Zero Configuration Networking: Using the Java APIs, Part 2
Pages: 1, 2, 3, 4, 5, 6

Adding, Updating, and Removing Additional Records

A standard DNS-SD service is described by two DNS records: an SRV record, giving target host and port number, and a TXT record, containing zero or more key/value attributes. For almost all applications, advertising a service with these two records is all that's needed. However, there are certain applications—iChat being the prime example—that have extra requirements. For the benefit of applications like this, DNS-SD provides some additional specialized APIs to add, update, and remove additional records.

DNS-SD allows applications to add additional DNS records to an existing service registration using DNSSDRegistration's addRecord method. iChat attaches a small JPEG image to each advertised service, containing the user's icon or picture, and because this is too large to fit in a TXT record attribute, iChat attaches it as an additional record. Adding records like this is something that should not be done indiscriminately because of the cost in increased network traffic, but in the case of iChat, it is the most appropriate way to communicate a user's icon to all the other iChat clients on the local network.

Calling addRecord( ) returns a DNSRecord object, which supports two operations, update( ) and remove( ). If you need to change the data in the record (as iChat does when the user changes the icon), then you can use update( ) to provide new data to replace the old data in the record.

When adding a record, you need to specify the DNS type of the record. The original DNS types are listed in RFC 1035, and newer types are given in later RFCs. For example, the SRV record type (type 33) is specified in RFC 2782. You can also find the list of currently defined DNS types at http://www.iana.org/assignments/dns-parameters. On many systems, you can also find the defined types listed in one of the C header files, such as /usr/include/nameser.h or /usr/include/dns_sd.h. The current IANA list of DNS types is shown in Table 8-2.

Table 8-2. DNS resource record types

TYPE	Value	Meaning	Reference
A	1	A host address	RFC1035
NS	2	An authoritative name server	RFC1035
MD	3	A mail destination (OBSOLETE; use MX)	RFC1035
MF	4	A mail forwarder (OBSOLETE; use MX)	RFC1035
CNAME	5	The canonical name for an alias	RFC1035
SOA	6	Marks the start of a zone of authority	RFC1035
MB	7	A mailbox domain name (EXPERIMENTAL)	RFC1035
MG	8	A mail group member (EXPERIMENTAL)	RFC1035
MR	9	A mail rename domain name (EXPERIMENTAL)	RFC1035
NULL	10	A null RR (EXPERIMENTAL)	RFC1035
WKS	11	A well-known service description	RFC1035
PTR	12	A domain name pointer	RFC1035
HINFO	13	Host information	RFC1035
MINFO	14	Mailbox or mail list information	RFC1035
MX	15	Mail exchange	RFC1035
TXT	16	Text strings	RFC1035
RP	17	For Responsible Person	RFC1183
AFSDB	18	For AFS Data Base location	RFC1183
X25	19	For X.25 PSDN address	RFC1183
ISDN	20	For ISDN address	RFC1183
RT	21	For Route Through	RFC1183
NSAP	22	For NSAP address, NSAP style A record	RFC1706
NSAP-PTR	23
SIG	24	For security signature	RFC2535 RFC3755 RFC4034
KEY	25	For security key	RFC2535 RFC3755 RFC4034
PX	26	X.400 mail mapping information	RFC2163
GPOS	27	Geographical Position	RFC1712
AAAA	28	IP6 Address	Thomson
LOC	29	Location Information	Vixie
NXT	30	Next Domain (OBSOLETE)	RFC2535, RFC3755
EID	31	Endpoint Identifier	Patton
NIMLOC	32	Nimrod Locator	Patton
SRV	33	Server Selection	RFC2782
ATMA	34	ATM Address	Dobrowski
NAPTR	35	Naming Authority Pointer	RFC2168, RFC2915
KX	36	Key Exchanger	RFC2230
CERT	37	CERT	RFC2538
A6	38	A6	RFC2874
DNAME	39	DNAME	RFC2672
SINK	40	SINK	Eastlake
OPT	41	OPT	RFC2671
APL	42	APL	RFC3123
DS	43	Delegation Signer	RFC3658
SSHFP	44	SSH Key Fingerprint	RFC-ietf-secsh-dns-05.txt
IPSECKEY	45	IPSECKEY	RFC4025
RRSIG	46	RRSIG	RFC3755
NSEC	47	NSEC	RFC3755
DNSKEY	48	DNSKEY	RFC3755
UINFO	100	IANA-Reserved
UID	101	IANA-Reserved
GID	102	IANA-Reserved
UNSPEC	103	IANA-Reserved
TKEY	249	Transaction Key	RFC2930
TSIG	250	Transaction Signature	RFC2845
IXFR	251	Incremental transfer	RFC1995
AXFR	252	Transfer of an entire zone	RFC1035
MAILB	253	Mailbox-related RRs (MB, MG, or MR)	RFC1035
MAILA	254	Mail agent RRs (OBSOLETE; see MX)	RFC1035
ANY	255	A request for any record(s)	RFC1035

When adding or updating records, it is your responsibility to make sure that the byte array data you provide is properly formatted for the DNS record type in question. You can specify the DNS time to live (TTL), though for most applications, it's most sensible to simply pass zero and let DNS-SD use its default TTL.

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