udhcpc6: add comments about option 39, no code changes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

networking/udhcp/d6_common.h

@@ -87,6 +87,42 @@ struct d6_option {
 #define D6_OPT_IA_PD         25
 #define D6_OPT_IAPREFIX      26
 
+/* RFC 4704 "The DHCPv6 Client FQDN Option"
+ * uint16	option-code	OPTION_CLIENT_FQDN (39)
+ * uint16	option-len	1 + length of domain name
+ * uint8	flags
+ * char[]	domain-name	partial or fully qualified domain name
+ *
+ * Flags format is |MBZ|N|O|S|
+ * The "S" bit indicates whether the server SHOULD or SHOULD NOT perform
+ * the AAAA RR (FQDN-to-address) DNS updates.  A client sets the bit to
+ * 0 to indicate that the server SHOULD NOT perform the updates and 1 to
+ * indicate that the server SHOULD perform the updates.  The state of
+ * the bit in the reply from the server indicates the action to be taken
+ * by the server; if it is 1, the server has taken responsibility for
+ * AAAA RR updates for the FQDN.
+ * The "O" bit indicates whether the server has overridden the client's
+ * preference for the "S" bit.  A client MUST set this bit to 0.  A
+ * server MUST set this bit to 1 if the "S" bit in its reply to the
+ * client does not match the "S" bit received from the client.
+ * The "N" bit indicates whether the server SHOULD NOT perform any DNS
+ * updates.  A client sets this bit to 0 to request that the server
+ * SHOULD perform updates (the PTR RR and possibly the AAAA RR based on
+ * the "S" bit) or to 1 to request that the server SHOULD NOT perform
+ * any DNS updates.  A server sets the "N" bit to indicate whether the
+ * server SHALL (0) or SHALL NOT (1) perform DNS updates.  If the "N"
+ * bit is 1, the "S" bit MUST be 0.
+ *
+ * If a client knows only part of its name, it MAY send a name that is not
+ * fully qualified, indicating that it knows part of the name but does not
+ * necessarily know the zone in which the name is to be embedded.
+ * To send a fully qualified domain name, the Domain Name field is set
+ * to the DNS-encoded domain name including the terminating zero-length
+ * label.  To send a partial name, the Domain Name field is set to the
+ * DNS-encoded domain name without the terminating zero-length label.
+ * A client MAY also leave the Domain Name field empty if it desires the
+ * server to provide a name.
+ */
 #define D6_OPT_CLIENT_FQDN   39
 
 #define D6_OPT_TZ_POSIX      41

networking/udhcp/d6_dhcpc.c

@@ -98,8 +98,12 @@ static const char opt_req[] = {
 
 static const char opt_fqdn_req[] = {
 	(D6_OPT_CLIENT_FQDN >> 8), (D6_OPT_CLIENT_FQDN & 0xff),
-	0, 2,
-	0, 0
+	0, 2, /* optlen */
+	0, /* flags: */
+	/* S=0: server SHOULD NOT perform AAAA RR updates */
+	/* O=0: client MUST set this bit to 0 */
+	/* N=0: server SHOULD perform updates (PTR RR only in our case, since S=0) */
+	0 /* empty DNS-encoded name */
 };
 
 /*** Utility functions ***/