gunicorn edit header before passing wsgi

case-1-no-config.md

Run server

$ gunicorn main:app

Request/Response

No header

» http localhost:8000/hello/world
HTTP/1.1 200 OK
Connection: close
Content-Length: 39
Content-Type: text/html; charset=UTF-8
Date: Wed, 16 Sep 2020 16:51:30 GMT
Server: gunicorn/20.0.4

<b>Hello world from localhost:8000</b>!

With header

» http localhost:8000/hello/world x-forwarded-host:example.com
HTTP/1.1 200 OK
Connection: close
Content-Length: 39
Content-Type: text/html; charset=UTF-8
Date: Wed, 16 Sep 2020 16:52:00 GMT
Server: gunicorn/20.0.4

<b>Hello world from localhost:8000</b>!

case-2-with-config.md

Run server

$ gunicorn main:app -c config.py

Request/Response

No header

» http localhost:8000/hello/world
HTTP/1.1 200 OK
Connection: close
Content-Length: 39
Content-Type: text/html; charset=UTF-8
Date: Wed, 16 Sep 2020 16:53:12 GMT
Server: gunicorn/20.0.4

With header

» http localhost:8000/hello/world x-forwarded-host:example.com
HTTP/1.1 200 OK
Connection: close
Content-Length: 36
Content-Type: text/html; charset=UTF-8
Date: Wed, 16 Sep 2020 16:53:31 GMT
Server: gunicorn/20.0.4

<b>Hello world from example.com</b>!

config.py

"""Configuration
"""

forwarded_allow_ips = "*"
proxy_protocol = True
proxy_allow_ips = "*"


def pre_request(worker, req):
    """Catch x-forwarded-host
    """
    xf_hosts = [h[1] for h in req.headers if h[0] == "X-FORWARDED-HOST"]
    if not xf_hosts:
        return
    xf_host = xf_hosts[0]
    headers = []
    print(f"Cache X-Forwarded-Host {xf_host}")
    for h in req.headers:
        if h[0] != "HOST":
            headers.append(h)
            continue
        headers.append(("HOST", xf_host))
    req.headers = headers

main.py

from bottle import (
    Bottle, run, request, template,
)


app = Bottle()


@app.route('/hello/<name>')
def index(name):
    host = request.headers.get("Host")
    return template(
        '<b>Hello {{name}} from {{host}}</b>!',
        name=name,
        host=host,
    )


if __name__ == "__main__":
    run(app, host='localhost', port=8000)

requirements.txt

gunicorn
bottle