Respect the case of realm in the dns -> realm mapping configuration

While unusual, some Kerberos installations use lower case realm names. We
should respect what the configuration says.

Always making them upper case was making the session code fail to find the
existing session for the realm (eg. it was trying to find the session for
EXAMPLE.COM and not example.com) and issue a TGS-REQ as if we were trying to do
cross-realm authentication.

It makes sense to keep the canonicalization of the DNS name though.

config/krb5conf.go

@@ -462,7 +462,7 @@ func (d *DomainRealm) parseLines(lines []string) error {
 		}
 		p := strings.Split(line, "=")
 		domain := strings.TrimSpace(strings.ToLower(p[0]))
-		realm := strings.TrimSpace(strings.ToUpper(p[1]))
+		realm := strings.TrimSpace(p[1])
 		d.addMapping(domain, realm)
 	}
 	return nil

config/krb5conf_test.go

@@ -51,6 +51,10 @@ const (
         admin_server = kerberos.example.com
         auth_to_local = RULE:[1:$1@$0](.*@EXAMPLE.COM)s/.*//
  }
+ lowercase.org = {
+  kdc = kerberos.lowercase.org
+  admin_server = kerberos.lowercase.org
+ }
 
 
 [domain_realm]
@@ -61,6 +65,7 @@ const (
   .example.com = EXAMPLE.COM
  hostname1.example.com = EXAMPLE.COM
  hostname2.example.com = TEST.GOKRB5
+ .testlowercase.org = lowercase.org
 
 
 [appdefaults]
@@ -323,7 +328,7 @@ func TestLoad(t *testing.T) {
 	assert.Equal(t, "FILE:/home/gokrb5/client.keytab", c.LibDefaults.DefaultClientKeytabName, "[libdefaults] default_client_keytab_name not as expected")
 	assert.Equal(t, []string{"aes256-cts-hmac-sha1-96", "aes128-cts-hmac-sha1-96"}, c.LibDefaults.DefaultTktEnctypes, "[libdefaults] default_tkt_enctypes not as expected")
 
-	assert.Equal(t, 2, len(c.Realms), "Number of realms not as expected")
+	assert.Equal(t, 3, len(c.Realms), "Number of realms not as expected")
 	assert.Equal(t, "TEST.GOKRB5", c.Realms[0].Realm, "[realm] realm name not as expectd")
 	assert.Equal(t, []string{"10.80.88.88:749"}, c.Realms[0].AdminServer, "[realm] Admin_server not as expectd")
 	assert.Equal(t, []string{"10.80.88.88:464"}, c.Realms[0].KPasswdServer, "[realm] Kpasswd_server not as expectd")
@@ -513,6 +518,7 @@ func TestResolveRealm(t *testing.T) {
 		{"hostname2.example.com", "TEST.GOKRB5"},
 		{"one.two.three.example.com", "EXAMPLE.COM"},
 		{".test.gokrb5", "TEST.GOKRB5"},
+		{"foo.testlowercase.org", "lowercase.org"},
 	}
 	for _, tt := range tests {
 		t.Run(tt.domainName, func(t *testing.T) {