domain redirect

Which version you're running on. If running on 11.5.x or above, use the rewrite profile:

• clientside : https://www.blah-test.com/
• serverside : https://www.mainpage.de/

else,

when HTTP_REQUEST { 
if { ([HTTP::host] equals "www.blah-test.com") } { 
HTTP::host [string map {www.blah-test.com www.mainpage.de} [HTTP:host]]
} 
} 
when HTTP_RESPONSE {
if { [HTTP::header values Location] contains "www.mainpage.de" } {
HTTP::header replace Location [string map {www.mainpage.de www.blah-test.com} [HTTP::header value Location]]
}
}