[Bug 1849658] Re: resolved fallback to TCP fails for truncated UDP replies

[Dan Streetman]

bionic verification note: as mentioned in description, the commit
introducing this wasn't present in bionic so this bug isn't reproducable
with version 237-3ubuntu10.31; however that commit was added to version
237-3ubuntu10.32 in bug 1849733, so the verification here doesn't need
to check version ..ubuntu10.31, it only needs to verify this bug wasn't
introduced in version ..ubuntu10.32


ubuntu at lp1849733-b:~$ dpkg -l systemd|grep ii
ii  systemd        237-3ubuntu10.32 amd64        system and service manager
ubuntu at lp1849733-b:~$ telnet toomany100.ddstreet.org
Trying 10.254.201.100...
^C
ubuntu at lp1849733-b:~$ dpkg -l systemd|grep ii
ii  systemd        237-3ubuntu10.32 amd64        system and service manager
ubuntu at lp1849733-b:~$ dig +noanswer +noedns toomany.ddstreet.org
;; Truncated, retrying in TCP mode.

; <<>> DiG 9.11.3-1ubuntu1.10-Ubuntu <<>> +noanswer +noedns toomany.ddstreet.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6871
;; flags: qr rd ra; QUERY: 1, ANSWER: 40, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;toomany.ddstreet.org.		IN	A

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Fri Nov 15 15:53:48 UTC 2019
;; MSG SIZE  rcvd: 678

ubuntu at lp1849733-b:~$ sudo systemd-resolve --flush-caches 
ubuntu at lp1849733-b:~$ dig +noanswer +noedns toomany.ddstreet.org
;; Truncated, retrying in TCP mode.

; <<>> DiG 9.11.3-1ubuntu1.10-Ubuntu <<>> +noanswer +noedns toomany.ddstreet.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46778
;; flags: qr rd ra; QUERY: 1, ANSWER: 40, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;toomany.ddstreet.org.		IN	A

;; Query time: 0 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Fri Nov 15 15:53:56 UTC 2019
;; MSG SIZE  rcvd: 678

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https://bugs.launchpad.net/bugs/1849658

Title:
  resolved fallback to TCP fails for truncated UDP replies

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  Fix Committed
Status in systemd source package in Disco:
  Fix Committed
Status in systemd source package in Eoan:
  Fix Committed
Status in systemd source package in Focal:
  Fix Released

Bug description:
  [impact]

  for DNS UDP replies larger than 512 bytes, fallback to TCP is used.
  For example 'host toomany.ddstreet.org'.

  Due to a bug in resolved in refcounting DNS stream types, the refcount
  underflows for type 0 streams (which resolved uses to talk to upstream
  nameservers), resulting in resolved being unable to fallback to TCP to
  handle truncated UDP replies.

  [test case]

  ubuntu at sf247344-upstream:~$ dig +noanswer +noedns toomany.ddstreet.org
  ;; Truncated, retrying in TCP mode.

  ; <<>> DiG 9.11.3-1ubuntu1.9-Ubuntu <<>> +noanswer +noedns toomany.ddstreet.org
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 2683
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 40, AUTHORITY: 0, ADDITIONAL: 0

  ;; QUESTION SECTION:
  ;toomany.ddstreet.org.		IN	A

  ;; Query time: 0 msec
  ;; SERVER: 127.0.0.53#53(127.0.0.53)
  ;; WHEN: Thu Oct 24 11:40:29 UTC 2019
  ;; MSG SIZE  rcvd: 678

  ubuntu at sf247344-upstream:~$ sudo resolvectl flush-caches
  ubuntu at sf247344-upstream:~$ dig +noanswer +noedns toomany.ddstreet.org

  ; <<>> DiG 9.11.3-1ubuntu1.9-Ubuntu <<>> +noanswer +noedns toomany.ddstreet.org
  ;; global options: +cmd
  ;; connection timed out; no servers could be reached

  [regression potential]

  very low, as this only properly sets the stream type in the DnsStream
  object; any regression would be a failure to be able to use TCP for
  DNS requests or replies.

  [other info]

  https://github.com/systemd/systemd/pull/13838

  The commit adding stream types is not present in x/b, so this is
  needed only for disco and later.

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