How to enable X-Forwarded-Host ?

The following is part of my baseline HTTP iRule:

when HTTP_REQUEST {
 
	# Remove untrusted HTTP X-Forwarded headers
	# from HTTP requests and set our own:
	#
	# - X-Forwarded-For
	# - X-Forwarded-Host
	# - X-Forwarded-Port
	# - X-Forwarded-Proto
	# - X-Real-IP
	#
 
	#  X-Forwarded headers clean-up
	#
	HTTP::header remove X-Forwarded-For
	HTTP::header remove X-Forwarded-Host
	HTTP::header remove X-Forwarded-Port
	HTTP::header remove X-Forwarded-Proto
	HTTP::header remove X-Real-IP
 
	# Set our own X-Forwarded and X-Real-IP headers
 
	# X-FORWARDED-FOR and X-REAL-IP
	#
	HTTP::header insert X-Forwarded-For [IP::client_addr]
	HTTP::header insert X-Real-IP [IP::client_addr]
 
	# X-FORWARDED-HOST
	#
	if { [HTTP::host] ne "" } {
		HTTP::header insert X-Forwarded-Host [HTTP::host]
	} else {
	    HTTP::header insert X-Forwarded-Host [clientside {IP::local_addr}]
	}
 
	# X-FORWARDED-PORT
	#
	HTTP::header insert X-Forwarded-Port [TCP::local_port]
 
	# X-FORWARDED-PROTO
	#
	if { [PROFILE::exists clientssl] == 1 } {
		HTTP::header insert X-Forwarded-Proto "https"
	} else {
		HTTP::header insert X-Forwarded-Proto "http"
	}
}