I am using Google Kubernetes Engine to deploy some applications that need to connect to a DB on premises. In order to do that, I have configured a VPN tunnel and created a VPC.
Then, I created a GKE cluster (1 node) that is using that VPC and I can confirm that the DB is accessible by connecting to the node and try to ping the DB server
~ $ sudo toolbox ping 10.197.100.201
Spawning container root-gcr.io_google-containers_toolbox-20180309-00 on
/var/lib/toolbox/root-gcr.io_google-containers_toolbox-20180309-00.
Press ^] three times within 1s to kill container.
PING 10.197.100.201 (10.197.100.201): 56 data bytes
64 bytes from 10.197.100.201: icmp_seq=0 ttl=62 time=45.967 ms
64 bytes from 10.197.100.201: icmp_seq=1 ttl=62 time=44.186 ms`
However, if I try to do the same from a Pod, I am not able to connect.
root@one-shot-pod:/# traceroute 10.197.100.201
traceroute to 10.197.100.201 (10.197.100.201), 30 hops max, 60 byte
packets
1 10.0.0.1 (10.0.0.1) 0.046 ms 0.009 ms 0.007 ms
2 * * *
3 * * *```
What am I missing?
After some investigation, I found the root cause of the problem. Basically, the communication wasn't working properly because there is something called ip masquerade (https://cloud.google.com/kubernetes-engine/docs/how-to/ip-masquerade-agent) that is used for NAT translation.
As GKE has some default addresses that are configured to not be masquerade (on the version that I was using, the defaults were: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16) and the destination ip was 10.197.100.201 (part of 10.0.0.0/8) and that ip was outside the cluster, the solution was modifying the nonMasqueradeCIDRs and remove 10.0.0.0/8 and use 10.44.0.0/14 (GKE cluster CIDR) instead.
In order to do that, I used the following configmap:
apiVersion: v1
data:
config: |-
nonMasqueradeCIDRs:
- 10.44.0.0/14
- 172.16.0.0/12
- 192.168.0.0/16
resyncInterval: 60s
kind: ConfigMap
metadata:
name: ip-masq-agent
namespace: kube-system
After that, to apply the config, you can upload the configmap using the follwing command:
kubectl create configmap ip-masq-agent --from-file <configmap file> --namespace kube-system