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Description of problem:
sudo outputs an erealloc3 error whenever all retrieved sssd rules fail sudoHost netgroup match

Version-Release number of selected component (if applicable):
sudo-1.8.6p3-5.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1. Use the attached LDIF file to fill a LDAP directory.
2. Use the attached sssd.conf as the base for SSSD configuration.
3. Execute "su -c 'sudo -u user2 whoami' user1" as root.
  
Actual results:
sudo: internal error, tried to erealloc3(0)

Expected results:
sudo: no tty present and no askpass program specified

(NOTE: the expected result above is probably still incorrect due to Bug 875740)

Additional info:
The LDAP backend works as expected.
The attached patch fixes the problem.

Created attachment 644899 [details]
Patch fixing the problem

Created attachment 644900 [details]
Base LDIF file

Created attachment 644901 [details]
sssd.conf

Hi Nikolai, thank you for your reproducer.
But in my case "su -c 'sudo -u user2 whoami' user1" pass without errors.

In ldap entry "dn: cn=netgroup_server,ou=Netgroups,dc=example,dc=com"
you have "nisNetgroupTriple: (server.sss-test.test,,)".

server.sss-test.test is hostname of the machine, where ldap server run?

Verified fixed in 1.8.6p3-6.el6.
Relevant sssd sudo suite test passes:

:: [   PASS   ] :: attrs_host_netgroup_mismatch

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2013-0363.html