Unit 11: Kerberos ticket policy

Prerequisites:

In this module you will explore how to manage properties of Kerberos authentication and authorization in FreeIPA.

Kerberos ticket policies

FreeIPA’s primary authentication mechanism is based on Kerberos infrastructure. Each user has an associated Kerberos principal and potential aliases. Each FreeIPA service has its own Kerberos service and, optionally, alias names as well. Depending on who initiates a communication flow, in Kerberos terminology there are client and a target (server) principals. Any Kerberos principal can be used in both client and server roles, though typically FreeIPA services are used as targets and users are used as clients.

A client first authenticates to a Key Distribution Center (KDC) and obtains a ticket granting ticket (TGT). With the ticket granting ticket in possession, the client then asks a KDC for a service ticket to the target Kerberos principal. After the service ticket is issued by the KDC, the client can initiate its application-specific communication to the target application server.

When a KDC issues a ticket, there are few properties that can be controlled with the help of a Kerberos policy in FreeIPA. Each ticket has its own life time and a potential renewal age: a ticket can be renewed before its life time has ended but until the renewal age expired.

The combination of Kerberos ticket life time and renewal age altogether comprises a Kerberos ticket policy. The policy itself is not stored in a single place because individual parts of it are applied to different objects and at different stages of authentication and authorization processes. Instead, it is split into several parts which are associated with the corresponding Kerberos principals, whether they are used as clients or as targets.

For users, Kerberos ticket life time and renewal age can be managed with Kerberos ticket policy commands described in ipa help krbtpolicy manual.

If no specific policy is associated with a user, a default one is applied. To manage the default policy the same ipa krbtpolicy-* commands are used, without an explicit user name.

Display the default Kerberos ticket policy:

[admin@client]$ kinit admin
Password for admin@IPADEMO.LOCAL:
[admin@client]$ ipa krbtpolicy-show
  Max life: 86400
  Max renew: 604800

Modify the default policy to 8 hours max life, 1-day max renewal:

[admin@client]$ ipa krbtpolicy-mod --maxlife=28800 --maxrenew=86400
  Max life: 28800
  Max renew: 86640

Display effective Kerberos ticket policy for user admin:

[admin@client]$ ipa krbtpolicy-show admin
  Max life: 28800
  Max renew: 86640

Modify per-user policy for user ‘admin’:

[admin@client]$ ipa krbtpolicy-mod admin --maxlife=3600
  Max life: 3600

Reset per-user policy for user admin:

[admin@client]$ ipa krbtpolicy-reset admin
  Max life: 28800
  Max renew: 86640

Currently (FreeIPA 4.8.4), FreeIPA does not allow a Kerberos service principal to have a custom Kerberos ticket policy. As result, only default Kerberos ticket policy is applicable to service principals.

Kerberos authentication indicators

A Kerberos client may have different means to prove possession of a client principal credentials to a KDC. There are several so-called ‘pre-authentication mechanisms’ that are used for this purpose. FreeIPA KDC is able to record which pre-authentication method was used when issuing the ticket granting ticket. The specific label is called an authentication indicator.

Authentication indicators are associated with the following pre-authentication mechanisms:

Authentication indicator

Pre-authentication mechanism

radius

RADIUS

otp

FreeIPA two factor authentication (password + OTP)

pkinit

PKINIT, smart-card or certificate authentication

hardened

Hardened Password (by SPAKE or FAST)

Hardened authentication indicator is set by FreeIPA KDC when a Kerberos client has used one of two pre-authentication mechanisms that allow protecting an exchange between the client and the KDC:

  • FAST is a Kerberos pre-authentication mechanism defined in RFC 6113, section 5.4. It is used to securely pass so-called FAST factors to the KDC. Such factors might represent a traditional password-based exchange, or two-factor authentication, or something else. There are multiple types of FAST armors supported by FreeIPA.

  • SPAKE is a new pre-authentication mechanism, being standardized by the IETF Kitten working group. Its purpose is to increase security of Kerberos pre-authentication exchanges by making offline brute-force attacks infeasible and to enable use of multi-factor authentication without relying on FAST. SPAKE implementation in FreeIPA currently only supports a single factor authentication.

In the context of authentication indicators, FAST and SPAKE pre-authentication methods give higher level of protection than an exchange using encrypted timestamp method, traditional for Kerberos 5.

Each authentication indicator conveys the fact that KDC was able to pre-authenticate the initial ticket granting ticket exchange using chosen mechanism. This fact can further be used to differentiate the issued ticket life time and renewal age.

