We have an ASP.NET MVC 5 app hosted behind a Netscaler. The app uses Google reCAPTCHA but its outgoing requests by reCAPTCHA fail because the IP addresses are blocked. I researched the IPs it was trying to hit and they're similar to but not listed here: https://code.google.com/archive/p/recaptcha/wikis/FirewallsAndRecaptcha.wiki
This post talks about this issue as well: Recaptcha IP addresses
The IPs change frequently and seem to not be listed anywhere. We can't use the hostname, as we're required to whitelist specific IPs on Netscaler.
Any reliable way to get reCAPTCHA working with Netscaler? If not, what alternative is there to reCAPTCHA?
You can implement recaptcha as an auth method. if you do that your,security team might allow the traffic without ip filters.
Have a look here. https://support.citrix.com/article/CTX216091