Beyond Linux® From Scratch - Version 2015-03-06-multilib

Chapter 4. Security

MIT Kerberos V5-.1

Introduction to MIT Kerberos V5

MIT Kerberos V5 is a free implementation of Kerberos 5. Kerberos is a network authentication protocol. It centralizes the authentication database and uses kerberized applications to work with servers or services that support Kerberos allowing single logins and encrypted communication over internal networks or the Internet.

This package is known to build and work properly using an LFS-7.7 platform.

Package Information

MIT Kerberos V5 Dependencies

Optional

DejaGnu-1.5.2 (for full test coverage), GnuPG-2.0.26 (to authenticate the package), keyutils-1.5.9, OpenLDAP-2.4.40, Python-2.7.9 (used during the testsuite) and rpcbind-0.2.2 (used during the testsuite)

[Note]

Note

Some sort of time synchronization facility on your system (like ntp-4.2.8p1) is required since Kerberos won't authenticate if there is a time difference between a kerberized client and the KDC server.

User Notes: http://wiki.linuxfromscratch.org/blfs/wiki/mitkrb

Installation of MIT Kerberos V5

MIT Kerberos V5 is distributed in a TAR file containing a compressed TAR package and a detached PGP ASC file. You'll need to unpack the distribution tar file, then unpack the compressed tar file before starting the build.

After unpacking the distribution tarball and if you have GnuPG-2.0.26 installed, you can authenticate the package. First, check the contents of the file krb5-.1.tar.gz.asc. 

gpg2 --verify krb5-.1.tar.gz.asc krb5-.1.tar.gz

You will probably see output similar to: 

gpg: Signature made Mon Aug 11 22:53:10 2014 GMT using RSA key ID 749D7889
gpg: Can't check signature: No public key

You can import the public key with: 

gpg2 --pgp2 --keyserver pgp.mit.edu --recv-keys 0x749D7889

Now re-verify the package with the first command above. You should get a indication of a good signature, but the key will still not be certified with a trusted signature. Trusting the downloaded key is a separate operation but it is up to you to determine the level of trust.

Build MIT Kerberos V5 by running the following commands: 

cd src &&
sed -e "s@python2.5/Python.h@& python2.7/Python.h@g" \
    -e "s@-lpython2.5]@&,\n  AC_CHECK_LIB(python2.7,main,[PYTHON_LIB=-lpython2.7])@g" \
    -i configure.in &&
sed -e 's@\^u}@^u cols 300}@' \
    -i tests/dejagnu/config/default.exp &&
autoconf &&
./configure --prefix=/usr            \
            --sysconfdir=/etc        \
            --localstatedir=/var/lib \
            --with-system-et         \
            --with-system-ss         \
            --with-system-verto=no   \
            --enable-dns-for-realm &&
make

To test the build, issue: make check. You need at least Tcl-8.6.3, which is used to drive the testsuite. Furthermore, DejaGnu-1.5.2 must be available for some of the tests to run. If you have a former version of MIT Kerberos V5 installed, it may happen that the test suite pick up the installed versions of the libraries, rather than the newly built ones. If so, it is better to run the tests after the installation.

Now, as the root user: 

make install &&

for LIBRARY in gssapi_krb5 gssrpc k5crypto kadm5clnt kadm5srv \
               kdb5 kdb_ldap krad krb5 krb5support verto ; do
    chmod -v 755 /usr/lib/lib$LIBRARY.so
done          &&
unset LIBRARY &&

mv -v /usr/lib/libkrb5.so.3*        /lib &&
mv -v /usr/lib/libk5crypto.so.3*    /lib &&
mv -v /usr/lib/libkrb5support.so.0* /lib &&

ln -v -sf ../../lib/libkrb5.so.3.3        /usr/lib/libkrb5.so        &&
ln -v -sf ../../lib/libk5crypto.so.3.1    /usr/lib/libk5crypto.so    &&
ln -v -sf ../../lib/libkrb5support.so.0.1 /usr/lib/libkrb5support.so &&

mv -v /usr/bin/ksu /bin &&
chmod -v 755 /bin/ksu   &&

install -v -dm755 /usr/share/doc/krb5-.1 &&
cp -vfr ../doc/*  /usr/share/doc/krb5-.1

Command Explanations

sed -e ...: The first sed fixes Python detection. The second one increases the width of the virtual terminal used for some tests, to prevent some spurious characters to be echoed, which is taken as a failure.

