How To:Set up Kerberos authentication

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This article is a part of the How Tos collection.

Please see amanda-auth(7) for authoritative documentation on Amanda configuration of Kerberos.

Contents

Kerberos v4

Note that support for Kerberos 4 will be removed after Amanda-2.6.1 is released.

Configuration

The configure script defaults to:

#  define SERVER_HOST_PRINCIPLE "amanda"
#  define SERVER_HOST_INSTANCE  ""
#  define SERVER_HOST_KEY_FILE  "/.amanda"
#  define CLIENT_HOST_PRINCIPLE "rcmd"
#  define CLIENT_HOST_INSTANCE  HOSTNAME_INSTANCE
#  define CLIENT_HOST_KEY_FILE  KEYFILE
#  define TICKET_LIFETIME       128

You can override these with configure options if you so desire, with:

    --with-server-principal=ARG    server host principal  [amanda]
    --with-server-instance=ARG     server host instance   []
    --with-server-keyfile=ARG      server host key file   [/.amanda]
    --with-client-principal=ARG    client host principal  [rcmd]
    --with-client-instance=ARG     client host instance   [HOSTNAME_INSTANCE]
    --with-client-keyfile=ARG      client host key file   [KEYFILE]
    --with-ticket-lifetime=ARG     ticket lifetime        [128]


The configure script will search under /usr/kerberos/lib, /usr/cygnus/lib, /usr/lib, and /opt/kerberos/lib for libkrb.a. (in that order) for the kerberos bits. If it finds them, kerberos support will be added in, if it doesn't, it won't. If the kerberos bits are found under some other hierarchy, you can specify this via the --with-krb4-security=DIR, where DIR is where the kerberos bits live. It'll look under the 'lib' directory under this hierarchy for libkrb.a.

Installation

The kerberized AMANDA service uses a different port on the client hosts. The /etc/services line is:

kamanda      10081/udp


And the /etc/inetd.conf line is:

kamanda dgram udp wait root /usr/local/libexec/amanda/amandad amandad -auth=krb4

Note that you're running this as root, rather than as your dump user. AMANDA will set its uid down to the dump user at times it doesn't need to read the srvtab file, and give up root permissions entirely before it goes off and runs dump. Alternately you can change your srvtab files to be readable by user amanda.

Amanda configuration file

The following dumptype options apply to krb4:

auth "krb4"    # use krb4 auth for this host
               # (you can mingle krb hosts & bsd .rhosts in one conf)
kencrypt       # encrypt this filesystem over the net using the krb4
               # session key.  About 2x slower.  Good for those root
               # partitions containing your keyfiles.  Don't want to
               # give away the keys to an ethernet sniffer!
               # This is currently always enabled.  There is no
               # way to disable it.  This is a bug.

Kerberos v5

Building

You must specify --with-krb5-security to configure, otherwise there will be no attempt to look for kerberos binaries. You may specify a path that the system should look for the kerberos libraries, or leave it to the default.

By default, when --with-krb5-security is specified with with no path, the configure script will search under /usr/kerberos/lib, /usr/cygnus/lib, /usr/lib, and /opt/kerberos/lib for libkrb.a. (in that order) for the kerberos bits. If it finds them, kerberos support will be added in, if it doesn't, it won't. If the kerberos bits are found under some other hierarchy, you can specify this via the --with-krb5-security=DIR, where DIR is where the kerberos bits live. It'll look under the 'lib' directory under this hierarchy for libkrb.a.

The krb5 driver script defaults to:

/*
 * The lifetime of our tickets in minutes.
 */
#define AMANDA_TKT_LIFETIME     (12*60)

/*
 * The name of the service in /etc/services.
 */
#define AMANDA_KRB5_SERVICE_NAME        "k5amanda"


You can currently only override these by editing the source.

The principal and keytab file that the amanda uses are genearlly set in the amanda.conf file (see below). You can hardcode this in the source if you really want to and that's described in common-src/krb5-security.c

Installation

The kerberized AMANDA service uses a different port on the client hosts. The /etc/services line is:

k5amanda      10082/tcp


And the /etc/inetd.conf line is:

k5amanda stream tcp nowait root /usr/local/libexec/amanda/amandad amandad -auth=krb5

Note that you're running this as root, rather than as your dump user. AMANDA will set it's uid down to the dump user at times it doesn't need to read the keytab file, and give up root permissions entirely before it goes off and runs dump. Alternately you can change your keytab files to be readable by user amanda. You should understand the security implications of this before changing the permissions on the keytab.

Amanda configuration file

The following dumptype options apply to krb5:

auth "krb5"    # use krb5 auth for this host
               # (you can mingle krb hosts & bsd .rhosts in one conf)

The following two configuration directives are required in the amanda.conf file for kerberos 5 dumps to work:

krb5keytab
krb5principal

For example:

krb5keytab	"/etc/krb5.keytab-amanda"
krb5principal	"amanda/saidin.omniscient.com"

The principal in the second option must be contained in the first. The keytab should be readable by the amanda user. (and definitely not world readable!) This is (obviously) on the server. In MIT's kadmin, the following:

addprinc -randkey amanda/saidin.omniscient.com
ktadd -k /etc/krb5.keytab-amanda amanda/saidin.omniscient.com

will do the trick. You will obviously want to change the principal name to reflect something appropriate for the conventions at your site.

You must also configure each client to allow the amanda principal in for dumps. This is described in section 4.

Destination Host Permissions file

There are several ways to go about authorizing a server to connect to a client.

The normal way is via a .k5amandausers file or a .k5login file in the client user's home directory. The determination of which file to use is based on the way you ran configure on AMANDA. By default, AMANDA will use .k5amandahosts, but if you configured with --without-amandahosts, AMANDA will use .k5login. (similar to the default for .rhosts/.amandahosts-style security). The .k5login file syntax is a superset of the default krb5 .k5login. The routines to check it are implemented in amanda rather than using krb5_kuserok because the connections are actually gssapi based.

This .k5amandahosts/.k5login is a hybrid of the .amandahosts and a .k5login file. You can just list principal names, as in a .k5login file and the principal will be permitted in from any host. If you do NOT specify a realm, then there is no attempt to validate the realm (this is only really a concern if you have cross-realm authentication set up with another realm or something else that allows you multiple realms in your kdc. If you do specify a realm, only that principal@realm will be permitted to connect.

You may prepend this with a hostname and whitespace, and only that principal (with optional realm as above) will be permitted to access from that hostname.

Here are examples of valid entries in the .k5amandahosts:

service/amanda
service/amanda@TEST.COM
dumpmaster.test.com service/amanda
dumpmaster.test.com service/amanda@TEST.COM

Rather than using a .k5amandahosts or .k5login file, the easiest way is to use a principal named after the destination user (such as amanda@TEST.COM in our example) and not have either a .k5amandahosts or .k5login file in the destination user's home directory.

There is no attempt to verify the realm in this case yet is only a concern if you have cross-realm authentication setup.

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