Dynamic DNS

[Aaron Peschel]

Looking for some advice on getting Dynamic DNS working on Azure.

For some background: typically with a Linux based OS, DHCP is used to register a host with DNS, rather than each host registering with the DNS server directly. However, Azure does not support this behavior and requires that hosts register with the DNS server directly. This leads to difficulty in trying to get Linux hosts working with DDNS on their platform. There is some documentation available here that we have referred to: https://docs.microsoft.com/en-us/azure/virtual-network/virtual-networks-name-resolution-ddns However, these instructions appear to be Ubuntu specific and do not necessarily work on other distros.

I've been digging through documentation on systemd-networkd and dhcpcd.conf, and they mention having the ability to use hooks similar to dhcpclient. I haven't been able to pin it down, but maybe something in /libexec/dhcpcd-hooks would work? However, this directory doesn't exist in CoreOS, and I'm not sure if it's the correct method or even supported.

Anyone here have suggestions on how to get a ( dhcpclient -> nsupdate -> dns server ) equivalent working on CoreOS?

[paul...@coreos.com]

Hi Aaron,

We have a community guide here, that suggests some default configurations for Azure. Does this work?

https://coreos.com/os/docs/latest/booting-on-azure.html

If not, holler, and I'll see if I can dig up an answer. 

Cheers,

[Aaron Peschel]

Hi Paul,

I took a look through the default configurations there. Unfortunately, they do not appear to take dynamic DNS into account. Ideally I would like a way to hook into the dhcp client CoreOS uses to point it to the DNS server rather than the DHCP server for DNS updates.

-Aaron

[paul...@coreos.com]

Hi Aaron,

CoreOS inherits from Gentoo, so their docs on DHCP are likely relevant:

https://wiki.gentoo.org/wiki/Dhcpcd

I also asked our devs, to see if they had any thoughts. They replied with a comment + question:

> DDNS is already set up.

> We'd need to know if they are using ASM (old APIs) or ARM (new APIs)

> If they are using ARM, they might need to request a domain name. If they are using ASM, it will already be set up.

Cheers,

[Aaron Peschel]

Hello Paul,

We're using ARM with custom DNS. I'm not clear on what your dev means by these lines:

> DDNS is already set up

Is he referring to the default Azure DNS? With custom DNS, Azure DHCP doesn't forward DDNS to the DNS server. With a custom DNS server, Azure requires hosts to reach out to the DNS server directly for updates.

> If they are using ARM, they might need to request a domain name.

I'm not aware of any method to set a domain name for a VNet in azure.

Thanks,

-Aaron

[paul...@coreos.com]

Hey Aaron,

Yeah poking around a little bit more, it sounds like our rec is to do the thing you initially wrote in to ask about. Woops!

We don't have anyone in-house with the Azure expertise you're asking for. But, some-what naively poking around, does this doc have what you're looking for?

https://coreos.com/os/docs/latest/configuring-dns.html

Q to the community, is there anyone else out there running a custom DNS on Azure?