msktutil - Online in the Cloud

PROGRAM:

NAME

msktutil - fetches and manages kerberos keytabs in an Active Directory environment

SYNOPSIS

msktutil [command 1] [command 2] [command 3] ...

DESCRIPTION

msktutil is a Unix/Linux keytab client for Microsoft Active Directory environments. This
program is capable of creating accounts in Active Directory, adding service principals to
those accounts, and creating local keytab files so that kerberizied services can utilize
Active directory as a Kerberos realm. msktutil will create and manage machine accounts by
default. The --use-service-account option lets msktutil operate on service accounts.
msktutil requires that the Kerberos client libraries are properly installed and configured
to use Active Directory as a realm.

Whenever a principal is added or a keytab is updated, the secret password for the
corresponding account is changed. By default, the password is not stored, so it needs to
be reset each time msktutil is executed. All entries in the keytab will be automatically
updated whenever the password is reset. The previous entries will be left in the keytab,
so sessions using the older key versions will not break. This behavior is similar to the
way Windows hosts handle machine password changes.

CREDENTIALS

There are two common methods of using this program. The first is to "kinit" with
Administrator-like credentials which have permission to create computer objects in your
Active Directory server. If you invoke the program with such credentials, you can create
a new computer account or service account from scratch.

The second is to pre-create the accounts with such credentials, and then invoke msktutil
on a machine without any special permissions. When the computer account or service
account exists already, msktutil will attempt to authenticate as that account using either
the existing keytab, or if that fails, a default password. When that default password is
not specified with the option --old-account-password, msktutil will use the default
machine password. It will then change the password and update the keytab appropriately.
This is usually the more convenient option when joining many computers to the domain.

To pre-create a computer account, you may use the Active Directory Users and Computers
GUI, select "new computer" from the right click menu, and type the short DNS name, then
right click on the newly created object and select "Reset account" to set the password to
the default value. Another alternative is to invoke msktutil with the --precreate
argument. Both methods accomplish the same thing.

To pre-create a service account, you may use the Active Directory Users and Computers GUI,
select "new user" from the right click menu, fill in all required data, set the password
to a specific value and use setspn.exe to set the desired servicePrincipalName(s). You
may also select "must change password at next logon".

PASSWORD EXPIRY

Be aware that Windows machines will, by default, automatically change their account
password every 30 days, and thus many domains have a 90-day password expiry window, after
which your keytab will stop working. There are two ways to deal with this:

a) (Preferred): Make sure you're running a daily cron job to run msktutil --auto-update,
which will change the password automatically 30 days after it was last changed and update
the keytab.

b) (Not preferred): disable password expiry for the account via the --dont-expire-password
option (or otherwise setting DONT_EXPIRE_PASSWORD flag in userAccountControl in AD).

PASSWORD POLICY ISSUES

This section only applies to msktutil --use-service-account.

While machine account passwords may be changed at any time, service accounts are user
accounts and your Active Directory domain may have special password policies for those
user accounts. E.g., "minimum password age" is typically set to 1 day, which means that
you will have to wait for that time to pass until you may invoke msktutil --update --use-
service-account.

OTHER NOTES

Unlike other kerberos implementations, Active Directory has only a single key for all of
the principals associated with an account. So, if you create a HTTP/hostname service
principal, it will share the same key as the host/hostname principal. If you want to
isolate (security-wise) different service principals, you may want to create a dedicated
service account for them (with --use-service-account) and a separate keytab file (with
--keytab).

Also note: kinit -k 'host/computername' *will not work*, by default, even when that is a
valid service principal existing in your keytab. Active Directory does not allow you to
authenticate as a service principal, so do not use that as a test of whether the service
principal is working. If you actually want to authenticate as the computer account user,
kinit -k 'computername$' instead.

