I would recommend not using nslookup for DNS troubleshooting - it can behave it some unusual ways and presents its results in too abstract a manner to be able to pick apart the nuances of some of the more complicated (and subtle) problems.
Back to the problem, the cause is with the configuration of the st-laurence.wilts.sch.uk domain or, specifically, the DNS configuration of the authoritative servers for that domain...
If we ask Nominet (managers of the .uk namespace) who is responsible for the st-laurence.wilts.sch.uk domain we get:
$ dig @ns1.nic.uk st-laurence.wilts.sch.uk ns
; <<>> DiG 9.3.2-P2.3 <<>> @ns1.nic.uk st-laurence.wilts.sch.uk ns
; (2 servers found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63722
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 0
;; QUESTION SECTION:
;st-laurence.wilts.sch.uk. IN NS
;; AUTHORITY SECTION:
st-laurence.wilts.sch.uk. 172800 IN NS ns0.ifl.net.
st-laurence.wilts.sch.uk. 172800 IN NS ns1.ifl.net.
;; Query time: 42 msec
;; SERVER: 2a01:40:1001:35::2#53(2a01:40:1001:35::2)
;; WHEN: Wed May 4 22:40:33 2011
;; MSG SIZE rcvd: 855
So, ns0.ifl.net and ns1.ifl.net. If we ask them (IFL) the same question we get the same results (as expected). If, however, we ask them for the A record for
www.st-laurence.wilts.sch.uk we get the following:
$ dig @ns0.ifl.net www.st-laurence.wilts.sch.uk
; <<>> DiG 9.3.2-P2.3 <<>> @ns0.ifl.net www.st-laurence.wilts.sch.uk
; (1 server found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 63874
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;www.st-laurence.wilts.sch.uk. IN A
;; ANSWER SECTION:
www.st-laurence.wilts.sch.uk. 0 IN CNAME web6.e4education.co.uk.
;; AUTHORITY SECTION:
. 86400 IN SOA ns0.ifl.net. dnsmaster.rmplc.co.uk. 2011040500 28800 7200 604800 86400
;; Query time: 57 msec
;; SERVER: 194.238.48.51#53(194.238.48.51)
;; WHEN: Wed May 4 22:44:44 2011
;; MSG SIZE rcvd: 142
Okay, so
www.st-laurence.wilts.sch.uk is an alias for web6.e4education.co.uk. No problem with that, however the problem is that ns0.ifl.net thinks that that domain (record) doesn't exist (hence the NXDOMAIN status). The reason for this? Well, the authority section holds the answer - ns0.ifl.net thinks *it* is authoritive for the root of the DNS hence if it doesn't know how to get to the domain then nobody does. This is a misconfiguration (unintentional I'm sure but you never know...).
The inconsistency of results between Plusnet's DNS server and somebody elses (OpenDNS etc) is down to the knowledge (or not) of the e4education.co.uk domain i.e. the domain of the record (web6.e4education.co.uk) that
www.st-laurence.wilts.sch.uk is an alias of. Plusnet appear to be accepting ns0.ifl.net's view of the world and running straight into the misconfiguration brickwall. However, other DNS's are politely ignoring ns0.ifl.net's claims about authority (for the root!) and starting the iterative process again to crawl the e4education.co.uk tree. In particular, assuming no prior knowledge, they are returning to the root (the real root, not ns0.ifl.net's usurped authority), being referred to Nominet, and finding that the authority for e4education.co.uk resides with ns.rackspace.com and ns2.rackspace.com:
$ dig @ns1.nic.uk e4education.co.uk ns
; <<>> DiG 9.3.2-P2.3 <<>> @ns1.nic.uk e4education.co.uk ns
; (2 servers found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24666
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 0
;; QUESTION SECTION:
;e4education.co.uk. IN NS
;; AUTHORITY SECTION:
e4education.co.uk. 172800 IN NS ns2.rackspace.com.
e4education.co.uk. 172800 IN NS ns.rackspace.com.
;; Query time: 43 msec
;; SERVER: 2a01:40:1001:35::2#53(2a01:40:1001:35::2)
;; WHEN: Wed May 4 22:50:55 2011
;; MSG SIZE rcvd: 83
And from the Rackspace servers can then find the target:
$ dig @ns.rackspace.com web6.e4education.co.uk a
; <<>> DiG 9.3.2-P2.3 <<>> @ns.rackspace.com web6.e4education.co.uk a
; (1 server found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19022
;; flags: qr aa rd; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 2
;; QUESTION SECTION:
;web6.e4education.co.uk. IN A
;; ANSWER SECTION:
web6.e4education.co.uk. 86400 IN A 78.46.108.137
web6.e4education.co.uk. 86400 IN A 178.63.100.205
;; AUTHORITY SECTION:
e4education.co.uk. 86400 IN NS ns.rackspace.com.
e4education.co.uk. 86400 IN NS ns2.rackspace.com.
;; ADDITIONAL SECTION:
ns.rackspace.com. 86400 IN A 69.20.95.4
ns2.rackspace.com. 86400 IN A 65.61.188.4
;; Query time: 45 msec
;; SERVER: 69.20.95.4#53(69.20.95.4)
;; WHEN: Wed May 4 22:51:51 2011
;; MSG SIZE rcvd: 152
So, to summarise: the DNS servers at ns0.ifl.net and ns1.ifl.net are incorrectly configured (and appear to have been since 05/04/2011 if their SOA serials are following convention) so St Laurence school ought to be giving them a prod. However, the question of why Plusnet are taking the word of ns0.ifl.net for a record outside of the server's bailiwick ought to be also addressed - the reason being that in misconfiguration cases like these it will unecessarilly stop Plusnet in its tracks.
[Edit: I've reported the misconfiguration to IPL (RM plc)]
Mathew
