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FreeIPAv2:Automatic Ticket Renewal

FreeIPAv2 Development Documentation

Please note that this content was made during FreeIPAv2 development and it may no longer be relevant for current versions of FreeIPA. We left the content here for study and archaeological purposes.

Please check our Documentation for a recent list of topics.

Requirements

Kerberos as authentication mechanism allows authenticating different kinds of principals: users, host, services etc. As a result of the authentication a so called TGT (ticket granting ticket) is issued. This ticket is used to obtain tickets to contact other services, so that they do not need to contact the central server to confirm authentication. The TGT is valid for a limited period of time. When the TGT expires a new one need to issued. If allowed by the TGT and requested on issue, the ticket can also be renewed before it is expired. The ticket cannot be renewd past the maximum renewal time. This page deals with acquiring and/or renewing TGTs.

Host Ticket

In IPA v2 every host enrolled with IPA will have a kerberos keytab that will be provisioned to the machine during the machine enrollment. This keytab will be used to authenticate the host when the host itself needs to perform an operation against the IPA server or any other client in the network.

Because the keytab contains the secret it is always possible to acquire a new TGT at any time. If a TGT is expired a new one will be requested before performing an operation that requires exchange of kerberos credentials.

User Ticket

Users logging into the IPA enrolled client are going to use a PAM module that will proxy requests down to the IPA Data Provider. The result will be the generation of a credential cache for the user containing the User TGT.

Credentials Expiration

Sometimes the user may need to run long jobs that take hours to complete. If the job tries to access other kerberos enabled services like NFS on behalf of the user, access will be denied if the Ticket is expired, and the job may abort. This situation is causes a lot of problems in the customer environment and is one of the show stoppers for broader kerberos adoption.

The requirement to solve this problem was added to the IPA PRD.

Other Considerations

There several important considerations that should be taken into the account:

  • The solution should not jeopardize security
  • The solution should not require end user intervention
  • The user ticket should not be renewed by default, but only if explicitly configured to do so.

Design

There are several different approaches that address the requirements.

Kerberos Renewal Approach

The Kerberos protocol allows to renew a ticket if it is marked as renewable (and original ticket was requested as renewable). The TGTs in addition to the “renewable” flag has a max renew time – when asking for renewals the expiration time will not be set beyond this time limit. Renewal must occur before the ticket expires. Potentially this can be defined on per users bases and stored, per principal, in LDAP using this objectclass.

  objectClasses: ( 2.16.840.1.113719.1.301.6.16.1 
  NAME 'krbTicketPolicyAux' 
  AUXILIARY 
  MAY ( krbTicketFlags $ krbMaxTicketLife $ krbMaxRenewableAge ) 
  X-ORIGIN 'user defined' ) 

Currently in IPA v2 there is no UI to add this information, but it can be manipulated using LDAP operations.
It would be possible to leverage this class to set per principal defaults, to allow or disallow renewals on a per principal basis. A possible scenario to manage these options is this:

  • Any kerberos principal entry would be populated with a krbTicketPolicyAux objectclass.
  • There would be a management interface that would allow bulk manipulation of kerberos ticket flags, ticket maximum time and renewable age.
  • There would be an option in the IPA client policy that will indicate that the clients should attempt to renew the tickets automatically. The IPA clients affected by this policy will attempt automatic renewal of the tickets.
  • The IPA data provider will have additional logic:
    • If the client is configured to automatically renew tickets, a “check kerberos renewal” task will be created.
    • The task will periodically generate an event and when that happens it will check if it is time to renew tickets.
    • If a ticket is close to expiration it will try to renew it.
    • Only ticket for logged in users are checked for, a list of references to the credential caches for each user is kept up to date based on login/logout events.
    • The renewal must happen before the ticket is expired, it is usually wise to renew a ticket when half the time of validity is passed:
     (now > releasetime + (expiretime - releasetime)/2)

or:

     (now > expiretime - configured_amount_of_time)

The exact method can be configured by a specific policy.

If we implement this functionality there will be no need to capture and store user passwords on the client (but will be limited to renewal time). We can also make the IPA client policy to specify a group of users for which the tickets should be renewed. In this case the handler first will check if the user who owns a ticket is a member of a group. It is pretty simple and doable. The only long pole in this solution is management of the options and ages in the IPA UI. But may be it is not that needed. The policy management effectively provides the capability of specifying the set of hosts (by making a policy destined to these hosts) and a user group (in the policy) for which the tickets need to be automatically renewed. It might then be sufficient not to use explicit attributes on a per user entry but rather configure the IPA data provider to always request renewable tickets with renewal period a long enough to complete nightly jobs or long "over the weekend jobs". With the current configuration in v1 the renewal age is set to 14 days maximum which should be more than enough for any unattended job in normal environments. Another factor to keep in mind is that in normal circumstances the user can obtain a completely new ticket simply performing an authentication. A common method to force a new authentication is to lock the machine and provide the user password to the screensaver. This will generate a new PAM authentication which will allow us to obtain a completely new TGT extending renewal time to 14 days from that point on.

