Public bug reported:
RFC2308 states:
6 - Negative answers from the cache
When a server, in answering a query, encounters a cached negative
response it MUST add the cached SOA record to the authority section
of the response with the TTL decremented by the amount of time it was
stored in the cache. This allows the NXDOMAIN / NODATA response to
time out correctly.
The effect is that the negative cache results returned by dnsmasq cannot
be cached by other resolvers such as systemd-resolved.
A good example of why this is a problem:
Two clients with systemd-resolved send DNS queries to dnsmasq for the same
record
The first one makes a query and gets NXDOMAIN with SOA.
This results in systemd-resolved caching the negative result.
The second client makes a query and gets NODATA without the SOA.
This results in systemd-resolved not caching the negative result.
Consider a domain name that only has an A record published.
When connecting to that domain a lookup happens for the AAAA and A record.
One can end up in a situation where systemd-resolved has the A record cached
locally, but it still goes out to the network to perform the AAAA lookup only
to get the same negative cache result that is missing the SOA
I see this behavior in 18.04 and 22.04
First query to dnsmasq can be cached:
dig @10.0.1.21 a.google.com
; <<>> DiG 9.16.1-Ubuntu <<>> @10.0.1.21 a.google.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 3107
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;a.google.com. IN A
;; AUTHORITY SECTION:
google.com. 60 IN SOA ns1.google.com.
dns-admin.google.com. 449437361 900 900 1800 60
;; Query time: 15 msec
;; SERVER: 10.0.1.21#53(10.0.1.21)
;; WHEN: Thu May 19 15:00:12 EDT 2022
;; MSG SIZE rcvd: 91
Cached response from dnsmasq cannot be cached:
dig @10.0.1.21 a.google.com
; <<>> DiG 9.16.1-Ubuntu <<>> @10.0.1.21 a.google.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 61408
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;a.google.com. IN A
;; Query time: 0 msec
;; SERVER: 10.0.1.21#53(10.0.1.21)
;; WHEN: Thu May 19 15:00:13 EDT 2022
;; MSG SIZE rcvd: 41
** Affects: dnsmasq (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1974230
Title:
Negative cache results from dnsmasq do not include SOA as required in
RFC2308
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