Support reading but ignoring PROXY protocol from a subset of clients

[Castaglia]

It would be nice to be able to configure something like the following for mod_proxy_protocol:

<Limit PROXY>
  # Configure list of trusted senders
</Limit>

I.e. to be able to configure the list of trusted clients, the ones we trust to send PROXY properly. If the connected peer is not in the configured list, but DOES send the PROXY protocol, then mod_proxy_protocol would read that data, but not actually switch the internal data structures to use the reported addresses.

[Castaglia]

Another way of implementing this, rather than via <Limit>, might be use a <Class>, and mod_ifsession. Which should Just Work(tm) -- except that we'd still want mod_proxy_protocol to read the PROXY protocol bytes, but ignore them. That, I think, is more doable, and easier.

[Castaglia]

The requested functionality has been implemented, but using an ProxyProtocolIgnore directive rather than <Limit>:

  <Class untrusted-senders>
    From 1.2.3.4
    From 5.6.7.8
  </Class>

  <IfModule mod_proxy_protocol.c>
    # We always to read PROXY protocol headers from clients...
    ProxyProtocolEngine on

    # ...but we don't always trust the senders
    <IfClass untrusted-senders>
      ProxyProtocolIgnore on
    </IfClass>
  </IfModule>