How to configure Docker port mapping to use Nginx as an upstream proxy?

@T0xicCode's answer is correct, but I thought I would expand on the details since it actually took me about 20 hours to finally get a working solution implemented.

If you're looking to run Nginx in its own container and use it as a reverse proxy to load balance multiple applications on the same server instance then the steps you need to follow are as such:

Link Your Containers

When you docker run your containers, typically by inputting a shell script into User Data, you can declare links to any other running containers. This means that you need to start your containers up in order and only the latter containers can link to the former ones. Like so:

#!/bin/bashsudo docker run -p 3000:3000 --name API mydockerhub/apisudo docker run -p 3001:3001 --link API:API --name App mydockerhub/appsudo docker run -p 80:80 -p 443:443 --link API:API --link App:App --name Nginx mydockerhub/nginx

So in this example, the API container isn't linked to any others, but the App container is linked to API and Nginx is linked to both API and App.

The result of this is changes to the env vars and the /etc/hosts files that reside within the API and App containers. The results look like so:

/etc/hosts

Running cat /etc/hosts within your Nginx container will produce the following:

172.17.0.5  0fd9a40ab5ec127.0.0.1   localhost::1 localhost ip6-localhost ip6-loopbackfe00::0 ip6-localnetff00::0 ip6-mcastprefixff02::1 ip6-allnodesff02::2 ip6-allrouters172.17.0.3  App172.17.0.2  API

ENV Vars

Running env within your Nginx container will produce the following:

API_PORT=tcp://172.17.0.2:3000API_PORT_3000_TCP_PROTO=tcpAPI_PORT_3000_TCP_PORT=3000API_PORT_3000_TCP_ADDR=172.17.0.2APP_PORT=tcp://172.17.0.3:3001APP_PORT_3001_TCP_PROTO=tcpAPP_PORT_3001_TCP_PORT=3001APP_PORT_3001_TCP_ADDR=172.17.0.3

I've truncated many of the actual vars, but the above are the key values you need to proxy traffic to your containers.

To obtain a shell to run the above commands within a running container, use the following:

sudo docker exec -i -t Nginx bash

You can see that you now have both /etc/hosts file entries and env vars that contain the local IP address for any of the containers that were linked. So far as I can tell, this is all that happens when you run containers with link options declared. But you can now use this information to configure nginx within your Nginx container.

Configuring Nginx

This is where it gets a little tricky, and there's a couple of options. You can choose to configure your sites to point to an entry in the /etc/hosts file that docker created, or you can utilize the ENV vars and run a string replacement (I used sed) on your nginx.conf and any other conf files that may be in your /etc/nginx/sites-enabled folder to insert the IP values.

OPTION A: Configure Nginx Using ENV Vars

This is the option that I went with because I couldn't get the /etc/hosts file option to work. I'll be trying Option B soon enough and update this post with any findings.

The key difference between this option and using the /etc/hosts file option is how you write your Dockerfile to use a shell script as the CMD argument, which in turn handles the string replacement to copy the IP values from ENV to your conf file(s).

Here's the set of configuration files I ended up with:

Dockerfile

FROM ubuntu:14.04MAINTAINER Your Name <you@myapp.com>RUN apt-get update && apt-get install -y nano htop git nginxADD nginx.conf /etc/nginx/nginx.confADD api.myapp.conf /etc/nginx/sites-enabled/api.myapp.confADD app.myapp.conf /etc/nginx/sites-enabled/app.myapp.confADD Nginx-Startup.sh /etc/nginx/Nginx-Startup.shEXPOSE 80 443CMD ["/bin/bash","/etc/nginx/Nginx-Startup.sh"]

nginx.conf

daemon off;user www-data;pid /var/run/nginx.pid;worker_processes 1;events {    worker_connections 1024;}http {    # Basic Settings    sendfile on;    tcp_nopush on;    tcp_nodelay on;    keepalive_timeout 33;    types_hash_max_size 2048;    server_tokens off;    server_names_hash_bucket_size 64;    include /etc/nginx/mime.types;    default_type application/octet-stream;    # Logging Settings    access_log /var/log/nginx/access.log;    error_log /var/log/nginx/error.log;    # Gzip Settingsgzip on;    gzip_vary on;    gzip_proxied any;    gzip_comp_level 3;    gzip_buffers 16 8k;    gzip_http_version 1.1;    gzip_types text/plain text/xml text/css application/x-javascript application/json;    gzip_disable "MSIE [1-6]\.(?!.*SV1)";    # Virtual Host Configs      include /etc/nginx/sites-enabled/*;    # Error Page Config    #error_page 403 404 500 502 /srv/Splash;}

