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

[Dan Streetman]

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

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

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

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

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

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

Status in systemd:
  Fix Released
Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Bionic:
  New
Status in systemd source package in Focal:
  New
Status in systemd source package in Hirsute:
  Won't Fix
Status in systemd source package in Impish:
  Won't Fix

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]

  enable debug logging in systemd-resolved, with 'sudo systemctl edit
  systemd-resolved' and then add:

  [Service]
  Environment=SYSTEMD_LOG_LEVEL=debug

  then save that file and restart systemd-resolved (or reboot).

  Make sure to flush the cache and reset server features before
  reproducing:

  $ sudo resolvectl flush-caches
  $ sudo resolvectl reset-server-features

  Lookup 'toomany.ddstreet.org' and check systemd-resolved logs to see
  if it used TCP fallback or not:

  ...
  Aug 24 12:17:48 lp1940715-f systemd-resolved[1199]: Reply truncated, retrying via TCP.
  ...
  Aug 24 12:17:48 lp1940715-f systemd-resolved[1199]: Verified we get a response at feature level TCP from DNS server 10.202.51.1.
  Aug 24 12:17:48 lp1940715-f systemd-resolved[1199]: Added positive unauthenticated cache entry for toomany.ddstreet.org IN A 1799s on eth0/INET/10.202.51.1

  A correct lookup using larger EDNS0 response size looks like:

  ...
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Using feature level UDP+EDNS0 for transaction 43808.
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Using DNS server 10.202.51.1 for transaction 43808.
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Announcing packet size 1472 in egress EDNS(0) packet.
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Emitting UDP, link MTU is 1500, socket MTU is 0, minimal MTU is 40
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Sending query packet with id 43808 of size 49.
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Processing query...
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Received dns UDP packet of size 689, ifindex=131, ttl=0, fragsize=0
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Processing incoming packet of size 689 on transaction 43808 (rcode=SUCCESS).
  Aug 24 12:28:13 lp1940715-f systemd-resolved[174]: Added positive unauthenticated non-confidential cache entry for toomany.ddstreet.org IN A 1175s on eth0/INET/10.202.51.1
  ...

  [regression potential]

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

  [scope]

  this is needed in impish and earlier

  this is fixed in jammy at commit
  22545a74c3a24395bc6e894fe6a681deb66d434c

  this is fixed upstream by commit
  526fce97afe130f71dba3bd4646196bbb1188b82 which in included in v250, so
  this is fixed in kinetic and later

  the upstream PR is:
  https://github.com/systemd/systemd/pull/20528

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