msktutil(1)msktutil(1)NAMEmsktutil - fetches and manages kerberos keytabs in an Active Directory
environment
SYNOPSISmsktutil [command 1] [command 2] [command 3] ...
DESCRIPTIONmsktutil 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 pass‐
word for the corresponding account is changed. By default, the pass‐
word is not stored, so it needs to be reset each time msktutil is exe‐
cuted. All entries in the keytab will be automatically updated when‐
ever 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 pass‐
word is not specified with the option --old-account-password, msktutil
will use the default machine password. It will then change the pass‐
word 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 cre‐
ated 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 set‐
spn.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 work‐
ing. There are two ways to deal with this:
a) (Preferred): Make sure you're running a daily cron job to run msktu‐
til --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_PASS‐
WORD 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 sin‐
gle 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 prin‐
cipal, 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 userPrin‐
cipalName are treated as valid account names to kinit as.
msktutil will use kerberized LDAP operations to talk to domain con‐
trollers. 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 man‐
age 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 ser‐
vice account.
-u, --update
Forces a password change and updates all related service princi‐
pal 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 reg‐
ularly.
--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 MSKTU‐
TIL_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 com‐
puter account name and the SAM Account Name. Note that a '$'
will be automatically appended to the SAM Account Name.
Defaults to the machine's hostname, excluding the realm, with
dots replaced with dashes.
That is: if the realm is EXAMPLE.COM, and the hostname is
FOO.EXAMPLE.COM, the default computer name is FOO. If the host‐
name is FOO.BAR.EXAMPLE.COM, the default computer name is FOO-
BAR.
--account-name <name>
An alias for --computer-name that can be used when operating on
service accounts. Note that a '$' 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 use‐
ful 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
-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 princi‐
pal 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 spec‐
ified, 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 vari‐
able. 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 sys‐
tems 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 mod‐
ify certain attributes that require Administrator credentials
(description, userAccountControl, userPrincipalName, in a
default AD setup).
--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 environ‐
ment 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 cre‐
dentials.
--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 userAccount‐
Control attribute. Generally requires administrator creden‐
tials.
--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-supportedEncryp‐
tionTypes 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 equiv‐
alent to MIT's krb5.conf parameter allow_weak_crypto.
--no-pac
Specifies that service tickets for this account should not con‐
tain 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 creden‐
tials.
-s, --service <principal>
Specifies a service principal to add to the account (and thus
keytab, if appropriate). The service is of the form <ser‐
vice>/<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 on the computer account or service
account to be <principal>. Note that the realm will automati‐
cally be appended to the value given. The userPrincipalName is
an additional name which can be used to kinit. This is gener‐
ally unnecessary, since you can always authenticate as the name
given by --accountname (i.e. computername$ for computer
accounts) whether or not userPrincipalName is set. Generally
requires administrator credentials.
--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.
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 --accountname srv-http --no-pac
This will create an HTTP/hostname.example.com@REALM principal, and as‐
sociate that principal with a service account called 'srv-http'. Cor‐
responding 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 compat‐
ible 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 interoper‐
ability with Samba. As Samba (version 3) only supports arcfour-
encrypted Kerberos tickets the --enctypes option must be used to select
only that encryption type.
AUTHOR
Ken Dreyer, Mark Pröhl, Olaf Flebbe
0.5.1 msktutil(1)
