Host component of bind_url should be used in dynamic rules but isn't

[Kirill888]

Bug description

Not sure if this is a "bug" or misunderstanding of configuration, or a missing feature to be honest.

My use case

• Externally managed traefik proxy that handles some other services besides jupyterhub
• External access via https with SNI (routing based on Host even when using TLS)
• Using FileProvider, generating only dynamic rules

Dynamic routing rules generated by traefik-proxy ignore domain supplied in .public_url configuration.

Expected behaviour

I expect routing rules to include Host() component if that was configured in JupyterHub.bind_url, instead Host rule is only included when jupyterhub runs with host based routing.

If bind_url="https://example.com", I expect dynamic routing rules to be

Host(`example.com`)&&(PathPrefix(`/{ROUTESPEC}/`)||Path(`/{ROUTESPEC}`))  

Actual behaviour

generated dynamic routing rules only include PathPrefix and Path rules. Confirmed by code inspection here:

traefik-proxy/jupyterhub_traefik_proxy/traefik_utils.py

Lines 44 to 64 in 9dcc5b6

	if routespec.startswith("/"):
	# Path-based route, e.g. /proxy/path/
	host = ""
	path = routespec
	else:
	# Host-based routing, e.g. host.tld/proxy/path/
	host, slash, path = routespec.partition("/")
	path = slash + path
	path_no_slash = path.rstrip("/")
	path_rule = f"PathPrefix(`{path}`)"
	if path_no_slash:
	# include exact Path('/prefix') so that both /prefix/ and /prefix
	# are served correctly
	path_rule = f"( {path_rule} || Path(`{path_no_slash}`) )"
	if host:
	rule = f"Host(`{host}`) && {path_rule}"
	else:
	rule = path_rule

I think generate_rule function should take host argument extracted from .public_url configuration and if that is not empty include it in the rule.

How to reproduce

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Your personal set up

• OS:

• Version(s):

Full environment

# paste output of `pip freeze` or `conda list` here

Configuration

# jupyterhub_config.py

Logs

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