If you really need to be able to authenticate as 'host/computername', you can also use the
--upn argument to set the userPrincipalName attribute (generally requires administrator
credentials, not computer account credentials). Both 'computername$' and the value of
userPrincipalName are treated as valid account names to kinit as.

msktutil will use kerberized LDAP operations to talk to domain controllers. To obtain a
LDAP service ticket, the DNS service will be used to construct the domain controllers LDAP
principal name. If DNS is mis-configured, this construction may fail. To work around
this issue, you may specify the fully qualified DNS name of your domain controller with
the --server option and additionally use the --no-reverse-lookups option.

Samba (www.samba.org) provides the net command that can be used to manage kerberos keytabs
as well. Using msktutil and commands like "net ads join" or "net ads keytab" together can
lead to trouble. With the --set-samba-secret option, msktutil can be used as a
replacement for net.

Active Directory includes authorization data (e.g. information about group memberships) in
Kerberos tickets. This information is called PAC and may lead to very large ticket sizes.
Especially HTTP services are known to produce failures if that size exceeds the HTTP
header size. If your service does not make use of that PAC information (which is true for
most Unix/Linux-services) you may just disable it with the --no-pac option.

MODES

-v, --version
Displays version information

--help Displays a help message

-c, --create
Creates a keytab for the current host or a given service account. Equivalent to
--update --service host.

-f, --flush
Flushes out all principals for the current accountname from the keytab, and makes
corresponding changes to the machine or service account.

-u, --update
Forces a password change and updates all related service principal entries from the
servicePrincipalName and userPrincipalName attributes. Updates dNSDomainName for
machine accounts and always updates msDS-supportedEncryptionTypes attributes with
current values, and applies other changes as specified.

--auto-update
Checks if the password is at least 30 days old (from pwdLastSet attribute), and
that the account does not have password expiry disabled. If those conditions are
met, acts just like --update. Will also update if the keytab failed to
authenticate but the default password did work (e.g. after resetting the account in
AD). Otherwise, exits without doing anything (even if attribute modifying options
are given). This option is intended for use from a daily crontab to ensure that
the password is rotated regularly.

--precreate
Pre-create (or update) an account for the given host with default password. Does
not use or update local keytab. Requires -h or --computer-name argument. Implies
--user-creds-only. Generally requires administrator credentials.

CONNECTION/SETUP OPTIONS

-b, --base <base>
Specifies a relative LDAP base when creating a new account. For example,
specifying '-b OU=Unix' for a computer named SERVER in an Active Directory domain
example.com would create a computer account in the LDAP path:
CN=SERVER,OU=Unix,DC=EXAMPLE,DC=COM. This option can also be specified by setting
the MSKTUTIL_LDAP_BASE environment variable to the desired value.

If not specified, the default value is read from AD (and the default there, unless
modified by an admin, is CN=Computers for machine accounts and CN=Users for service
accounts).

--computer-name <name>
Specifies that the new account should use <name> for the computer account name and
the SAM Account Name. Note that a '$' will be automatically appended to the SAM
Account Name. Defaults to the machine's hostname, excluding the realm, with dots
replaced with dashes.

That is: if the realm is EXAMPLE.COM, and the hostname is FOO.EXAMPLE.COM, the
default computer name is FOO. If the hostname is FOO.BAR.EXAMPLE.COM, the default
computer name is FOO-BAR.

--account-name <name>
An alias for --computer-name that can be used when operating on service accounts.
Note that a '$' will not be automatically appended to the SAM Account Name when
using service accounts.

--old-account-password <password>
Use supplied account password for authentication. This is useful if the keytab
does not yet exist but the password of the computer account is known. This
password will be changed by msktutil in order to create or update the keytab

--password <new_password>
Specify the new account password instead of generating a random one. Consider the
password policy settings when defining the string.

-h, --hostname <name>
Overrides the current hostname to be used to be <name>. If this is not specified,
the local host name will be used. Note that the local name lookup service will be
to qualify and resolve names into fully-qualified names, including a domain
extension. This affects the default hostname for other arguments, and the default
computer-name. The hostname is also used to set the dNSDomainName attribute.

