ssh/knownhosts: fix bracket normalisation

1. When using the bracketed syntax, the port is mandatory, even if
   it's the default one. Otherwise, OpenSSH rejects it with:
   "address [abcd:abcd:abcd:abcd]: missing port in address".
   See sshd(8): SSH_KNOWN_HOSTS FILE FORMAT.

2. Brackets are not necessary when using the default port,
   even for IPv6 addresses.

Fixes golang/go#53463

ssh/knownhosts/knownhosts.go

@@ -439,14 +439,12 @@ func New(files ...string) (ssh.HostKeyCallback, error) {
 func Normalize(address string) string {
 	host, port, err := net.SplitHostPort(address)
 	if err != nil {
-		host = address
+		host = strings.Trim(address, "[]")
 		port = "22"
 	}
 	entry := host
 	if port != "22" {
 		entry = "[" + entry + "]:" + port
-	} else if strings.Contains(host, ":") && !strings.HasPrefix(host, "[") {
-		entry = "[" + entry + "]"
 	}
 	return entry
 }

ssh/knownhosts/knownhosts_test.go

@@ -247,7 +247,7 @@ func TestLine(t *testing.T) {
 		"server.org":                             "server.org " + edKeyStr,
 		"server.org:22":                          "server.org " + edKeyStr,
 		"server.org:23":                          "[server.org]:23 " + edKeyStr,
-		"[c629:1ec4:102:304:102:304:102:304]:22": "[c629:1ec4:102:304:102:304:102:304] " + edKeyStr,
+		"[c629:1ec4:102:304:102:304:102:304]:22": "c629:1ec4:102:304:102:304:102:304 " + edKeyStr,
 		"[c629:1ec4:102:304:102:304:102:304]:23": "[c629:1ec4:102:304:102:304:102:304]:23 " + edKeyStr,
 	} {
 		if got := Line([]string{in}, edKey); got != want {
@@ -326,8 +326,8 @@ func TestNormalize(t *testing.T) {
 		"[127.0.0.1]:23":           "[127.0.0.1]:23",
 		"127.0.0.1:23":             "[127.0.0.1]:23",
 		"[a.b.c]:22":               "a.b.c",
-		"[abcd:abcd:abcd:abcd]":    "[abcd:abcd:abcd:abcd]",
-		"[abcd:abcd:abcd:abcd]:22": "[abcd:abcd:abcd:abcd]",
+		"[abcd:abcd:abcd:abcd]":    "abcd:abcd:abcd:abcd",
+		"[abcd:abcd:abcd:abcd]:22": "abcd:abcd:abcd:abcd",
 		"[abcd:abcd:abcd:abcd]:23": "[abcd:abcd:abcd:abcd]:23",
 	} {
 		got := Normalize(in)