I have the following setup:
Dataflow has variable number of workers and hence, I will have different IP address. Is there any way by which I can setup an outbound IP for all the dataflows (when they send a REST request) so that I can filter those requests out at GKE's end.
I have explored Cloud VPN but that might not work as I do not have control over GKE project. I just want an IP that I can provide to GKE load balancer to white list.
If you can't really ask the other project owners to setup a VPC and create a firewall rule to allow only a specific tag (such as dataflow), then you could use a VM with a static external IP as a proxy to the Kubernetes cluster.
This is because you can not know up front what IPs will the Dataflow workers get, so indeed you will get workers within a very large IP range. I think using a specific network tag would be very helpful in this case, since all the Dataflow workers will get that tag.