NOTE: It's important to include daemon off; in your nginx.conf file to ensure that your container doesn't exit immediately after launching.

api.myapp.conf

upstream api_upstream{    server APP_IP:3000;}server {    listen 80;    server_name api.myapp.com;    return 301 https://api.myapp.com/$request_uri;}server {    listen 443;    server_name api.myapp.com;    location / {        proxy_http_version 1.1;        proxy_set_header Upgrade $http_upgrade;        proxy_set_header Connection 'upgrade';        proxy_set_header Host $host;        proxy_set_header X-Real-IP $remote_addr;        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;        proxy_set_header X-Forwarded-Proto $scheme;        proxy_cache_bypass $http_upgrade;        proxy_pass http://api_upstream;    }}

Nginx-Startup.sh

#!/bin/bashsed -i 's/APP_IP/'"$API_PORT_3000_TCP_ADDR"'/g' /etc/nginx/sites-enabled/api.myapp.comsed -i 's/APP_IP/'"$APP_PORT_3001_TCP_ADDR"'/g' /etc/nginx/sites-enabled/app.myapp.comservice nginx start

I'll leave it up to you to do your homework about most of the contents of nginx.conf and api.myapp.conf.

The magic happens in Nginx-Startup.sh where we use sed to do string replacement on the APP_IP placeholder that we've written into the upstream block of our api.myapp.conf and app.myapp.conf files.

This ask.ubuntu.com question explains it very nicely:Find and replace text within a file using commands

GOTCHA On OSX, sed handles options differently, the -i flag specifically. On Ubuntu, the -i flag will handle the replacement 'in place'; it will open the file, change the text, and then 'save over' the same file. On OSX, the -i flag requires the file extension you'd like the resulting file to have. If you're working with a file that has no extension you must input '' as the value for the -i flag.

GOTCHA To use ENV vars within the regex that sed uses to find the string you want to replace you need to wrap the var within double-quotes. So the correct, albeit wonky-looking, syntax is as above.

So docker has launched our container and triggered the Nginx-Startup.sh script to run, which has used sed to change the value APP_IP to the corresponding ENV variable we provided in the sed command. We now have conf files within our /etc/nginx/sites-enabled directory that have the IP addresses from the ENV vars that docker set when starting up the container. Within your api.myapp.conf file you'll see the upstream block has changed to this:

upstream api_upstream{    server 172.0.0.2:3000;}

The IP address you see may be different, but I've noticed that it's usually 172.0.0.x.

You should now have everything routing appropriately.

GOTCHA You cannot restart/rerun any containers once you've run the initial instance launch. Docker provides each container with a new IP upon launch and does not seem to re-use any that its used before. So api.myapp.com will get 172.0.0.2 the first time, but then get 172.0.0.4 the next time. But Nginx will have already set the first IP into its conf files, or in its /etc/hosts file, so it won't be able to determine the new IP for api.myapp.com. The solution to this is likely to use CoreOS and its etcd service which, in my limited understanding, acts like a shared ENV for all machines registered into the same CoreOS cluster. This is the next toy I'm going to play with setting up.

OPTION B: Use /etc/hosts File Entries

This should be the quicker, easier way of doing this, but I couldn't get it to work. Ostensibly you just input the value of the /etc/hosts entry into your api.myapp.conf and app.myapp.conf files, but I couldn't get this method to work.

UPDATE: See @Wes Tod's answer for instructions on how to make this method work.

Here's the attempt that I made in api.myapp.conf:

upstream api_upstream{    server API:3000;}

Considering that there's an entry in my /etc/hosts file like so: 172.0.0.2 API I figured it would just pull in the value, but it doesn't seem to be.

