Redirecting and masquerading the result for the client.

Technically speaking, the HTTP::header replace Host command replaces the Host header in the ingress request, which the client wouldn't see. If the back end server is somehow reflecting that value, then that's a different issue. There isn't usually a Host header in an HTTP response, so where might the client be seeing this replaced value? I ask because the fix is dependent on where the value is coming through.

Depending on how complex the masquerade, you have a few options from a fairly simple iRule and STREAM replacement, to a much more elaborate ProxyPass or 11.4 rewrite profile. So is it just the Host header that needs to be replaced, or the URI as well?