Hi,
I prepared design page for the one-way trusts feature:
https://fedorahosted.org/sssd/wiki/DesignDocs/OneWayTrusts
The text is copied below as well for your convenience. I have a couple
of questions to discuss:
- Is it worth it to put the keytabs into a new subdirectory so that
a special SELinux label can be applied? I think yes, since then
we'd have additional layer of protection of the trust secrets
- Is there any reasonable fallback beyond re-fetching the keytab we
can do? I can't think of any..
- Should the IPA tools signal SSSD either via UNIX signals or via
the DBus API once a trust has been added or removed to minimize
the race condition and improve user experience? Currently I hope
not, the proposed mechanism should be enough but testing is still
required.
When the design page is agreed on, I'll turn the three subsections of
the implementation phase into smaller tickets and close the tracker.
= Feature Name =
Related ticket(s):
* https://fedorahosted.org/sssd/ticket/2579
=== Problem statement ===
The next IPA release will support one-way trusts. SSSD needs to add support
for this feature in its server mode.
=== Use cases ===
One-way trust to Active Directory where FreeIPA realm trusts Active Directory
forest using cross-forest trust feature of AD but the AD forest does not trust
FreeIPA realm. Users from AD forest can access resources in FreeIPA realm.
=== Overview of the solution ===
At a high level, SSSD needs to examine the trust objects whether they
are one-way or two way trusts. For each one-way trust, SSSD needs to fetch
and store the keytab and use the keytab to secure the connection. For
two-way trusts, we can keep using the existing code that reuses the IPA
realm and the system keytab for both IPA and AD connectins. Care must be
taken to remove keytabs of trusts that were removed as well.
Fetching the keytab would be done by calling the `ipa-getkeytab` utility
for every one-way trust. The keytab would only be (re)fetched if it's
missing or if attempting to use the keytab failed. On the IPA server,
we must make sure that the IPA server identity is allowed to read the keytab.
Because handling multiple keytabs increases the risk of failing connections in
case the trust wasn't setup correctly, we need to modify the failover code to
not set the whole back end offline in case connecting to a subdomain AD server
fails. Instead, the subdomain will be marked as inactive for a short period of
time, during which it would act as offline. The proper way of solving this
problem would be to rework the failover module so that it can act per domain,
not only per back end, however, that change is out of scope for this release.
=== Implementation details ===
This section describes all the required changes in detail.
==== Reading the subdomains in the IPA subdomain handler ====
The IPA subdomain handler will include the attribute `ipaNTTrustDirection`
when reading the trust objects. Currently this attribute is not readable
by the host principal, so the IPA ACIs must be relaxed (ipa ticket?). If
the trust direction is set to an OR of `lsa.LSA_TRUST_DIRECTION_OUTBOUND`
and `lsa.LSA_TRUST_DIRECTION_INBOUND`, then it's a two-way trust and we'll
just use the existing code that re-uses the IPA keytab for the AD trusted
domain as well. If the attribute is only `lsa.LSA_TRUST_DIRECTION_OUTBOUND`,
we handle the trust as a one-way trust. The trust type can be stored in
`ipa_ad_server_ctx`.
Each `sss_domain_info` structure will be created as `inactive` in the
subdomain code. After enumerating the trusted domains, the subdomain
handler will check if a keytab already exists for every one-way trusted
domain. If yes, the domain is ready to use and can be enabled. If there is
no keytab, the subdomain handler will fork out a call to `ipa-getkeytab`,
fetch the keytab and store it under `/var/lib/sss/db`. The `ipa-getkeytab`
call will be done using Kerberos credentials the host has. IPA ACIs must
be modified accordingly to allow the IPA server principals to fetch the
trust keytabs, but noone else.
The directory `/var/lib/sss/db` is already only accessible to the sssd
user only. As an additional security measure, we might want to store the
keytabs under a new directory so that the keytabs also receive a SELinux
context stricter than the default `sssd_var_lib_t`. If fetching the keytab
succeeds, the domain would be enabled. The SELinux policy must also be
adjusted to allow calling `ipa-getkeytab` by the `sssd_be` process.
If any trust relationships were removed, the corresponding keytabs must
be removed from the disk as well.
==== Changes to the AD id_ctx instantiation ====
With two-way trust, we can keep using the default IPA principal and keytab.
