[Bug 1897744] Re: VerifyHostKeyDNS not working due to missing trust-ad flag

[Christian Ehrhardt ]

I just re-debugged into the same case - now marked as dup.
You might consider using https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1898590/comments/7 as test steps to verify that once uploaded to focal-proposed.

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https://bugs.launchpad.net/bugs/1897744

Title:
  VerifyHostKeyDNS not working due to missing trust-ad flag

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Focal:
  In Progress

Bug description:
  [impact]

  without trust-ad resolv.conf option, glibc will strip AD from systemd-
  resolved responses. one thing this will prevent working is ssh
  VerifyHostKeyDNS

  [test case]

  setup a target system to ssh into, and create a SSHFP DNS record for
  its public key. on a different source system, setup dns to enable
  DNSSEC, and attempt to ssh to the target system using the
  VerifyHostKeyDNS=yes option.

  setup of the SSHFP is out of scope for this bug, but e.g.:
  https://en.wikipedia.org/wiki/SSHFP_record
  https://tools.ietf.org/html/rfc4255

  [regression potential]

  regressions would likely involve DNS lookup failures, probably if
  DNSSEC is enabled but possibly even without, and likely when the
  application requesting the dns lookup processes the response AD.

  [scope]

  this is needed only in focal.

  glibc first stripped the AD in version 2.31, so this is not needed in
  bionic or earlier.

  this was added upstream in commit a742f9828ea which was included in
  v246, so this is fixed already in groovy.

  [original description]

  Hi,

  1)
  Description:	Ubuntu 20.04.1 LTS
  Release:	20.04

  2)
  systemd:        245.4-4ubuntu3.2

  3)
  I set VerifyHostKeyDNS to YES and hosts are automatically verified via sshfp.

  4)
  I still get the security question
  Matching host key fingerprint found in DNS.
  Are you sure you want to continue connecting (yes/no/[fingerprint])?

  The issue is known and fixed in systemd v246.
  https://github.com/systemd/systemd/pull/16072

  Best regards
  Daniel

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