FR: Only accept PROXY protocol from certain IPs

[stratself]

Is your feature request related to a problem? Please describe.

I'm looking to front ejabberd with some routers that send the PROXY protocol packets. As a further point of restriction/containment I'd like ejabberd to only receive it from specific IP addresses.

Currently this is not possible as PROXY protocol tagged packets can be accepted from anywhere. As access rules are handled after IPs in the proxy protocol are parsed, this may have security implications if ejabberd isn't exposed properly.

Describe the solution you'd like

Something like this in the listen modules:

listen:
  -
    port: 5223
    module: ejabberd_c2s
    access: c2s
    use_proxy_protocol: true
    proxy_protocol_acl:
        - 192.168.1.6/32
        - 192.168.2.0/24

And enforce it as a requirement when use_proxy_protocol: true, so operators have to go out of their way to enable access from anyone via defining [ 0.0.0.0/0, ::/0 ]. Perhaps this could be a gradual migration, but I hope there'll be a point where explicit IP declaration is necessary.

Since all the other ACLType doesn't apply at this level I guess a list of CIDR would do.

Describe alternatives you've considered

Having a middleware or firewall rules between the router and ejabberd, which may be clunkier to set up esp with Docker environments

Additional context

Previous discussed in the MUC.

Section 2 of the spec says that "The receiver SHOULD ensure proper access filtering so that only trusted proxies are allowed to use this protocol."