Outgoing Services
#################################################
$cmd 60001 allow tcp from any to any 21 setup limit src-addr 10
$cmd 60002 allow tcp from any to any 22 setup limit src-addr 8
$cmd 60003 allow tcp from any to any 25 setup limit src-addr 10
$cmd 60004 allow tcp from any to any 587 setup limit src-addr 20
$cmd 60005 allow tcp from any to any 53 setup limit src-addr 3
$cmd 60006 allow udp from any to any 53 limit src-addr 3
$cmd 60007 allow tcp from any to any 80 setup limit src-addr 20
$cmd 60008 allow tcp from any to any 110 setup limit src-addr 20
$cmd 60009 allow tcp from any to any 143 setup limit src-addr 10
$cmd 60010 allow tcp from any to any 443 setup limit src-addr 10
$cmd 60011 allow tcp from any to any 2222 setup limit src-addr 12
$cmd 60012 allow tcp from any to any 35000-35999 in setup limit src-addr 10
$cmd 60013 allow tcp from any to any 993 setup limit src-addr 10
$cmd 60014 allow tcp from any to any 995 setup limit src-addr 10
$cmd 60015 allow tcp from any to any 465 setup limit src-addr 10
$cmd 60016 allow tcp from any to any 585 setup limit src-addr 10
#################################################
# Deny Port scanning (Nmap)
#################################################
$cmd 00600 deny log logamount 50 ip from any to any ipoptions rr
$cmd 00610 deny log logamount 50 ip from any to any ipoptions ts
$cmd 00620 deny log logamount 50 ip from any to any ipoptions lsrr
$cmd 00630 deny log logamount 50 ip from any to any ipoptions ssrr
$cmd 00640 deny log logamount 50 tcp from any to any tcpflags syn,fin
$cmd 00650 deny log logamount 50 tcp from any to any tcpflags syn,rst
#################################################
# Deny and Log
#################################################
$cmd deny log all from any to any
Reboot your system to enable the firewall.