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Inclusive Rule set Example

The following rule set is an example of how to code a very secure inclusive type of firewall. An inclusive firewall only allows services matching pass rules through and blocks all others by default. All firewalls have at the minimum two interfaces which have to have rules to allow the firewall to function.

All Unix flavored systems including FBSD are designed to use interface lo0 and IP address 127.0.0.1 for internal communication within the FBSD operating system. The firewall rules must contain rules to allow free, unmolested movement of these special internally used packets.

The interface which faces the public Internet is the one which you code your rules to authorize and control access out to the public Internet and access requests arriving from the public Internet. This can be your ‘user ppp’ tun0 interface or your NIC that is cabled to your DSL or cable modem.

In cases where one or more NIC's are cabled to private LANs (local area networks) behind the firewall, those interfaces must have a rule coded to allow free, unmolested movement of packets originating from those LAN interfaces.

The rules should be first organized into three major sections; all the free and unmolested interfaces, public interface outbound, and the public interface inbound.

The order of the rules in each of the public interface sections should be in order of the most used rules being placed before less often used rules with the last rule in the section being a block log all packets on that interface and direction.

The outbound section in the following rule set only contains ‘pass’ rules which contain selection values that uniquely identify the service that is authorized for public Internet access. All the rules have the ‘quick’, ‘on’, proto, port, and keep state option coded. The ‘proto tcp’ rules have the ‘flag’ option included to identify the session start request as the triggering packet to activate the stateful facility.

The inbound section has all the blocking of undesirable packets first for two different reasons. First, these things being blocked may be part of an otherwise valid packet which may be allowed in by the later authorized service rules. Second, by having a rule that explicitly blocks selected packets that I receive on an infrequent bases and don’t want to see in the log, this keeps them from being caught by the last rule in the section which blocks and logs all packets which have fallen through the rules. The last rule in the section which blocks and logs all packets is how you create the legal evidence needed to prosecute the people who are attacking your system.

Another thing you should take note of is there is no response returned for any of the undesirable stuff; their packets just get dropped and vanish. This way the attackers have no knowledge if their packets have reached your system. The less the attackers can learn about your system the more secure it is. For the inbound 'nmap OS fingerprint' attempts rule I log the first occurrence, because this is something an attacker would do.

Any time you see log messages on a rule with ‘log first’ you should do an ipfstat –h command to see the number of times the rule has been matched so you know if you are being flooded, (IE: under attack).

The following rule set is a complete, very secure ‘inclusive’ type of firewall rule set that I have used on my system. You cannot go wrong using this rule set for your own. Just comment out any pass rules for services to don’t want to authorize.

If you see messages in your log that you want to stop seeing just add a block rule in the inbound section.

You have to change the ‘dc0’ interface name in every rule to the interface name of the NIC that connects your system to the public Internet. For ‘user ppp’ it would be ‘tun0’.

Add the following statements to /etc/ipf.rules

#################################################################
# No restrictions on Inside LAN Interface for private network
# Not needed unless you have LAN
#################################################################

#pass out quick on xl0 all
#pass in quick on xl0 all

#################################################################
# No restrictions on Loopback Interface
#################################################################
pass in quick on lo0 all
pass out quick on lo0 all

#################################################################
# Interface facing Public Internet (Outbound Section)
# Interrogate session start requests originating from behind the
# firewall on the private network
# or from this gateway server destined for the public Internet.
#################################################################

# Allow out access to my ISP's Domain name server.
# xxx must be the IP address of your ISP’s DNS.
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
pass out quick on dc0 proto tcp from any to xxx port = 53 flags S keep state
pass out quick on dc0 proto udp from any to xxx port = 53 keep state

# Allow out access to my ISP's DHCP server for cable or DSL networks.
# This rule is not needed for ‘user ppp’ type connection to the
# public Internet, so you can delete this whole group.
# Use the following rule and check log for IP address.
# Then put IP address in commented out rule & delete first rule
pass out log quick on dc0 proto udp from any to any port = 67 keep state
#pass out quick on dc0 proto udp from any to z.z.z.z port = 67 keep state

# Allow out non-secure standard www function
pass out quick on dc0 proto tcp from any to any port = 80 flags S keep state

# Allow out secure www function https over TLS SSL
pass out quick on dc0 proto tcp from any to any port = 443 flags S keep state

