** Description changed: + [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.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1796501 Title: systemd-resolved tries to mitigate DVE-2018-0001 even if DNSSEC=yes To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1796501/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