--localstatedir=/var/lib: This parameter is used so that the Kerberos variable run-time data is located in /var/lib instead of /usr/var.

--with-system-et: This switch causes the build to use the system-installed versions of the error-table support software.

--with-system-ss: This switch causes the build to use the system-installed versions of the subsystem command-line interface software.

--with-system-verto=no: This switch fixes a bug in the package: it does not recognize its own verto library installed previously. This is not a problem, if reinstalling the same version, but if you are updating, the old library is used as system's one, instead of installing the new version.

--enable-dns-for-realm: This switch allows realms to be resolved using the DNS server.

mv -v /usr/bin/ksu /bin: Moves the ksu program to the /bin directory so that it is available when the /usr filesystem is not mounted.

--with-ldap: Use this switch if you want to compile OpenLDAP database backend module.

Configuring MIT Kerberos V5

Config Files

/etc/krb5.conf and /var/lib/krb5kdc/kdc.conf

Configuration Information

Kerberos Configuration
[Tip]

Tip

You should consider installing some sort of password checking dictionary so that you can configure the installation to only accept strong passwords. A suitable dictionary to use is shown in the CrackLib-2.9.2 instructions. Note that only one file can be used, but you can concatenate many files into one. The configuration file shown below assumes you have installed a dictionary to /usr/share/dict/words.

Create the Kerberos configuration file with the following commands issued by the root user: 

cat > /etc/krb5.conf << "EOF"
# Begin /etc/krb5.conf

[libdefaults]
    default_realm = <LFS.ORG>
    encrypt = true

[realms]
    <LFS.ORG> = {
        kdc = <belgarath.lfs.org>
        admin_server = <belgarath.lfs.org>
        dict_file = /usr/share/dict/words
    }

[domain_realm]
    .<lfs.org> = <LFS.ORG>

[logging]
    kdc = SYSLOG[:INFO[:AUTH]]
    admin_server = SYSLOG[INFO[:AUTH]]
    default = SYSLOG[[:SYS]]

# End /etc/krb5.conf
EOF

You will need to substitute your domain and proper hostname for the occurrences of the <belgarath> and <lfs.org> names.

default_realm should be the name of your domain changed to ALL CAPS. This isn't required, but both Heimdal and MIT recommend it.

encrypt = true provides encryption of all traffic between kerberized clients and servers. It's not necessary and can be left off. If you leave it off, you can encrypt all traffic from the client to the server using a switch on the client program instead.

The [realms] parameters tell the client programs where to look for the KDC authentication services.

The [domain_realm] section maps a domain to a realm.

Create the KDC database: 

kdb5_util create -r <LFS.ORG> -s

Now you should populate the database with principals (users). For now, just use your regular login name or root. 

kadmin.local
kadmin.local: add_policy dict-only
kadmin.local: addprinc -policy dict-only <loginname>

The KDC server and any machine running kerberized server daemons must have a host key installed: 

kadmin.local: addprinc -randkey host/<belgarath.lfs.org>

After choosing the defaults when prompted, you will have to export the data to a keytab file: 

kadmin.local: ktadd host/<belgarath.lfs.org>

This should have created a file in /etc named krb5.keytab (Kerberos 5). This file should have 600 (root rw only) permissions. Keeping the keytab files from public access is crucial to the overall security of the Kerberos installation.

