Setting up an OpenVPN tunnel using a CentOS-based system as the server and a router flashed with Tomato firmware as the client – Part 3

 

Important
This is an edited version of a post that originally appeared on a blog called The Michigan Telephone Blog, which was written by a friend before he decided to stop blogging. It is reposted with his permission. Comments dated before the year 2013 were originally posted to his blog. The link to Amazon.com in this article is an affiliate link, and if you make a purchase through that link I will receive a small commission on the sale.

Continued from Part 2

Okay, time for the hard part… well, at least it was for me.  Please understand, the instructions at The ‘Point and Click’ Home VPN HowTo Guide, combined with the knowledge I acquired from the book OpenVPN: Building and Integrating Virtual Private Networks (Amazon affiliate link) by Markus Feilner, enabled me to get a tunnel going using a software client with no sweat. But getting it to work with the Tomato VPN client, and in particular, to get it to work the way we needed it to, was a whole other thing. It turned out, as so often happens that some simple configuration changes were all that was needed – but finding the correct configuration changes to make were pure grief.

EDIT: The above-mentioned book has been updated and expanded under a new title — see Mini-review of Beginning OpenVPN 2.0.9 by Markus Feilner and Norbert Graf (Packt Publishing)

I want to digress just a moment to speak about those elitists that seem to frequent certain forums and IRC channels, and just love to tell newbies (often in not so polite terms) that all the answers can be found by using Google. I’ve pretty much figured out that most of the time, the person saying that doesn’t know the answer either, but they are like the schoolyard bully that gets their kicks by picking on others. In this case, Google may have had the answers somewhere, but they sure weren’t showing up in the first few pages of results. Instead, what was showing up was many others who were having the same problems and asking the same questions, but not getting answers! And usually after the first four or five pages of results, the results became even less relevant, if that were possible. Finally, after taking a more or less scattershot approach to seeking help, I came up with a “recipe” that works in this situation. Of course I cannot guarantee it will work for you… heck, I can’t guarantee it will work for me next week… but at least it’s working as I write this. But the next time someone suggests that you should just Google it (or something like that), I suggest you ask them what search terms they would suggest using that will bring you the answer to your question. When whatever they suggest doesn’t work, let them know and ask for more suggestions, and just keep repeating until you find the answer or they get sick of talking to you. If we all did that, I suspect some of the online bullies would discover that maybe they should keep quiet if they don’t know the answer.

Okay, so to begin, let’s make sure we are all on the same page. I assume you’ve installed the Webmin OpenVPN + CA module (again, following the instructions at The ‘Point and Click’ Home VPN HowTo Guide) but if you had difficulty locating the module, try here — as I write this, it appears that the newest version of the module is the OpenVPNadmin WebMin pre-release 2.5 version. And when you try to download it, if you get “Unauthorized access to downloads!”, try a different browser, or make sure JavaScript and cookies are enabled.  You will probably need to download it to your computer first, then upload it to Webmin, if your experience is anything like mine was.

When you go into the module, you’ll initially see a page like this:

OpenVPN administration page
OpenVPN administration page

This is “home base” for this module, so when I say “return to the admin page”, this is the page I mean. It’s also the page you use to create a new Certification Authority (normally you only need to do that once, but if you really want to you can make more). In the upper left corner there is a link labeled “Module Config” – click on it and make sure all the paths are correct:

OpenVPN + CA configuration page
OpenVPN + CA configuration page

A couple notes on this page:  First, although it indicates that “Server Hint for Clients” is a required field, there is no indication of what actually goes there, nor any default value.  Second, the information under “If you use bridge device” is only applicable if you use TAP mode (remember, we’re using TUN), but I do know the defaults are not correct.  Just in case you ever want to try to get TAP mode working, here is what these paths should be (at least on our installation):

Command to start Bridge:
/usr/libexec/webmin/openvpn/br_scripts/bridge_start

Command to stop Bridge:
/usr/libexec/webmin/openvpn/br_scripts/bridge_end

Path to DOWN-ROOT-PLUGIN:
/usr/share/openvpn/plugin/lib/openvpn-down-root.so

Do check the paths on this page, because the defaults aren’t totally correct.  Now let’s return to the admin page.  I probably should have mentioned that you don’t really need to change any of the defaults for making a certificate for just your own use – those values are probably only important if you are creating a certificate that will be used by the public.  Once you have made a certificate, you can click on the Certification Authority List, where you’ll see this page:

OpenVPN Certification Authority page
OpenVPN Certification Authority page

I really don’t know why you can create a new Certification Authority on both this page and the main admin page, but oh well… once you have created one it will show up in the list at the top of the page.  You can click on it to view what you created, but there’s nothing you can change there. The main thing of importance is the “Keys List” link, where you can actually create new keys and view existing ones:

OpenVPN key list and key creation page
OpenVPN key list and key creation page

Okay, now here’s the important thing to remember about key creation: You need one key for your server, and one key for each client.  You will note we have a server key, and client key for the software client (used for testing) and one more for the OpenVPN client on the Asus router.

As you may have surmised, except for the key name field, you can leave everything else at the defaults. However, there is one thing on this page that will lead you astray.  It says, “Server key doesn’t need password!“, which might lead you to think that a client key does need one, but that’s not true.  And if you use a password with your Tomato-based client key, you will never make a successful connection.  The keys are far stronger protection than passwords anyway, so the only reason you might even consider using one is for a software client where you want to keep anyone other than an authorized user from connecting to the VPN tunnel.  Anyway, don’t use a password for your server key or your Tomato OpenVPN client key!!!

Okay, now it’s time to navigate back to the admin page, then click on “VPN list”. Once you have created a server key, you can create your actual server. So click on the New VPN Server button and a page will come up that permits you to create a server configuration.  This is where things get really tricky.  Here is how ours is filled out:

OpenVPN Server page
OpenVPN Server page

Now the above is actually the edit page you get once the server is created and you have returned to edit it.  Note that some values cannot be changed after the initial entry, so be sure you get them right the first time.  In particular, make sure to select tun and not tap. If you do make a mistake, you can always delete the entire server configuration and start over, but that’s a bit of a pain. There are some differences in our configuration and the one at The ‘Point and Click’ Home VPN HowTo Guide, and generally speaking there are good reasons for the differences. But that said, nobody is claiming our configuration is perfect, just that it works! If you see something here you think should be changed, feel free to leave a comment.

Note in particular the text fields at the bottom of the page. Getting those right is the key to making this thing work! So, here’s what’s in each of those:

Additional Configurations:

push “route 192.168.0.0 255.255.255.0”
push redirect-gateway
push “dhcp-option WINS 192.168.0.50”
script-security 2 system

up (script execute after VPN up):

route add -net 192.168.5.0 netmask 255.255.255.0 gw 10.8.0.2 tun0
echo 1 > /proc/sys/net/ipv4/conf/tun0/proxy_arp
echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp

down-pre (script execute before VPN down):

route delete -net 192.168.5.0 netmask 255.255.255.0 gw 10.8.0.2 tun0

Of course, in the Additional Configurations section you want the line push “dhcp-option WINS 192.168.0.50” to point to a WINS server on your network, if you have one (if you don’t, just leave that line out).

In the up and down-pre scripts, if you want to be able to reach the local network behind the WAN port of the router on the client side, you should add additional route add and route delete lines in a format similar to those above, but substituting that network’s address range.

Now, once again, I’m NOT saying this is a perfect configuration, just that it works for us. However, if you notice something that just doesn’t look right, feel free to leave a comment. Much of the added configuration was put in so that traffic from the primary LAN to addresses in the 192.168.5.x address space would actually go through the tunnel – that was the hardest part. It was (relatively) easy to create a tunnel in the first place, but getting packets to and from their intended destinations was another matter entirely.

When you have configured your server and clicked “Save”, it should show up in your server list:

OpenVPN Server List page
OpenVPN Server List page

Now click on “Clients List” – there will be a button labeled “New VPN Client” (no screenshot, it’s just a button to click at this point).  Click that button and you will be presented with a client configuration page.  Here’s the one we created for our Tomato OpenVPN client running on the Asus router (again, this is actually the edit page, since the client is already created on our system):

OpenVPN Client Configuration page
OpenVPN Client Configuration page

Most of the settings here will be the defaults but there are two things to pay particular attention to:  First, the “Remote IP” must be the external address of your server – remember that this is the configuration that goes to the client. And second, notice the ccd file content box at the bottom – it is crucial that this be filled in correctly. In our case, we used this line:

iroute 192.168.5.0 255.255.255.0

If you miss this – and I speak from experience on this one – the packets from the Internet or your local LAN just aren’t going to get back to the client end of the tunnel! And remember to add an additional, similar line if you also want to be able to reach the network connected to the WAN port at the client router, substituting the base address of that network for the 192.168.5.0.