-k, --keytab <file>
Specifies to use <file> for the keytab. This option can also be specified by
setting the MSKTUTIL_KEYTAB environment variable to the name of the desired keytab
file. This keytab is both read from, in order to authenticate as the given
account, and written to, after updating the account password. Default:
/etc/krb5.keytab --keytab-auth-as <name> Specifies which principal name we should
try to use, when we authenticate from a keytab. Normally, msktutil will try to use
the account name or the host principal for the current host. If this option is
specified, instead msktutil will try to use the given principal name first, and
only fall back to the default behavior if we fail to authenticate with the given
name. This option can be useful if you do not know the current password for the
relevant account, do not have a keytab with the account principal, but you do have
a keytab with a service principal associated with that account.

--server <server>
Specifies to use <server> as the domain controller. This affects both kerberos and
ldap operations. The server can also be specified by setting the MSKTUTIL_SERVER
environment variable. Default: looked up in DNS from the realm name.

--server-behind-nat
When the server is behind a firewall that performs Network Address Translation,
KRB-PRIV messages fail validation. This is because the IP adddress in the
encrypted part of the message cannot be rewritten in the NAT process. This option
ignores the resulting error for the password change process, allowing systems
outside the NAT firewall to join the domain managed by servers inside the NAT
firewall.

--realm <realm>
Specifies to use <realm> as kerberos realm. Default: use the default_realm from
[libdefaults] section of krb5.conf.

--site <site>
Find and use domain controller in specific AD site. This option is ignored if
option --server is used.

-N, --no-reverse-lookup
Do not attempt to canonicalize the name of the domain controller via DNS reverse
lookups. You may need to do this if your client cannot resolve the PTR records for
a domain controller or your DNS servers store incorrect PTR records. Default: Use
DNS reverse lookups to canonicalize DC names.

--user-creds-only
Don't attempt to authenticate with a keytab: only use user's credentials (from e.g.
kinit). You may need to do this to modify certain attributes that require
Administrator credentials (description, userAccountControl, userPrincipalName, in a
default AD setup).

--auto-update-interval <days>
Number of <days> when --auto-update will change the account password. Defaults to
30 days.

--verbose
Enables verbose status messages. May be specified more then once to get LDAP
debugging.

OBJECT TYPE/ATTRIBUTE-SETTING OPTIONS

--use-service-account
Create and maintain service accounts instead of machine accounts.

--delegation
Enables the account to be trusted for delegation. This option can also be enabled
by setting the MSKTUTIL_DELEGATION environment variable. This modifies the
userAccountControl attribute. Generally requires administrator credentials.

--description <text>
Sets the account's description attribute to the given text (or removes if text is
''). Generally requires administrator credentials.

--disable-delegation
Disables the account from being trusted for delegation. This modifies the
userAccountControl attribute. Generally requires administrator credentials.

--disable-no-pac
Unsets the flag that disables the KDC's including of a PAC in the machine's service
tickets. This modifies the userAccountControl attribute. Generally requires
administrator credentials.

--dont-expire-password
Sets the DONT_EXPIRE_PASSSWORD bit in the userAccountControl attribute, which
disables password expiry for this account. If you don't run a cron job to
periodically rotate the keytab, you will want to set this flag. Generally requires
administrator credentials.

--do-expire-password
Unsets the DONT_EXPIRE_PASSWORD flag in the userAccountControl attribute.
Generally requires administrator credentials.

--enctypes <integer>
Sets the supported encryption types in the msDs-supportedEncryptionTypes field.

You may OR together the following values:
0x1=des-cbc-crc
0x2=des-cbc-md5
0x4=rc4-hmac-md5
0x8=aes128-cts-hmac-sha1
0x10=aes256-cts-hmac-sha1

This value is used to determine which encryption types AD will offer to use, and
which encryption types to put in the keytab.

If the value is set to 0x3 (that is: only the two DES types), it also attempts to
set the DES-only flag in userAccountControl.

