[Bug 1940715] [NEW] systemd-resolved restricts edns0 advertised max size to 512

[Dan Streetman]

Public bug reported:

[impact]

when talking to upstream nameservers, systemd-resolved limits its
advertised max packet size as 512 in its edns0 opt. However, one of the
primary benefits of edns0 is to allow using packet sizes larger than
512, which is the pre-edns0 max packet size.

this results in systemd-resolved failing to handle responses larger than
512 with udp/edns0, and having to fall back to tcp. This is not optimal
(since tcp dns imposes significantly higher overhead) and may even cause
failures, if a firewall allows udp dns but blocks tcp dns traffic.

[test case]

TBD

[regression potential]

failure to correctly look up dns records, or other problems while
performing dns lookups with systemd-resolved

[scope]

this is needed for all releases

this still needs fixing upstream

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: systemd (Ubuntu Bionic)
     Importance: Low
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Focal)
     Importance: Low
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Hirsute)
     Importance: Low
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Impish)
     Importance: Undecided
         Status: New

** Also affects: systemd (Ubuntu Hirsute)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Impish)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Focal)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu Bionic)
       Status: New => In Progress

** Changed in: systemd (Ubuntu Focal)
       Status: New => In Progress

** Changed in: systemd (Ubuntu Hirsute)
       Status: New => In Progress

** Changed in: systemd (Ubuntu Bionic)
   Importance: Undecided => Low

** Changed in: systemd (Ubuntu Focal)
   Importance: Undecided => Low

** Changed in: systemd (Ubuntu Hirsute)
   Importance: Undecided => Low

** Changed in: systemd (Ubuntu Bionic)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

** Changed in: systemd (Ubuntu Focal)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

** Changed in: systemd (Ubuntu Hirsute)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

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https://bugs.launchpad.net/bugs/1940715

Title:
  systemd-resolved restricts edns0 advertised max size to 512

Status in systemd package in Ubuntu:
  New
Status in systemd source package in Bionic:
  In Progress
Status in systemd source package in Focal:
  In Progress
Status in systemd source package in Hirsute:
  In Progress
Status in systemd source package in Impish:
  New

Bug description:
  [impact]

  when talking to upstream nameservers, systemd-resolved limits its
  advertised max packet size as 512 in its edns0 opt. However, one of
  the primary benefits of edns0 is to allow using packet sizes larger
  than 512, which is the pre-edns0 max packet size.

  this results in systemd-resolved failing to handle responses larger
  than 512 with udp/edns0, and having to fall back to tcp. This is not
  optimal (since tcp dns imposes significantly higher overhead) and may
  even cause failures, if a firewall allows udp dns but blocks tcp dns
  traffic.

  [test case]

  TBD

  [regression potential]

  failure to correctly look up dns records, or other problems while
  performing dns lookups with systemd-resolved

  [scope]

  this is needed for all releases

  this still needs fixing upstream

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