Reject empty listening addresses

Using an empty listening address will result in chasquid listening on a
random port, which is a dangerous misconfiguration.

That is most likely done to prevent it from listening at all.

To prevent this misconfiguration, explicitly reject empty listening
addresses early and with a warning, so there is no ambiguity.

Users can still prevent chasquid from listening by just commenting out
the entry in the config (and not passing any systemd file descriptors).

See https://github.com/albertito/chasquid/issues/45 for more details and
discussion, including alternatives considered.

Thanks to xavierg who reported this via IRC.

chasquid.go

@@ -169,8 +169,13 @@ func main() {
 func loadAddresses(srv *smtpsrv.Server, addrs []string, ls []net.Listener, mode smtpsrv.SocketMode) int {
 	naddr := 0
 	for _, addr := range addrs {
-		// The "systemd" address indicates we get listeners via systemd.
-		if addr == "systemd" {
+		if addr == "" {
+			// An empty address is invalid, to prevent accidental
+			// misconfiguration.
+			log.Errorf("Invalid empty listening address for %v", mode)
+			log.Fatalf("If you want to disable %v, remove it from the config", mode)
+		} else if addr == "systemd" {
+			// The "systemd" address indicates we get listeners via systemd.
 			srv.AddListeners(ls, mode)
 			naddr += len(ls)
 		} else {

test/t-20-bad_configs/c-10-empty_listening_addr/.expected-error

@@ -0,0 +1 @@
+Invalid empty listening address for submission

test/t-20-bad_configs/c-10-empty_listening_addr/chasquid.conf

@@ -0,0 +1,6 @@
+mail_delivery_agent_bin: "test-mda"
+mail_delivery_agent_args: "%to%"
+data_dir: "../.data"
+mail_log_path: "../.logs/mail_log"
+
+submission_address: ""

test/t-20-bad_configs/run.sh

@@ -18,7 +18,7 @@ mkdir -p c-04-no_cert_dirs/certs/
 
 # Generate certs for the tests that need them.
 for i in c-05-no_addrs c-06-bad_maillog c-07-bad_domain_info \
-	c-08-bad_sts_cache c-09-bad_queue_dir ;
+	c-08-bad_sts_cache c-09-bad_queue_dir c-10-empty_listening_addr ;
 do
 	CONFDIR=$i/ generate_certs_for testserver
 done