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INTRODUCTION
MUNGE (MUNGE Uid 'N' Gid Emporium) is an authentication service for creating
and validating credentials. It is designed to be highly scalable for use
in an HPC cluster environment. It allows a process to authenticate the
UID and GID of another local or remote process within a group of hosts
having common users and groups. These hosts form a security realm that is
defined by a shared cryptographic key. Clients within this security realm
can create and validate credentials without the use of root privileges,
reserved ports, or platform-specific methods.
RATIONALE
The need for MUNGE arose out of the HPC cluster environment. Consider the
scenario in which a local daemon running on a login node receives a client
request and forwards it on to remote daemons running on compute nodes within
the cluster. Since the user has already logged on to the login node, the
local daemon just needs a reliable means of ascertaining the UID and GID
of the client process. Furthermore, the remote daemons need a mechanism to
ensure the forwarded authentication data has not been subsequently altered.
A common solution to this problem is to use Unix domain sockets to
determine the identity of the local client, and then forward this
information on to remote hosts via trusted rsh connections. But this
presents several new problems. First, there is no portable API for
determining the identity of a client over a Unix domain socket. Second,
rsh connections must originate from a reserved port; the limited number
of reserved ports available on a given host directly limits scalability.
Third, root privileges are required in order to bind to a reserved port.
Finally, the remote daemons have no means of determining whether the
client identity is authentic.
OVERVIEW
A process creates a credential by requesting one from the local
MUNGE service. The encoded credential contains the UID and GID of
the originating process. This process sends the credential to another
process within the security realm as a means of proving its identity.
The receiving process validates the credential with the use of its local
MUNGE service. The decoded credential provides the receiving process with
a reliable means of ascertaining the UID and GID of the originating process.
This information can be used for accounting or access control decisions.
The contents of the credential (including any optional payload data) are
encrypted with a key shared by all munged daemons within the security realm.
The integrity of the credential is ensured by a message authentication
code (MAC). The credential is valid for a limited time defined by
its time-to-live (TTL). The daemon ensures unexpired credentials
are not replayed on a particular host. Decoding of a credential can
be restricted to a particular user and/or group ID. The payload data
can be used for purposes such as embedding the destination's address to
ensure the credential is only valid on a specific host. The internal
format of the credential is encoded in a platform-independent manner.
And the credential itself is base64 encoded to allow it to be transmitted
over virtually any transport.
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