June 12, 2018
The various changes necessary to add support for GSSAPI authenticated profile fetching have now been moved from our local dice configuration into the general lcfg area. The new support can be enabled by specifying the
LCFG_AUTH_GSSAPI macro at the start of an LCFG source profile. Most of the resources necessary are configured in the
lcfg/options/lcfg-client-gssapi.h header file but sites wanting to use this will still need a little additional configuration.
For the installer, support is enabled by setting the lcfg.kauth=1 option on the kernel command line. Those sites using the
ed/options/pxeserver.h header will automatically get a menu option which supports this mode. The installer can be be enabled for a client with something like this:
#endif /* GSSAPI support enabled */
#endif /* LINUX_EL7 */
It is important to note that the
lcfg-client-gssapi.h header does not remove http access for the XML profile. That could either be turned off completely for the apache virtual host or a per-client deny all rule could be put into the associated
.htaccess file. Also, the default access control for the XML profiles is based on an apache group named lcfgadmins which must be populated on the LCFG servers, for example:
Note that this is a list of
admin principals, this is done so the credentials can also be used to register machines in the KDC using the kdcregister tool.
As part of this work I’ve also added the necessary methods to support using kdcregister in the LCFG installer. Most sites use this and have up to now been duplicating the configuration. It can be enabled by specifying the
LCFG_OPTIONS_KERBEROS_CLIENT_REGISTER macro prior to including the
lcfg/options/kerberos-client.h header file.
May 17, 2018
In the LCFG installer we have previously only needed to prompt the administrator for a Kerberos principle and password when using the kdcregister tool. With a switch to GSSAPI authentication for LCFG profile fetching we now need the administrator to also authenticate for that purpose. Everyone knows that system administrators are busy people who really don’t want to be prompted twice for the same principle/password so we need to avoid that if at all possible. For most services once the user has authenticated and acquired a ticket granting ticket (TGT) they don’t need to authenticate again but kdcregister needs a kadmin/admin ticket which causes us a slight problem as it typically (depending on site policy) cannot be acquired using the TGT. This is a deliberate security measure to make it harder to become an admin user with enhanced privileges to modify entries in the KDC. The simple solution taken for the LCFG installer is to prompt the user for principle and password and then use them behind the scenes to authenticate twice using kinit in slightly different ways using two different credential cache files. As we have to do a number of LCFG profile fetches the
KRB5CCNAME environment variable gets set to the path to the credentials cache which holds the TGT so all profile fetches (and anything else which might need it) will just work. For this to work with the kdcregister tool the code had to be modified (see bug#1068 for details) to support a new command line option so that the path to the credentials cache can be specified. Previously kdcregister has always used the default credentials cache which was either the path in
KRB5CCNAME or whatever the krb5.conf had specified. With those modifications in place the LCFG installer is now fully ready for fetching LCFG profiles in a secure way which doesn’t require any extra authentication steps for administrators. For this part of the project to be totally completed there are still quite a few resource changes which need to be pushed out over the next few weeks but those can only happen once sites have deployed new installer images.
April 25, 2018
Having added support to the LCFG installer for prompting the user for a Kerberos principal at the very beginning of the process it would be nice to be able to use that for all other steps which need authentication. In particular, the call to kdcregister to create the host keytab file should not need to also prompt the user for the principal and password. I quickly discovered this is a slightly awkward problem because the kdcregister command is issued via the baseinstall component which is called using the
om command. When calling any component method om will firstly clear the environment to make it safe. This means that the path to the credentials cache, which is usually found from the
KRB5CCNAME environment variable, is not available to kdcregister. I considered various approaches to solving this problem and eventually concluded that om needed a new feature which would be similar to the env_keep option support by sudo. This can then be used to specify a list of environment variables which will not be cleared. Using this feature is straightforward, the LCFG schema for om, which is inherited by most components, now includes an
om_env_keep resource. The feature is supported in version 0.13.1 of the lcfg-om package which will be in the next weekly stable release.
April 18, 2018
Having completed the work to add support for GSSAPI auth to the client for fetching profiles I’ve now moved on to the LCFG installer. Currently the installer attempts to fetch the LCFG profile for the machine just prior to the
(I)nstall, (D)ebug, (S)hell, (P)atchup, (R)eboot prompt. That fetching is done by calling the client component install method which in turn calls rdxprof in one shot mode. Having previously ported the client component to the Perl
LCFG::Component framework I had hoped this would “just work” but it turned out that a number of bootstrapping issues were only being avoided previously due to many hardwired paths in the shell ngeneric code. The Perl framework takes a different approach and prefers to use the LCFG sysinfo resources wherever possible, this improves platform independence and maintainability but presents a bootstrapping problem at the first stage of the install when we have not yet downloaded any profile and thus have no sysinfo resources… I wasn’t keen on performing major surgery on the Perl component framework so I decided that the simplest solution to this problem was to get the installer to call rdxprof directly. With this change the installer worked again but still required support for Kerberos authentication.
Adding support for Kerberos authentication has been done in a fairly simple way. I’ve added support for two new install kernel command line options:
lcfg.realm. When the
lcfg.kauth option is specified the user is prompted to enter their principal name and the kinit program is run to do the authentication. The user may specify the full principal name, if the realm is not specified then either the
lcfg.realm option or the upper-cased domain name is used (e.g.
@LCFG.ORG). If the authentication fails then the user is prompted to re-enter the principal name (which defaults to the previously entered string) and password. Once the Kerberos authentication has succeeded the credentials will be automatically used by rdxprof when required for fetching the LCFG profile.
