MIT Kerberos Documentation

replay cache

A replay cache (or “rcache”) keeps track of all authenticators recently presented to a service. If a duplicate authentication request is detected in the replay cache, an error message is sent to the application program.

The replay cache interface, like the credential cache and keytab interfaces, uses type:value strings to indicate the type of replay cache and any associated cache naming data to use.

Background information

Some Kerberos or GSSAPI services use a simple authentication mechanism where a message is sent containing an authenticator, which establishes the encryption key that the client will use for talking to the service. But nothing about that prevents an eavesdropper from recording the messages sent by the client, establishing a new connection, and re-sending or “replaying” the same messages; the replayed authenticator will establish the same encryption key for the new session, and the following messages will be decrypted and processed. The attacker may not know what the messages say, and can’t generate new messages under the same encryption key, but in some instances it may be harmful to the user (or helpful to the attacker) to cause the server to see the same messages again a second time. For example, if the legitimate client sends “delete first message in mailbox”, a replay from an attacker may delete another, different “first” message. (Protocol design to guard against such problems has been discussed in RFC 4120.)

Even if one protocol uses further protection to verify that the client side of the connection actually knows the encryption keys (and thus is presumably a legitimate user), if another service uses the same service principal name, it may be possible to record an authenticator used with the first protocol and “replay” it against the second.

The replay cache mitigates these attacks somewhat, by keeping track of authenticators that have been seen until their five-minute window expires. Different authenticators generated by multiple connections from the same legitimate client will generally have different timestamps, and thus will not be considered the same.

This mechanism isn’t perfect. If a message is sent to one application server but a man-in-the-middle attacker can prevent it from actually arriving at that server, the attacker could then use the authenticator (once!) against a different service on the same host. This could be a problem if the message from the client included something more than authentication in the first message that could be useful to the attacker (which is uncommon; in most protocols the server has to indicate a successful authentication before the client sends additional messages), or if the simple act of presenting the authenticator triggers some interesting action in the service being attacked.

Default rcache type

There is currently only one implemented kind of replay cache, called dfl. It stores replay data in one file, occasionally rewriting it to purge old, expired entries.

The default type can be overridden by the KRB5RCACHETYPE environment variable.

The placement of the replay cache file is determined by the following:

  1. The KRB5RCACHEDIR environment variable;
  2. If KRB5RCACHEDIR is unspecified, on UNIX, the library will fall back to the environment variable TMPDIR, and then to a temporary directory determined at configuration time such as /tmp or /var/tmp; on Windows, it will check the environment variables TEMP and TMP, and fall back to the directory C:\.

Performance issues

Several known minor performance issues that may occur when replay cache is enabled on the Kerberos system include: delays due to writing the authenticator data to disk slowing down response time for very heavily loaded servers, and delays during the rewrite that may be unacceptable to high-performance services.

For use cases where replays are adequately defended against for all protocols using a given service principal name, or where performance or other considerations outweigh the risk of replays, the special replay cache type “none” can be specified:


It doesn’t record any information about authenticators, and reports that any authenticator seen is not a replay.