Case-Insensitive Response Compression May Cause Problems With Mixed-Case Data and Non-Conforming Clients

[Tony Finch]

I have been testing this and I have found some interesting behaviour
related to negative cacheing and empty non-terminals.

If I query for a name in upper case, and there is no answer, the negative
cache entry is stored in upper case. If I later query for a different type
at the same name and get a positive answer all in lower case, the initial
answer is lower case, but subsequent answers returned from the cache have
some upper case labels.

My test server is running 9.10.0a1 and for this test the relevant part of
the config is:

	view rec {
		match-recursive-only yes;
		zone cam.ac.uk {
			type static-stub;
			server-addresses { ::1; };
		};
	};
	view auth {
		recursion no;
		allow-recursion { none; };
		zone cam.ac.uk {
			type slave;
			masters { ucam; };
	};

Here's a transcript of a shell session illustrating the oddity.

$ rndc flush
$ # create a negative cache entry with upper case labels
$ dig _kerberos._udp.UXSUP.CSI.CAM.AC.UK

; <<>> DiG 9.10.0a1 <<>> _kerberos._udp.UXSUP.CSI.CAM.AC.UK
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 52286
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;_kerberos._udp.UXSUP.CSI.CAM.AC.UK. IN A

;; AUTHORITY SECTION:
cam.ac.uk.              10800   IN      SOA     authdns0.csx.cam.ac.uk. hostmaster.ucs.cam.ac.uk. 1391763551 14400 3600 604800 14400

;; Query time: 890 msec
;; SERVER: ::1#53(::1)
;; WHEN: Fri Feb 07 14:38:11 GMT 2014
;; MSG SIZE  rcvd: 136

$ # get a positive answer in lower case
$ dig _kerberos._udp.UXSUP.CSI.CAM.AC.UK srv

; <<>> DiG 9.10.0a1 <<>> _kerberos._udp.UXSUP.CSI.CAM.AC.UK srv
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 27438
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;_kerberos._udp.UXSUP.CSI.CAM.AC.UK. IN SRV

;; ANSWER SECTION:
_kerberos._udp.uxsup.csi.cam.ac.uk. 86400 IN SRV 0 0 88 ghast.csi.cam.ac.uk.

;; Query time: 0 msec
;; SERVER: ::1#53(::1)
;; WHEN: Fri Feb 07 14:38:14 GMT 2014
;; MSG SIZE  rcvd: 136

$ # get a positive answer from the cache with mixed case
$ dig _kerberos._udp.UXSUP.CSI.CAM.AC.UK srv

; <<>> DiG 9.10.0a1 <<>> _kerberos._udp.UXSUP.CSI.CAM.AC.UK srv
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44019
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;_kerberos._udp.UXSUP.CSI.CAM.AC.UK. IN SRV

;; ANSWER SECTION:
_kerberos._udp.UXSUP.CSI.cam.ac.uk. 86398 IN SRV 0 0 88 ghast.csi.cam.ac.uk.

;; Query time: 0 msec
;; SERVER: ::1#53(::1)
;; WHEN: Fri Feb 07 14:38:16 GMT 2014
;; MSG SIZE  rcvd: 136


Tony.
-- 
f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
Forties, Cromarty: East, veering southeast, 4 or 5, occasionally 6 at first.
Rough, becoming slight or moderate. Showers, rain at first. Moderate or good,
occasionally poor at first.