This tutorial will show you how to configure ddclient on Raspbian and Ubuntu. Many tutorials don't explain what to do if your server is behind a router, but this one will. I recently set up a backup server on an internet connection that has a dynamic IP address. So far, I've been spoiled at home because my ISP (PlusNet) makes switching to a static IP address easy and cheap, so obviously I did that. This time though, I didn't have that option, and I didn't want to configure a dynamic dns client on that router either, so I had to set it up on the server itself.

Installation

Some DNS providers offer their own dynamic DNS clients, but most of them are proprietary. The one I am going to use is called ddclient, it's free and open source, and written in Perl. Use this command to install ddclient:

sudo apt-get install ddclient

Enable dynamic DNS with your DNS provider

If you're not using namecheap, then this section will be different, but the concept is the same regardless of DNS provider. Log into namecheap and select the relevant domain, then choose "Dynamic DNS" under Miscellaneous settings. Use the radio buttons to enable dynamic DNS, and then make a note of the password. It should go without saying, but be really careful what you do with this password - anyone with access to it could change any DNS record for your domain that they wanted to. Unfortunately, namecheap doesn't let you restrict the dynamic DNS to just one subdomain. Next, navigate to "All host records" add a DNS A record for your domain (use the @ symbol for this), or subdomain. Use the dummy IP address 127.0.0.1 for now, the first time we run ddclient this will be updated to your actual Wide Area Network (WAN) IP address.

ddclient configuration

Now you have everything you need to configure ddclient. The main configuration file for ddclient is at /etc/ddclient.conf, you can open this file to edit it with a text editor of your choice - this command will open it in nano:

sudo nano /etc/ddclient.conf

Here is a sample "normal" configuration file for ddclient:

protocol=namecheap
server=dynamicdns.park-your-domain.com
login=yourdomain.com
password='password'
subdomain

• protocol is set by your dynamic DNS provider. For namecheap the value is "namecheap"
• server is the hostname of the dynamic DNS server. The dynamic DNS servers used by namecheap are located at "dynamicdns.park-your-domain.com"
• login is your domain name
• password is the string we obtained earlier from the namecheap web interface. Leave the single quotation marks around the string.
• The last line is the subdomain to be modified. In my case this was backup, for backup.samhobbs.co.uk. If you wanted to update your root domain, you would put an @ symbol on this line instead

WAN IP discovery

The above configuration would work fine if ddclient was installed on a router, since the router knows your WAN IP address. However, it doesn't work if your server is behind a router because the server only knows its Local Area Network (LAN) IP address. There is a configuration parameter called use, which determines the method ddclient uses to find the WAN IP. Important! - if you specify this parameter, it must go above the rest of the configuration in the file. If you specify it below, it won't work! This caused me quite a lot of grief. The default value for use is if, which uses information from the netwrok interface (think ifconfig). If you have multiple network interfaces, you can specify which one like use=if, if=eth0 for ethernet, if=lo for the loopback address, if=wlan0 for wireless LAN etc. However, none of these will work for us because none of them will give the WAN IP. There are two more types of value you can set: web, and router firmware values like fw and linksys.

Getting your WAN IP from your router's status page

Although I haven't opted for the router firmware method, I think it's quite interesting and worth discussing. Router firmware settings look something like this:

use=fw, fw=192.168.1.1/status.htm, fw-login=admin, fw-password=admin, fw-skip='IP Address'

...where fw= sets the location of the status page for that particular router containing the WAN IP address. If the status page is not available to unauthenticated users, you must set the username and password to allow ddclient to authenticate with the router. fw-skip tells ddclient to ignore any IP address on the status page you specified before a certain string, in this case 'IP Address'. Some popular router manufacturers have their own settings for ease of use, for example if you have a Linksys router you can use this line:

use=linksys, fw=linksys, fw-login=admin, fw-password=admin

Note that since a lot of routers won't let more than one user log in as admin at a time, you could potentially prevent ddclient from updating your dynamic IP address if you are logged in yourself at the same time.

Getting your WAN IP address from a web service

The web method involves ddclient querying one of the many "what is my ip" type web services on the internet, and extracting your IP address from the page returned. You can tell ddclient to use this method by using this line:

use=web

Similarly to other methods, you can also specify which website to use with the web-skip parameter. Some options with preset values are dnspark, dyndns and loopia, although you can use any site you like. For example, you could use somedomain.com by setting use=somedomain.com, with an appropriate web-skip-pattern=foo to ignore IP addresses before the string "foo" if necessary..

Secure submission

Remember how I said anyone with your dynamic dns password can change your DNS records? Sending your password via http (not https) is a bad idea. This parameter will force https:

ssl=yes

Again, this needs to go above the protocol parameter in your config file. For this to work, you need a perl library that can use SSL. Install it with this command:

sudo apt-get install libio-socket-ssl-perl

Testing your configuration

You can check if the pre-defined use values can detect your WAN IP by running this command:

sudo ddclient -query

If your server is connected with an ethernet cable, the output should look something like this:

use=if, if=lo address is 127.0.0.1
use=if, if=p2p1 address is 192.168.1.119
use=if, if=wlan0 address is NOT FOUND
use=web, web=dnspark address is 1.2.3.4
use=web, web=dyndns address is 1.2.3.4
use=web, web=loopia address is 1.2.3.4

To test your ddclient configuration with really verbose output, printing all possible configuration parameters and their values, you can use this command:

sudo ddclient -debug -verbose -noquiet

I won't print a sample output because it's too long, but somewhere near the bottom you should see a line like this:

SUCCESS:  updating backup: good: IP address set to 1.2.3.4

While we've got all this information, It's worth checking to make sure you are actually using SSL to connect to your dynamic DNS provider. Look for lines like this:

CONNECT:  dynamicdns.park-your-domain.com
CONNECTED:  using SSL

Run ddclient as a daemon

Since we don't just want the IP address to update once, we still need to set up ddclient to run as a daemon so it can check for a change of IP address periodically and notify the dynamic DNS provider if necessary. To start the daemon we need to open another configuration file, /etc/default/ddclient and set:

run_daemon="true"

You will notice there is a daemon_interval parameter there too, I think the default value of 300 seconds (5 minutes) is reasonable, so I didn't change it. Save and close the file, and then run:

sudo service ddclient start

to start the daemon, and:

sudo service ddclient status

to check its status. ddclient keeps a cache of your IP address, and it will only update the record with your dynamic DNS provider if your IP address has changed. Since some ISPs seem to only allocate new IP addresses when the modem is power cycled, and some dynamic DNS providers will time out if you don't update the record in a while, there is one thing left to do - we need to add a cron job to force an update weekly, just in case. Choose whether you want to force an update daily or weekly, and then create a file called ddclient in the relevant directory, e.g. /etc/cron.daily or /etc/cron.weekly:

sudo nano /etc/cron.daily/ddclient

Fill in this information:

#!/bin/sh
/usr/sbin/ddclient -force

Then make the script executable:

sudo chmod +x /etc/cron.daily/ddclient

Done :)

Useful links

• ddclient wiki