Connection refused for NextJS inside Docker swarm

[indapublic]

Summary

I have my private self-hosted docker image with nextjs inside. This image exposes 3000 port.

I tried to run this image via docker and docker swarm.

1. Docker

docker run -p 3000:3000 path-to-my-image

Result:

docker ps:

0.0.0.0: 3000->3000/tcp, ::: 3000->3000/tcp

ps inside container:

PID   USER     TIME  COMMAND
    1 nextjs    0:00 next-router-wo
   22 nextjs    0:01 next-render-worker-app
   23 nextjs    0:01 next-render-worker-pages
   51 nextjs    0:00 sh
   57 nextjs    0:00 ps

netstat -tulp|grep 3000 inside container:

tcp        0      0 cf748210f6f3:3000       0.0.0.0:*               LISTEN      1/next-router-wo

curl http://127.0.0.1:3000 outputs correct response. So all is fine.

2. Docker swarm

stack.yaml

version: "3.7"

services:
  app:
    image: path-to-my-image
    ports:
      - 3000:3000
    deploy:
      mode: replicated
      replicas: 1
      placement:
        constraints: [node.role == manager]

docker service ls shows service is online, 1 replica

ps inside container:

PID   USER     TIME  COMMAND
    1 nextjs    0:00 next-router-wo
   22 nextjs    0:01 next-render-worker-app
   23 nextjs    0:01 next-render-worker-pages
   50 nextjs    0:00 sh
   56 nextjs    0:00 ps

netstat -tulp|grep 3000 inside container:

tcp        0      0 3e53ca570e77:3000       0.0.0.0:*               LISTEN      1/next-router-wo

But curl http://127.0.0.1:3000 fails:

curl: (7) Failed to connect to 127.0.0.1 port 3000 after 0 ms: Connection refused

Tried another port, another machines, tried node and pm2. All is same.

I can’t understand the reason and where to dig next.

Thank you in advance.

Additional information

Did two repositories (and public docker images): 

1. github.com/indapublic/docker-swarm-expose-test (plain js, works) 

2. github.com/indapublic/docker-swarm-expose-test-nextjs (next js, doesn't work)

Example

No response

[NolanTrem]

By explicitly setting HOSTNAME in my next.js project's Docker image, I was able to solve this.

I found that when I run:

docker run -p 3000:3000 myimage

• Docker maps the host machine’s port 3000 directly to the container’s port 3000
• If Next.js is listening on 0.0.0.0 inside the container, then from your host machine, you can hit http://127.0.0.1:3000 (or http://localhost:3000) successfully

With Docker Swarm when you do:

version: "3.7"
services:
  app:
    image: myimage
    ports:
      - "3000:3000"
    deploy:
      mode: replicated
      replicas: 1
      placement:
        constraints: [node.role == manager]

Then run:

docker stack deploy -c stack.yaml myapp

• I see the container is running and is listening on port 3000 (inside the container)
• But accessing http://127.0.0.1:3000 (or curl localhost:3000) from the manager node gives “connection refused” or "connection reset"

This seems to come to the fact that Swarm publishes that port via its ingress network , so you typically have to use the node’s actual IP (e.g., 192.168.x.x:3000) from outside the container or from another machine.

Also, if your Next.js server is not listening on 0.0.0.0, but only on localhost, that can cause issues in Swarm. The container needs to bind to all interfaces so Swarm’s ingress traffic can route through.

I think this comes down to two main things:

1. Swarm Routing Mesh

   • When you expose a port in Swarm (3000:3000), Swarm sets up routing that listens on the host IP (generally 0.0.0.0 on the node).
   • Traffic arrives at that external IP:port combination, then is forwarded inside to the container’s network interface on port 3000.
   • If you try to curl 127.0.0.1:3000 inside the container, it’s not the same interface that’s receiving the published traffic from outside.

2. Next.js May Default to localhost

   • Some Node or Next.js setups default to listening on 127.0.0.1 or localhost only.
   • For container-based deployment (especially in Swarm), you typically need to bind to 0.0.0.0. That ensures the service is reachable on any network interface, including the swarm overlay network.
   • If Next.js only listens on 127.0.0.1, connections from the ingress are dropped.

TLDR; Set HOSTNAME=0.0.0.0 (or use -H 0.0.0.0 in your startup command) and then curl your node’s IP, you should see a successful response.

[jingjiezhou2021]

P.S. Even if you pass -H 0.0.0.0, Docker automatically sets a HOSTNAME environment variable (the container ID). Next.js will sometimes pick up this HOSTNAME and bind to that internal container IP instead of 0.0.0.0.
The practical fix is always explicitly set the environment variable HOSTNAME=0.0.0.0 in your Dockerfile or compose file

[riteshreg]

facing same issue