Calling an application outside cluster from a pod

Question

There is a web service app running on a Compute Engine and a GKE cluster in the same network.

Is it possible for a pod in the cluster to call the web service app using internal IP address of web service app?

Your answer will be appreciated.

Thanks.

Answer 1

TL;DR

Yes it's possible.

Assuming that you are talking about the Internal IP address of your VM you will need to create a rule allowing traffic from pod address range to your VM.

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Example

Assuming that:

• There is a Compute Engine instance named: nginx and it's configured to run on port 80.
• There is a Kubernetes Engine within the same network as your GCE instance.

You will need to check the pod ip address range of your GKE cluster. You can do it by either:

• Cloud Console (Web UI)
• $ gcloud container clusters describe CLUSTER-NAME --zone=ZONE | grep -i "clusterIpv4Cidr"

The firewall rule could be created by either:

• Cloud Console (Web UI)
• gcloud command like below:

gcloud compute --project=PROJECT-ID firewall-rules create pod-to-vm \
--direction=INGRESS --priority=1000 --network=default \
--action=ALLOW --rules=tcp:80 --source-ranges=clusterIpv4Cidr \
--target-tags=pod-traffic

Disclaimer!

1. Enter the value from last command (describe cluster) in the place of clusterIpv4Cidr
2. You will need to add pod-traffic to your VM's network tags!

After that you can spawn a pod and check if you can communicate with your VM:

• $ kubectl run -it ubuntu --image=ubuntu -- /bin/bash
• $ apt update && apt install -y curl dnsutils

You can communicate with your VM with GKE pods by either:

• IP address of your VM:

root@ubuntu:/# curl IP_ADDRESS
REDACTED
<p><em>Thank you for using nginx.</em></p>
REDACTED

• Name of your VM (nginx):

root@ubuntu:/# curl nginx
REDACTED
<p><em>Thank you for using nginx.</em></p>
REDACTED

You can also check if the name is correctly resolved by running:

root@ubuntu:/# nslookup nginx
Server:     DNS-SERVER-IP
Address:    DNS-SERVER-IP#53

Non-authoritative answer:
Name:   nginx.c.PROJECT_ID.internal
Address: IP_ADDRESS

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Additional resources:

• Stackoverflow.com: Unable to connect from gke to gce