MWRPFirewall

still a work in progress - dna.
Be a few days before I get it all up

Introduction

Netfilter? what's that?
iptables

The S45firewall script
What should the firewall do?
Example router port configurations and firewall scripts

OpenWRT default configuration and firewall

Port configuration
Firewall configuration

Private WAN interface, public LAN interface

Port configuration
Firewall configuration

Introduction

The OpenWRT Linux distribution used on the Linksys WRT54G(s) has a basic firewall as part of the distribution. This firewall is configured for typical home AP use. The firewall uses iptables to load rules into the netfilter part of the Linux kernel. As a firewall this is pretty neat, there is no running process, you can verify it is configured how you want and you can log what is going on.
This Wiki page makes a brief introduction to netfilter and iptables and details what a MW Node firewall should do. Examples are then given for six different configurations. These examples should be able to be used with only minor changes (such as the interfaces in your particular router).

A future evolution will reduce these configurations to a single script with a small number of input parameters (probably through environment variables).

Netfilter? what's that?

Well, there is a lot of stuff written about netfilter. Probably the best place to start is in the External link

netfilter FAQ page .

In a nutshell there is a packet switch/filter built into the linux kernel. As packets are received they pass through the netfilter and may be acted upon at various points.

                        / \
                       /   \
-> NIC - PREROUTING - routing ---- FOREWARD ---------- POSTROUTING - NIC ->
                       \   /                       |
                        \ /                        |
                         |                         |
                         |                         | 
                       INPUT --- applications --- OUTPUT

There are two tables of interest, the filter table where the INPUT, OUTPUT and FOREWARD chains are defined and the nat table where the PREROUTING and POSTROUTING chains live.

The routing decission determines the path a packet takes through netfilter. If IP forewarding is "on" then packets that are not addressed to the router itself are passed through to the FORWARD chain. Note, only packets for networks this router is configured for are passed through here.

Well, if you want to get technical this is what it really looks like: netfilter diagram

At each of the chains in the path rules can be defined that tell netfilter what to do with packets that match a rule. This could be as simple as ACCEPTing the packet or it could send it to a different chain for further processing.

iptables

iptables is a tool that is used to manipulate the filtering rules. It is very flexible and has lots of options. Check the man page for details.

In our firewall scripts we use iptables to clear the netfilter tables and create the rules we want to apply.

We use the following tables/chains:

filter / INPUT for packets inbound to our router
filter / OUTPUT for packets outbound from our router
filter / FOREWARD for packets we are forwarding from one segment to another
nat / PREROUTING for SNAT from our private segment to public segment
nat / POSTROUTING for DNAT for port forwarding into our private segment

The S45firewall script

The default firewall script ( /etc/init.d/S45firewall ) serves as the base for developing variations for the other described router configurations. There are a couple of bad configuration examples and it is a little more verbose than it needs to be ( for such a simple use ) but nevertheless it is a good starting point.

This script is used when the WAN port is connected to the Internet ( unsecure side) and allows the following:

ssh connection from WAN
Port forwarding (bad example conflicts with ssh from WAN)
forwarding to DMZ machine (again bad example)
INPUT rules to allow access to the router from the LAN and allow ICMP/GRE packets
OUTPUT rules to allow anything out from the router
FORWARD rules to allow LAN to LAN and LAN to WAN
FORWARD rules to support the specified port forwarding and DMZ configuration
PREROUTING rules for port forwarding and DMZ DNAT address conversion
POSTROUTING rules for LAN to WAN SNAT (well, MASQ actually) address conversion

annotated S45firewall script

#!/bin/sh
. /etc/functions.sh
WAN=$(nvram get wan_ifname)
LAN=$(nvram get lan_ifname)

clear the iptables and creates a new "user" chain for each table/chain combination

## CLEAR TABLES
for T in filter nat mangle; do
 iptables -t $T -F
 iptables -t $T -X
done

iptables -N input_rule
iptables -N output_rule
iptables -N forwarding_rule

iptables -t nat -N prerouting_rule
iptables -t nat -N postrouting_rule

Optional things are added to the "user" chains

## Allow SSH from WAN
# iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 22 -j ACCEPT
# iptables        -A input_rule      -i $WAN -p tcp --dport 22 -j ACCEPT

This example conflicts with the above one. The rule entered first will take precdence

## Port forwarding
# iptables -t nat -A prerouting_rule -i $WAN -p tcp --dport 22 -j DNAT --to 192.168.1.2
# iptables        -A forwarding_rule -i $WAN -p tcp --dport 22 -d 192.168.1.2 -j ACCEPT

This example sends incoming ports to the 192.168.1.2 machine, it is not a true DMZ

which should be on a separate network segment

## DMZ (should be placed after port forwarding / accept rules)
# iptables -t nat -A prerouting_rule -i $WAN -j DNAT --to 192.168.1.2
# iptables        -A forwarding_rule -i $WAN -d 192.168.1.2 -j ACCEPT

The default tables/chains have the general policy set along with actions to deal with junk

## INPUT
##  (connections with the router as destination)

 # base case
 iptables -P INPUT DROP
 iptables -A INPUT -m state --state INVALID -j DROP
 iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
 iptables -A INPUT -p tcp --syn --tcp-option \! 2 -j  DROP

we alow packets from the private segment and ICMP(ping) and GRE(router chatter) from anywhere

