Configuring a Cisco Router For Secure dyndns Dynamic DNS
While I normally dislike using dynamic IP addresses on cisco devices, they are sometimes a necessary evil. I finally got around to setting up a Cisco router at home this weekend, and decided that I needed to setup Dynamic DNS so I could get into it remotely, even though it is on a dynamic IP address on my Verizon FiOS connection. In doing the research I came across this posting which not only explains how to configure IOS to use the dyndns service, but it also shows how to do it securely using https.
To be honest, I went ahead and used the http (insecure) method as I did not want to mess with certificates, but in many situations you may need the security provided by https. I am not really worried about anybody hijacking the dyndns for my home IP address. 
There is also a Cisco page on how to setup ddns, but it is really confusing and for my purposes, the only useful parts were all the way at the end of the page. I believe most of this page deals with getting the Cisco router to update DNS on behalf of clients. In our case all we want is for the router to update the DNS for a single IP address of it’s outside interface.
One very important learning I got out of this process was on how to escape a question mark (?) when you are trying to insert it into a text string on a Cisco device. Normally IOS treats a question mark as a request for help. To escape it you must type ctrl-v right before pressing ?. I never knew that before. I had tried to insert ? into description fields before but always gave up and chose to use other punctuation to get my point across.
UPDATE: After running this config for a day I got a nastygram from dyndns that I was updating too frequently. I had to add the minimum and maximum lines to my config so that it only updates once every 28 days if your IP does not change:
interface FastEthernet0/0
ip ddns update hostname bitplumber.homeip.net
ip ddns update dyndnsip ddns update method dyndns
HTTP
add http://usernamehere:passwordhere@members.dyndns.org/nic/update?system=dyndns&hostname=<h>&myip=<a>
interval maximum 28 0 0 0
interval minimum 28 0 0 0
-Eric
Interesting. Ctrl-V is often (on Unix-like systems) the escape code to enter “compose” mode, usually used to generate ligatures like Æ or accented characters like é.
too frequently?
no-ip.com does it by default every 5 minutes, been using it for years now without problems