With FreeIPA 4.8.3 or later, Kerberos ticket policy allows an administrator to set different life time and renewal age for ticket granting tickets obtained with different pre-authentication methods. Each policy setting may include authentication indicator to say that the life time or renewal age applies to TGTs with which include this indicator:

[admin@client]$ ipa krbtpolicy-mod --help
  Usage: ipa [global-options] krbtpolicy-mod [USER] [options]

  Modify Kerberos ticket policy.
  Options:
    -h, --help            show this help message and exit
    --maxlife=INT         Maximum ticket life (seconds)
    --maxrenew=INT        Maximum renewable age (seconds)
    --otp-maxlife=INT     OTP token maximum ticket life (seconds)
    --otp-maxrenew=INT    OTP token ticket maximum renewable age (seconds)
    --radius-maxlife=INT  RADIUS maximum ticket life (seconds)
    --radius-maxrenew=INT
                          RADIUS ticket maximum renewable age (seconds)
    --pkinit-maxlife=INT  PKINIT maximum ticket life (seconds)
    --pkinit-maxrenew=INT
                          PKINIT ticket maximum renewable age (seconds)
    --hardened-maxlife=INT
                          Hardened ticket maximum ticket life (seconds)
    --hardened-maxrenew=INT
                          Hardened ticket maximum renewable age (seconds)
   ....

For example, we can allow admin user to renew its ticket for two days if it was obtained with hardened authentication indicator:

[admin@client]$ ipa krbtpolicy-mod admin --hardened-maxrenew=$((2*24*60*60))
  Hardened max renew: 172800
[admin@client]$ ipa krbtpolicy-show admin
  Max life: 28800
  Max renew: 86640
  Hardened max renew: 172800

There is no way to see authentication indicators for already issued tickets with existing Kerberos utilities. However, MIT Kerberos tracing facilities can be used to see what pre-authentication method was used to obtain a ticket:

