Allow OutboundIp / Outbound Interface for IHS liberty plugin

General Description

It is common for reverse proxies to use a specific outbound interface for communications to upstream, which is not the default route.

IHS with the websphere plugin is the only reverse proxy which does not support this very common feature. Indeed, I would call it a must-have to have a web server be called 'reverse proxy'-capable.

mod_proxy supports this OOTB: https://httpd.apache.org/docs/trunk/mod/mod_proxy.html#proxysourceaddress. However, according to IBM support, this is not a supported scenario (for whatever reason).

nginx supports this OOTB: proxy_bind directive for any location. This was mentioned in IBM red papers a long time ago.

squid has tcp_outgoing_address.

haproxy has the source keyword.

Use case

1. To harden Liberty or WebSphere, or for performance reasons (taking load off of Liberty/WAS), you might want to serve static files from IHS and let upstream only do the dynamic responses.

2. For security concerns, because Liberty settings have had security issues in the past (directory browsing etc.).

3. For merely load balancing via reverse proxy to different upstream endpoints (taking off load from specific IP interfaces).

4. When upstream is only reachable from specific IPs on the system due to firewall settings or the Liberty tcpOption "addressIncludeList"

(probably many more).

Effects of not implementing this feature

* Load balancing in high security environments can become impossible (see use cases).

* system misconfiguration cannot be circumvented.

* load problems on interfaces.

* acceptance problems (people will want to switch away from IBM IHS or not consider it as an option) when in need of such an option.