Add -dane_ee_no_namechecks s_client(1) option

The DANE API supports a DANE_FLAG_NO_DANE_EE_NAMECHECKS option, but
there was no way to exercise/enable it via s_client.  This commit
addresses that gap.

Reviewed-by: Rich Salz <rsalz@openssl.org>

apps/s_client.c

@@ -555,7 +555,7 @@ typedef enum OPTION_choice {
 #ifndef OPENSSL_NO_CT
     OPT_CT, OPT_NOCT, OPT_CTLOG_FILE,
 #endif
-    OPT_DANE_TLSA_RRDATA
+    OPT_DANE_TLSA_RRDATA, OPT_DANE_EE_NO_NAME,
 } OPTION_CHOICE;
 
 OPTIONS s_client_options[] = {
@@ -589,6 +589,8 @@ OPTIONS s_client_options[] = {
     {"dane_tlsa_domain", OPT_DANE_TLSA_DOMAIN, 's', "DANE TLSA base domain"},
     {"dane_tlsa_rrdata", OPT_DANE_TLSA_RRDATA, 's',
      "DANE TLSA rrdata presentation form"},
+    {"dane_ee_no_namechecks", OPT_DANE_EE_NO_NAME, '-',
+     "Disable name checks when matching DANE-EE(3) TLSA records"},
     {"reconnect", OPT_RECONNECT, '-',
      "Drop and re-make the connection with the same Session-ID"},
     {"showcerts", OPT_SHOWCERTS, '-', "Show all certificates in the chain"},
@@ -783,6 +785,7 @@ int s_client_main(int argc, char **argv)
     STACK_OF(OPENSSL_STRING) *ssl_args = NULL;
     char *dane_tlsa_domain = NULL;
     STACK_OF(OPENSSL_STRING) *dane_tlsa_rrset = NULL;
+    int dane_ee_no_name = 0;
     STACK_OF(X509_CRL) *crls = NULL;
     const SSL_METHOD *meth = TLS_client_method();
     const char *CApath = NULL, *CAfile = NULL;
@@ -1257,6 +1260,9 @@ int s_client_main(int argc, char **argv)
                 goto end;
             }
             break;
+        case OPT_DANE_EE_NO_NAME:
+            dane_ee_no_name = 1;
+            break;
         case OPT_NEXTPROTONEG:
 #ifndef OPENSSL_NO_NEXTPROTONEG
             next_proto_neg_in = opt_arg();
@@ -1701,6 +1707,8 @@ int s_client_main(int argc, char **argv)
                        "records.\n", prog);
             goto end;
         }
+        if (dane_ee_no_name)
+            SSL_dane_set_flags(con, DANE_FLAG_NO_DANE_EE_NAMECHECKS);
     } else if (dane_tlsa_rrset != NULL) {
         BIO_printf(bio_err, "%s: DANE TLSA authentication requires the "
                    "-dane_tlsa_domain option.\n", prog);

doc/apps/s_client.pod

@@ -24,6 +24,7 @@ B<openssl> B<s_client>
 [B<-no-CApath>]
 [B<-dane_tlsa_domain domain>]
 [B<-dane_tlsa_rrdata rrdata>]
+[B<-dane_ee_no_namechecks>]
 [B<-attime timestamp>]
 [B<-check_ss_sig>]
 [B<-crl_check>]
@@ -225,6 +226,24 @@ whitespace is ignored in the associated data field.  For example:
   DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
   ...
 
+=item B<-dane_ee_no_namechecks>
+
+This disables server name checks when authenticating via DANE-EE(3) TLSA
+records.
+For some applications, primarily web browsers, it is not safe to disable name
+checks due to "unknown key share" attacks, in which a malicious server can
+convince a client that a connection to a victim server is instead a secure
+connection to the malicious server.
+The malicious server may then be able to violate cross-origin scripting
+restrictions.
+Thus, despite the text of RFC7671, name checks are by default enabled for
+DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
+to do so.
+In particular, SMTP and XMPP clients should set this option as SRV and MX
+records already make it possible for a remote domain to redirect client
+connections to any server of its choice, and in any case SMTP and XMPP clients
+do not execute scripts downloaded from remote servers.
+
 =item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>,
 B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>,
 B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>,