[Bug 1849733] Re: resolved incorrectly limits TCP reply to edns0 payload

[Dan Streetman]

ubuntu at lp1849733-b:~$ dpkg -l systemd|grep ii
ii  systemd        237-3ubuntu10.31 amd64        system and service manager
ubuntu at lp1849733-b:~$ telnet toomany100.ddstreet.org
telnet: could not resolve toomany100.ddstreet.org/telnet: Temporary failure in name resolution

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...


** Tags removed: verification-needed verification-needed-bionic
** Tags added: verification-done verification-done-bionic

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https://bugs.launchpad.net/bugs/1849733

Title:
  resolved incorrectly limits TCP reply to edns0 payload

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 Released

Bug description:
  [impact]

  glibc's getaddrinfo() uses EDNS0 to talk to resolved, and it sets its
  payload limit to 1200.  When the response is larger than 1200,
  resolved will limit the response and set the truncate flag.  This
  causes getaddrinfo() to switch to TCP and request again, but glibc
  incorrectly keeps the EDNS0 RR opt, with the same 1200 payload limit.
  Most dns nameservers ignore EDNS0 payload limit for TCP, since per RFC
  it applies only to UDP, but resolved does not and again marks the
  response as truncated.  This prevents getaddrinfo() from being able to
  resolve any records with a response over 1200 bytes.

  [test case]

  use ping or telnet, which use getaddrinfo(), to lookup an A record
  with a lot of results, like toomany100.ddstreet.org

  $ telnet toomany100.ddstreet.org
  telnet: could not resolve toomany100.ddstreet.org/telnet: Temporary failure in name resolution

  [regression potential]

  any regression would likely result in failure to correctly lookup a
  hostname or to provide the correct response to a local client.

  [other info]

  note that on Bionic, this also requires backporting TCP pipelining
  support in the stub resolver.

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