1. How Kerberos Works
2. No Key To Generate Kerberos Tickets
3. Scom Alert No Key To Generate Kerberos Ticket
4. Scom Alert No Key To Generate Kerberos Ticket

Version 5 of the Kerberos protocol was originally implemented usingthe Data Encryption Standard (DES) as a block cipher for encryption.While it was considered secure at the time, advancements in computationalability have rendered DES vulnerable to brute force attacks on its 56-bitkeyspace. As such, it is now considered insecure and should not beused (RFC 6649).

The issue is I want to be able to generate a Kerberos ticket upon logging in, or at least so I don't have to enter my password in, at all. I used GSSAPI auth to get to localhost, so I got in with a kerberos ticket. Can I pass that one along, perhaps? I suspect this is your problem, actually.

1. This event generates every time Key Distribution Center issues a Kerberos Ticket Granting Ticket (TGT). This event generates only on domain controllers. If TGT issue fails then you will see Failure event with Result Code field not equal to “ 0x0 ”.
2. Oct 16, 2017  When you are diagnosing an Event ID 27 while processing a ticket-granting service (TGS) request for the target server, the account did not have a suitable key to generate a Kerberos ticket. You can use Klist to query the Kerberos ticket cache to determine if any tickets are missing, if the target server or account is in error, or if the encryption type is not supported.
3. This event generates every time Key Distribution Center issues a Kerberos Ticket Granting Ticket (TGT). This event generates only on domain controllers. If TGT issue fails then you will see Failure event with Result Code field not equal to “0x0”. This event doesn't generate for Result Codes: 0x10, 0x17 and 0x18.
4. Create a Kerberos principal and keytab files for each encryption type you use. Obtain the key of the principal by running the subcommand getprinc principalname. Provide the path of the keytab file as the value of the KEYTAB parameter in the Kerberos secegokerberos.conf plug-in configuration file.

History¶

DES was used in the original Kerberos implementation, and was theonly cryptosystem in krb5 1.0. Partial support for triple-DES (3DES) wasadded in version 1.1, with full support following in version 1.2.The Advanced Encryption Standard (AES), which supersedes DES, gainedpartial support in version 1.3.0 of krb5 and full support in version 1.3.2.However, deployments of krb5 using Kerberos databases created with olderversions of krb5 will not necessarily start using strong crypto forordinary operation without administrator intervention.

Types of keys¶

• The database master key: This key is not exposed to user requests,but is used to encrypt other key material stored in the kerberosdatabase. The database master key is currently stored as K/Mby default.
• Password-derived keys: User principals frequently have keysderived from a password. When a new password is set, the KDCuses various string2key functions to generate keys in the databasefor that principal.
• Keytab keys: Application server principals generally use randomkeys which are not derived from a password. When the databaseentry is created, the KDC generates random keys of various enctypesto enter in the database, which are conveyed to the application serverand stored in a keytab.
• Session keys: These are short-term keys generated by the KDC whileprocessing client requests, with an enctype selected by the KDC.

Linux generate ssh key dsa free. For details on the various enctypes and how enctypes are selected by the KDCfor session keys and client/server long-term keys, see Encryption types.When using the kadmin interface to generate new long-term keys,the -e argument can be used to force a particular set of enctypes,overriding the KDC default values.

Note

When the KDC is selecting a session key, it has no knowledge about thekerberos installation on the server which will receive the service ticket,only what keys are in the database for the service principal.In order to allow uninterrupted operation toclients while migrating away from DES, care must be taken to ensure thatkerberos installations on application server machines are configured tosupport newer encryption types before keys of those new encryption typesare created in the Kerberos database for those server principals.

