Caddy is not throwing an error on MacOS if address is already in use

[fschoell]

Caddy starts up without issues on MacOS even if there is already another process listening on the same port.

Example:

# test1/Caddyfile
:80 {
  respond "hello world server1"
}

# test2/Caddyfile
:80 {
  respond "hello world server2"
}

Starting up server 1:

$ caddy run --config test1/Caddyfile
2024/08/12 11:58:43.773	INFO	using config from file	{"file": "test1/Caddyfile"}
2024/08/12 11:58:43.773	INFO	adapted config to JSON	{"adapter": "caddyfile"}
2024/08/12 11:58:43.775	INFO	admin	admin endpoint started	{"address": "localhost:2019", "enforce_origin": false, "origins": ["//[::1]:2019", "//127.0.0.1:2019", "//localhost:2019"]}
2024/08/12 11:58:43.775	WARN	http.auto_https	server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server	{"server_name": "srv0", "http_port": 80}
2024/08/12 11:58:43.775	INFO	tls.cache.maintenance	started background certificate maintenance	{"cache": "0xc000676280"}
2024/08/12 11:58:43.775	INFO	http.log	server running	{"name": "srv0", "protocols": ["h1", "h2", "h3"]}
2024/08/12 11:58:43.775	INFO	autosaved config (load with --resume flag)	{"file": "/Users/work/Library/Application Support/Caddy/autosave.json"}
2024/08/12 11:58:43.775	INFO	serving initial configuration
2024/08/12 11:58:43.799	INFO	tls	storage cleaning happened too recently; skipping for now	{"storage": "FileStorage:/Users/work/Library/Application Support/Caddy", "instance": "394620b5-a9e0-4018-b6c0-60875ef7ed91", "try_again": "2024/08/13 11:58:43.799", "try_again_in": 86399.999999331}
2024/08/12 11:58:43.800	INFO	tls	finished cleaning storage units

Starting up server 2:

$ caddy run --config test2/Caddyfile
2024/08/12 11:59:44.663	INFO	using config from file	{"file": "test2/Caddyfile"}
2024/08/12 11:59:44.664	INFO	adapted config to JSON	{"adapter": "caddyfile"}
2024/08/12 11:59:44.665	INFO	admin	admin endpoint started	{"address": "localhost:2019", "enforce_origin": false, "origins": ["//localhost:2019", "//[::1]:2019", "//127.0.0.1:2019"]}
2024/08/12 11:59:44.665	WARN	http.auto_https	server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server	{"server_name": "srv0", "http_port": 80}
2024/08/12 11:59:44.665	INFO	tls.cache.maintenance	started background certificate maintenance	{"cache": "0xc00068b780"}
2024/08/12 11:59:44.665	INFO	http.log	server running	{"name": "srv0", "protocols": ["h1", "h2", "h3"]}
2024/08/12 11:59:44.665	INFO	autosaved config (load with --resume flag)	{"file": "/Users/work/Library/Application Support/Caddy/autosave.json"}
2024/08/12 11:59:44.665	INFO	serving initial configuration
2024/08/12 11:59:44.687	INFO	tls	storage cleaning happened too recently; skipping for now	{"storage": "FileStorage:/Users/work/Library/Application Support/Caddy", "instance": "394620b5-a9e0-4018-b6c0-60875ef7ed91", "try_again": "2024/08/13 11:59:44.687", "try_again_in": 86399.999999446}
2024/08/12 11:59:44.687	INFO	tls	finished cleaning storage units

Sending a request is answered by server 1:

$ curl http://localhost
hello world server1%

Expected behaviour: Server 2 should not start up but throw an error, such as it does on Linux systems:

$ caddy run --config test2/Caddyfile
2024/08/12 11:56:25.354	INFO	using config from file	{"file": "test1/Caddyfile"}
2024/08/12 11:56:25.355	INFO	adapted config to JSON	{"adapter": "caddyfile"}
2024/08/12 11:56:25.356	INFO	admin	admin endpoint started	{"address": ":2020", "enforce_origin": false, "origins": ["//:2020"]}
2024/08/12 11:56:25.356	WARN	admin	admin endpoint on open interface; host checking disabled	{"address": ":2020"}
2024/08/12 11:56:25.356	WARN	http.auto_https	server is listening only on the HTTP port, so no automatic HTTPS will be applied to this server	{"server_name": "srv0", "http_port": 80}
2024/08/12 11:56:25.356	INFO	tls.cache.maintenance	started background certificate maintenance	{"cache": "0xc00072be80"}
2024/08/12 11:56:25.356	INFO	tls.cache.maintenance	stopped background certificate maintenance	{"cache": "0xc00072be80"}
Error: loading initial config: loading new config: http app module: start: listening on :80: listen tcp :80: bind: address already in use

Versions:

$  caddy --version
v2.8.4 h1:q3pe0wpBj1OcHFZ3n/1nl4V4bxBrYoSoab7rL9BMYNk=

MacOS: 14.6.1

[mholt]

I've noticed that quirk about macOS. Kinda weird/cool I guess! It also doesn't require privileges to bind lower ports.

Not sure that this is a Caddy issue. 🤷‍♂️