Background

The 'SRV' Resource Record (RR) allows administrators to use several servers for a single domain, to move services from host to host with little fuss, and to designate some hosts as primary servers for a service and others as backups.

Clients ask for a specific service/protocol for a specific domain (the word domain is used here in the strict RFC 1034 sense), and get back the names of any available servers.


Details

RR Type: SRV
ID: 33 (0x0021)
Defining RFC: RFC 2782
Description: Service Locator
Function: Generalized service location record, used for newer protocols instead of creating protocol-specific records such as MX.
Status: Active

'SRV' RR Syntax

name ttl class type rdata {priority, weight, port, target}

RR Field Example Description
name _kerberos._tcp The name field is derivied from the combining of _service._proto.name

Service - The symbolic name of the desired service, as defined in Assigned Numbers [STD 2] or locally. An underscore (_) is prepended to the service identifier to avoid collisions with DNS labels that occur in nature.
Some widely used services, notably POP, don't have a single universal name. If Assigned Numbers names the service indicated, that name is the only name which is legal for SRV lookups. The Service is case insensitive.

Proto - The symbolic name of the desired protocol, with an underscore (_) prepended to prevent collisions with DNS labels that occur in nature. _TCP and _UDP are at present the most useful values for this field, though any name defined by Assigned Numbers or locally may be used (as for Service). The Proto is case insensitive.

Name - The domain this RR refers to. The SRV RR is unique in that the name one searches for is not this name.
ttl This is the records time to live value (in seconds). If there is no TTL specified, the zone's default $TTL Directive will be used.
class IN Specifies the class to be 'Internet'.
type SRV Specifies the RDATA field will contain data in the SRV RDATA format.
rdata priority 0 The priority of this target host. A client MUST attempt to contact the target host with the lowest-numbered priority it can reach; target hosts with the same priority SHOULD be tried in an order defined by the weight field. The range is 0-65535. This is a 16 bit unsigned integer in network byte order.
weight 100 A server selection mechanism. The weight field specifies a relative weight for entries with the same priority. Larger weights SHOULD be given a proportionately higher probability of being selected. The range of this number is 0-65535. This is a 16 bit unsigned integer in network byte order. Domain administrators SHOULD use Weight 0 when there isn't any server selection to do, to make the RR easier to read for humans (less noisy). In the presence of records containing weights greater than 0, records with weight 0 should have a very small chance of being selected.

In the absence of a protocol whose specification calls for the use of other weighting information, a client arranges the SRV RRs of the same Priority in the order in which target hosts, specified by the SRV RRs, will be contacted. The following algorithm SHOULD be used to order the SRV RRs of the same priority:

  • To select a target to be contacted next, arrange all SRV RRs (that have not been ordered yet) in any order, except that all those with weight 0 are placed at the beginning of the list.
  • Compute the sum of the weights of those RRs, and with each RR associate the running sum in the selected order. Then choose a uniform random number between 0 and the sum computed (inclusive), and select the RR whose running sum value is the first in the selected order which is greater than or equal to the random number selected. The target host specified in the selected SRV RR is the next one to be contacted by the client. Remove this SRV RR from the set of the unordered SRV RRs and apply the described algorithm to the unordered SRV RRs to select the next target host. Continue the ordering process until there are no unordered SRV RRs. This process is repeated for each Priority.

port 88 The port on this target host of this service. The range is 0-65535. This is a 16 bit unsigned integer in network byte order. This is often as specified in Assigned Numbers but need not be.
target dc01.corp.yourdomain.com. The domain name of the target host. There MUST be one or more address records for this name, the name MUST NOT be an alias (in the sense of RFC 1034 or RFC 2181). Implementors are urged, but not required, to return the address record(s) in the Additional Data section. Unless and until permitted by future standards action, name compression is not to be used for this field.

A Target of "." means that the service is decidedly not available at this domain.

Examples

Snippets from a fictitious forward lookup 'corp.yourdomain.com' zone file

Typical 'SRV' Record Entry

 
;
;   Zone records
;
_kerberos._tcp IN SRV 0  100  88        dc01.corp.yourdomain.com.
 
dc01 IN A 192.168.0.10


Web Service load-balancing Example

Snippets from a fictitious forward lookup 'yourdomain.com' zone file
 
;
;   Zone records
;
 
; load-balance the workload between the old-server (@ 25%) and the new-server (@ 75%)
_http._tcp IN SRV 0  1  80        old-server.yourdomain.com.
_http._tcp IN SRV 0  3  80        new-server.yourdomain.com.
 
; if neither the old-server or new-server are available, switch to using the backup-server
_http._tcp IN SRV 1  0  80        backup-server.yourdomain.com.
 
old-server IN A 192.168.0.50
new-server IN A 192.168.0.200
backup-server IN A 192.168.0.100


See Also




This content was last updated on December 11, 2020
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