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📛 NAME

anvil - Postfix session count and request rate control

🚀 Quick Reference

Use Case	Command/Request	Description
🔌 Register connection	request=connect, ident=string	📊 Get simultaneous connection count & rate per time unit
🔌 Register disconnect	request=disconnect, ident=string	✅ Acknowledge session end
📨 Register message delivery	request=message, ident=string	📈 Get message delivery rate per time unit
👥 Register recipient	request=recipient, ident=string	📈 Get recipient address rate per time unit
🔒 New TLS session	request=newtls, ident=string	📈 Get new TLS negotiation rate (Postfix 2.3+)
🔒 TLS report (no update)	request=newtls_report, ident=string	📈 Query TLS rate without incrementing counter
🔑 AUTH request	request=auth, ident=string	📈 Get AUTH rate per time unit
⚙️ Reload config	postfix reload	🔄 Apply changes to main.cf immediately

📋 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

Available in 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) or postlogd(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.

Parameter	Default	Description
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	🏷️ A prefix that is prepended to the process name in syslog records, so that, for example, "smtpd" becomes "prefix/smtpd".
service_name	read-only	🔧 Postfix 3.3+ The master.cf service name of a Postfix daemon process.

📚 SEE ALSO

• smtpd(8), Postfix SMTP server
• postconf(5), configuration parameters
• master(5), generic daemon options

📄 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

📛 NAME 🚀 Quick Reference 📋 SYNOPSIS 📖 DESCRIPTION 🔌 CONNECTION COUNT/RATE CONTROL 📨 MESSAGE RATE CONTROL 👥 RECIPIENT RATE CONTROL 🔒 TLS SESSION NEGOTIATION RATE CONTROL 🔑 AUTH RATE CONTROL 🛡️ SECURITY 🔍 DIAGNOSTICS 🐛 BUGS ⚙️ CONFIGURATION PARAMETERS 📚 SEE ALSO 📄 README FILES 📜 LICENSE 📜 HISTORY ✍️ AUTHOR(S)