python flask does not work behind HTTP proxy - flask

Question

I suspect the issue is the HTTP proxy in the server. But I am not sure.
I set up a hello world Flask app on Ubuntu, I was able to access the page by
elinks http://localhost:5000, # and
elinks http://127.0.0.1:5000, # but not
But NOT
elinks http://<server_ip_in_LAN>:5000 # I was also not able to remote access the page on another machine
Then I looked at my proxy settings, in /etc/environment, it has the following:
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
http_proxy="http://proxy-ip:8080/"
https_proxy="http://proxy-ip:8080/"
ftp_proxy="http://proxy-ip:8080/"
git_proxy="http://proxy-ip:8080/"
no_proxy="localhost,127.0.0.1,localaddress,.localdomain.com"
HTTP_PROXY="http://proxy-ip:8080/"
HTTPS_PROXY="http://proxy-ip:8080/"
FTP_PROXY="http://proxy-ip:8080/"
GIT_PROXY="http://proxy-ip:8080/"
NO_PROXY="localhost,127.0.0.1,localaddress,.localdomain.com"
Further, I use ufw to control the firewall, port 5000 is allowed from anywhere.
And I was able to see the following by running nmap -Pn localhost
$ nmap -Pn localhost
Starting Nmap 7.01 ( https://nmap.org ) at 2017-08-04 21:09 EDT
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00016s latency).
Other addresses for localhost (not scanned): ::1
Not shown: 994 closed ports
PORT STATE SERVICE
22/tcp open ssh
80/tcp open http
3000/tcp open ppp
5000/tcp open upnp
5432/tcp open postgresql
8080/tcp open http-proxy
Nmap done: 1 IP address (1 host up) scanned in 0.09 seconds
However, if I run
nmap -Pn <server_ip_in_LAN>,
the "5000/tcp open upnp" line was missing, implying the port number seems only open to my localhost, but not open to LAN.
Why? How can I solve it?
Thanks in advance.

Answer 1

Flask often use internal host:
127.0.0.1
. So you can connect by server_ip_in_LAN by change host IP:
app.run(host= '0.0.0.0')

Answer 2

I have faced the similar kind of issue when I was trying to set my first pycharm project for flask.
Things you need to check
HTTP Proxy(if you are behind a proxy)
Verify the proxy details if proxy added
Check for port if not already used.
for flask specific(if you want to run on specific host and port)
app.run(host='0.0.0.0',port='5000', debug=True)
It gets started on 0.0.0.0 you can change it to localhost.