Add configurable SASL Kerberos hostname override (Java-style kerberos.domain.name)

CONFIGURATION.md

@@ -91,6 +91,7 @@ ssl.certificate.verify_cb                |  *  |                 |
 sasl.mechanisms                          |  *  |                 |        GSSAPI | high       | SASL mechanism to use for authentication. Supported: GSSAPI, PLAIN, SCRAM-SHA-256, SCRAM-SHA-512, OAUTHBEARER. **NOTE**: Despite the name only one mechanism must be configured. <br>*Type: string*
 sasl.mechanism                           |  *  |                 |        GSSAPI | high       | Alias for `sasl.mechanisms`: SASL mechanism to use for authentication. Supported: GSSAPI, PLAIN, SCRAM-SHA-256, SCRAM-SHA-512, OAUTHBEARER. **NOTE**: Despite the name only one mechanism must be configured. <br>*Type: string*
 sasl.kerberos.service.name               |  *  |                 |         kafka | low        | Kerberos principal name that Kafka runs as, not including /hostname@REALM <br>*Type: string*
+sasl.kerberos.domain.name               |  *  |                 |               | low        | Override for the broker hostname part in the service principal (the portion after `service/`). Use this if the Kerberos ticket should target a fixed FQDN instead of the broker's advertised hostname. <br>*Type: string*
 sasl.kerberos.principal                  |  *  |                 |   kafkaclient | low        | This client's Kerberos principal name. (Not supported on Windows, will use the logon user's principal). <br>*Type: string*
 sasl.kerberos.kinit.cmd                  |  *  |                 | kinit -R -t "%{sasl.kerberos.keytab}" -k %{sasl.kerberos.principal} \|\| kinit -t "%{sasl.kerberos.keytab}" -k %{sasl.kerberos.principal} | low        | Shell command to refresh or acquire the client's Kerberos ticket. This command is executed on client creation and every sasl.kerberos.min.time.before.relogin (0=disable). %{config.prop.name} is replaced by corresponding config object value. <br>*Type: string*
 sasl.kerberos.keytab                     |  *  |                 |               | low        | Path to Kerberos keytab file. This configuration property is only used as a variable in `sasl.kerberos.kinit.cmd` as ` ... -t "%{sasl.kerberos.keytab}"`. <br>*Type: string*

src/rdkafka_conf.c

@@ -996,6 +996,12 @@ static const struct rd_kafka_property rd_kafka_properties[] = {
      "Kerberos principal name that Kafka runs as, "
      "not including /hostname@REALM",
      .sdef = "kafka"},
+    {_RK_GLOBAL, "sasl.kerberos.domain.name", _RK_C_STR,
+     _RK(sasl.domain_name),
+     "Override for the broker hostname portion of the service principal "
+     "(the string after \"service/\"). Set this if the Kerberos ticket "
+     "should always target a fixed FQDN regardless of the broker's "
+     "advertised hostname."},
     {_RK_GLOBAL, "sasl.kerberos.principal", _RK_C_STR, _RK(sasl.principal),
      "This client's Kerberos principal name. "
      "(Not supported on Windows, will use the logon user's principal).",

src/rdkafka_sasl_cyrus.c

@@ -522,6 +522,7 @@ static int rd_kafka_sasl_cyrus_client_new(rd_kafka_transport_t *rktrans,
              rktrans},
             {SASL_CB_CANON_USER, (void *)rd_kafka_sasl_cyrus_cb_canon, rktrans},
             {SASL_CB_LIST_END}};
+        const char *principal_host;
 
         state                       = rd_calloc(1, sizeof(*state));
         rktrans->rktrans_sasl.state = state;
@@ -544,8 +545,13 @@ static int rd_kafka_sasl_cyrus_client_new(rd_kafka_transport_t *rktrans,
 
         memcpy(state->callbacks, callbacks, sizeof(callbacks));
 
+        principal_host = (rk->rk_conf.sasl.domain_name &&
+                          *rk->rk_conf.sasl.domain_name)
+                             ? rk->rk_conf.sasl.domain_name
+                             : hostname;
+
         mtx_lock(&rktrans->rktrans_rkb->rkb_rk->rk_conf.sasl.lock);
-        r = sasl_client_new(rk->rk_conf.sasl.service_name, hostname, NULL,
+        r = sasl_client_new(rk->rk_conf.sasl.service_name, principal_host, NULL,
                             NULL, /* no local & remote IP checks */
                             state->callbacks, 0, &state->conn);
         mtx_unlock(&rktrans->rktrans_rkb->rkb_rk->rk_conf.sasl.lock);

src/rdkafka_sasl_win32.c

@@ -512,9 +512,18 @@ static int rd_kafka_sasl_win32_client_new(rd_kafka_transport_t *rktrans,
         state                       = rd_calloc(1, sizeof(*state));
         rktrans->rktrans_sasl.state = state;
 
-        _snwprintf(state->principal, RD_ARRAYSIZE(state->principal), L"%hs/%hs",
-                   rktrans->rktrans_rkb->rkb_rk->rk_conf.sasl.service_name,
-                   hostname);
+        {
+                const char *principal_host;
+                if (rk->rk_conf.sasl.domain_name &&
+                    *rk->rk_conf.sasl.domain_name)
+                        principal_host = rk->rk_conf.sasl.domain_name;
+                else
+                        principal_host = hostname;
+
+                _snwprintf(state->principal, RD_ARRAYSIZE(state->principal),
+                           L"%hs/%hs", rk->rk_conf.sasl.service_name,
+                           principal_host);
+        }
 
         state->cred = rd_kafka_sasl_sspi_cred_new(rktrans, errstr, errstr_size);
         if (!state->cred)