server-alias annotation to allow multiple hosts for the same ingress?

[hostalp]

Hello,
is there any functionality similar to what's provided by the other ingress controller via the annotation nginx.ingress.kubernetes.io/server-alias?

Allows the definition of one or more aliases in the server definition of the NGINX configuration using the annotation nginx.ingress.kubernetes.io/server-alias: "<alias 1>,<alias 2>". This will create a server with the same configuration, but adding new values to the server_name directive.

It's an easy way how to add multiple hosts to the configuration.
I searched the documentation a little, but haven't found anything so I have to use the alternative approach with YAML anchors to reuse the same http configuration section by multiple hosts.
It would be nice to have this feature available though.

[brianehlert]

The use case is that you want to define a list of hostnames instead of being constrained to a single hostname for an Ingress or a VirtualServer.
This implies that these hostnames then all have an identical configuration including the backend.

An example that immediately jumps to mind is external.dns.name and internal.dns.name to align with the behavior that NGINX does hostname validation by default.

Does that align with the thinking? Do you have a different example?

[hostalp]

Another example is the use of www.dns.name and dns.name in a single ingress resource.

[brianehlert]

Any alias would be appended after the hostname.
The existing host would be configured in nginx.conf as the server_name with any aliases following.

See:

• https://serverfault.com/questions/718801/server-aliases-on-nginx
• https://nginx.org/en/docs/http/server_names.html

It would be relatively straightforward to use an alias annotation that post-pends after the canonical host name given in the main body of the configuration.

I am guessing that the historic reason for not adding this is that hostname collisions could happen. Today that is resolved by using the oldest object wins methodology. But it could be possible to introduce an alias on one ingress object that clashes with an alias defined on another.
The question becomes: how this is resolved or not resolved and reported.

In the case of a business merge or similar, I could see an alias quickly being some alternative DNS name.

I am also curious how this might relate to projects such as external-dns or loadbalancers. Or if in those cases it is not worried about.
Asking since I don't know

[vepatel]

For both Ingress and VirtualServer

[shaun-nx]

For VirtualServer, would this be a new spec block or will the existing spec.host field become a list?

For example:

apiVersion: k8s.nginx.org/v1
kind: VirtualServer
metadata:
  name: cafe
spec:
  hosts:
    - host: cafe.example.com
    - host: foo.example.com

[brianehlert]

To have a canonical primary a way to approach this would probably be to be explicit about alias rather than array or host names.
I say that because an alias pattern would be used different by the customer than I think a list of hosts would.
Alias implies that the domain would be the same. Which I feel is highly appropriate. An array of host names implies randomness.
Consider this pattern:

apiVersion: k8s.nginx.org/v1
kind: VirtualServer
metadata:
  name: cafe
spec:
  hosts:
    - host: example.com
    - alias: www.example.com
    - alias: example.net
    - alias: www.example.net

In the nginx.conf this would translate to:

server {
    listen       80;
    server_name  example.com  www.example.com example.net www.example.net;
    ...
}

YAML syntax might need to be updated a bit if VirtualServer supports an array of host names. In which case consider this:

apiVersion: k8s.nginx.org/v1
kind: VirtualServer
metadata:
  name: cafe
spec:
  hosts:
    - host: example.com
       alias: www.example.com
       alias: example.net
       alias: www.example.net
    - host: contoso.com
      alias: www.contoso.com

[hostalp]

You may do something like that with your custom resources (VirtualServer), but the YAML structure of the standard Ingress resource is given.
For that reason you may consider an approach similar to the nginx.org/rewrites annotation https://github.com/nginxinc/kubernetes-ingress/tree/main/examples/ingress-resources/rewrites
E.g. something like:

nginx.org/server-aliases: "host=example.com alias=www.example.com[;host=contoso.com alias=www.contoso.com;...]"

How to deal with multiple aliases per host is up to you. It could be a comma delimited list like aliases=www.example.com,example.net or some other form such as multiple alias parameters per host entry.

[brianehlert]

yeah, I can see that.
As an annotation, that looks complicated to me for the use case.
I mean the syntax is fine, but the usability seems overly complicated.
I could almost see an annotation per host name. The possibility for multiples of the same annotation. (I don't even know if that is a thing, I have never looked).
I have a feeling that the logic under the hood will end up influencing this a bit.