With one-way trust, the keytab retrieved from the IPA server must be
used. Also, the principal must be passed into the `ad_create_default_options`
function. The custom values must be set before we proceed to instantiate
LDAP provider options. The only AD provider option we need to set is
AD_KRB5_REALM.
In the LDAP provider, we must take care that the following sdap options
are set correctly:
* SDAP_SASL_AUTHID - must be set to the NetBIOS name of the IPA domain. (A
domain `TRUST.COM` would set this value to `TRUST$`).
* SDAP_SASL_REALM - must be set to the AD realm
* SDAP_KRB5_KEYTAB - must be set to the per-domain keytab retrieved from IPA
The AD provider eventually calls `sdap_set_sasl_options()` from the
LDAP provider, we need to make sure this function receives the correct
values. During experimentation we were able to show that using multiple
different SASL users and different realms doesn't cause any problems in
SASL or LDAP libraries.
The only place that will keep using the IPA realm is the failover
instantiation. We need to keep using this hack until failover is per-backend.
==== Subdomain offline status changes ====
At the moment, the whole back end can be either online or offline and
the status applies to both the main domain and the subdomains alike. As
an effect, a failure to connect to a subdomain server would also make the
main domain operate offline. In many subdomain setups, it's actually more
convenient not to, because the subdomain server might be on a different
network segment, behind a different firewall etc. Instead, the domain
should only be made inactive.
The `sss_domain_info` structure would convert the 'bool disabled' parameter
into an `enum sss_domain_state`. The supported values would be:
* ''disabled'' - the domain should not be used by either responder or
provider. It was removed or disabled on the server.
* ''active'' - the domain can be used by a responder and the data provider
would forward its request to the backend
* ''inactive'' - the domain can be used by a responder, but the data
provider would just shortcut as if the domain was offline. For now, this
option will be used by subdomains only.
The implementation would include renaming the existing `be_mark_offline()`
function to be called `be_mark_dom_offline()` and modifying its behaviour. The
existing code that sets the offline status and runs the offline callbacks
would be called for parent domains only. For subdomains, we would mark the
subdomain as inactive and schedule a tevent request that would unconditionally
reset inactive domain to active. The request would be scheduled after
`offline_timeout` seconds to be consistent with main domains from user's
perspective. Likewise, the `be_reset_offline()` function will be extended to
reset inactive domains to active as well as the SIGUSR1 and SIGUSR2 signal
handlers. Finally, all calls to the `be_is_offline()` function should be
inspected and the invocations that are per-domain should be converted to a new
function `be_dom_is_offline()` that would check the offline status for parent
domains and the offline state for subdomains. We should also make sure the
backend offline status structure is opaque as currently its internals are
readable by external users as well. Making the offline status opaque would
make it safer to perform modifications to the offline code.
In both offline and inactive cases, the ID handlers would reply with
`DP_ERR_OFFLINE`. The crucial difference between offline and inactive at this
point would be that inactive domains are re-activated undonditionally. When we
modify the failover code to handle domains separately, we'll be able to
leverage per-domain online checks or online/offline callbacks as well.
==== Detecting re-established trusts ====
In cases the `sdap_kinit_send()` request fails, the sdap code would return a
special error code instead of blindly returning `EIO` as it does at the
moment. When the `ipa_get_ad_acct` request receives this error code, it would
re-run the subdomain request in order to check if the trust relationship still
exists and in order to re-fetch the keytab again. In case the trust is still
established, we would re-set the inactive domain status and re-run the account
request. In case the trust went away, the subdomain code should remove the
trusted domain (however, this must be tested). If the account request returns
any error after reinitializing the ketab, the subdomain would be marked
inactive in order to avoid fetching the keytab again and again. In order to be
able to run the subdomain request separately from the subdomain back end
handler, the subdomain code must be wrapped into a subdomain request as the
code currently assumes it's being called from the subdomain backend handler
only.
=== Future work ===
Handling failover and offline status on per-domain basis instead of
per-backend basis should be done in the next release.
=== Configuration changes ===
none
=== How To Test ===
Establish a one-way trust relationship with an AD domain. Make sure both
IPA and AD users are resolvable. It's prudent to test combinations of
one-way and two-way trusts with different forests. Make sure removing a
trust relationship removes the keytab from the filesystem. Make sure that
SSSD handles re-establishing a trust relationship.
=== Authors ===
* Jakub Hrozek <jhro...@redhat.com>
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