Note: Windows 2008R2 refuses to use DES by default; you thus cannot use DES-only
keys unless you have enabled DES encryption for your domain first. Recent versions
of MIT kerberos clients similarly refuse to use DES by default.

Default: sets the value to 0x1C: that is, use anything but DES.

--allow-weak-crypto
Enables the usage of DES keys for authentication. This is equivalent to MIT's
krb5.conf parameter allow_weak_crypto.

--no-pac
Specifies that service tickets for this account should not contain a PAC. This
modifies the userAccountControl attribute. See Microsoft Knowledge Base article
#832575 for details. This option can also be specified by setting the
MSKTUTIL_NO_PAC environment variable. Generally requires administrator
credentials.

-s, --service <principal>
Specifies a service principal to add to the account (and thus keytab, if
appropriate). The service is of the form <service>/<hostname>. If the hostname is
omitted, assumes current hostname. May be specified multiple times.

--remove-service <principal>
Specifies a service principal to remove from the account (and keytab if
appropriate).

--upn <principal>
Sets the userPrincipalName attribute of the computer account or service account to
be <principal>.

The userPrincipalName can be used in addition to the sAMAccountName (e.g.
computername$ for computer accounts) for kinit.

<principal> can be provided in short form (e.g. host/hostname.example.com) or in
long form (e.g. host/hostname.example.com@EXAMPLE.COM). In short form the default
realm will automatically be appended.

This operation requires administrator privileges.

--set-samba-secret
Use Samba's net changesecretpw command to locally set the machine account password
in Samba's secrets.tdb. $PATH need to include Samba's net command. Samba needs to
be configured appropriately.

--no-tls
Don't use TLS on LDAP. LDAP traffic in Active Directory is already encrypted by
SASL/GSSAPI so there is no need for TLS.

EXAMPLES

For unprivileged users the most common invocations are:

msktutil --update --service host --service HTTP

This will update a computer account in Active Directory with a new password, write out a
new keytab, and ensure that it has both "host" and "HTTP" service principals are on it for
the hostname.

msktutil --auto-update

This is useful in a daily cron job to check and rotate the password automatically when
it's 30 days old.

For users with admin privileges in AD, some common uses:

msktutil --create --service host --service HTTP

This will create a computer account in Active Directory with a new password, write out a
new keytab, and ensure that it has both "host" and "HTTP" service principals are on it for
the hostname.

msktutil --precreate --host computer1.example.com

This will pre-create an account for computer1 with the default password using your
credentials. This can be done on a central host, e.g. to script the addition of many
hosts. You can then use msktutil --create on the hosts themselves (without special
credentials) to join them to the domain.

msktutil --host afs --service afs --enctypes 0x03

This will create an afs/cell.name@REALM principal, and associate that principal with a
computer account called 'afs'. The principal will be marked as DES-only, which is
required for AFS.

msktutil --create --use-service-account --service HTTP/hostname.example.com --keytab /etc/apache/krb5.keytab --account-name srv-http --no-pac

This will create an HTTP/hostname.example.com@REALM principal, and associate that
principal with a service account called 'srv-http'. Corresponding Kerberos keys will be
written to the keytab file /etc/apache/krb5.keytab. The size of Kerberos tickets for that
service will stay small because no PAC information will be included.

msktutil --create --service host/hostname --service host/hostname.example.com --set-samba-secret --enctypes 0x4

This will create a computer account in Active Directory that is compatible with Samba.
The command creates a new password, write out a new keytab, and ensure that it includes
both "host/hostname" and "host/hostname.example.com" as service principals (which is
equivalent to what setspn.exe -R would do on windows). The new computer password will be
stored in Samba's secrets.tdb database to provide interoperability with Samba. As Samba
(version 3) only supports arcfour-encrypted Kerberos tickets the --enctypes option must be
used to select only that encryption type.

Use msktutil online using onworks.net services