I also had a couple of ancillary issues with my Elastic Load Balancer sourcing from all AZ's so that may have been the issue when I tried this route. Instead I had to learn how to handle replacing strings in Linux, so that was fun. I'll give this a try in a while and see how it goes.

I tried using the popular Jason Wilder reverse proxy that code-magically works for everyone, and learned that it doesn't work for everyone (ie: me). And I'm brand new to NGINX, and didn't like that I didn't understand the technologies I was trying to use.

Wanted to add my 2 cents, because the discussion above around linking containers together is now dated since it is a deprecated feature. So here's an explanation on how to do it using networks. This answer is a full example of setting up nginx as a reverse proxy to a statically paged website using Docker Compose and nginx configuration.

TL;DR;

Add the services that need to talk to each other onto a predefined network. For a step-by-step discussion on Docker networks, I learned some things here:https://technologyconversations.com/2016/04/25/docker-networking-and-dns-the-good-the-bad-and-the-ugly/

Define the Network

First of all, we need a network upon which all your backend services can talk on. I called mine web but it can be whatever you want.

docker network create web

Build the App

We'll just do a simple website app. The website is a simple index.html page being served by an nginx container. The content is a mounted volume to the host under a folder content

DockerFile:

FROM nginxCOPY default.conf /etc/nginx/conf.d/default.conf

default.conf

server {    listen       80;    server_name  localhost;    location / {        root   /var/www/html;        index  index.html index.htm;    }    error_page   500 502 503 504  /50x.html;    location = /50x.html {        root   /usr/share/nginx/html;    }}

docker-compose.yml

version: "2"networks:  mynetwork:    external:      name: webservices:  nginx:    container_name: sample-site    build: .    expose:      - "80"    volumes:      - "./content/:/var/www/html/"    networks:      default: {}      mynetwork:        aliases:          - sample-site

Note that we no longer need port mapping here. We simple expose port 80. This is handy for avoiding port collisions.

Run the App

Fire this website up with

docker-compose up -d

Some fun checks regarding the dns mappings for your container:

docker exec -it sample-site bashping sample-site

This ping should work, inside your container.

Build the Proxy

Nginx Reverse Proxy:

Dockerfile

FROM nginxRUN rm /etc/nginx/conf.d/*

We reset all the virtual host config, since we're going to customize it.

docker-compose.yml

version: "2"networks:  mynetwork:    external:      name: webservices:  nginx:    container_name: nginx-proxy    build: .    ports:      - "80:80"      - "443:443"    volumes:      - ./conf.d/:/etc/nginx/conf.d/:ro      - ./sites/:/var/www/    networks:      default: {}      mynetwork:        aliases:          - nginx-proxy

Run the Proxy

Fire up the proxy using our trusty

docker-compose up -d

Assuming no issues, then you have two containers running that can talk to each other using their names. Let's test it.

docker exec -it nginx-proxy bashping sample-siteping nginx-proxy

Set up Virtual Host

Last detail is to set up the virtual hosting file so the proxy can direct traffic based on however you want to set up your matching:

sample-site.conf for our virtual hosting config:

  server {    listen 80;    listen [::]:80;    server_name my.domain.com;    location / {      proxy_pass http://sample-site;    }  }

Based on how the proxy was set up, you'll need this file stored under your local conf.d folder which we mounted via the volumes declaration in the docker-compose file.

Last but not least, tell nginx to reload it's config.

docker exec nginx-proxy service nginx reload

These sequence of steps is the culmination of hours of pounding head-aches as I struggled with the ever painful 502 Bad Gateway error, and learning nginx for the first time, since most of my experience was with Apache.

This answer is to demonstrate how to kill the 502 Bad Gateway error that results from containers not being able to talk to one another.

I hope this answer saves someone out there hours of pain, since getting containers to talk to each other was really hard to figure out for some reason, despite it being what I expected to be an obvious use-case. But then again, me dumb. And please let me know how I can improve this approach.

Using docker links, you can link the upstream container to the nginx container. An added feature is that docker manages the host file, which means you'll be able to refer to the linked container using a name rather than the potentially random ip.