service
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... SRV RR allows administrators to use several servers for a single
domain, to move services from host to host with little fuss, and to
...
... host with little fuss, and to
designate some hosts as primary servers for a service and others as
backups.
...
... (There is an example near the end of this document.)
Service
The symbolic name of the desired service, as defined in Assigned
...
... Service
The symbolic name of the desired service, as defined in Assigned
Numbers or locally.
...
... Numbers or locally.
Some widely used services, notably POP, don't have a single
universal name. If Assigned Numbers names the service ...
... 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
...
... SRV
lookups. Only locally defined services may be named locally.
The Service is case insensitive.
...
... lookups. Only locally defined services may be named locally.
The Service is case insensitive.
Proto
...
... 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.
...
... The port on this target host of this service. The range is
0-65535. This is often as specified in Assigned Numbers but
...
... and we don't know how widespread this behaviour is.
- Where one service is provided by several hosts, one can either
provide A records ...
...
- If a host is intended to provide a service only when the main
server(s) is/are down, it probably shouldn't be listed in A
records.
...
... Hosts that are referenced by backup A records must use the port
number specified in Assigned Numbers for the service.
Currently there's a practical limit of 512 bytes for DNS ...
... are very round: A reply packet has a 30-byte overhead plus the name
of the service ("telnet.tcp.asdf.com" for instance); each SRV RR adds
...
... asking a separate server when the client selects a server and
contacts it. For short-lived services like SMTP an extra step in the
connection establishment ...
... connection establishment seems too expensive, and for long-lived
services like telnet, the load figure may well be thrown off a minute
after the connection ...
... port number happens
at the client, often using a file such as /etc/services.
Moving this information to the DNS ...
... update
these files on every single computer of the net every time a new
service is added, and makes it possible to move standard services out
of the "root ...
... these files on every single computer of the net every time a new
service is added, and makes it possible to move standard services out
of the "root-only" port range ...
...
If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV
RR which specifies the requested Service and Protocol in the
reply:
...
...
- Port numbers SHOULD NOT be used in place of the symbolic service
or protocol names (for the same reason why variant names cannot
be allowed: Applications would have to do two or more lookups ...
... SRV RR whose Protocol was
TCP and whose Service was SMTP would override RFC 974(-> 2821prop)'s rules
...
... to block internal clients from accessing specific external
services, slightly harder to block internal users from running
unautorised services, and more important for the router ...
... services, slightly harder to block internal users from running
unautorised services, and more important for the router
operations and DNS operations ...
... hosts from being referenced
as servers (as, indeed, some sites become unwilling secondary
MXes today). This could lead to denial of service.
- With SRV ...