Which KDC Will Client Choose for Authentication

I never knew what for but I’ve always known it when it comes


In kerberized environment, every client should authenticate himself to KDC. For toy play, one KDC is enough, and it can serve all requests without overloading. But in production environment, with thousands of machines and clients, one KDC is not enough, a small auth traffic can lead to an application QPS downgrade. So, can auth requests be distributed to other KDCs in a cluster, and if so, how.

Talk is cheap, ___

Following is the snippet of how a client choose a KDC for authentication. sendReq It is simple, but the code itself is not easy to find in java src.
var4, retrieved from configuration var3 according to realm, is a string concatenation of KDCs. var5 is the iterator after the KDCs string processed.
The question is answered, client will sequentially try the KDC by iterating var4 through var5. So the former KDC in the list, the sooner client will pick it up for authentication.
And the sequence of the KDCs is determined by the krb5.conf,

  kdc = kdc1.com
  kdc = kdc2.com
  kdc = kdc3.com
  kdc = kdc4.com
  kdc = kdc5.com
  kdc = kdc6.com

It is an example with 6 KDCs, so a client will first try kdc1.com, if fails then kdc2 and so on.


Back to question in the beginning,

Q1: How a client determine which KDC for authentication?
A1: By the sequence of KDCs listed in /etc/krb5.conf
Q2: Can auth requests be distributed to other KDCs in a cluster?
A2: Yes, by giving out /etc/krb5.conf with different KDCs sequence. But it is not a very elegant solution, because it has to distribute serveral krb5.confs which is difficult for management from administrator perspective. Here it is another suggest which i’m using it in my production. Set up LVS-HAProxy-KeepAlive architecture before KDCs, let LVS do the load balancing that redirects auth requests to HAProxy, HAProxy strategically choose the backend KDCs and forward auth requests, KeepAlive to ensure one of HAProxy is working.
A3: Why KDCs do not have built in load balancing?
Q3: From my personal understanding, since master KDC is always the up-to-date one, while slave KDCs may have some lags from sync with master KDC which may lead to authentication failure if one of them is chosen. Sounds like a trade-off here, but this can be avoided as long as we kprod or transfer full DB snapshot to each slave KDC every time we update the master KDCs (like adding a pricipal, deleting a principal).