Express does not recognise used ports allocated by other processes (not started by express)

[zukoaang]

Hey,
There's a weird bug, that express does not recognise used ports by other processes (not started by node/express).

For example:
create a simple express app that listen on port 9000, that return something like 'hello express' in the response.
in the other hand, start another listening service on the same port, like using python:
python -m SimpleHTTPServer 9090

then open the browser, and create a request to localhost:9000, and you will see response from express
then, create another request to 127.0.0.1:9000, and you will see response from python (ftp serve)

some meta:
OS: MacOS 10.14
node -v: v11.13.0
npm -v: 6.7.0
express version (package.json): ^4.16.2

(found with @https://github.com/20matan)

Thanks

[20matan]

*typo in port 9090, should use the same port for both processes

[LinusU]

Could it be that localhost is resolving to an ipv6 address that makes them route to different places?

I think that express binds on 0.0.0.0 (all interfaces) by default, whereas python binds to 127.0.0.1 (the loopback interface) by default, which is probably also relevant...

[20matan]

@LinusU I don't think there's a difference in the route, because if you do this process in the other way - first run the express app and then the python, the python throws "address already in use" error.

I think it's a bug in express (or even node?) in the way of checking whether the port is already allocated

[dougwilson]

Based on the description, this sounds like a Node.js bug, as Express.js doesn't implement listening functionality -- the app.listen is just straight through to the Node.js core server.listen method.

Can you try this example server and see if you get the same behavior?

var http = require('http')
var server = http.createServer((req, res) => res.end('hello node'))
server.listen(9000)

[LinusU]

because if you do this process in the other way - first run the express app and then the python, the python throws "address already in use" error.

I think this relates to what I wrote about default interfaces. When you start python first it binds to 127.0.0.1, and leaves all other interfaces open. When you then start Express it will bind on "all available interfaces", which will be everything except 127.0.0.1.

When you start Express first, it will bind to all interfaces, including 127.0.0.1. When then starting python the 127.0.0.1 interface will already be taken.

That's also why 127.0.0.1 will route to python, whereas localhost will route to express. Python is only listening to 127.0.0.1, and express is listening to all other interfaces...

edit: to clarify, I don't think that this is a bug, it's just how binding to interfaces works. Being able to bind to different programs to the same port, but on different interfaces, is a feature, not a bug.

[dougwilson]

Closing, as this is a Node.js issue, rather than an Express.js issue.