HTTP-01 always arrives on port 80 - the CA chooses the port, not the operator -
so it never collides with an admin UI on 443. The conflict only exists for
TLS-ALPN-01, which is the challenge type that does use 443.
Given that, the server keeps a permanent listener on 80 that answers challenges
from a webroot and redirects everything else to the admin UI. Same arrangement
as the webroot plugins for Apache and nginx, and better than letting the ACME
client bind 80 per renewal: nothing binds and unbinds, so a renewal cannot fail
because the port was briefly busy, and the client needs only write access to a
directory instead of the privilege to bind a low port. Port 80 also gets a use
it would want anyway.
The ACME client stays an external program. lego is also a Go library, but
importing it would put a large dependency tree into a server that deliberately
has none, and the CLI does the same job from a timer.
Tokens are validated by *shape* before any filesystem call, so traversal never
reaches the disk - a stronger guarantee than sanitising a path and trusting the
sanitiser.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
