I have a public Github repo and an on-prem Jenkins server (this could not be changed)
I want a Jenkins job to start every time a PR is opened in the github repo.
Now, if my Jenkins had public access I will simply set up a webhook on github, but since github can not access my Jenkins what can I do to trigger the job?
Is there an option to set a webhook the other way around? meaning the Jenkins will check every few seconds/minutes if a PR was opened and if so it will trigger the job.
You can configure the GitHub Pull Request Builder Plugin to poll for PR refspecs, although it works somewhat differently than using webhooks.
Alternatively, assuming you have connectivity from the on-premise environment to the outside, you can setup ngrok / webhook relay / smee to have a publicly-accessible address that forwards GitHub webhooks. CloudBees have a video on forwarding webhooks using ngrok.
If a DMZ is possible then using tunnels such as GitWebhookProxy, stunnel, or SSH are also good options.