Exit the kadmin program (use quit or exit) and return back to the shell prompt. Start the KDC daemon manually, just to test out the installation: 

/usr/sbin/krb5kdc

Attempt to get a ticket with the following command: 

kinit <loginname>

You will be prompted for the password you created. After you get your ticket, you can list it with the following command: 

klist

Information about the ticket should be displayed on the screen.

To test the functionality of the keytab file, issue the following command: 

ktutil
ktutil: rkt /etc/krb5.keytab
ktutil: l

This should dump a list of the host principal, along with the encryption methods used to access the principal.

At this point, if everything has been successful so far, you can feel fairly confident in the installation and configuration of the package.

Additional Information

For additional information consult the documentation for krb5-.1 on which the above instructions are based.

Init Script

If you want to start Kerberos services at boot, install the /etc/rc.d/init.d/krb5 init script included in the blfs-bootscripts-20150304 package using the following command: 

make install-krb5

Contents

Installed Programs: gss-client, gss-server, k5srvutil, kadmin, kadmin.local, kadmind, kdb5_ldap_util (optional), kdb5_util, kdestroy, kinit, klist, kpasswd, kprop, kpropd, kproplog, krb5-config, krb5kdc, krb5-send-pr, ksu, kswitch, ktutil, kvno, sclient, sim_client, sim_server, sserver, uuclient and uuserver
Installed Libraries: libgssapi_krb5.so, libgssrpc.so, libk5crypto.so, libkadm5clnt_mit.so, libkadm5clnt.so, libkadm5srv_mit.so, libkadm5srv.so, libkdb_ldap.so (optional), libkdb5.so, libkrad.so, libkrb5.so, libkrb5support.so, libverto.so, and some plugins under the /usr/lib/krb5 tree
Installed Directories: /usr/include/{gssapi,gssrpc,kadm5,krb5}, /usr/lib/krb5, /usr/share/{doc/krb5-.1,examples/krb5,gnats}, and /var/lib/{,run}/krb5kdc

Short Descriptions

k5srvutil

is a host keytable manipulation utility.

kadmin

is an utility used to make modifications to the Kerberos database.

kadmind

is a server for administrative access to a Kerberos database.

kdb5_util

is the KDC database utility.

kdestroy

removes the current set of tickets.

kinit

is used to authenticate to the Kerberos server as a principal and acquire a ticket granting ticket that can later be used to obtain tickets for other services.

klist

reads and displays the current tickets in the credential cache.

kpasswd

is a program for changing Kerberos 5 passwords.

kprop

takes a principal database in a specified format and converts it into a stream of database records.

kpropd

receives a database sent by kprop and writes it as a local database.

krb5-config

gives information on how to link programs against libraries.

krb5kdc

is the Kerberos 5 server.

ksu

is the super user program using Kerberos protocol. Requires a properly configured /etc/shells and ~/.k5login containing principals authorized to become super users.

kswitch

makes the specified credential cache the primary cache for the collection, if a cache collection is available.

ktutil

is a program for managing Kerberos keytabs.

kvno

prints keyversion numbers of Kerberos principals.

sclient

used to contact a sample server and authenticate to it using Kerberos 5 tickets, then display the server's response.

sserver

is the sample Kerberos 5 server.

libgssapi_krb5.so

contain the Generic Security Service Application Programming Interface (GSSAPI) functions which provides security services to callers in a generic fashion, supportable with a range of underlying mechanisms and technologies and hence allowing source-level portability of applications to different environments.

libkadm5clnt.so

contains the administrative authentication and password checking functions required by Kerberos 5 client-side programs.

libkadm5srv.so

contain the administrative authentication and password checking functions required by Kerberos 5 servers.

libkdb5.so

is a Kerberos 5 authentication/authorization database access library.

libkrad.so

contains the internal support library for RADIUS functionality.

libkrb5.so

is an all-purpose Kerberos 5 library.

Last updated on 2015-02-26 23:16:43 -0500