In-a-Dyn

DDNS What and How

Joachim Wiberg

February 17, 2020

In-a-Dyn DDNS

What is it?

  • DDNS = Dynamic DNS
    Updated automatically

  • DDNS Provider = ISP or Registrar
    Usually an Internet Service Provider

  • DNS Name for an IP address
    Most ISP customers have a DHCP or PPPoE/PPPoA address

  • In-a-Dyn is a DDNS client – sync your IP with your name

Requirements

  • ISPs are everywhere, where their customers are.
    Many are also DDNS providers.

  • A DDNS client should
    • support as many providers as possible
    • support updating more than one DNS entry, or
    • even multiple DDNS providers from one IP (DNS backup)

  • In-a-Dyn:
    → supports over 30 different providers
    → supports both IPv4 and IPv6
    → supports HTTPS w/ OpenSSL or GnuTLS

How does it work?

A good DDNS client actively tracks changes to your external IP address,
and automatically updates your DDNS provider(s).

  • Most DDNS providers have a “check ip” server, e.g. https://checkip.dyndns.com

     jocke@fra:~$ curl checkip.dyndns.com
 <html><head><title>Current IP Check</title></head><body>Current IP Address: 138.68.65.229</body></html>

  • Update the DNS entry, usually with a HTTP GET request. Can be done manually, of course, but most users prefer an automatic service do this for them

Common Use-Case

How do we set up a VPN to each site when ISPs can change address of our sites at any time? Note, even if you swap it around and let the sites connect to your head office, it may also have a DHCP address.

  • User has multiple connected sites
  • Each site has a DHCP address

Solution

  1. Sites connect to DDNS provider update their DNS entry
  2. DDNS provider publishes update DNS record to Internet
  3. Connection (VPN usually) can be initiated from either end
    → Usually on-demand service VPNs are used
    → Works with Roadwarriors too

The Protocol

Wow, Dyn.com gets lots of customers, let’s copy them!

  • Simple idea, invented by Dyn.com:
    1. Use HTTP GET to get IP address
    2. Use HTTP GET to update the DNS
  • Every other ISP/DNS provider copied protocol:
    • FreeDNS (really good, free alternative!)
    • Loopia
    • D.U.I.A.
    • …
  • Some providers today use HTTP Post to update
    • Cloudflare
    • Yandex
    • DNSPod

Problems

  • Sounds simple, let’s wrap curl in a shell script!

  • Well … scripts work for many use-cases but there are things the DDNS providers don’t want you to know

  • Many DDNS providers block your account
    • for “too frequent” updates – their discretion :-(
    • updating without actual IP change – no curl cron jobs :-(
  • No Internet, we’ve all been there, but what if:
    • the provider doesn’t respond
    • the provider responds weirdly
    • need validation + retry mechanisms + logging

Testing

  • How to test this?
    • HTTPS certificates
    • IPv4 vs IPv6
    • Providers blocking too frequent updates
    • etc.
  • In-a-Dyn:
    → has a built-in .conf file validator
    → supports custom providers
    → automatic testing of Dyn.com and FreeDNS

Questions?