[admin@client]$ KRB5_TRACE=/dev/stderr kinit admin
[29708] 1583503381.62516: Getting initial credentials for admin@IPADEMO.LOCAL
[29708] 1583503381.62518: Sending unauthenticated request
[29708] 1583503381.62519: Sending request (176 bytes) to IPADEMO.LOCAL
[29708] 1583503381.62520: Initiating TCP connection to stream AA.BB.CC.DD:88
[29708] 1583503381.62521: Sending TCP request to stream AA.BB.CC.DD:88
[29708] 1583503381.62522: Received answer (515 bytes) from stream AA.BB.CC.DD:88
[29708] 1583503381.62523: Terminating TCP connection to stream AA.BB.CC.DD:88
[29708] 1583503381.62524: Response was from master KDC
[29708] 1583503381.62525: Received error from KDC: -1765328359/Additional pre-authentication required
[29708] 1583503381.62528: Preauthenticating using KDC method data
[29708] 1583503381.62529: Processing preauth types: PA-PK-AS-REQ (16), PA-PK-AS-REP_OLD (15), PA-PK-AS-REQ_OLD (14), PA-FX-FAST (136), PA-ETYPE-INFO2 (19), PA-PKINIT-KX (147), PA-SPAKE (151), PA-ENC-TIMESTAMP (2), PA_AS_FRESHNESS (150), PA-FX-COOKIE (133)
[29708] 1583503381.62530: Selected etype info: etype aes256-cts, salt "SOME-VALUE", params ""
[29708] 1583503381.62531: Received cookie: SOME-VALUE
[29708] 1583503381.62532: PKINIT client has no configured identity; giving up
[29708] 1583503381.62533: Preauth module pkinit (147) (info) returned: 0/Success
[29708] 1583503381.62534: PKINIT client received freshness token from KDC
[29708] 1583503381.62535: Preauth module pkinit (150) (info) returned: 0/Success
[29708] 1583503381.62536: PKINIT client has no configured identity; giving up
[29708] 1583503381.62537: Preauth module pkinit (16) (real) returned: 22/Invalid argument
[29708] 1583503381.62538: PKINIT client ignoring draft 9 offer from RFC 4556 KDC
[29708] 1583503381.62539: Preauth module pkinit (15) (real) returned: -1765328360/Preauthentication failed
[29708] 1583503381.62540: PKINIT client ignoring draft 9 offer from RFC 4556 KDC
[29708] 1583503381.62541: Preauth module pkinit (14) (real) returned: -1765328360/Preauthentication failed
[29708] 1583503381.62542: SPAKE challenge received with group 1, pubkey 327144B7EC68505214E5A3606FD2091A7C47CBB60020D7D94B8C4878456B879E
Password for admin@IPADEMO.LOCAL:
[29708] 1583503386.372820: SPAKE key generated with pubkey F0AD6539C037C28758B692FA38FF8F924D5E52C593E485B3700DBF7FD2856477
[29708] 1583503386.372821: SPAKE algorithm result: B53EC5E8C1A22F36F91FD584915F19B3F06CDF3CE460704E2C900AE83DF53EDC
[29708] 1583503386.372822: SPAKE final transcript hash: AC42F4221481B9C9ED3169568A09BBDA9FAACE46DE13F6DCAFF8261003115A9C
[29708] 1583503386.372823: Sending SPAKE response
[29708] 1583503386.372824: Preauth module spake (151) (real) returned: 0/Success
[29708] 1583503386.372825: Produced preauth for next request: PA-FX-COOKIE (133), PA-SPAKE (151)
[29708] 1583503386.372826: Sending request (435 bytes) to IPADEMO.LOCAL
[29708] 1583503386.372827: Initiating TCP connection to stream AA.BB.CC.DD:88
[29708] 1583503386.372828: Sending TCP request to stream AA.BB.CC.DD:88
[29708] 1583503386.372829: Received answer (1419 bytes) from stream AA.BB.CC.DD:88
[29708] 1583503386.372830: Terminating TCP connection to stream AA.BB.CC.DD:88
[29708] 1583503386.372831: Response was from master KDC
[29708] 1583503386.372832: AS key determined by preauth: aes256-cts/AE1D
[29708] 1583503386.372833: Decrypted AS reply; session key is: aes256-cts/12C3
[29708] 1583503386.372834: FAST negotiation: available
[29708] 1583503386.372835: Initializing KCM:123456 with default princ admin@IPADEMO.LOCAL
[29708] 1583503386.372836: Storing admin@IPADEMO.LOCAL -> krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL in KCM:123456
[29708] 1583503386.372837: Storing config in KCM:123456 for krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL: fast_avail: yes
[29708] 1583503386.372838: Storing admin@IPADEMO.LOCAL -> krb5_ccache_conf_data/fast_avail/krbtgt\/IPADEMO.LOCAL\@IPADEMO.LOCAL@X-CACHECONF: in KCM:123456
[29708] 1583503386.372839: Storing config in KCM:123456 for krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL: pa_type: 151
[29708] 1583503386.372840: Storing admin@IPADEMO.LOCAL -> krb5_ccache_conf_data/pa_type/krbtgt\/IPADEMO.LOCAL\@IPADEMO.LOCAL@X-CACHECONF: in KCM:123456

As can be seen above, pre-authentication type, pa_type, 151 (SPAKE) was used in for pre-authentication. A look at the credential cache content will show that the renewal age policy applied corresponds to the hardened variant:

[admin@client]$ klist -f -d -e
Ticket cache: KCM:123456
Default principal: admin@IPADEMO.LOCAL

Valid starting       Expires              Service principal
06.03.2020 09.03.06  06.03.2020 17.03.06  krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL
     renew until 07.03.2020 09.03.01, Flags: FRIA
     Etype (skey, tkt): aes256-cts-hmac-sha1-96, aes256-cts-hmac-sha1-96 , AD types:

As result, admin user can now use its ticket up to 8 hours and renew it until 48 hours since the original ticket was obtained.

Finally, we can reset the default Kerberos ticket policy to the installation default:

[admin@client]$ ipa krbtpolicy-reset
  Max life: 86400
  Max renew: 604800

Enforcing authentication indicators

Authentication indicators from the ticket granting ticket are copied by the KDC into service tickets issued with the help of the TGT presented by a Kerberos client. The indicators can be seen by the applications receiving a communication encrypted with the service ticket. This allows an application administrator to permit restricted access to only those clients who used specific pre-authentication mechanisms to obtain their initial ticket granting ticket. For example, an application might decide to only allow access to a specialized resource to people who used smart-card authentication initially, even if the application itself only supports Kerberos authentication.

At the moment, there are no known applications that implement authentication indicator-based authorization. Instead, FreeIPA provides a check for an authentication indicator at KDC side. This means that a lack of a specific authentication indicator in TGT can result in denying an issuance of a requested service ticket. A consequence is that an application will never see any user with a ticket that does not contain a specified authentication indicator.

