wrong redirect uri for oauth using docker

[chriamue]

When running backend in a docker container using backend as its dns,
a frontend proxying to the backend hosting at example.com,
gitlab url conains backend:5000 which should be example.com:

&redirect_uri=https%3A%2F%2Fbackend%3A5000%2Fauth%2Fauthorized%2Fgitlab&scope=openid+read_user

The config.py contains this for oauth:

OAUTH_REMOTE_APP_GITLAB = dict(
        consumer_key=os.getenv('OAUTH_GITLAB_CONSUMER_KEY', ''),
        consumer_secret=os.getenv('OAUTH_GITLAB_CONSUMER_SECRET', ''),
        base_url='https://gitlab.com/api/v4/user',
        access_token_url='https://gitlab.com/oauth/token',
        access_token_method='POST',
        authorize_url='https://gitlab.com/oauth/authorize',
        request_token_url=None,
        request_token_params={'scope': 'openid read_user'},
    )

Adding
SERVER_NAME='example.com' breaks

The problem is url_for in bundles.oauth.views.OAuthController.login,
which creates http://backend:5000.

return provider.authorize(callback=url_for(
            'o_auth_controller.authorized', remote_app=remote_app,
            _external=True, _scheme='https'))

Is there a way to customize the generated url_for.
Setting SERVER_NAME='example.com' breaks, frontend be able to reach the server using backend:5000.

[chriamue]

A workaround is to change headers host in proxy:

const express = require('express')
const proxy = require('express-http-proxy')
const app = express()
...
app.use(/^\/api|auth\//, proxy(`http://${backendHost}:${backendPort}`, {
proxyReqOptDecorator: function(proxyReqOpts, srcReq) {
    proxyReqOpts.headers['HOST'] = srcReq.headers['host'];
    return proxyReqOpts;
  },
...

[briancappello]

What happens if you set EXTERNAL_SERVER_NAME='example.com' (also with _external=True)?