dial tcp <REMOTE-IP>:6379: connect: connection refused - go

Question

I'm building an application on GKE (Google Kubernetes Engine) and a system using GCE instances of Redis.
When I try to connect from the application pod on GKE to Redis on GCE, I get connection refused.(dial tcp <REMOTE-IP>:6379: connect: connection refused)
The application is written in Go, and the redis library is go-redis(v8).
Why can't I connect?
The source code for the connection part and the part where the error occurs is as follows.
redisServerName = os.Getenv("REDIS_SERVER") // "sample.com:6379"
redisClient = redis.NewClient(&redis.Options{
Addr: redisServerName,
Password: "",
DB: 0,
})
p, err := redisClient.Ping(ctx).Result()
log.Println(p, err)
The hostname is resolved, so it is not a DNS problem, and the redis-cli commands are executable, so it does not seem to be a Firewall problem.
# redis-cli -h <REMOTE_IP> ping
PONG
Postscript
Here is the result of running the command from the Pod with the Go application running
/# redis-cli -h redis.sample.com
redis.sample.com:6379> // can connect
/# nc redis.sample.com 6379
// There is NO response.

Answer 1

I assert that every application in a container will have the same layer 4 (for redis, TCP) access to the network. Since Redis provides no significant access control, this means that if one app on your container has network access to redis server, all other apps on the same container will too. And if one can't contact redis, neither will the other.
On the same container. This is where testing gets tricky, because it isn't helpful or feasible to reproduce your k8s and gke config here.
ICMP Ping and tcp/6379 are different. Just because ping works, doesn't mean Redis can, and vise versa. And different containers will have different network access in k8s and gke.
Do this test on the app container, to take everything possible out of he equation.
apk add redis only pulls in a few packages, only 8MB and provides redis-cli when I tested, but you don't need any client app for redis; it's simple enough to be done with, say, netcat
. You don't have to issue a valid redis cmd, either - if you get an -ERR unknown command response, you know network works:
/ # echo "hi, redis!" |nc localhost 6379
-ERR unknown command `hi,`, with args beginning with: `redis!`,
If it works there and not in Go, it's probably because the environment variable REDIS_SERVER isn't set properly. So you might want to test that at the command line, as well.
nc $REDIS_SERVER 6379