Skip to main content

7. Fictional Example

7.1 Example Description​

Disclaimer

This example uses fictional service "foobar" as an aid in understanding SRV records. If ever service "foobar" is implemented, it is not intended that it will necessarily use SRV records.

This is (part of) the zone file for example.com, a still-unused domain:

7.2 Zone File​

$ORIGIN example.com.
@               SOA server.example.com. root.example.com. (
                    1995032001 3600 3600 604800 86400 )
                NS  server.example.com.
                NS  ns1.ip-provider.net.
                NS  ns2.ip-provider.net.
; foobar - use old-slow-box or new-fast-box if either is
; available, make three quarters of the logins go to
; new-fast-box.
_foobar._tcp    SRV 0 1 9 old-slow-box.example.com.
                 SRV 0 3 9 new-fast-box.example.com.
; if neither old-slow-box or new-fast-box is up, switch to
; using the sysdmin's box and the server
                 SRV 1 0 9 sysadmins-box.example.com.
                 SRV 1 0 9 server.example.com.
server           A   172.30.79.10
old-slow-box     A   172.30.79.11
sysadmins-box    A   172.30.79.12
new-fast-box     A   172.30.79.13
; NO other services are supported
*._tcp          SRV  0 0 0 .
*._udp          SRV  0 0 0 .

7.3 Client Query​

In this example, a client of the "foobar" service in the "example.com." domain needs:

  • An SRV lookup of "_foobar._tcp.example.com."
  • Possibly A lookups of "new-fast-box.example.com." and/or the other hosts named

7.4 Response Size Analysis​

The size of the SRV reply is approximately 365 bytes:

Breakdown:

  • 30 bytes: general overhead
  • 20 bytes: for the query string, "_foobar._tcp.example.com."
  • 130 bytes: for 4 SRV RR's (20 bytes each plus the lengths of "new-fast-box", "old-slow-box", "server" and "sysadmins-box" - "example.com" in the query section is quoted here and doesn't need to be counted again)
  • 75 bytes: for 3 NS RRs (15 bytes each plus the lengths of "server", "ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is quoted and only needs to be counted once)
  • 120 bytes: for the 6 address records (assuming IPv4 only) mentioned by the SRV and NS RR's
Tags:
Last updated on