[Bug 1796501] Re: systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes
Bryan Quigley
bryan.quigley at canonical.com
Thu Nov 14 21:50:16 UTC 2019
Verified in bionic with 237-3ubuntu10.32
Dnssec and not are now consistent, tested general functionality with
dnssec=yes and not set as well.
** Tags removed: verification-needed verification-needed-bionic
** Tags added: verification-done verification-done-bionic
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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:
Fix Released
Status in systemd source package in Bionic:
Fix Committed
Status in systemd source package in Cosmic:
Won't Fix
Status in systemd source package in Disco:
Fix Committed
Bug description:
[impact]
an NXDOMAIN response from a dns server when systemd-resolved is
configured as DNSSEC=yes breaks dns resolution as it downgrades from
DNSSEC.
[test case]
see comment 9
[regression potential]
as with the original patch that introduced this problem, this has the
potential to break dns resolution.
[other info]
original 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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