Security Concerns

The only drawback of this solution is that an arbitrary program can be developed to automatically renew the kerberos tickets bypassing IPA policy checks since they are enforced on a client. But still this seems to be a viable solution for v2 even considering this issue.


Alternative Approach

The alternative approach calls for caching the user passwords on the client. During the authentication the user password will be captured and stored locally on the client to allow offline authentication. Normally the user password is hashed before being stored, so that it can be used only to verify authentication, but not be used to impersonate the user in case the machine is compromised. In case renewal of kerberos credentials is more important than other security concerns, then the clear text password can be stored in protected kernel memory. This way it will be automatically cleared when the machine is shut off or hibernated, and the system will never swap it to disk as the memory pages will be locked. As an extra measure it may be reversibly encrypted with an appropriate secret.

In this case the logic will be the same as the previous one. the only differences being that the user passwords are captured and stored on the client, and that renewal could happen even if the ticket is actually expired as we posses the user secret.

Security Concerns

The main issue with this approach is the need to cache passwords on the client, this is a quite relevant security concern. This approach allows to overcome the problem of being able to renew credentials past the renewal time and before a ticket is expired but should not be used unless there is a critical need for it (very long, completely unattended jobs on very secure systems.

Suggested Solution

After some discussion we decided that we will use a slightly modified ticket renewal approach.

Solution for Host Use Case

There are several components of the IPA client that would require kerberos authentication:

  • IPA Data Provider – used to do LDAP lookups
  • XML-RPC client – (formerly known as the Policy Downloader) used to download policies and execute certificate related requests
  • Audit client – component responsible for uploading the logs collected from different processes

Each component will implement its own independent kerberos authentication logic. This authentication logic will be capable of:

  • Authenticating with keytab if the TGT is not available or expired
  • Renewing TGT if it is about to expire

There is no need to use a shared ticket cache. For simplicity each process will keep its own ticket cache in memory and re-authenticate or renew ticket as needed.

To avoid the re-implementation of the same logic multiple times a common library will be created. This library will be implemented SSSD developers and shared with others team members who are working on the client components.

Solution for User Use Case

For the user case we decided that it would be an overhead to implement the logic described above inside the IPA client. We agreed that by default the IPA Data Provider when performs authentication will not request renewable kerberos tickets for users. If the user needs a renewable ticket he would be able to request it using “kinit -r

For this approach to work the kerberos ticket policies should be enforced on the server side. There are several places where and how the policies can be specified.

The research showed the following rules:

  • One can specify maximum ranges for renewal and lifetime in the /ver/kerberos/krb5kdc/kdc.conf file. It is in the realms section. max_life setting defines maximum life of the ticket. max_renewable_life setting defines the period during which the ticket is renewable. In IPA v1 the values are 7d and 14 days. This means that if the client ask for a ticket valid for the 7d it will get it.
  • If the settings are removed from the kdc.conf file the default hard coded values are 24h for lifetime and 7d for renewal.
  • There is an entry in the kerberos hive with cn equal name of the realm. This entry has krbTicketPolicyAux object class applied to it but no attributes that specify the timeouts. If those attributes added they can only further restrict the rules defined in the configuration file. They can't extend the lifetime beyond what has been set in kds.conf or, if entries are not defined, the hard coded values.
  • Each individual principal can have the krbTicketPolicyAux object class applied to it. Some principals already have it applied but not users. So in v2 we will add the krbTicketPolicyAux object class to user object and expose the ticket lifetime attributes in UI and CLI. This would allow to alter the policies defined at the higher level but up to the limits imposed by configuration.

Based on the rules above we will use the following defaults:

  • kdc.conf as current (no changes)
    • max_life = 7d
    • max_reneable_life = 14d
  • Kerberos realm entry
    • krbMaxTicketLife = 86400 (it is 1 day)
    • krbMaxRenewableAge = 604800 (it is 7 days)
  • User entries will have
    • objectClass = krbTicketPolicyAux (applied during upgrade)
    • krbMaxTicketLife - missing
    • krbMaxRenewableAge - missing

By adding the attributes to the user entry the customer would be able to override (extend) the ticket policies on per user basis up to 7 days and 14 days. On the client side the IPA client will always ask for the renewable ticket with 7 days lifetime and 14 days renewable life time. This is equivalent to:

 kinit -r 14d -l 7d <user>

These two values will be defined in the IPA client policy and will be centrally changeable if ever customer would want to alter values in kdc.conf and match the new kdc,conf values with the values used on the client.

In UI the kerberos fields will be special “protected” fields non editable until explicitly requested. The CLI can be used to effectively build a “bulk update” of the attributes if such functionality is needed. We might explore using same approach for the management of the password policy on per user basis instead of one size fits all as it is currently in v1.x.