NameSurfer Suite
SRV records
FusionLayer

NameSurfer 7.6.4.1


The SRV (Location of Service) resource record type can be used to to specify location of the servers for specific protocol and domain name.

Name specifies protocol and it has a form _Service._Proto, where Service is a name, for example http and Proto is protocol name, for example tcp or udp.

Resource record contents is specified as a list of whitespace-separated words:

  • Priority which is an unsigned integer in a range 0-65535. Smaller number means higher priority. The client will always be directed to the server with smallest priority if one of them is available.
  • Weight which is an unsigned integer in a range 0-65535. This number specifies the probability for this service of being selected. If you have only one server for this service it is recommended to specify 0 as weight. Otherwise probability for a particular server of being selected is proportional to the weight, which means that server with low weight will be selected rarely and a zero-weight server have very small chance of being selected.
  • Port number is a target port number of this service.
  • Target is a domain name of the service target host. Target name "." banns that the service is not available at this domain.
For example, one can specify two web servers for a domain. In this case RR name could be _http._tcp and resource records could be 
0 1 80 old-slow-machine.example.com
0 3 80 new-fast-machine.example.com
1 0 80 backup-machine.example.com
If one of old-slow-machine and new-fast-machine is available web clients will access these machines and faster machine is supposed to be 3 times havier loaded than the old one. If both are down clients are directed to backup machine.

Records like 

*._udp          SRV  0 0 0 .
specifies that no UDP based services are available.

Functionality of this DNS feature depends very much on clients software. If client does not support SRV records than these priority and load balancing mechanism will not function.