How Kerberos Works

Upgrade procedure¶

This procedure assumes that the KDC software has already been upgradedto a modern version of krb5 that supports non-DES keys, so that theonly remaining task is to update the actual keys used to service requests.The realm used for demonstrating this procedure, ZONE.MIT.EDU,is an example of the worst-case scenario, where all keys in the realmare DES. The realm was initially created with a very old version of krb5,and supported_enctypes in kdc.conf was set to a valueappropriate when the KDC was installed, but was not updated as the KDCwas upgraded:

This resulted in the keys for all principals in the realm being forcedto DES-only, unless specifically requested using kadmin.

Before starting the upgrade, all KDCs were running krb5 1.11,and the database entries for some “high-value” principals were:

The krbtgt/REALM key appears to have never been changed since creation(its kvno is 1), and all three database entries have only a des-cbc-crc key.

The krbtgt key and KDC keys¶

Perhaps the biggest single-step improvement in the security of the cellis gained by strengthening the key of the ticket-granting service principal,krbtgt/REALM—if this principal’s key is compromised, so is theentire realm. Since the server that will handle service ticketsfor this principal is the KDC itself, it is easy to guarantee that itwill be configured to support any encryption types which might beselected. However, the default KDC behavior when creating new keys is toremove the old keys, which would invalidate all existing tickets issuedagainst that principal, rendering the TGTs cached by clients useless.Instead, a new key can be created with the old key retained, so thatexisting tickets will still function until their scheduled expiry(see Changing the krbtgt key).

Note

The new krbtgt@REALM key should be propagated to slave KDCsimmediately so that TGTs issued by the master KDC can be used toissue service tickets on slave KDCs. Slave KDCs will refuse requestsusing the new TGT kvno until the new krbtgt entry has been propagatedto them.

It is necessary to explicitly specify the enctypes for the new databaseentry, since supported_enctypes has not been changed. Leavingsupported_enctypes unchanged makes a potential rollback operationeasier, since all new keys of new enctypes are the result of explicitadministrator action and can be easily enumerated.Upgrading the krbtgt key should have minimal user-visible disruption otherthan that described in the note above, since only clients which list thenew enctypes as supported will use them, per the procedurein Session key selection.Once the krbtgt key is updated, the session and ticket keys for userTGTs will be strong keys, but subsequent requestsfor service tickets will still get DES keys until the service principalshave new keys generated. Application serviceremains uninterrupted due to the key-selection procedure on the KDC.

After the change, the database entry is now:

Since the expected disruptions from rekeying the krbtgt principal areminor, after a short testing period, it isappropriate to rekey the other high-value principals, kadmin/admin@REALMand kadmin/changepw@REALM. These are the service principals used forchanging user passwords and updating application keytabs. The kadminand password-changing services are regular kerberized services, so thesession-key-selection algorithm described in Session key selectionapplies. It is particularly important to have strong session keys forthese services, since user passwords and new long-term keys are conveyedover the encrypted channel.

It is not necessary to retain a single-DES key for these services, sincepassword changes are not part of normal daily workflow, and disruptionfrom a client failure is likely to be minimal. Furthermore, if a kerberosclient experiences failure changing a user password or keytab key,this indicates that that client will become inoperative once servicesare rekeyed to non-DES enctypes. Such problems can be detected earlyat this stage, giving more time for corrective action.

Adding strong keys to application servers¶

Before switching the default enctypes for new keys over to strong enctypes,it may be desired to test upgrading a handful of services with thenew configuration before flipping the switch for the defaults. Thisstill requires using the -e argument in kadmin to get non-defaultenctypes:

Be sure to remove the old keys from the application keytab, per bestpractice.

Adding strong keys by default¶

Once the high-visibility services have been rekeyed, it is probablyappropriate to change kdc.conf to generate keys with the newencryption types by default. This enables server administrators to generatenew enctypes with the change subcommand of k5srvutil,and causes user passwordchanges to add new encryption types for their entries. It will probablybe necessary to implement administrative controls to cause all userprincipal keys to be updated in a reasonable period of time, whetherby forcing password changes or a password synchronization service thathas access to the current password and can add the new keys.

Note

The krb5kdc process must be restarted for these changes to take effect.

