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In the last section we were able to successfully build our image and then start up a container based

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on it.

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We saw the message listening on Port ADT but when we actually tried to visit local host ADHD inside

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the browser we got a pretty nasty error message.

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So let's figure out what went wrong and how to fix it.

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All right so here's a diagram of what's happening right now.

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Our browser is making a request to local host ADHD which is essentially a reference to your current

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machine on Port ADHD by default.

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No traffic that is coming into your computer or into your local host network is routed into the container

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so the container essentially has its own isolated set of ports that can receive traffic but by default

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no incoming traffic to your computer is going to be directed into a container in order to make sure

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that any request from either your computer or some outside computer will be redirected into the container.

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We have to setup a explicit port mapping a port mapping essentially says any time that someone makes

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a request to a given port on your local network take that request and automatically forward it to some

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port inside the container.

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So in other words if anyone makes a request to local host 880 take that request automatically forward

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it into the container on Port 880 where the node application can then receive it and process the request

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and eventually respond to it.

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Now one thing that I want to make sure is really clear right now from the get go this is only talking

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about incoming requests your docker container can by default make requests on its own behalf to the

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outside world.

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We've already seen that in action.

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Anytime you've been installing a dependency.

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When we ran npm install during the docker build process just a moment ago NPM reaches to the outside

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world and reaches out across the Internet.

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And so there's no limitation by default on your containers ability to reach out.

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It's strictly a limitation on the ability for incoming traffic to get in to the container.

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So in order to set up this mapping in order to kind of forward this traffic to a specific port inside

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the container we're going to make a slight adjustment to the way in which we start the container up

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or specifically the docker run command.

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So this is not a change that we're going to make to the docker file.

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We do not setup port forwarding inside the doctor file the port forwarding stuff is strictly a runtime

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constraint.

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In other words it's something that we only change when we run a container or start a container up.

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So here's what we're gonna do to setup Docker run with port mapping.

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We're going to add on a dash P flack the syntax overall is Docker run then DSP then we're going to specify

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that any incoming traffic on our local network to this port right here should be automatically forwarded

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on to this port inside the container right here.

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And notice how there's a separation between these two ports are these two numbers of a colon.

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And then as usual after that we're going to specify either the image I.D. or the image name which is

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what we are now using.

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So let's flip over to our terminal and we're going to give this a quick shot.

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I'm going to first stop the running container by pressing control see and then I'm going to start the

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container back up.

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But this time I'm going to set up some port mapping so I'll say anytime a request comes in to 80 80

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on my local machine redirect it to port 80 80 inside the container and then I'll make sure that I still

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specify the image that I want to use for this container as my Docker I.D. you'll put whatever yours

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is right there.

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Simple web and I'm going to get that on what to just one line like so OK so let's start this now from

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the outside it looks like everything is still identical to how it was before.

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But if you flip on over to your browser open up a new tab again and go to local host ADHD you'll very

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quickly see the text.

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Hi there appear on the screen.

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This is the response that we would expect to see from the running node application.

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So that means that we are now correctly forwarding incoming requests into the container the node application

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is processing the request and then sending a response back out.

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So that's pretty much it.

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That is Port mapping in a nutshell.

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Now very quickly one last little thing I want to show you because this will be done very frequently

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on real projects.

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The port from the source machine or the source network that we are mapping from to the port that we

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are mapping to inside the container do not have to be identical.

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So we could very easily say Hey any time that a request comes into local host five thousand redirect

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that request to 880 inside the container.

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And like I said this is actually something that we're going to do quite frequently in production applications

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so now this is essentially saying OK browsers make your request a five thousand redirect request to

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80 80 inside the container.

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Again these two ports do not have to be identical.

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So let's try doing that again with the docker run command.

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I'm going to stop the thing by passing control see and I will do doctor run dash P and I'll say that

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I want to map port five thousand on my local network to 80 80 inside the container and then I'll do

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Steven Greider simple web looks like everything started up successfully.

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Now if I go back over to my browser and I tried to refresh local host colon 880 all very quickly see

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an error message.

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But if I instead go to local host five thousand well there's the node server it is now taking traffic

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or incoming requests to local OES 5000 and redirecting them to 1880 inside the container.

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Now last thing you might be kind of curious about you know this is kind of an obvious but I'm going

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to say simultaneously a silly question can we change the port inside the container.

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Well yes of course we can change this to be you know six thousand seven thousand whatever you want it

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to be just if you change the port inside the container you need to make sure that your actual web server

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application so inside my index dot J as file we specified that our node application should listen on

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Port 88.

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So if you're going to change the target port inside the container.

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Well of course you've got to make sure the change the port that your application is listening to as

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well.

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You would have to change this to 6000 or 7000 or whatever it might be.

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So that's it.

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That is Port mapping in a nutshell.

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The one big thing to remember is that we have to specify it at runtime not inside the docker file.

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So we've got one more step towards success.

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We are actually able to visit our running application inside the browser.

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So like I said we were going to run into a handful of little errors along the way.

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We saw a case in which we were not copying over the Pakistan Jason File we saw using a bad base image

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and we also saw a lack of port forwarding.

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Again these are all little things that I kind of expect that you'll see as soon as you start using Docker

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on your own personal project.

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Now even though our application is successfully running there is one last little thing that I want to

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point out so we're going to go over it one last little fix in the next section.

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So quick break and I'll see you in just a minute.