There are some packet filtering products that block requests using Nmap's default HTTP user agent. You can use a different user agent value by setting the argument http.useragent:
$ nmap -p80 --script http-sqli-finder --script-args http.useragent="Mozilla 42" <target>
To set the user agent in your NSE script, you can pass the header field, as follows:
options = {header={}} options['header']['User-Agent'] = "Mozilla/9.1 (compatible; Windows NT 5.0 build 1420;)" local req = http.get(host, port, uri, options)