At this point, all service administrators can update their services and theservers behind them to take advantage of strong cryptography.If necessary, the server’s krb5 installation should be configured and/orupgraded to a version supporting non-DES keys. See Encryption types forkrb5 version and configuration settings.Only when the service is configured to accept non-DES keys shouldthe key version number be incremented and new keys generated(k5srvutilchange&&k5srvutildelold).

When a single service principal is shared by multiple backend servers ina load-balanced environment, it may be necessary to schedule downtimeor adjust the population in the load-balanced pool in order to propagatethe updated keytab to all hosts in the pool with minimal service interruption.

Removing DES keys from usage¶

This situation remains something of a testing or transitory state,as new DES keys are still being generated, and will be used if requestedby a client. To make more progress removing DES from the realm, the KDCshould be configured to not generate such keys by default.

No Key To Generate Kerberos Tickets

Note

An attacker posing as a client can implement a brute force attack againsta DES key for any principal, if that key is in the current (highest-kvno)key list. This attack is only possible if allow_weak_crypto = trueis enabled on the KDC. Setting the +requires_preauth flag on aprincipal forces this attack to be an online attack, much slower thanthe offline attack otherwise available to the attacker. However, settingthis flag on a service principal is not always advisable; see the entry inadd_principal for details.

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The following KDC configuration will not generate DES keys by default:

Note

As before, the KDC process must be restarted for this change to takeeffect. It is best practice to update kdc.conf on all KDCs, not just themaster, to avoid unpleasant surprises should the master fail and a slaveneed to be promoted.

It is now appropriate to remove the legacy single-DES key from thekrbtgt/REALM entry:

After the maximum ticket lifetime has passed, the old database entryshould be removed.

After the KDC is restarted with the new supported_enctypes,all user password changes and application keytab updates will notgenerate DES keys by default.

Once all principals have been re-keyed, DES support can be disabled on theKDC (allow_weak_crypto = false), and client machines can removeallow_weak_crypto = true from their krb5.conf configurationfiles, completing the migration. allow_weak_crypto takes precedence overall places where DES enctypes could be explicitly configured. DES keys willnot be used, even if they are present, when allow_weak_crypto = false.

Scom Alert No Key To Generate Kerberos Ticket

Support for legacy services¶

If there remain legacy services which do not support non-DES enctypes(such as AFS), allow_weak_crypto must remain enabled on the KDC.Client machines need not have this setting, though—applicationswhich require DES can use API calls to allow weak crypto on a per-requestbasis, overriding the system krb5.conf. However, having allow_weak_cryptoset on the KDC means that any principals which have a DES key in the databasecould still use those keys. To minimize the use of DES in the realm andrestrict it to just legacy services which require DES, it is necessaryto remove all other DES keys. The realm has been configured such thatat password and keytab change, no DES keys will be generated by default.The task then reduces to requiring user password changes and havingserver administrators update their service keytabs. Administrativeoutreach will be necessary, and if the desire to eliminate DES issufficiently strong, the KDC administrators may choose to randkeyany principals which have not been rekeyed after some timeout period,forcing the user to contact the helpdesk for access.

The Database Master Key¶

This procedure does not alter K/M@REALM, the key used to encrypt keymaterial in the Kerberos database. (This is the key stored in the stash fileon the KDC if stash files are used.) However, the security risk ofa single-DES key for K/M is minimal, given that access to materialencrypted in K/M (the Kerberos database) is generally tightly controlled.If an attacker can gain access to the encrypted database, they likelyhave access to the stash file as well, rendering the weak cryptographybroken by non-cryptographic means. As such, upgrading K/M to a strongerencryption type is unlikely to be a high-priority task.

Scom Alert No Key To Generate Kerberos Ticket

Is is possible to upgrade the master key used for the database, ifdesired. Using kdb5_util‘s add_mkey, use_mkey, andupdate_princ_encryption commands, a new master key can be addedand activated for use on new key material, and the existing entriesconverted to the new master key.