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According to http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#x-headers, the X-Forwarded-Host can contain more than one value:

Be careful when using these headers on the origin server, since they will contain more than one (comma-separated) value if the original request already contained one of these headers

Can REMOTE_HOST contain more than one value? Or it always be the single value?

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That's true for X-Forwarded-Host just because there might be multiple proxies in between the original client and the server. The REMOTE_HOST is just a resolved name of REMOTE_ADDR, or NULL, if the resolution fails (i.e. no PTR exists or DNS is temporarily unavailable or the query times out). It's always a single value.

REMOTE_ADDR is always a single value usually referring to the direct client connecting the server. However, with mod_remoteip you can alter this behaviour to get the originating useragent even where that useragent is behind a load balancer, front end server, or proxy server. That's useful when using load balancers and server side reverse proxies, while X-Forwarded-Host is suitable for client side proxies.

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