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Testing RedHat6/CentOS6 krb5.conf compatible enctypes - httpd: GSSAPI Error: Unspecified GSS failure. Minor code may provide more information (Credential cache is empty)

Russ Allbery eagle at
Mon Sep 19 10:27:30 PDT 2011

Fletcher Cocquyt <fcocquyt at> writes:

> We are testing CentOS6 builds and found our existing krb5.conf enctypes
> have been deprecated resulting in the kerberos auth for ssh failing
> until we update the krb5.conf as follows:

>  # default_tkt_enctypes = des-cbc-crc # DES is deprecated in RHEL 5.6 and 6.
>  # default_tgs_enctypes = des-cbc-crc # DES is deprecated in RHEL 5.6 and 6.
>  default_tkt_enctypes = rc4-hmac
>  default_tgs_enctypes = rc4-hmac
>  permitted_enctypes = rc4-hmac

You should never restrict to only rc4-hmac as an enctype at Stanford
unless you're interacting specifically with Windows XP systems under very,
very specific circumstances.  We introduced 256-bit AES and 3DES (both of
which are stronger than RC4) prior to introducing RC4, so any system that
has RC4 keys also has AES and 3DES keys except for some Windows
cross-realm situations.

In general, you never want to set those krb5.conf configuration options at
all.  If you don't set them, the Kerberos library will negotiate
appropriate encryption types.  (Be aware that if you use the Kerberos
ticket caches directly from Java, you'll need to have the US encryption
module installed, but that's fairly easy to do and worthwhile anyway.)

> Now the question is, are these new enctypes perfectly backward
> compatible?

It's a completely different enctype with no relationship to DES, so in
that sense it's not backward compatible at all.  But my guess is that what
you're asking is whether every Stanford service that had DES keys has RC4
keys.  The answer is no: RC4 keys are a relatively new addition only for
Windows XP support.  If you allow 3DES and 256-bit AES, you'll be on much
firmer ground, but there are still some services (although no ITS services
apart from AFS, which is special) that are DES-only.

> We¹re testing the new enctypes on CentOS/RedHat 5 and so far the only
> error turning up is on our webservers:

> httpd: GSSAPI Error: Unspecified GSS failure.  Minor code may provide more
> information (Credential cache is empty)

This implies that you can't get Kerberos tickets, which probably means
that your local Kerberos keytab doesn't have RC4 keys, which is not
surprising if it's older than a couple of years.  If you just remove those
krb5.conf settings, I suspect WebAuth will start working fine.

Alternately, you can add:

    allow_weak_crypto = true

to the [libdefaults] section of krb5.conf, which will enable DES keys
again, although in the long run we do want to phase out use of DES.

Russ Allbery <eagle at>
Technical Lead, ITS Infrastructure Delivery Group, Stanford University

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