In particular, the ability to look up DNS resource record data types (including, but not limited to, host names) in the absence of a conventional managed DNS server, is becoming essential.
Multicast DNS (mDNS) provides the ability to do DNS-like operations on the local link in the absence of any conventional unicast DNS server. In addition, mDNS designates a portion of the DNS namespace to be free for local use, without the need to pay any annual fee, and without the need to set up delegations or otherwise configure a conventional DNS server to answer for those names (Source: Abstract of the Multicast DNS internet draft from 2006).
DNS Service Discovery is a way of using standard DNS programming interfaces, servers, and packet formats to browse the network for services (Source: www.dns-sd.org).
References and recommended reading: RFC 1034, RFC 1035, Internet drafts for mDNS and DNS-SD.
Files | |
| file | mdns.c |
| Multicast Domain Name System (MDNS) implementation file for the Netzer application. | |
| file | mdns.h |
| MDNS Client Module Header file for the application. | |
Modules | |
| Resolving records | |
| The module include functions for resolving unique records like hostname or service records. | |
Defines | |
| #define | MDNS_PORT 5353u |
| The MDNS UDP port. | |
| #define | DNS_PORT 53u |
| The DNS UDP port. | |
| #define | DNS_TYPE_A (0x0001u) |
| DNS type for hostnames. | |
| #define | DNS_TYPE_PTR (0x000cu) |
| DNS type for pointer records. | |
| #define | DNS_TYPE_MX (0x000fu) |
| DNS type for mailbox records. | |
| #define | DNS_TYPE_SRV (0x0021u) |
| DNS type for service records. | |
| #define | DNS_TYPE_TXT (0x0010u) |
| DNS type for text records. | |
| #define | DNS_TYPE_ANY (0x00FFu) |
| DNS type for all records. | |
Enumerations | |
| enum | _MDNS_STATE { MDNS_HAS_STOPPED = 0, MDNS_IS_STARTING_UP, MDNS_IS_RUNNING } |
| Possible states of the MDNS responder service. More... | |
Functions | |
| BOOL | bInitMDNSResponder (WORD HTTPPort) |
| The function initializes the MDNS service. | |
| void | StartMDNSResponder (void) |
| The service is started by a call of this function. | |
| void | StopMDNSResponder (void) |
| The service is stopped by a call of this function. | |
| void | MDNSResponder (void) |
| The function implements the state machine of the MDNS responder. | |
| void | MDNSProcessFrame (NODE_INFO *remoteNode, IP_ADDR *localIP) |
| In the MDNS module the state machine and the receiption of frames are divorced. | |
| MDNS_STATE | eGetMDNSState (void) |
| The function retrieves the state of the MDNS responder. | |
| enum _MDNS_STATE |
Possible states of the MDNS responder service.
The state of the service can be requested by the eGetMDNSState function.
| BOOL bInitMDNSResponder | ( | WORD | HTTPPort | ) |
The function initializes the MDNS service.
The application must call this function at least once before starting and using the service. After the sucessful init the application can start the service with StartMDNSResponder. Every time the network interface changes, the application must call this function to ensure the correct functionality of the service.
| HTTPPort | The currently used port of the HTTP service. |
| TRUE | The initialization was successful. | |
| FALSE | The initialization was not successful and therefore the service can not be used. |
| MDNS_STATE eGetMDNSState | ( | void | ) |
The function retrieves the state of the MDNS responder.
For a simplified API only a few states from the internal MDNS state machine are returned.
| void MDNSProcessFrame | ( | NODE_INFO * | remoteNode, | |
| IP_ADDR * | localIP | |||
| ) |
In the MDNS module the state machine and the receiption of frames are divorced.
The receiption of frames is handled stateless and answers will be promptly generated. For quick processing the UDP module should check for the MDNS socket or event he MDNS port in received messages and call this function directly. With the given parameters the function filters for relevant frames.
| remoteNode | Extracted information about the remote node (MAC and IP). | |
| localIP | The IP address where the frame was send to (can be a unicast or multicast address). |
| void MDNSResponder | ( | void | ) |
The function implements the state machine of the MDNS responder.
The application must call this function periodically. The state machine handles the starting and the resolve procedures.
| void StartMDNSResponder | ( | void | ) |
The service is started by a call of this function.
The application should check the state of the service with eGetMDNSState if the service was started successfully. The starting procedure can take some milliseconds or seconds. The procedure includes a so called probing, where all the unique DNS records (see MDNS draft for a detailed discussion about unique and shared records). will be announced to the network. The procedures fails, if the network knows some of this unique records (i.e. the hostname is already taken by another host).
| void StopMDNSResponder | ( | void | ) |
The service is stopped by a call of this function.
This function is called automatically in case the starting procedure fails. If the starting procedure was successful and the service is running before the call so-called "goodbye" announcements (see MDNS draft for details) for shared records will be send.
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