Man 8 munged

Name

munged -- MUNGE daemon

Synopsis

munged [OPTION] ...

Description

The munged daemon is responsible for authenticating local MUNGE clients and servicing their credential encode & decode requests.

All munged daemons within a security realm share a common key. All hosts within this realm are expected to have common users/UIDs and groups/GIDs. The key is used to cryptographically protect the credentials; it is created with the mungekey command.

When a credential is created, munged embeds metadata within it including the effective UID and GID of the requesting client (as determined by munged) and the current time (as determined by the local clock). It then compresses the data, computes a message authentication code, encrypts the data, and base64-encodes the result before returning the credential to the client.

When a credential is validated, munged first checks the message authentication code to ensure the credential has not been subsequently altered. Next, it checks the embedded UID/GID restrictions to determine whether the requesting client is allowed to decode it. Then, it checks the embedded encode time against the current time; if this difference exceeds the embedded time-to-live, the credential has expired. Finally, it checks whether this credential has been previously decoded on this host; if so, the credential has been replayed. If all checks pass, the credential metadata and payload are returned to the client.

Options

• -h, --help
  Display a summary of the command-line options.

• -L, --license
  Display license information.

• -V, --version
  Display version information.

• -f, --force
  Force the daemon to run if at all possible. This overrides warnings for an existing local domain socket, a lack of entropy for the PRNG, and insecure file/directory permissions. Use with caution as overriding these warnings can affect security.

• -F, --foreground
  Run the daemon in the foreground.

• -M, --mlockall
  Lock pages into memory to prevent the daemon from being swapped to disk. This can prevent authentication delays under extreme memory pressure. Requires sufficient memory lock limits. See the MEMORY LOCKING section below. [Added in 0.5.11]

• -s, --stop
  Stop the daemon bound to the socket and wait for it to shut down. Use with the --socket option to target a daemon bound to a non-default socket location. This option exits with a zero status if the specified daemon was successfully stopped, or a non-zero status otherwise. [Added in 0.5.14]

• -S, --socket path
  Specify the local domain socket for communicating with clients.

• -v, --verbose
  Be verbose. [Added in 0.5.14]

• --auth-server-dir directory
  Specify an alternate directory in which the daemon will create the pipe used to authenticate clients. The recommended permissions for this directory are 0711. This option is only valid on platforms where client authentication is performed via a file-descriptor passing mechanism.

• --auth-client-dir directory
  Specify an alternate directory in which clients will create the file used to authenticate themselves to the daemon. The recommended permissions for this directory are 1733. This option is only valid on platforms where client authentication is performed via a file-descriptor passing mechanism.

• --benchmark
  Disable recurring timers in order to reduce some noise while benchmarking. This affects the PRNG entropy pool, supplementary group mapping, and credential replay hash. Do not enable this option when running in production.

• --group-check-mtime boolean
  Specify whether the modification time of /etc/group should be checked before updating the supplementary group membership mapping. If this value is non-zero, the check will be enabled and the mapping will not be updated unless the file has been modified since the last update. [Added in 0.5.7]

• --group-update-time seconds
  Specify the number of seconds between updates to the supplementary group membership mapping; this mapping is used when restricting credentials by GID. A value of 0 causes it to be computed initially but never updated (unless triggered by a SIGHUP). A value of -1 causes it to be disabled. [Added in 0.5.7]

• --key-file path
  Specify an alternate pathname to the key file.

• --listen-backlog integer
  Specify the socket's listen backlog limit; note that the kernel may impose a lower limit. A value of 0 uses the software default. A value of -1 specifies SOMAXCONN, the maximum listen backlog queue length defined in <sys/socket.h>. [Added in 0.5.16]

• --log-file path
  Specify an alternate pathname to the log file. [Added in 0.5.13]

• --max-ttl integer
  Specify the maximum time-to-live (in seconds) for credentials. This value caps the TTL during both encoding and decoding. The hard-coded upper bound is 3600 seconds (1 hour); the default is 3600. Reducing this value will limit the maximum retention time for replay cache entries, which is viable if clocks within the MUNGE realm can be kept in sync with minimal skew. [Added in 0.5.12]

• --num-threads integer
  Specify the number of threads to spawn for processing credential requests.

• --origin address
  Specify the origin address that will be encoded into credential metadata. This can be a hostname or IPv4 address; it can also be the name of a local network interface, in which case the first IPv4 address found assigned to that interface will be used. The default value is the IPv4 address of the hostname returned by gethostname(). Failure to resolve the address will result in an error; if overridden, the origin will be set to the null address. [Added in 0.5.14]

• --pid-file path
  Specify an alternate pathname for storing the Process ID of the daemon. [Added in 0.5.12]

• --seed-file path
  Specify an alternate pathname to the PRNG seed file. [Added in 0.5.13]

• --syslog
  Redirect log messages to syslog when the daemon is running in the background. [Added in 0.5.11]

• --trusted-group group
  Specify the group name or GID of the "trusted group". This is used for permission checks on a directory hierarchy. Directories with group write permissions are allowed if they are owned by the trusted group (or the sticky bit is set). [Added in 0.5.13]

Memory Locking

The --mlockall option locks pages into memory to prevent swapping under memory pressure.

Memory usage scales with the number of worker threads, size of the user and group databases (when supplementary group mapping is enabled), and the rate of credential decoding (which grows the replay cache).

Many systems default to an insufficient memory lock limit. A minimum of 16 MB is required for basic operation; 64-256 MB is recommended for typical configurations. Increase it in one of the following ways:

systemd-based systems

Create /etc/systemd/system/munge.service.d/memlock.conf:

  [Service]
  LimitMEMLOCK=256M

Then reload and restart:

  systemctl daemon-reload
  systemctl restart munge.service

PAM limits

Add to /etc/security/limits.conf (values in KB):

  munge  soft  memlock  262144
  munge  hard  memlock  262144

On systemd-based systems, PAM limits are ignored for services. Use the systemd configuration instead.

Shell environment (for testing)

Set the limit before starting munged (value in KB):

  ulimit -l 262144
  munged --mlockall

Security Considerations

The replay cache is unbounded and grows with credential decode rate. Replay entries are retained until credentials expire based on their time-to-live (TTL). Credentials have a default TTL of 5 minutes; clients can specify up to the maximum TTL configured on the encoding munged (1 hour by default, configurable via --max-ttl). During decoding, the credential's TTL is capped at the decoding munged's maximum TTL. When --mlockall is specified, replay entries are locked in physical memory.

On systems with untrusted local users, an attacker could encode credentials with the maximum TTL (1 hour by default) and decode them repeatedly to fill the replay cache. Once the memory lock limit is reached, munged will likely terminate when attempting to allocate additional memory, causing a denial of service.

Use this option only on trusted systems where local users are controlled, or set the memory lock limit to a reasonable upper bound rather than unlimited. Monitor munged's locked memory usage on high-volume systems to ensure the limit is not approached during normal operation. Reducing --max-ttl can limit replay cache growth but requires tight clock synchronization.

Signals

• SIGHUP
  Immediately update the supplementary group membership mapping instead of waiting for the next scheduled update; this mapping is used when restricting credentials by GID.

• SIGTERM
  Terminate the daemon.

Notes

All clocks within a security realm must be kept in sync within the credential time-to-live setting.

While munged prevents a given credential from being decoded on a particular host more than once, nothing prevents a credential from being decoded on multiple hosts within the security realm before it expires.