The official IPsec Howto for Linux

L2TP/IPsec Linux setup

Background
You are referring to host-to-host openswan configuration if i got your question correctly. Hyppo Drums 1 1 4. Shared secret authetication in Openswan IPSec Creating a tunnel between two seperate networks using openswan shared secret is the easiest and fast method. For this tutorial we will be using secret for shared secret or PSK authentication, but if you have been provided with a certificate or RSA key you will want to use rsasig. Will wait for your reply! Now look at the scenario where two hosts establish a connection with each other. The content of the file ifcfg , which was created for the IPSEC internetwork connection in the first network, is shown in Listing 6.

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Howto Configure PFSense Site-to-Site IPSec VPN Tunnel For Remote Access

Fortunately, The GNOME Project has released a project they call NetworkManager that greatly simplifies network management on Linux and works on most desktops and even on the command line, if you are so inclined.

If you are using another Debian derivative, such as Ubuntu, you probably already have NetworkManager installed as well. You can use the command line tool to check what devices are managed by NetworkManager ;.

Despite being developed mainly by governmental and corporate interests, L2TP is an open protocol standard. This is also an open standard with open source implementations. A popular open source Linux implementation of IPsec is strongSwan and packages can be found in many popular distribution repositories.

Openswan used to be the IPsec project of choice but has been deprecated in Debian and descendants. PSK is supported starting with NetworkManager-strongswan My current release of Mint is using 1. Depending on your authentication method you might get a dialog asking for your VPN password and will be given the options to save your password for future use. Once your VPN is established you should receive a pop-up status message and the NetworkManager applet will change to have a little lock on it:.

Feel free to leave your questions and suggestions in the comments section below. My hacking career began at age 12, when my father brought home a Sinclair ZX computer kit and a lifelong fascination with computing was born. I became a privacy and security advocate after learning of the US Government's criminal investigation against Phil Zimmermann.

I soon tired of the rat race and moved to the Northwoods of Minnesota where I have over 2 million acres of protected wilderness to roam. Here I indulge in my passions for wilderness and technology and as an advocate for computer security and privacy. In case somebody come across this post, here is my specific case and solution https: Different technologies can be used to encrypt your communication.

For understanding IPSec and its working, you can refer the below link. There are other technologies as well that protect data communication over wire, some of them are mentioned below. However we cannot reliably use them for our purpose of interconnecting two branch offices which are geographically isolated through internet. VPN is a very useful technology that is widely deployed in organizations that require secure remote access to remote network.

Some noteworthy points about Virtual Private Networks are mentioned below. In other words an entire IP packet is encrypted for security. IPSec is used for authentication as well as encryption of the complete communication that happens between two hosts on the internet.

As IPSec works in network layer, traffic generated by all applications are by default encrypted and sent, hence there is no modification required to be done on the existing application to make it compatible with IPSec.

We will be using one such IPSec implementation in Linux for creating a tunnel between two private networks through the internet. There was a project called as Free-Swan, which was the first implementation of IPSec on Linux, but due to some reason, the project did not last long the last version of free-swan was released at In the above shown figure i have tried to depict, the VPN setup that we will be configuring now.

There are two networks showin in the above diagram. These two networks are geographically isolated from each other and of course they are private network addresses and cannot be routed through internet to communicate with each other. We will be interconnecting these two networks together, so that the hosts on network A can communicate with hosts on network B just like they communicate to any local network.

For making this work we will be having two VPN servers. This kind of a setup is called as gateway to gateway or sometimes site to site VPN. Both of them will need a public internet IP address, to communicate with each other through the internet. This configuration is very much necessary for clients on both the networks to reach the other network, as well as proper working of routing.

The first step is to configure IP forwarding. The name itself suggests that it is used to forward packets destined for other hosts. It is basically done when you need to make a linux machine act as a router. In our case of establishing a VPN tunnel between two networks, both the VPN servers will be acting as a router to reach the network on the other side. Hence we need to enable it. IP forwarding can be enabled in Linux by the below command on the fly.

Hence we need to make our ip forwarding permanent. This can be done by modifying sysctl. To make that change in sysctl effective, you can run the below command. Please do not forget to enable ip forwarding on both the VPN server's.

Let's add some Iptables rule that will modify the source IP address of a packet before that packet is send out. This is very much useful because this helps in modifying the source ip of the packet.

On network A VPN server enter the below command. The above command on network A VPN server will modify the source address of a packet originating from On the VPN server on the other side, apply the same above command with the source address of Now we are set to install and configure openswan ipsec server on both the VPN servers.

Openswan ipsec tunnels allows you to authenticate the traffic going through the tunnel in two methods. The two methods are mentioned below. We will see both the configuration one by one. Lets see shared secret for encryption in openswan ipsec first. Creating a tunnel between two seperate networks using openswan shared secret is the easiest and fast method. For this to work properly we will begin with installing openswan on the linux machine the package is available for almost all linux distributions.

