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 can have only one remote client at a time. If
a server 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 (postfix)
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)