In order to enable the check, add authentication indicator to a service using ipa service-mod command. We can create a new service and associate the pkinit authentication indicator with it:

[admin@client]$ ipa service-add my-service/`hostname`
---------------------------------------------------
Added service "my-service/client.ipademo.local@IPADEMO.LOCAL"
---------------------------------------------------
  Principal name: my-service/client.ipademo.local@IPADEMO.LOCAL
  Principal alias: my-service/client.ipademo.local@IPADEMO.LOCAL
  Managed by: client.ipademo.local
[admin@client]$ ipa-getkeytab -k my-service.keytab -p my-service/`hostname`
Keytab successfully retrieved and stored in: my-service.keytab

A new service, my-service/client.ipademo.local, was created and a set of random Kerberos keys was associated with it by retrieving a keytab with the keys. If we want to request a service ticket to the service, the service must have Kerberos keys.

MIT Kerberos provides a nice tool to request Kerberos service tickets: kvno. At this point our service has no authentication indicator associated and a request will succeed:

[admin@client]$ KRB5_TRACE=/dev/stderr kvno -S my-service `hostname`
[29770] 1583505522.60592: Getting credentials admin@IPADEMO.LOCAL -> my-service/client.ipademo.local@IPADEMO.LOCAL using ccache KCM:123456
[29770] 1583505522.60593: Retrieving admin@IPADEMO.LOCAL -> my-service/client.ipademo.local@IPADEMO.LOCAL from KCM:!23456 with result: -1765328243/Matching credential not found
[29770] 1583505522.60594: Retrieving admin@IPADEMO.LOCAL -> krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL from KCM:!23456 with result: 0/Success
[29770] 1583505522.60595: Starting with TGT for client realm: admin@IPADEMO.LOCAL -> krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL
[29770] 1583505522.60596: Requesting tickets for my-service/client.ipademo.local@IPADEMO.LOCAL, referrals on
[29770] 1583505522.60597: Generated subkey for TGS request: aes256-cts/8F4D
[29770] 1583505522.60598: etypes requested in TGS request: aes256-cts, aes128-cts, aes256-sha2, aes128-sha2, des3-cbc-sha1, rc4-hmac, camellia128-cts, camellia256-cts
[29770] 1583505522.60600: Encoding request body and padata into FAST request
[29770] 1583505522.60601: Sending request (1655 bytes) to IPADEMO.LOCAL
[29770] 1583505522.60602: Initiating TCP connection to stream AA.BB.CC.DD:88
[29770] 1583505522.60603: Sending TCP request to stream AA.BB.CC.DD:88
[29770] 1583505522.60604: Received answer (1626 bytes) from stream AA.BB.CC.DD:88
[29770] 1583505522.60605: Terminating TCP connection to stream AA.BB.CC.DD:88
[29770] 1583505522.60606: Response was from master KDC
[29770] 1583505522.60607: Decoding FAST response
[29770] 1583505522.60608: FAST reply key: aes256-cts/71CF
[29770] 1583505522.60609: TGS reply is for admin@IPADEMO.LOCAL -> my-service/client.ipademo.local@IPADEMO.LOCAL with session key aes256-cts/8B3E
[29770] 1583505522.60610: TGS request result: 0/Success
[29770] 1583505522.60611: Received creds for desired service my-service/client.ipademo.local@IPADEMO.LOCAL
[29770] 1583505522.60612: Storing admin@IPADEMO.LOCAL -> my-service/client.ipademo.local@IPADEMO.LOCAL in KCM:123456
my-service/client.ipademo.local@IPADEMO.LOCAL: kvno = 1

Let’s associate pkinit authentication indicator with the service:

[admin@client]$ ipa service-mod my-service/`hostname` --auth-ind pkinit
------------------------------------------------------
Modified service "my-service/client.ipademo.local@IPADEMO.LOCAL"
------------------------------------------------------
  Principal name: my-service/client.ipademo.local@IPADEMO.LOCAL
  Principal alias: my-service/client.ipademo.local@IPADEMO.LOCAL
  Authentication Indicators: pkinit
  Managed by: client.ipademo.local

Since our credentials cache already contains Kerberos ticket to my-service/client.ipademo.local from the previous step, kvno will not attempt to obtain a new ticket if we just request it again. Instead, we need to destroy our credentials cache or specify a different one and re-try again:

