Re: ls input/output error ("NFS: readdir(/) returns -5") on krb5 NFSv4 client using SHA2

[Salvatore Bonaccorso <carnil@debian.org>]

Hi,

On Tue, Nov 18, 2025 at 11:43:29PM +0000, Tyler W. Ross wrote:
> On 11/18/25 10:52 AM, Scott Mayhew wrote:
> > Oh!  I see the problem.  If the automatically acquired service ticket
> > for a normal user is using aes256-cts-hmac-sha1-96, then I'm assuming
> > the machine credential is also using aes256-cts-hmac-sha1-96.
> > Run 'klist -ce /tmp/krb5ccmachine_IPA.TWRLAB.NET' to check.  You can't
> > use 'kvno -e' to choose a different encryption type.  Why are you doing
> > that?
> 
> Aha! Thank you!

Thanks to all helping to debug this issue when reported downstream in
Debian, your time invested is very much appreciated!

> That's exactly the case: the machine credential is
> aes256-cts-hmac-sha1-96.
> 
> So, taking a step back for context/background: this issue was escalated to
> me by someone attempting to use constrained delegation via gssproxy. In the
> course of troubleshooting that, we found (by examining the krb5kdc logs on
> the IPA server) that the NFS service ticket acquired by gssproxy had an
> aes256-cts-hmac-sha384-192 session key.
> 
> Not understanding that the machine and user tickets must having matching
> enctypes, I ended up down this rabbit hole thinking the problem
> was with the SHA2 enctypes. Sorry to bring you all with me on that
> misadventure.
> 
> 
> 
> The actual issue at hand then seems to be that gssproxy is requesting (and
> receiving) a service ticket with an unusable (for the NFS mount) enctype,
> when performing constrained delegation/S4U2Proxy.
> 
> krb5kdc logs of gssproxy performing S4U2Self and S4U2Proxy:Nov 18 18:06:51
> directory.ipa.twrlab.net krb5kdc[8463](info): TGS_REQ (8 etypes
> {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17),
> aes256-cts-hmac-sha384-192(20), aes128-cts-hmac-sha256-128(19),
> UNSUPPORTED:des3-hmac-sha1(16), DEPRECATED:arcfour-hmac(23),
> camellia128-cts-cmac(25), camellia256-cts-cmac(26)}) 10.108.2.105: ISSUE:
> authtime 1763506600, etypes {rep=aes256-cts-hmac-sha1-96(18),
> tkt=aes256-cts-hmac-sha384-192(20), ses=aes256-cts-hmac-sha1-96(18)},
> host/nfsclient.ipa.twrlab.net@IPA.TWRLAB.NET for
> host/nfsclient.ipa.twrlab.net@IPA.TWRLAB.NET
> Nov 18 18:06:51 directory.ipa.twrlab.net krb5kdc[8463](info):
> ... PROTOCOL-TRANSITION s4u-client=jsmith@IPA.TWRLAB.NET
> Nov 18 18:06:51 directory.ipa.twrlab.net krb5kdc[8463](info): closing down
> fd 4
> Nov 18 18:06:51 directory.ipa.twrlab.net krb5kdc[8465](info): TGS_REQ (4
> etypes {aes256-cts-hmac-sha384-192(20), aes128-cts-hmac-sha256-128(19),
> aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17)}) 10.108.2.105:
> ISSUE: authtime 1763506600, etypes {rep=aes256-cts-hmac-sha1-96(18),
> tkt=aes256-cts-hmac-sha384-192(20), ses=aes256-cts-hmac-sha384-192(20)},
> host/nfsclient.ipa.twrlab.net@IPA.TWRLAB.NET for
> nfs/nfssrv.ipa.twrlab.net@IPA.TWRLAB.NET
> Nov 18 18:06:51 directory.ipa.twrlab.net krb5kdc[8465](info): ...
> CONSTRAINED-DELEGATION s4u-client=jsmith@IPA.TWRLAB.NET
> Nov 18 18:06:51 directory.ipa.twrlab.net krb5kdc[8465](info): closing down
> fd 11
> 
> 
> On the Fedora 43 client, gssproxy also acquires an
> aes256-cts-hmac-sha384-192 service ticket, but the machine credential is
> aes256-cts-hmac-sha384-192 and everything works as-ex
> pected.

I'm looping in here the gssproxy maintainer as well. Simon, this is
about https://bugs.debian.org/1120598 . I assume there is nothing on
gssroxy side which can be done to warn about the situation, quoting
again:

> The actual issue at hand then seems to be that gssproxy is requesting (and
> receiving) a service ticket with an unusable (for the NFS mount) enctype,
> when performing constrained delegation/S4U2Proxy.

?

Regards,
Salvatore