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anvil - Postfix session count and request rate control
anvil [generic Postfix daemon options]
The Postfix anvil(8) server maintains statistics about client connection counts or client request rates. This information can be used 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.
In the following text, ident specifies a (service, client) combination. The exact syntax of that information is application-dependent; the anvil(8) server does not care.
To register a new connection send the following request to the anvil(8) server:
request=connect
ident=string
The anvil(8) server answers with the number of simultaneous connections and the number of connections per unit time for the (service, client) combination specified with ident:
status=0
count=number
rate=number
To register a disconnect event send the following request to the anvil(8) server:
request=disconnect
ident=string
The anvil(8) server replies with:
status=0
To register a message delivery request send the following request to the anvil(8) server:
request=message
ident=string
The anvil(8) server answers with the number of message delivery requests per unit time for the (service, client) combination specified with ident:
status=0
rate=number
To register a recipient request send the following request to the anvil(8) server:
request=recipient
ident=string
The anvil(8) server answers with the number of recipient addresses per unit time for the (service, client) combination specified with ident:
status=0
rate=number
The features described in this section are available with Postfix 2.3 and later.
To register a request for a new (i.e. not cached) TLS session send the following request to the anvil(8) server:
request=newtls
ident=string
The anvil(8) server answers with the number of new TLS session requests per unit time for the (service, client) combination specified with ident:
status=0
rate=number
To retrieve new TLS session request rate information without updating the counter information, send:
request=newtls_report
ident=string
The anvil(8) server answers with the number of new TLS session requests per unit time for the (service, client) combination specified with ident:
status=0
rate=number
To register an AUTH request send the following request to the anvil(8) server:
request=auth
ident=string
The anvil(8) server answers with the number of auth requests per unit time for the (service, client) combination specified with ident:
status=0
rate=number
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 requests. No persistent state is kept because standard system library routines are not sufficiently robust for update-intensive applications.
Although the in-memory 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 process can have only one remote client at a time. If a server process reports multiple simultaneous clients, state is kept only for the last reported client.
The anvil(8) server automatically discards client request information after it expires. To prevent the anvil(8) server from discarding client request rate information too early or too late, a rate limited service should always register connect/disconnect events even when it does not explicitly limit them.
On low-traffic mail systems, changes to main.cf are picked up automatically as anvil(8) processes run for only a limited amount of time. On other mail systems, 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.
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 Wietse Venema Google, Inc. 111 8th Avenue New York, NY 10011, USA