IRULE to take number from URL dynamically and change to port for web server

when HTTP_REQUEST {
  if {[string tolower [HTTP::uri]] starts_with /login.htmld and [HTTP::query [HTTP::uri] rtport] >= 4500 and [HTTP::query [HTTP::uri] rtport] <= 4599 } {
    HTTP::redirect "https://website.com:[HTTP::query [HTTP::uri] rtport]/login.htmld"
  }
}

Should do the trick

Not sure if the rtport is placed in correct location on redirect. I am new to Irules however...

Not sure if the rtport is placed in correct location on redirect.

e.g.

[root@ve11a:Active:In Sync] config  tmsh list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  if { [string tolower [HTTP::uri]] starts_with "/login.htmld" and [URI::query [HTTP::uri] rtport] >= 4500 and [URI::query [HTTP::uri] rtport] <= 4599 } {
    HTTP::redirect "https://website.com:[URI::query [HTTP::uri] rtport]/login.htmld"
  }
}
}

[root@ve11a:Active:In Sync] config  curl -I http://172.28.24.10/login.htmld?rtport=4555
HTTP/1.0 302 Found
Location: https://website.com:4555/login.htmld
Server: BigIP
Connection: Keep-Alive
Content-Length: 0

Not sure if the rtport is placed in correct location on redirect.

e.g.

[root@ve11a:Active:In Sync] config  tmsh list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  if { [string tolower [HTTP::uri]] starts_with "/login.htmld" and [URI::query [HTTP::uri] rtport] >= 4500 and [URI::query [HTTP::uri] rtport] <= 4599 } {
    HTTP::redirect "https://website.com:[URI::query [HTTP::uri] rtport]/login.htmld"
  }
}
}

[root@ve11a:Active:In Sync] config  curl -I http://172.28.24.10/login.htmld?rtport=4555
HTTP/1.0 302 Found
Location: https://website.com:4555/login.htmld
Server: BigIP
Connection: Keep-Alive
Content-Length: 0

If the url changes will the HTTP::uri pickup if it were something new like /test1/login.htmld?

yes, HTTP::uri returns uri of request.

HTTP::uri