You can use the system package manage for installing openswan. Yum tutorial in Linux. The below yum command can be used to install openswan in linux. However you can achieve the same result in any distribution without much modification. Now the second step is configure our ipsec.

KLIPS is currently the more stable one, the one which is easier to use. IPSec works by encrypting packets at the network level, in other words an entire IP packet is encrypted along with its headers, and sometimes a new header is attached if you are masquerading IP packets. Inorder to enable IPSec packet to go through NAT devices, we need to enable this option by setting it to the value of " yes ".

This can cause problems if the server you are connecting to is using the same IP range internally. Say that IPSec server connects you to The client side or the server side?. The best method is to add all private subnet except those ranges used by the server. This is the first argument that mentions the name of the connection.

You can give any name as you wish, this is simply for identifying the tunnels. The ip address of the local IPsec server. Can be an IP address or a fully-qualified domain name. What are the subnet that will be reachable through this tunnel, on this side of the tunnel.

Simply adding routes on both the sides without adding the subnet will not make the hosts reachable. Let's take an example.

Or if you want to further simply the process of ipsec. But remember only one fact that for tunnel to work with preshared key passwords should be same on both vpn servers. Now we need to have the exact same configuration on the other side VPN server with the required changes in left, right, leftsubnet, rightsubnet, leftid, rightid.

To make the concept clear the value of right on the other vpn server must be the value of left on this VPN server. Now once the configuration is perfect, restart the ipsec service on both the sides. Your tunnel must work flawlessly if you did not make any configuration mistake.

You can check the tunnel by pinging any ip on the remote subnet. Now as discussed before, lets see the second method that can be used for authentication of our IPSec. We will create rsa keys on both the vpn servers first then we will see the ipsec. You can create an rsa key for your vpn server by the following command.

The above command should create a key for your VPN server inside ipsec. Run the same command on the other side VPN server. So now you have two rsa keys of bit size on both the servers. Now comes the main part of configuring ipsec. An example configuration file with rsasig authentication is shown below. Do the exact same configuration on both the servers and you are done. Now restart ipsec service on both the servers, to make the tunnel active. As suggested before you can try pinging the remote subnet to test the connection status.

That was maily for routing purpose, which will enable routing for client machines on both the networks to reach each others. So on client machines inside Same kind of route for reaching network A must be added on the clients inside network B.

Do you have any comment about having lefsubnet defined with the valid ip address and not the internal ip address? I have 8 valid address that are routed to my firewall. We have 2 interfaces: The internal machine has a one-to-one or masquerade NAT with the valid ip address but when it tries to connect throught the VPN, in reality it doesn't connect and tries to reach the destination going throught the normal Internet link and not the VPN.

I guess you are asking about having your leftsubnet option with 8 different IP addresses instead of private subnets correct me, and let me know if i got your query properly. So the only possible solution that you can do is probably mentioning the entire subnets those 8 ip addresses belong to, if not an issue You can block the rest from the firewall itself.

I believe your internal machines with private subnets are trying to reach the subnets on the other side of the VPN through your firewall let me know if am wrong. That wont happen until and unless you have those private subnets added inside leftsubnets option in the openswan config.

You also need to add a masquerade rule on your firewall that will masquerade all your private subnets which are trying to reach the other side of the VPN. Also please try adding a more specific route on your private hosts who are trying to reach hosts on the other side of the VPN not required if you already have a default route set to openswan device for all traffic.

I have one Ubuntu installed as Host machine and in virtual box i have installedd another Ubuntu. You are referring to host-to-host openswan configuration if i got your question correctly. Configuring that is quite simple. Install openswan on both the ubuntu machines.. Then run the below command, that will create a key pair.

However that will make it very slow.. The next step is to copy both the left and right side key's and put it inside a configuration file. This configuration file will be same on both the servers.

Now make a configuration file that will hold the left and right ip addresses along with our keys. The file will look something like the below.. Check the logs to confirm whether its working Hello Sarath, greetings from Ukraine! Thank you for your topic about Linux IPsec site-to-site, very informative!

But i have a very specific question on this subject. Also linux machine must provides NAT translating to another computer from its secondary network card to windows or linux machine - doesn't matter. I've been searching for my answer for several days on many forums and sites, but didn't find anything similar to my problem, and hopes only for your help! In addition to my previous question, i must clearify one more thing: And i have no idea, how to connect client linux pc to notebook through ASA and host.

Sorry for my bad English, i hope you understand my problem Configure the openswan server on linux as shown below Now you need to configure the same exact configuration withing ASA..

Quick Howto on configuring an ipsec tunnel