[Bug 2063443] Re: krb5.conf seems to ignore rdns = false

[Lukas Grässlin]

Let me mark it as invalid as I also already opened a bug ticket at the
debian bug tracker as we have the same results there as well. Probably
doesn't make sense to double track bugtickets then in this case, thanks!

** Changed in: krb5 (Ubuntu)
       Status: New => Invalid

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Title:
  krb5.conf seems to ignore rdns = false

Status in krb5 package in Ubuntu:
  Invalid

Bug description:
  We have a scenario where we need to disable reverse lookups for
  canonicalization in Kerberos as the customer's PTR records are not
  consistent and lead to wrongly requested SPNs otherwise (see
  https://web.mit.edu/kerberos/krb5-latest/doc/admin/princ_dns.html#reverse-
  dns-mismatches)

  Therefore we have set "rdns = false" in /etc/krb5.conf as follows:

  # cat /etc/krb5.conf

  [libdefaults]
  udp_preference_limit = 0
  default_realm = EMEA.EXAMPLE.COM
  rdns = false

  However this setting seems to get ignored for some reason. I tried to
  do some debugging and comparisons as we don't have the same issue for
  example on a RHEL8 (using krb5-libs-1.18.2-26.el8_9.x86_64) machine.

  On those RHEL machines the setting seems to do exactly what it's
  supposed to, in fact I can reproduce the issue we have on Ubuntu on
  RHEL as well if I remove the "rdns = false" line from the
  configuration.

  Kerberos authentication (in our test we use a simple ldapsearch with
  GSSAPI auth) fails then randomly as it sometimes gets a wrong SPN due
  to using a wrong PTR for canonicalization.

  On our Ubuntu 22.04 LTS (with libkrb5-3:amd64 1.19.2-2ubuntu0.3)
  however the setting does not seem to have any effect. I then re-did
  the same test with Debian oldstable, stable and sid as well, all with
  the exact same result.

  I actually ran a tcpdump while trying to do a Kerberos auth with the
  "rdns = false" setting in place and I can still see the reverse lookup
  being performed in the tcpdump (I anonymized some things so don't get
  confused about the IPs):

  10:47:58.382684 IP 1.2.3.4.55001 > 123.123.123.123.53: 12962+ [1au] A? domaincontroller01.emea.example.com. (55)
  10:47:58.382809 IP 1.2.3.4.37669 > 123.123.123.123.53: 38376+ [1au] AAAA? domaincontroller01.emea.example.com. (55)
  10:47:58.412041 IP 123.123.123.123.53 > 1.2.3.4.37669: 38376* 0/1/1 (143)
  10:47:58.412564 IP 123.123.123.123.53 > 1.2.3.4.55001: 12962* 1/9/10 A 5.6.7.8 (602)
  10:47:58.442326 IP 1.2.3.4.51232 > 123.123.123.123.53: 16995+ [1au] PTR? 8.7.6.5.in-addr.arpa. (55)
  10:47:58.471669 IP 123.123.123.123.53 > 1.2.3.4.51232: 16995 2/2/3 PTR emea.example.com., PTR DOMAINCONTROLLER.emea.example.com. (238)

  As you see there it does the PTR lookup and retrieves two entries
  (emea.example.com and DOMAINCONTROLLER.emea.example.com)

  If I do the same test on the RHEL8 machine I can actually see in
  tcpdump that with the "rdns = false" setting the reverse lookup is
  correctly NOT performed.

  I am a bit puzzled why this is the case as I have not seen other
  people reporting it (maybe everyone else has their reverse DNS under
  control ;)) even though as said in a quick test I was also able to get
  the same wrong result with multiple Debian releases and the mentioned
  Ubuntu release.

  The only thing I've found in the past where this exact issue was
  mentioned was many years ago:
  https://mailman.mit.edu/pipermail/krb5-bugs/2011-June/008684.html

  However this has been fixed ever since. I have not done yet any actual
  code comparison with the version that RHEL uses, also I'm not sure if
  the issue actually really exists in libkrb5 itself or if it might be a
  sideeffect from some other lib.

  How to reproduce on your own:
  Even if you don't have erroneous reverse DNS entries you could still try to reproduce it by just looking at tcpdump and checking if you see reverse lookups performed with and without the option.

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