Once you have saved this, it should appear in the VPN client list:

OpenVPN Client List page
OpenVPN Client List page

Once you have configured a client, the next step is to export its configuration, particularly the certificate and key files, to your computer using the “Export” link – all the necessary files will be packed up in a ZIP file.  The instructions at The ‘Point and Click’ Home VPN HowTo Guide tell you how to use this file with a software client, but with your Tomato firmware you will have to unzip the file and then open each of the three files (ca.crt, plus the client certificate and key files), in a text editor or viewer. Only those three files are used with the Tomato OpenVPN client; the others in the ZIP archive are not. Then you will need to cut and paste the contents of those files into the three fields that appear under VPN Tunneling | Client | Keys tab in the Tomato firmware (see the screenshot of that page in Part 1 if you’re not sure what file’s contents goes where). With regard to the Client Certificate, there is some extra information at the top of the file that does not need to be pasted into the text field – you only need the two lines —–BEGIN CERTIFICATE—– and —–END CERTIFICATE—– plus all the lines in between those two tags. On an Asus WL-520GU router it doesn’t matter much, but there have been cases with some other makes of router where leaving that excess information in was just enough excess data to “brick” the router! So if in doubt, leave the data above the —–BEGIN CERTIFICATE—– line out.

Once you have everything ready, you can go to the OpenVPN admin page in Webmin and start the server, then in your Tomato firmware start the client.  HOPEFULLY the client and server will connect and start communicating.  You can check from the server’s  admin page by clicking on “Active Connection” – hopefully you will see something like this:

OpenVPN Active Connections page
OpenVPN Active Connections page

If you do, congratulations! If not, you have some troubleshooting to do. On the server, take a look at the log file: /etc/openvpn/servers/servername/logs/openvpn.log. On the client, click on Logs (under Status in the left-hand menu) and look at the last several lines. OpenVPN usually doesn’t suffer in silence — when something isn’t working, chances are one or both logs will be filled with messages that may or may not be helpful. You can always try going to Google and using OpenVPN plus the actual error message text (enclosed in quotation marks); once in a while that will actually bring up something useful.

Still stuck? My plan is to add a Part 4 to this series, that will provide links to some of the most useful resources I’ve collected along this journey.  But you can certainly help — if I’ve made any errors in these instructions (not at all unlikely given the sleep I’ve missed during this project), and you find them and can figure out what was incorrect, PLEASE leave a comment. I will warn you that writing to me with your error message will probably get you nowhere – I can definitely play the part of the blind leading the blind, so to speak, but first I have to heal from all the bruises encountered on this journey! 🙂

I sincerely hope these posts have been useful to you! In Part 4 I’ll wrap this up with brief instructions for rotating the log file so it doesn’t grow forever, provide the aforementioned list of links, and give you a bit more insight into why I set this up using TUN mode even though I’d probably have preferred to use TAP.

5 thoughts on “Setting up an OpenVPN tunnel using a CentOS-based system as the server and a router flashed with Tomato firmware as the client – Part 3

  1. I two find this very helpful. I was wondering if you had any advice on setting plugins via the webmin interface. I have a radius plugin i would like to run however when i use the line
    plugin /etc/openvpn/radius-plugin/radiusplugin.so /etc/openvpn/radius-plugin/radiusplugin-1.cnf.
    In the “Additional Configurations” box it simply chops the line off on save. I have to manually edit the config after saving it and re-add the line.

  2. Thanks.

    In Tab y i don´t bring it. The problem is: Path to Bridge-Start or path to Bridge-End or path to Plugin not found.

    Can anybody help me? Thanks.

  3. NOTICE: All comments above this one were imported from the original Michigan Telephone Blog and may or may not be relevant to the edited article above.

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