March 28, 2018
This week I have been working on providing a way to configure the LCFG client profile fetcher via client component resources. In particular some sites will need to be able to specify SSL options (e.g.
verify_hostname) and also be able to set parameters for the authentication modules (e.g. gssapi might need the keytab file path to be specified). By default profile fetching will work for most sites without any additional configuration, furthermore as this is most easily expressed in terms of list and hash data structures I’ve decided to only support setting these parameters via a configuration file. Although it is currently configured entirely through the command line, rdxprof daemon supports loading configuration from a YAML file. I’ve altered the
SetOptions method so that when it encounters a
fetch entry in the configuration data hash it will pass this through to the
LCFG::Client::ProfileFetcher instance via a
configure method which knows how to handle the various options.
The current LCFG client component is written in bash which makes generating a config file in YAML more tricky than I would like. As we have a longstanding plan to rewrite all the core LCFG components into Perl this seemed like a good opportunity to get on with that job. I’ve previously been putting off this particular rewrite since the component is rather old and very complex. It manages the starting, stopping and signalling of the rdxprof daemon and as such it has a lot of code for handling PID files and checking for the liveness of processes. Given that we no longer support platforms such as SL6 and older this situation can be massively improved by switching to systemd for the management of rdxprof. I’ve introduced
/etc/sysconfig/rdxprof files which can be used by the component to control the daemon. To properly verify that the rdxprof daemon has successfully started the component creates a null callback and waits for it to be processed. I’ve moved the implementation of that into the LCFG::Client module itself so that the details are nicely hidden behind an API.
This is all implemented in perl-LCFG-Client version 4.3.4 and lcfg-client version 4.0.3. To make it easier to test I’ve added a
dice/options/lcfg-client.h header. If the
DICE_OPTIONS_LCFG_CLIENT_GSSAPI macro is defined then a new keytab will be created and the LCFG client will use it for authentication. The LCFG server is not yet quite ready for me to enable the use of gssapi but hopefully will be in the next couple of days.
Enabling gssapi for an LCFG client will be done something like this:
!client.url mSET(https://lcfg1.inf.ed.ac.uk/profiles https://lcfg2.inf.ed.ac.uk/profiles)
March 21, 2018
After improving support for Apache authentication in the LCFG server I have moved onto the client this week. The bulk of the work has been focused on the creation of a new
LCFG::Client::Fetcher module which encapsulates all the details associated with fetching XML profiles from various sources. As well as improving the authentication support I am taking the chance to overhaul a chunk of code which has not seen much love in either of the v3 or v4 projects. One particular issue is that currently the handling of the list of profile sources is spread around the client libraries, this means that even a small change can involve locating and altering many separate small pieces of code. This general work also includes adding support for IPv6, enhancing SSL security as well as making the code much more maintainable.
One big change in approach I’ve made is that the lists of local file and remote web server sources are now handled in a unified way where previously they were dealt with completely separately. The new Fetcher module has a single list of source objects (either
LCFG::Client::Fetch::Source::Remote) which come from the value of the
client.url resource. One advantage here is that it is now trivial to add an entirely new type of source (e.g. rsync or ldap) anything with an
LWP::Protocol module is a possibility. When configured to use both local files and remote sources the client has always preferred local files where possible, this behaviour is retained by using a priority system with file sources being guaranteed to have a higher default priority than any remote source.
The other part of recent development work is the addition of support for different authentication mechanisms. This is supported via modules in the
LCFG::Client::Fetch::Auth namespace, currently we have modules for basic (username/password) and gssapi authentication. As with the new source modules this approach means it is easy to support alternative mechanisms, including site-specific needs which might not be appropriate for merging into the upstream code base. Before making a request the Fetcher will call the relevant authentication module to initialise the environment. I am also working on supporting multiple mechanisms so that if one fails the next will be tried until one succeeds.
Most of the code for the client is now in place and I am working on documentation for the various new modules. Once that is done I need to consider how the necessary authentication information can make it from LCFG resources into the rdxprof application via the LCFG client component. Although I would rather not make such a big change it might be that I finally need to bite the bullet and rewrite the client component from bash into Perl.
March 13, 2018
I have recently begun work on the Review Security of LCFG Profile Access project. So far I have mostly been considering the various aspects of the project with the aim being to produce a list of ideas which can be discussed at some future Development Meeting.
The first aspect of the project I have looked at in more depth is the LCFG server which has support for generating Apache
.htaccess files. These can be used to limit access to each individual LCFG profile when fetched over http/https. We have traditionally supported both http and https protocols and relied on IP addresses to limit access but would like to move over to https-only along with using GSSAPI authentication, the LCFG client would then use a keytab to get the necessary credentials. To help with this change I have introduced a new schema (4) for the profile component and made some modifications to the LCFG server code which makes it easier to use the Apache mod_auth_gssapi module. In particular there is new
auth_tmpl_$ resource which allows the selection of a different template (e.g. the
apache_gssapi.tt template which is provided in the package) which more closely meets local requirements. There are also
auth_val_$_$ resources which can be used to specify any additional information that is required. For example:
!profile.version_profile mSET(4) /* not yet the default */
which results in the the LCFG server generating the following
Require user "host/foo.inf.ed.ac.uk@INF.ED.AC.UK"
Require group "admin"
profile.acl_ssl resource holds a list of users and groups (which have an ‘@’ prefix). In a real deployment it might make more sense to use an
lcfg/ principal rather
host/. The groupfile support is provided by the
mod_authz_groupfile module which needs to be loaded.
I have tested this with curl and it works as required. The LCFG client doesn’t currently have support for doing a kinit (or launching something like k5start in the background) prior to fetching the profile so it isn’t yet possible to actively use this authentication method.