# Allow out send & get email function
pass out quick on dc0 proto tcp from any to any port = 110 flags S keep state
pass out quick on dc0 proto tcp from any to any port = 25 flags S keep state

# Allow out Time
pass out quick on dc0 proto tcp from any to any port = 37 flags S keep state

# Allow out nntp news
pass out quick on dc0 proto tcp from any to any port = 119 flags S keep state

# Allow out gateway & LAN users non-secure FTP ( both passive & active modes)
# This function uses the IPNAT built in FTP proxy function coded in
# the NAT rules file to make this single rule function correctly.
# If you want to use the pkg_add command to install application packages
# on your gateway system you need this rule.
pass out quick on dc0 proto tcp from any to any port = 21 flags S keep state

# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
pass out quick on dc0 proto tcp from any to any port = 22 flags S keep state

# Allow out non-secure Telnet
pass out quick on dc0 proto tcp from any to any port = 23 flags S keep state

# Allow out FBSD svn function
pass out quick on dc0 proto tcp from any to any port = 3690 flags S keep state

# Allow out ping to public Internet
pass out quick on dc0 proto icmp from any to any icmp-type 8 keep state

# Allow out whois for LAN PC to public Internet
pass out quick on dc0 proto tcp from any to any port = 43 flags S keep state

# Block and log only the first occurrence of everything
# else that’s trying to get out.
# This rule enforces the block all by default logic.
block out log first quick on dc0 all

#################################################################
# Interface facing Public Internet (Inbound Section)
# Interrogate packets originating from the public Internet
# destined for this gateway server or the private network.
#################################################################

# Block all inbound traffic from non-routable or reserved address spaces
block in quick on dc0 from 192.168.0.0/16 to any    #RFC 1918 private IP
block in quick on dc0 from 172.16.0.0/12 to any     #RFC 1918 private IP
block in quick on dc0 from 10.0.0.0/8 to any        #RFC 1918 private IP
block in quick on dc0 from 127.0.0.0/8 to any       #loopback
block in quick on dc0 from 0.0.0.0/8 to any         #loopback
block in quick on dc0 from 169.254.0.0/16 to any    #DHCP auto-config
block in quick on dc0 from 192.0.2.0/24 to any      #reserved for doc's
block in quick on dc0 from 204.152.64.0/23 to any   #Sun cluster interconnect
block in quick on dc0 from 224.0.0.0/3 to any       #Class D & E multicast

##### Block a bunch of different nasty things. ############
# That I don’t want to see in the log

# Block frags
block in quick on dc0 all with frags

# Block short tcp packets
block in quick on dc0 proto tcp all with short

# block source routed packets
block in quick on dc0 all with opt lsrr
block in quick on dc0 all with opt ssrr

# Block nmap OS fingerprint attempts
# Log first occurrence of these so I can get their IP address
block in log first quick on dc0 proto tcp from any to any flags FUP

# Block anything with special options
block in quick on dc0 all with ipopts

# Block public pings
block in quick on dc0 proto icmp all icmp-type 8

# Block ident
block in quick on dc0 proto tcp from any to any port = 113

# Block all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
block in log first quick on dc0 proto tcp/udp from any to any port = 137
block in log first quick on dc0 proto tcp/udp from any to any port = 138
block in log first quick on dc0 proto tcp/udp from any to any port = 139
block in log first quick on dc0 proto tcp/udp from any to any port = 81

# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP’s DHCP server as it’s the only
# authorized source to send this packet type. Only necessary for
# cable or DSL configurations. This rule is not needed for
# ‘user ppp’ type connection to the public Internet.
# This is the same IP address you captured and
# used in the outbound section.
pass in quick on dc0 proto udp from z.z.z.z to any port = 68 keep state

# Allow in standard www function because I have apache server
pass in quick on dc0 proto tcp from any to any port = 80 flags S keep state

# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID/PW passed over public Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
#pass in quick on dc0 proto tcp from any to any port = 23 flags S keep state

# Allow in secure FTP, Telnet, and SCP from public Internet
# This function is using SSH (secure shell)
pass in quick on dc0 proto tcp from any to any port = 22 flags S keep state

# Block and log only first occurrence of all remaining traffic
# coming into the firewall. The logging of only the first
# occurrence stops a ‘denial of service’ attack targeted
# at filling up your log file space.
# This rule enforces the block all by default logic.
block in log first quick on dc0 all
################### End of rules file #####################################

 

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