Kerberos Flow Chart / Kerberos sessions keys / AS and TGS

1. Normal Flow
   
   
   
   
   
   
2. Forwarded Delegation
   
   
   
   
   
3. s4u Extension
   
   
   
   
   

A kerberos client is issued 2 type session keys:

1. Session key that is used for communication with TGS

2. Session key that is used for communication with service provider

The process of getting # 1 type session key is called "authentication". It's issued by Authentication Service (AS, for AD, AS resides on same machine as KDC). 

1. Client(machine) sends AS request(contains user ID in cleartext) on behalf of user to AS.
2. AS sends back session key type 1 back to client, encrypted with user's hash). AS also sends back TGT ( same session key plus other info such as session key valid duration) . TGT is not readable to client, it's encrypted with TGS' key (krbtgt hash)
3. Later on, user will use this session key to encrypt messages to TGS
4. AS (AD DC) never stores the session key. It's always the client to keep and present session key to TGS. This is achieved by sending TGT back to TGS (remember in step 2 above, TGT is encrypted with TGS' key so TGS can read the content and get the session key). In other words, session key becomes a timed common knowledge between TGS and client 

The process of getting #2 type session key can be roughly called "authorization" (actual access evaluation happens on resource provider, based on the Service Ticket presented). It's the process to get Service Ticket, which can be later presented to service provider. 

1. client sends a service request, one of the content of this request is its ID/time, encrypted with type #1 session key above. Along side the TGT (again this TGT is not readable to client, but it's readable to TGS/DC) 
2. TGS/DC decrypts TGT to get session key #1 so it can validate client ID /timestamp 
3. If matched, TGS sends back 2 things 
1. type #2 session key, encrypted with type #1 session key so client can read and use type #2 session key for communication with service provider
2. Service Ticket (not readable to client, but only for client to hold it and present to service provider). This is encrypted with service provider's key so only service provider can read it
3. In client's request, it specify a SPN for TGS to locate service provider's key
4. Client presents ST to service provider, along side its own ID, encrypted with type# session key

In short, neither TGS, nor service provide keeps any session keys. It's client's responsibility to obtain (Thru AS and TGS) to get those session keys, and present them to TGS/SP. 

2 TTL related registry on Windows client OS

1. Maximum time a client will cache positive results regardless what TTL a record holds:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters\MaxCacheEntryTtlLimit

Note: A record will be cache on client for the period specified in

• record TTL or
• above registry value, whichever is less. 

2. When register its own record, the TTL sent to DNS server:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\DefaultRegistrationTTL

See Also

• Negative results are cached very differently. Please see Negative cache TTL on Windows-based DNS
• DNS Zone intervals - mapping values in Windows GUI to nslookup result for SOA record
  

DNS Zone intervals - mapping values in Windows GUI to nslookup result for SOA record

 
See above, click to see enlarge picture.

This is a screenshot of SOA record.

What the intervals are:

• (Most important one) Minimum default TTL: Default TTL for all records in the zone that don't have their own TTL defined during creation. This parameter is also used as TTL of negative cache
• TTL : TTL for the SOA record itself
• Refresh - This is how often (in seconds) the slave name servers check with the primary name server to see if any changes have been made to the zone
• Retry - This is the time (in seconds) a slave (secondary) DNS server waits before retrying a failed zone transfer
• Expires  - Indicates when the zone data is no longer authoritative - time counted from the point when slave loses contact with master. Applies to Slaves or Secondary servers only. When this expires, slave will no longer respond to queries

Update #1: Regarding negative caching, it is lesser of  Minimum Default TTL, SOA TTL, and 15min (defined in MaxNegativeTTL,  can be viewed with powershell cmdlet get-dnsservercache)

Update #2: One can set different values on SOA TTL and Minimum Default TTL, but Windows will set both to same value, whichever is smaller. Tested on Windows 2016