anvil - Postfix session count and request rate control

anvil [generic Postfix daemon options]

The Postfix anvil(8) server maintains short-term statistics to defend against clients that hammer a server with either too many simultaneous sessions, or with too many successive requests within a configurable time interval. This server is designed to run under control by the Postfix master(8) server.

The anvil(8) server maintains no persistent database. Standard library utilities do not meet Postfix performance and robustness requirements.

When a remote client connects, a connection count (or rate) limited server should send the following request to the anvil(8) server:

request=connect

ident=string

This registers a new connection for the (service, client) combination specified with ident. The anvil(8) server answers with the number of simultaneous connections and the number of connections per unit time for that (service, client) combination:

status=0

count=number

rate=number

The rate is computed as the number of connections that were registered in the current "time unit" interval. It is left up to the server to decide if the remote client exceeds the connection count (or rate) limit.

When a remote client disconnects, a connection count (or rate) limited server should send the following request to the anvil(8) server:

request=disconnect

ident=string

This registers a disconnect event for the (service, client) combination specified with ident. The anvil(8) server replies with:

status=0

When a remote client sends a message delivery request, a message rate limited server should send the following request to the anvil(8) server:

request=message

ident=string

This registers a message delivery request for the (service, client) combination specified with ident. The anvil(8) server answers with the number of message delivery requests per unit time for that (service, client) combination:

status=0

rate=number

In order to prevent the anvil(8) server from discarding client request rates too early or too late, a message rate limited service should also register connect/disconnect events.

When a remote client sends a recipient address, a recipient rate limited server should send the following request to the anvil(8) server:

request=recipient

ident=string

This registers a recipient request for the (service, client) combination specified with ident. The anvil(8) server answers with the number of recipient addresses per unit time for that (service, client) combination:

status=0

rate=number

In order to prevent the anvil(8) server from discarding client request rates too early or too late, a recipient rate limited service should also register connect/disconnect events.

The anvil(8) server does not talk to the network or to local users, and can run chrooted at fixed low privilege.

The anvil(8) server maintains an in-memory table with information about recent clients of a connection count (or rate) limited service. Although state is kept only temporarily, this may require a lot of memory on systems that handle connections from many remote clients. To reduce memory usage, reduce the time unit over which state is kept.

Problems and transactions are logged to syslogd(8).

Upon exit, and every anvil_status_update_time seconds, the server logs the maximal count and rate values measured, together with (service, client) information and the time of day associated with those events. In order to avoid unnecessary overhead, no measurements are done for activity that isn't concurrency limited or rate limited.

Systems behind network address translating routers or proxies appear to have the same client address and can run into connection count and/or rate limits falsely.

In this preliminary implementation, a count (or rate) limited server can have only one remote client at a time. If a server reports multiple simultaneous clients, all but the last reported client are ignored.

Changes to main.cf are picked up automatically as anvil(8) processes run for only a limited amount of time. Use the command "postfix reload" to speed up a change.

The text below provides only a parameter summary. See postconf(5) for more details including examples.

anvil_rate_time_unit (60s)

The time unit over which client connection rates and other rates are calculated.

anvil_status_update_time (600s)

How frequently the anvil(8) connection and rate limiting server logs peak usage information.

config_directory (see 'postconf -d' output)

The default location of the Postfix main.cf and master.cf configuration files.

daemon_timeout (18000s)

How much time a Postfix daemon process may take to handle a request before it is terminated by a built-in watchdog timer.

ipc_timeout (3600s)

The time limit for sending or receiving information over an internal communication channel.

max_idle (100s)

The maximum amount of time that an idle Postfix daemon process waits for the next service request before exiting.

max_use (100)

The maximal number of connection requests before a Postfix daemon process terminates.

process_id (read-only)

The process ID of a Postfix command or daemon process.

process_name (read-only)

The process name of a Postfix command or daemon process.

syslog_facility (mail)

The syslog facility of Postfix logging.

syslog_name (postfix)

The mail system name that is prepended to the process name in syslog records, so that "smtpd" becomes, for example, "postfix/smtpd".

smtpd(8), Postfix SMTP server
postconf(5), configuration parameters
master(5), generic daemon options

Use "postconf readme_directory" or "postconf html_directory" to locate this information.

TUNING_README, performance tuning

The Secure Mailer license must be distributed with this software.

The anvil service is available in Postfix 2.2 and later.

Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA