[Bug 1796501] Re: systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes

[Bryan Quigley]

I grabbed the top 500 hosts in an Eaon LXD container with DNS=1.1.1.1
wget -O top500.csv https://moz.com/top-500/download/?table=top500Domains
cut -d, -f2 < top500.csv | cut -d\" -f2 > top500

I ran this script twice (with and without dnssec=yes):
while read p; do
  sleep 1
  echo "$p"
  resolvectl query $p > with_dnssec/$p
done <top500

The following domains failed only with DNSSEC=yes (and all failures included DVE- notices in journal).
people.com.cn
search.yahoo.com
news.yahoo.com

(oddly engadget wasn't on the list.. There may be a difference between
netword/network-manager?)

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https://bugs.launchpad.net/bugs/1796501

Title:
  systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes

Status in systemd package in Ubuntu:
  In Progress
Status in systemd source package in Bionic:
  In Progress
Status in systemd source package in Cosmic:
  In Progress
Status in systemd source package in Disco:
  In Progress

Bug description:
  I ask systemd-resolved through dig to resolve the SOA of test.asdf. (doesn't exist) but it returns SERVFAIL instead of NXDOMAIN. It seems to do the following steps:
  1. Ask upstream for SOA of test.asdf. with EDNS0, DO-bit and 4k size.
  2. Ask upstream for SOA of test.asdf. with EDNS0 and DO-bit.
  3. Ask upstream for SOA of test.asdf. with EDNS0.
  4. Ask upstream for SOA of test.asdf. without EDNS0.
  5. Repeat 1-4 for DS of test.asdf.
  6. Repeat 1-5 for asdf.
  7. Ask upstream for SOA of . with EDNS0, DO-bit and 4k size.
  8. Ask upstream for DNSKEY of . with EDNS0, DO-bit and 4k size.

  The upstream returns an unfragmented NXDOMAIN response for steps 1-6,
  an unfragmented NOERROR response for step 7 and a fragmented NOERROR
  response for step 8 which is the correct behaviour. DNSSEC records are
  included in the response if the DO-bit in the request was set.

  systemd-resolved should take the response from step 1 and start with
  validation instead of starting useless retries with reduced feture
  set. Step 3 and 4 are completely useless and probably lead to the
  SERVFAIL because I have configured it with DNSSEC=yes to prevent
  downgrade attacks.

  This regression seems to be caused by the patch resolved-Mitigate-
  DVE-2018-0001-by-retrying-NXDOMAIN-with.patch. The downgrade logic
  should only be executed if it is configured as DNSSEC=allow-downgrade
  or DNSSEC=no. See also
  https://github.com/systemd/systemd/pull/8608#issuecomment-396927885.

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