 # allow
 iptables -A INPUT -i \! $WAN  -j ACCEPT       # allow from lan/wifi interfaces
 iptables -A INPUT -p icmp     -j ACCEPT       # allow ICMP
 iptables -A INPUT -p gre      -j ACCEPT       # allow GRE

The input_rule chain has one target (above) to allow ssh form the WAN interface

 #
 # insert accept rule or to jump to new accept-check table here
 #
 iptables -A INPUT -j input_rule

 # reject (what to do with anything not allowed earlier)
 iptables -A INPUT -p tcp -j REJECT --reject-with tcp-reset
 iptables -A INPUT -j REJECT --reject-with icmp-port-unreachable

## OUTPUT
## (connections with the router as source)

 # base case
 iptables -P OUTPUT DROP
 iptables -A OUTPUT -m state --state INVALID -j DROP
 iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

 # allow
 iptables -A OUTPUT -j ACCEPT          #allow everything out

everything after the above line is unreachable in this chain

 #
 # insert accept rule or to jump to new accept-check table here
 #
 iptables -A OUTPUT -j output_rule

 # reject (what to do with anything not allowed earlier)
 iptables -A OUTPUT -p tcp -j REJECT --reject-with tcp-reset
 iptables -A OUTPUT -j REJECT --reject-with icmp-port-unreachable

## FORWARDING
## (connections routed through the router)

 # base case
 iptables -P FORWARD DROP
 iptables -A FORWARD -m state --state INVALID -j DROP
 iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
 iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

 # allow
 iptables -A FORWARD -i br0 -o br0 -j ACCEPT
 iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT

The forwarding_rule chain is where all the exciting things are happening

 #
 # insert accept rule or to jump to new accept-check table here
 #
 iptables -A FORWARD -j forwarding_rule

 # reject (what to do with anything not allowed earlier)
 # uses the default -P DROP

In this case postrouting SNAT is performed using the MASQ target

makes all packets from the private segment look like they come from the router

## MASQ
 iptables -t nat -A PREROUTING -j prerouting_rule
 iptables -t nat -A POSTROUTING -j postrouting_rule
 iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE

What should the firewall do?

The firewall has to do a couple of things to be usefull in a MW node:

Allow unrestricted access from your private segment into the MW segment.
NAT the addresses from your private segment to an address in your node segment.
Allow you to access your private segment from the MW network (only you, or your machines).
Allow administrative access (ssh) to the router from both the private and public segments.

In addition you may want to allow some services to be accessible:

DNS lookup
ssh to a server on the Internet
POP3 email from an email server
SMTP to an email server
HTTP to selected web sites
forward ports from the MW segment to a machine in either the private or DMZ segment

Note: By exposing these services clients connected to your Node will be "tunneling" through your private network to reach the Internet via your broadband connection. You may want to think about what you expose if you have a capped connection.

Example router port configurations and firewall scripts

OpenWRT default configuration and firewall

This configuration is useful for setting up a private wireless network but should not be used as part of a MW Node. Any machine connected to the LAN ports would be bridged with the Node and exposed to hacking.

       Public WAN         WRT PORT      Private LAN
                         +-------+
      -------------------|  WAN  |
                         +-------+
                 +-----  +-------+
                 |       | WLAN  |----------< Antenna (radio used as AP)
                 |       +-------+
                 |       +-------+
ports bridges    |       | LAN 1 |----------
  together       |       +-------+
                 |       +-------+
                 |       | LAN 2 |----------
                 |       +-------+
                 |       +-------+
                 |       | LAN 3 |----------
                 |       +-------+
                 |       +-------+
                 |       | LAN 4 |----------
                 +-----  +-------+

Port configuration

No changes required.

Firewall configuration

Use the S45firewall script as is out of the box.

Private WAN interface, public LAN interface

This is the simplest useful node configuration. It does not require any changes to the OpenWRT port assignment. In this configuration there is a single WAN port and a bridged LAN ( bridge the remaining 4 switch ports and the wirless port ).

       Private LAN        WRT PORT      Public LAN ( i.e. Melbourne wireless space )
                         +-------+
      -------------------|  WAN  |
                         +-------+
                 +-----  +-------+
                 |       | WLAN  |----------< Antenna (radio used as Node AP)
                 |       +-------+
                 |       +-------+
ports bridges    |       | LAN 1 |----------  to link radio
  together       |       +-------+
                 |       +-------+
                 |       | LAN 2 |----------  to node server
                 |       +-------+
                 |       +-------+
                 |       | LAN 3 |----------  Switch port 3
                 |       +-------+
                 |       +-------+
                 |       | LAN 4 |----------  Switch port 4
                 +-----  +-------+

In this configuration the router is using the WAN port to give you a connection from your private LAN space into the MW node. The firewall needs to be set up differently to the case where the LAN is the private side and the WAN is the public (Internet) side.

Port configuration

No changes in NVRAM required.

Firewall configuration

Version 2 (old) modified Mon, 26 Jul 2021 12:49:29 +0000 by dna0xff

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MWRPFirewall

Introduction

Netfilter? what's that?

iptables

The S45firewall script

What should the firewall do?

Example router port configurations and firewall scripts

OpenWRT default configuration and firewall

Port configuration

Firewall configuration

Private WAN interface, public LAN interface

Port configuration

Firewall configuration

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