pub struct SRV { /* fields omitted */ }RFC 2782, DNS SRV RR, February 2000
Introductory example
If a SRV-cognizant LDAP client wants to discover a LDAP server that
supports TCP protocol and provides LDAP service for the domain
example.com., it does a lookup of
_ldap._tcp.example.com
as described in [ARM]. The example zone file near the end of this
memo contains answering RRs for an SRV query.
Note: LDAP is chosen as an example for illustrative purposes only,
and the LDAP examples used in this document should not be considered
a definitive statement on the recommended way for LDAP to use SRV
records. As described in the earlier applicability section, consult
the appropriate LDAP documents for the recommended procedures.
The format of the SRV RR
Here is the format of the SRV RR, whose DNS type code is 33:
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
(There is an example near the end of this document.)
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; the example near the end
shows this clearly.
TTL
Standard DNS meaning [RFC 1035].
Class
Standard DNS meaning [RFC 1035]. SRV records occur in the IN
Class.
Creates a new SRV record data.
priority - lower values have a higher priority and clients will attempt to use these
first.weight - for servers with the same priority, higher weights will be chosen more often.port - the socket port number on which the service is listening.target - like CNAME, this is the target domain name to which the service is associated.
The newly constructed SRV record data.
Priority
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
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
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
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.
Formats the value using the given formatter. Read more
This method tests for self and other values to be equal, and is used by ==. Read more
This method tests for !=.
Feeds this value into the given [Hasher]. Read more
Feeds a slice of this type into the given [Hasher]. Read more
Performs copy-assignment from source. Read more