Confusing behaviour when starting a server on the admin port #7053
Description
With this:
% cat Caddyfile
http://localhost:2019 {
file_server browse
}
% caddy run
2025/06/08 15:17:36.279 INFO using adjacent Caddyfile
2025/06/08 15:17:36.279 INFO adapted config to JSON {"adapter": "caddyfile"}
2025/06/08 15:17:36.279 INFO admin admin endpoint started {"address": "localhost:2019", "enforce_origin": false, "origins": ["//localhost:2019", "//[::1]:2019", "//127.0.0.1:2019"]}
2025/06/08 15:17:36.280 INFO tls.cache.maintenance started background certificate maintenance {"cache": "0xc00037da80"}
2025/06/08 15:17:36.280 WARN http HTTP/2 skipped because it requires TLS {"network": "tcp", "addr": ":2019"}
2025/06/08 15:17:36.280 WARN http HTTP/3 skipped because it requires TLS {"network": "tcp", "addr": ":2019"}
2025/06/08 15:17:36.280 INFO http.log server running {"name": "srv0", "protocols": ["h1", "h2", "h3"]}
2025/06/08 15:17:36.280 INFO autosaved config (load with --resume flag) {"file": "/home/martin/.config/caddy/autosave.json"}
2025/06/08 15:17:36.280 INFO serving initial configuration
2025/06/08 15:17:36.281 INFO tls storage cleaning happened too recently; skipping for now {"storage": "FileStorage:/home/martin/.local/share/caddy", "instance": "cf308ad4-1b49-40a8-aca4-dd95b219400e", "try_again": "2025/06/09 15:17:36.281", "try_again_in": 86399.99999964}
2025/06/08 15:17:36.281 INFO tls finished cleaning storage units
2025/06/08 15:17:39.664 INFO admin.api received request {"method": "GET", "host": "localhost:2019", "uri": "/", "remote_ip": "127.0.0.1", "remote_port": "53406", "headers": {"Accept":["text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"],"Accept-Encoding":["gzip, deflate, br, zstd"],"Accept-Language":["en-IE,en;q=0.5"],"Connection":["keep-alive"],"Priority":["u=0, i"],"Sec-Fetch-Dest":["document"],"Sec-Fetch-Mode":["navigate"],"Sec-Fetch-Site":["none"],"Sec-Fetch-User":["?1"],"Upgrade-Insecure-Requests":["1"],"User-Agent":["Mozilla/5.0 (X11; Linux x86_64; rv:139.0) Gecko/20100101 Firefox/139.0"]}}
Going to http://localhost:2019 gives a 404.
The reason is that localhost:2019 is also used as an "admin port" by default. Simply using another port or admin off fixes it.
This is kind of obvious in hindsight, but not being familiar with Caddy this took me quite some time to figure out. I just saw port 2019 referenced somewhere and figured "well, that's a free port on my system, as good as any other".
Trying to use the admin port should probably give an error, or at least a big warning.