|Manage Your Cloud / Cloud Best Practices|
Eucalyptus checks message timestamps across components in the cloud infrastructure. This assures command integrity and provides better security.
Eucalyptus components receive and exchange messages using either Query or SOAP interfaces (or both). Messages received over these interfaces are required to have some form of a time stamp (as defined by AWS specification) to prevent message replay attacks. Because Eucalyptus enforces strict policies when checking timestamps in the received messages, for the correct functioning of the cloud infrastructure, it is crucial to have clocks constantly synchronized (for example, with ntpd) on all machines hosting Eucalyptus components. To prevent user command failures, it is also important to have clocks synchronized on the client machines.
Following the AWS specification, all Query interface requests containing the Timestamp element are rejected as expired after 15 minutes of the timestamp. Requests containing the Expires element expire at the time specified by the element. SOAP interface requests using WS-Security expire as specified by the WS-Security Timestamp element.
When checking the timestamps for expiration, Eucalyptus allows up to 20 seconds of clock drift between the machines. This is a default setting. You can change this value for the CLC at runtime by setting the bootstrap.webservices.clock_skew_sec property as follows:
For additional protection from the message replay attacks, the CLC implements a replay detection algorithm and rejects messages with the same signatures received within 15 minutes. Replay detection parameters can be tuned as described in Configure Replay Protection.