    ANVIL(8)                                                              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  ses-
           sions,  or with too many successive requests within a con-
           figurable 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 simultane-
           ous 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) combina-
           tion 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 ses-
           sion 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 with-
           out 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 activ-
           ity 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) lim-
           ited  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 lim-
           ited  amount  of time. On other mail systems, use the com-
           mand "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  an  incoming  connection
                  before terminating voluntarily.

           max_use (100)
                  The  maximal  number of incoming connections that a
                  Postfix daemon process will service  before  termi-
                  nating voluntarily.

           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 (see 'postconf -d' output)
                  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
           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

                                                                          ANVIL(8)