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ANVIL(8) System Manager's Manual ANVIL(8)

NAME

anvil - Postfix session count and request rate control

SYNOPSIS

anvil [generic Postfix daemon options]

DESCRIPTION


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.

CONNECTION COUNT/RATE CONTROL



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






MESSAGE RATE CONTROL




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






RECIPIENT RATE CONTROL




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






TLS SESSION NEGOTIATION RATE CONTROL




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






SECURITY




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.






DIAGNOSTICS




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.






BUGS




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.






CONFIGURATION PARAMETERS




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.



  

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

  

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

  

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

  

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

  

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

  

  
The maximum amount of time that an idle Postfix daemon process waits for
      an incoming connection before terminating voluntarily.

  

  
The maximal number of incoming connections that a Postfix daemon process
      will service before terminating voluntarily.

  

  
The process ID of a Postfix command or daemon process.

  

  
The process name of a Postfix command or daemon process.

  

  
The syslog facility of Postfix logging.

  

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








SEE ALSO


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






README FILES




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


TUNING_README, performance tuning






LICENSE




The Secure Mailer license must be distributed with this
  software.






HISTORY




The anvil service is available in Postfix 2.2 and later.






AUTHOR(S)


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