FIXED: Zyxel router DDNS error “Error validating DYNDNS svr answer. Check usr,pass,hostname!”

This is a quick post to hopefully help others suffering with this issue.

I setup DDNS (Dynamic DNS) on my Zyxel router, connected to a hostname that I had setup. No matter what I did with the configuration, I couldn’t get it to work.

In the end, I setup what I thought was right in the Dynamic DNS Configuration page, and used telnet to get into the router. I then ran the following command (obviously replace [email protected] (with your email address), 123456 (with your password), and (with your hostname)):
inadyn -u [email protected] -p 123456 --update_period_sec 3600 --dyndns_system [email protected] --alias

This worked!
I:INADYN: IP address for alias '' needs update to 'x.x.x.x'
I:INADYN: Alias '' to IP 'x.x.x.x' updated successful.

At this pointed, I closed the telnet window, and turned off the router for a while, restarting a while later, and getting a new IP address. To my surprise, the DDNS function updated!

I’m assuming something was not being picked up from the Zyxel Dynamic DNS Configuration page correctly, and so running it manually in the router fixed it. Either way, this seemed to worked.

Leave a comment if this works for you, as I don’t want to leave misleading advice here, but wanted to share as I struggled to find a fix online.


I'm passionate about technology, and particularly helping people make the most of it. I've spent the last 30 years helping others make the most of technology. My career started in IBM, but I choose to move into smaller business environments, to use a breadth of skills, and help businesses step change their IT services. My skills range from user based technology, through business systems (applications) to infrastructure. I also have a long background in IT security. I focus on what I consider to be "productive technology", i.e. adding genuine value to peoples lives. I'm not a big gamer, and don't hold much interest in what I consider to be disposable consumer technologies. During the day, you'll find me consulting with businesses or heading up an IT department. At the weekend, you'll find me sat at my Linux PC, writing PHP or Python code, or trying to help others on Twitter, this blog, or my YouTube channel: Artexic.