ANVIL(8)                                                 ANVIL(8)

NAME
       anvil - Postfix connection 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.

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  client_connection_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
       The following main.cf parameters are  especially  relevant
       to  this  program.  Use the postfix reload command after a
       configuration change.

       client_connection_rate_time_unit
              The unit of time over which  connection  rates  are
              calculated.

       client_connection_status_update_time
              Time  interval  for  logging the maximal connection
              count and connection rate information.

SEE ALSO
       smtpd(8) Postfix SMTP server

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

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

                                                         ANVIL(8)