[admin@client]$ kdestroy
[admin@client]$ kinit admin
Password for admin@IPADEMO.LOCAL:
[admin@client]$ KRB5_TRACE=/dev/stderr kvno -S my-service `hostname`
[29811] 1583506366.899807: Getting credentials admin@IPADEMO.LOCAL -> my-service/client.ipademo.local@IPADEMO.LOCAL using ccache KCM:123456
[29811] 1583506366.899808: Retrieving admin@IPADEMO.LOCAL -> my-service/client.ipademo.local@IPADEMO.LOCAL from KCM:123456 with result: -1765328243/Matching credential not found
[29811] 1583506366.899809: Retrieving admin@IPADEMO.LOCAL -> krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL from KCM:123456 with result: 0/Success
[29811] 1583506366.899810: Starting with TGT for client realm: admin@IPADEMO.LOCAL -> krbtgt/IPADEMO.LOCAL@IPADEMO.LOCAL
[29811] 1583506366.899811: Requesting tickets for my-service/client.ipademo.local@IPADEMO.LOCAL, referrals on
[29811] 1583506366.899812: Generated subkey for TGS request: aes256-cts/8737
[29811] 1583506366.899813: etypes requested in TGS request: aes256-cts, aes128-cts, aes256-sha2, aes128-sha2, des3-cbc-sha1, rc4-hmac, camellia128-cts, camellia256-cts
[29811] 1583506366.899815: Encoding request body and padata into FAST request
[29811] 1583506366.899816: Sending request (1655 bytes) to IPADEMO.LOCAL
[29811] 1583506366.899817: Initiating TCP connection to stream AA.BB.CC.DD:88
[29811] 1583506366.899818: Sending TCP request to stream AA.BB.CC.DD:88
[29811] 1583506366.899819: Received answer (447 bytes) from stream AA.BB.CC.DD:88
[29811] 1583506366.899820: Terminating TCP connection to stream AA.BB.CC.DD:88
[29811] 1583506366.899821: Response was from master KDC
[29811] 1583506366.899822: Decoding FAST response
[29811] 1583506366.899823: TGS request result: -1765328372/KDC policy rejects request
[29811] 1583506366.899824: Requesting tickets for my-service/client.ipademo.local@IPADEMO.LOCAL, referrals off
[29811] 1583506366.899825: Generated subkey for TGS request: aes256-cts/CC99
[29811] 1583506366.899826: etypes requested in TGS request: aes256-cts, aes128-cts, aes256-sha2, aes128-sha2, des3-cbc-sha1, rc4-hmac, camellia128-cts, camellia256-cts
[29811] 1583506366.899828: Encoding request body and padata into FAST request
[29811] 1583506366.899829: Sending request (1655 bytes) to IPADEMO.LOCAL
[29811] 1583506366.899830: Initiating TCP connection to stream AA.BB.CC.DD:88
[29811] 1583506366.899831: Sending TCP request to stream AA.BB.CC.DD:88
[29811] 1583506366.899832: Received answer (447 bytes) from stream AA.BB.CC.DD:88
[29811] 1583506366.899833: Terminating TCP connection to stream AA.BB.CC.DD:88
[29811] 1583506366.899834: Response was from master KDC
[29811] 1583506366.899835: Decoding FAST response
[29811] 1583506366.899836: TGS request result: -1765328372/KDC policy rejects request
kvno: KDC policy rejects request while getting credentials for my-service/client.ipademo.local@IPADEMO.LOCAL

Finally, we can remove the indicator from my-service/client.ipademo.local:

[admin@client]$ ipa service-mod my-service/`hostname` --auth-ind ''
------------------------------------------------------
Modified service "my-service/client.ipademo.local@IPADEMO.LOCAL"
------------------------------------------------------
  Principal name: my-service/client.ipademo.local@IPADEMO.LOCAL
  Principal alias: my-service/client.ipademo.local@IPADEMO.LOCAL
  Managed by: client.ipademo.local

Authentication indicators and FreeIPA services

Authentication indicators can become an effective way to enforce the use of a particular pre-authentication method. However, there are caveats. Since KDC-based enforcement does not allow anyone to obtain a service ticket to a Kerberos service if they do not possess an authentication indicator in question, great care has to be used when assigning authentication indicators to internal FreeIPA services.

Internal FreeIPA services include following Kerberos services on each IPA master or replica:

  • HTTP/master.ipademo.local@IPADEMO.LOCAL

  • ldap/master.ipademo.local@IPADEMO.LOCAL

  • DNS/master.ipademo.local@IPADEMO.LOCAL

  • cifs/master.ipademo.local@IPADEMO.LOCAL

These services are used by automated tools and internally by FreeIPA server applications themselves. The tools and servers currently cannot perform interactive authentication steps required by PKINIT and multi-factor authentication methods.