VPN Passthrough

Tom Eastep

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Virtual Private Networking (VPN)

It is often the case that a system behind the firewall needs to be able to access a remote network through Virtual Private Networking (VPN). The two most common means for doing this are IPsec and PPTP. The basic setup is shown in the following diagram:

A system with an RFC 1918 address needs to access a remote network through a remote gateway. For this example, we will assume that the local system has IP address and that the remote gateway has IP address

If PPTP is being used and you need to have two or more local systems connected to the same remote server at the same time, then you should be sure that the PPTP helpers modules are loaded (ip_conntrack_pptp and ip_nat_pptp or nf_conntrack_pptp and nf_nat_pptp). Using the default modules file, Shorewall (Lite) will attempt to load these modules when Shorewall (Lite) is started.

If IPsec is being used, you should configure IPsec to use NAT Traversal -- Under NAT traversal the IPsec packets (protocol 50 or 51) are encapsulated in UDP packets (normally with destination port 4500). Additionally, keep-alive messages are sent frequently so that NATing gateways between the end-points will retain their connection-tracking entries. This is the way that I connect to the HP Intranet and it works flawlessly without anything in Shorewall other than my ACCEPT loc->net policy. NAT traversal is available as a patch for Windows 2K and is a standard feature of Windows XP -- simply select "L2TP IPsec VPN" from the "Type of VPN" pulldown.

Alternatively, if you have an IPsec gateway behind your firewall then you can try the following: only one system may connect to the remote gateway and there are firewall configuration requirements as follows:

Table 1. /etc/shorewall/rules


The above may or may not work — your mileage may vary. NAT Traversal is definitely a better solution. To use NAT traversal:

Table 2. /etc/shorewall/rules with NAT Traversal


If you want to be able to give access to all of your local systems to the remote network, you should consider running a VPN client on your firewall. As starting points, see The /etc/shorewall/tunnels manpage.


Frequently Used Articles

- FAQs - Manpages - Configuration File Basics - Beginner Documentation - Troubleshooting

Shorewall 4.4/4.5/4.6 Documentation

Shorewall 4.0/4.2 Documentation

Shorewall 5.0/5.1/5.2 HOWTOs and Other Articles

- 6to4 and 6in4 Tunnels - Accounting - Actions - Aliased (virtual) Interfaces (e.g., eth0:0) - Anatomy of Shorewall - Anti-Spoofing Measures - AUDIT Target support - Bandwidth Control - Blacklisting/Whitelisting - Bridge/Firewall - Building Shorewall from GIT - Commands - Compiled Programs - Configuration File Basics - DHCP - DNAT - Docker - Dynamic Zones - ECN Disabling by host or subnet - Events - Extension Scripts - Fallback/Uninstall - FAQs - Features - Fool's Firewall - Forwarding Traffic on the Same Interface - FTP and Shorewall - Helpers/Helper Modules - Installation/Upgrade - IPP2P - IPSEC - Ipsets - IPv6 Support - ISO 3661 Country Codes - Kazaa Filtering - Kernel Configuration - KVM (Kernel-mode Virtual Machine) - Limiting Connection Rates - Linux Containers (LXC) - Linux-vserver - Logging - Macros - MAC Verification - Manpages - Manual Chains - Masquerading - Multiple Internet Connections from a Single Firewall - Multiple Zones Through One Interface - My Shorewall Configuration - Netfilter Overview - Network Mapping - No firewalling of traffic between bridge port - One-to-one NAT - Operating Shorewall - OpenVPN - OpenVZ - Packet Marking - Packet Processing in a Shorewall-based Firewall - 'Ping' Management - Port Forwarding - Port Information - Port Knocking (deprecated) - Port Knocking, Auto Blacklisting and Other Uses of the 'Recent Match' - PPTP - Proxy ARP - QuickStart Guides - Release Model - Requirements - Routing and Shorewall - Routing on One Interface - Samba - Shared Shorewall/Shorewall6 Configuration - Shorewall Events - Shorewall Init - Shorewall Lite - Shorewall on a Laptop - Shorewall Perl - Shorewall Setup Guide - SMB - SNAT - Split DNS the Easy Way - Squid with Shorewall - Starting/stopping the Firewall - Static (one-to-one) NAT - Support - Tips and Hints - Traffic Shaping/QOS - Simple - Traffic Shaping/QOS - Complex - Transparent Proxy - UPnP - Upgrade Issues - Upgrading to Shorewall 4.4 (Upgrading Debian Lenny to Squeeze) - VPN - VPN Passthrough - White List Creation - Xen - Shorewall in a Bridged Xen DomU - Xen - Shorewall in Routed Xen Dom0

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