Copyright © 2004, 2005 Thomas M. Eastep
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
2005-08-27
Table of Contents
The purpose of a Virtual Private Network (VPN) is to provide for secure communication between a set of hosts. Communication between a pair of hosts connected by a VPN occurs in stages:
Local-host-to-local-gateway. This communication is not encrypted; in the case where the traffic originates on the gateway itself, the communication is local to that system.
Local-gateway-to-remote-gateway. This communication is encrypted and can use a tunneling protocol such as GRE, AH or ESP or a standard protocol such as UDP or TCP. Some VPNs use multiple protocols; for example PPTP uses TCP port 1723 and GRE while IPSEC uses UDP port 500 together with ESP or AH.
Remote-gateway-to-remote-host. This is just the unencrypted traffic described in the first item as it is delivered to its destination.
Of course, one-way communication generally isn't useful so we need traffic in the other direction as well.
Remote-host-to-remote-gateway.
Remote-gateway-to-local-gateway.
Local-gateway-to-local-host.
When Netfilter is configured on a VPN gateway, each VPN packet goes through Netfilter twice! Let's first consider outbound traffic:
Local-host-to-local-gateway. This traffic has a source address in the local network or on the gateway itself. The destination IP address is that of a remote host; either the remote gateway itself or a host behind that gateway.
Local-gateway-to-remote-gateway. This (encrypted) traffic has a source IP address on the gateway and is addressed to the remote gateway.
Incoming traffic is similar.
When Shorewall is installed on a VPN gateway system, it categorizes the VPN-related traffic slightly differently:
Local-host-to-remote-host — same as Local-host-to-local-gateway above.
Local-gateway-to-remote-gateway.
Remote-gateway-to-local-gateway.
Remote-host-to-local-host — same as Local-gateway-to-local-host above.
Shorewall implements a set of features for dealing with VPN.
The /etc/shorewall/tunnels file. This file
is used to define remote gateways and the type of encrypted traffic
that will be passed between the Shorewall system and those remote
gateways. In other words, the tunnels file deals with Local-gateway-to-remote-gateway and Remote-gateway-to-local-gateway traffic.
The /etc/shorewall/zones file. An entry in
this file allows you to associated a name with the set of hosts behind
the remote gateway (or to the remote gateway itself if it is a
standalone system).
The /etc/shorewall/interfaces and
/etc/shorewall/hosts files. These files are used
to associate a set of remote hosts with the zone name defined in
/etc/shorewall/zones.
The /etc/shorewall/policy and
/etc/shorewall/rules files. These files are used
to define the connections that are permitted between the remote and
local hosts -- in other words, the Local-host-to-remote-host and Remote-host-to-local-host traffic.
Most VPN types are implemented using a virtual network device such as pppN (e.g., ppp0), tunN (e.g., tun0), etc. This means that in most cases, remote zone definition is similar to zones that you have already defined.
/etc/shorewall/zones:
#ZONE DISPLAY COMMENT
net Internet The big bad net
loc Local Local LAN
rem Remote Remote LAN/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTION
net eth0 detect norft1918,routefilter
loc eth1 detect
rem tun0 192.168.10.0/24The /etc/shorewall/hosts file comes into play when:
You have a number of remote networks.
The remote networks have different firewall requirements and you want to divide them into multiple zones.
There is no fixed relationship between the remote networks and virtual network devices (for example, the VPN uses PPTP and remote gateways connect on demand).
In this case, your configuration takes the following approach:
etc/shorewall/zones:
#ZONE DISPLAY COMMENT
net Internet The big bad net
loc Local Local LAN
rem1 Remote1 Remote LAN 1
rem2 Remote2 Remote LAN 2/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTION
net eth0 detect norft1918,routefilter
loc eth1 detect
- tun+ -/etc/shorewall/hosts:
#ZONE HOST(S) OPTIONS
rem1 tun+:10.0.0.0/24
rem2 tun+:10.0.1.0/24The /etc/shorewall/hosts file is also used with
kernel 2.6 native IPSEC.
The /etc/shorewall/tunnels file provides no
functionality that could not be implemented using entries in
/etc/shorewall/rules and I have elimination of the
/etc/shorewall/tunnels file as a long-term goal. The
following sections show how entries in
/etc/shorewall/tunnels can be replaced by rules for
some common tunnel types.
/etc/shorewall/tunnels:
#TYPE ZONE GATEWAY GATEWAY ZONE ipsec Z1 1.2.3.4 Z2
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST SOURCE # PORT PORT(S) ACCEPT $FW Z1:1.2.3.4 udp 500 ACCEPT Z1:1.2.3.4 $FW udp 500 ACCEPT $FW Z1:1.2.3.4 50 ACCEPT Z1:1.2.3.4 $FW 50 ACCEPT $FW Z1:1.2.3.4 51 ACCEPT Z1:1.2.3.4 $FW 51 ACCEPT $FW Z2:1.2.3.4 udp 500 ACCEPT Z2:1.2.3.4 $FW udp 500
The "noah" option causes the rules for protocol 51 to be eliminated. The "ipsecnat" causes UDP port 4500 to be accepted in both directions. If no GATEWAY ZONE is given then the last two rules above are omitted.
/etc/shorewall/tunnels:
#TYPE ZONE GATEWAY GATEWAY ZONE pptpserver Z1 1.2.3.4
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST SOURCE # PORT PORT(S) ACCEPT Z1:1.2.3.4 $FW tcp 1723 ACCEPT $FW Z1:1.2.3.4 47 ACCEPT Z1:1.2.3.4 $FW 47
Tunnel type "pptpclient" simply reverses the direction of the tcp port 1723 rule.
/etc/shorewall/tunnels:
#TYPE ZONE GATEWAY GATEWAY ZONE openvpn:P Z1 1.2.3.4
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST SOURCE # PORT PORT(S) ACCEPT Z1:1.2.3.4 $FW udp P ACCEPT $FW Z1:1.2.3.4 udp P
/etc/shorewall/tunnels:
#TYPE ZONE GATEWAY GATEWAY ZONE openvpnclient:P Z1 1.2.3.4
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST SOURCE # PORT PORT(S) ACCEPT Z1:1.2.3.4 $FW udp - P ACCEPT $FW Z1:1.2.3.4 udp P
/etc/shorewall/tunnels:
#TYPE ZONE GATEWAY GATEWAY ZONE openvpnserver:P Z1 1.2.3.4
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DEST SOURCE # PORT PORT(S) ACCEPT Z1:1.2.3.4 $FW udp P ACCEPT $FW Z1:1.2.3.4 udp - P