    ANVIL(8)                                                 ANVIL(8)

    NAME
           anvil - Postfix client count and rate management

    SYNOPSIS
           anvil [generic Postfix daemon options]

    DESCRIPTION
           The  Postfix  anvil server maintains short-term statistics
           to defend against clients that hammer a server with either
           too  many parallel connections or with too many successive
           connection attempts within a configurable  time  interval.
           This  server is designed to run under control by the Post-
           fix master server.

           The anvil server maintains no persistent  database.  Stan-
           dard library utilities do not meet Postfix performance and
           robustness requirements.

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

               request=connect
               ident=string

           This registers a new connection for the (service,  client)
           combination specified with ident. The anvil 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 server:

               request=disconnect
               ident=string

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

               status=0

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

           The anvil server maintains an in-memory table with  infor-
           mation  about  recent  clients  of  a connection count (or
           rate) limited service.  Although state is kept  only  tem-
           porarily, 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.

    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 can have only one remote client at a time.  If
           a  server  reports  multiple simultaneous clients, all but
           the last reported client are ignored.

    CONFIGURATION PARAMETERS
           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  pro-
                  cess.

           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  pro-
                  cess  name  in  syslog  records,  so  that  "smtpd"
                  becomes, for example, "postfix/smtpd".

    SEE ALSO
           smtpd(8), Postfix SMTP server
           postconf(5), configuration parameters

    README FILES
           TUNING_README, performance tuning

    LICENSE
           The  Secure  Mailer  license must be distributed with this
           software.

    HISTORY
           The anvil service was introduced with Postfix 2.1.

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

                                                             ANVIL(8)