KCM: Initial responder build and packaging

Adds the initial build of the Kerberos Cache Manager responder (KCM).

This is a deamon that is capable of holding and storing Kerberos
ccaches. When KCM is used, the kerberos libraries (invoked through e.g.
kinit) are referred to as a 'client' and the KCM deamon is referred to
as 'server'.

At the moment, only the Heimdal implementation of Kerberos implements the
KCM server:
    https://www.h5l.org/manual/HEAD/info/heimdal/Credential-cache-server-_002d-KCM.html
This patch adds a KCM server to SSSD.

In MIT, only the 'client-side' support was added:
    http://k5wiki.kerberos.org/wiki/Projects/KCM_client
This page also describes the protocol between the client and the server.

The client is capable of talking to the server over either UNIX sockets
(Linux, most Unixes) or Mach RPC (macOS). Our server only implements the
UNIX sockets way and should be socket-activated by systemd, although can
in theory be also ran explicitly.

The KCM server only builds if the configuration option "--with-kcm" is
enabled. It is packaged in a new subpackage sssd-kcm in order to allow
distributions to enable the KCM credential caches by installing this
subpackage only, without the rest of the SSSD. The sssd-kcm subpackage
also includes a krb5.conf.d snippet that allows the admin to just uncomment
the KCM defaults and instructs them to start the socket.

The server can be configured in sssd.conf in the "[kcm]" section.
By default, the server only listens on the same socket path the Heimdal
server uses, which is "/var/run/.heim_org.h5l.kcm-socket". This is,
however, configurable.

The file src/responder/kcm/kcm.h is more or less directly imported from
the MIT Kerberos tree, with an additional sentinel code and some
comments. Not all KCM operations are implemented, only those that also
the MIT client implements. That said, this KCM server should also be
usable with a Heimdal client, although no special testing was with this
hybrid.

The patch also adds several error codes that will be used in later
patches.

Related to:
    https://pagure.io/SSSD/sssd/issue/2887

Reviewed-by: Michal Židek <mzidek@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index dd1012015..c363d48a8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -155,6 +155,7 @@ WITH_SSSD_USER
 SSSD_RUNSTATEDIR
 WITH_SECRETS
 WITH_SECRETS_DB_PATH
+WITH_KCM
 
 m4_include([src/external/pkg.m4])
 m4_include([src/external/libpopt.m4])
@@ -193,13 +194,20 @@ m4_include([src/external/libresolv.m4])
 m4_include([src/external/intgcheck.m4])
 m4_include([src/external/systemtap.m4])
 m4_include([src/external/service.m4])
-m4_include([src/external/libcurl.m4])
 
 if test x$with_secrets = xyes; then
     m4_include([src/external/libhttp_parser.m4])
     m4_include([src/external/libjansson.m4])
 fi
 
+if test x$with_kcm = xyes; then
+    m4_include([src/external/libcurl.m4])
+fi
+# This variable is defined by external/libcurl.m4, but conditionals
+# must be always evaluated
+AM_CONDITIONAL([BUILD_WITH_LIBCURL],
+               [test x"$have_curlopt_unix_sockpath" = xyes])
+
 WITH_UNICODE_LIB
 if test x$unicode_lib = xlibunistring; then
     m4_include([src/external/libunistring.m4])