anvil(8) 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









AUTH RATE CONTROL





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









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.



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 terminating 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".








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
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA





      

    



  









  

  


    

      
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