-$Cambridge: exim/doc/doc-txt/NewStuff,v 1.44 2005/05/23 15:44:06 fanf2 Exp $
+$Cambridge: exim/doc/doc-txt/NewStuff,v 1.45 2005/05/23 16:58:55 fanf2 Exp $
New Features in Exim
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to be duplicated when the sender retries. Therefore you should not use
fakedefer if the message will be delivered normally.
+TF/04 There is a new ratelimit ACL condition which can be used to measure
+ and control the rate at which clients can send email. This is more
+ powerful than the existing smtp_ratelimit_* options, because those
+ options only control the rate of commands in a single SMTP session,
+ whereas the new ratelimit condition works across all connections
+ (concurrent and sequential) to the same host.
+
+ The syntax of the ratelimit condition is:
+
+ ratelimit = <m> / <p> / <options> / <key>
+
+ If the average client sending rate is greater than m messages per time
+ period p then the condition is true, otherwise it is false.
+
+ The parameter p is the smoothing time constant, in the form of an Exim
+ time interval e.g. 8h for eight hours. A larger time constant means it
+ takes Exim longer to forget a client's past behaviour. The parameter m is
+ the maximum number of messages that a client can send in a fast burst. By
+ increasing both m and p but keeping m/p constant, you can allow a client
+ to send more messages in a burst without changing its overall sending
+ rate limit. Conversely, if m and p are both small then messages must be
+ sent at an even rate.
+
+ The key is used to look up the data used to calcluate the client's
+ average sending rate. This data is stored in a database maintained by
+ Exim in its spool directory alongside the retry database etc. For
+ example, you can limit the sending rate of each authenticated user,
+ independent of the computer they are sending from, by setting the key
+ to $authenticated_id. The default key is $sender_host_address.
+
+ Each ratelimit condition can have up to two options. The first option
+ specifies what Exim measures the rate of, and the second specifies how
+ Exim handles excessively fast clients.
+
+ The per_mail option means that it measures the client's rate of sending
+ messages. This is the default if none of the per_* options is specified.
+
+ The per_conn option means that it measures the client's connection rate.
+
+ The per_byte option limits the sender's email bandwidth. Note that it
+ is best to use this option in the DATA ACL; if it is used in an earlier
+ ACL it relies on the SIZE parameter on the MAIL command, which may be
+ inaccurate or completely missing. You can follow the limit m in the
+ configuration with K, M, or G to specify limits in kilobytes,
+ megabytes, or gigabytes respectively.
+
+ The per_cmd option means that Exim recomputes the rate every time the
+ condition is processed, which can be used to limit the SMTP command rate.
+ The alias per_rcpt is provided for use in the RCPT ACL instead of per_cmd
+ to make it clear that the effect is to limit the rate at which recipients
+ are accepted. Note that in this case the rate limiting engine will see a
+ message with many recipients as a large high-speed burst.
+
+ If a client's average rate is greater than the maximum, the rate
+ limiting engine can react in two possible ways, depending on the
+ presence of the strict or leaky options. This is independent of the
+ other counter-measures (e.g. rejecting the message) that may be
+ specified by the rest of the ACL. The default mode is leaky, which
+ avoids a sender's over-aggressive retry rate preventing it from getting
+ any email through.
+
+ The strict option means that the client's recorded rate is always
+ updated. The effect of this is that Exim measures the client's average
+ rate of attempts to send email, which can be much higher than the
+ maximum. If the client is over the limit it will be subjected to
+ counter-measures until it slows down below the maximum rate.
+
+ The leaky option means that the client's recorded rate is not updated
+ if it is above the limit. The effect of this is that Exim measures the
+ client's average rate of successfully sent email, which cannot be
+ greater than the maximum. If the client is over the limit it will
+ suffer some counter-measures, but it will still be able to send email
+ at the configured maximum rate, whatever the rate of its attempts.
+
+ As a side-effect, the ratelimit condition will set the expansion
+ variables $sender_rate containing the client's computed rate,
+ $sender_rate_limit containing the configured value of m, and
+ $sender_rate_period containing the configured value of p.
+
+ Exim's other ACL facilities are used to define what counter-measures
+ are taken when the rate limit is exceeded. This might be anything from
+ logging a warning (e.g. while measuring existing sending rates in order
+ to define our policy), through time delays to slow down fast senders,
+ up to rejecting the message. For example,
+
+ # Log all senders' rates
+ warn
+ ratelimit = 0 / 1h / strict
+ log_message = \
+ Sender rate $sender_rate > $sender_rate_limit / $sender_rate_period
+
+ # Slow down fast senders
+ warn
+ ratelimit = 100 / 1h / per_rcpt / strict
+ delay = ${eval: 10 * ($sender_rate - $sender_rate_limit) }
+
+ # Keep authenticated users under control
+ deny
+ ratelimit = 100 / 1d / strict / $authenticated_id
+
+ # System-wide rate limit
+ defer
+ message = Sorry, too busy. Try again later.
+ ratelimit = 10 / 1s / $primary_hostname
+
+ # Restrict incoming rate from each host, with a default rate limit
+ # set using a macro and special cases looked up in a table.
+ defer
+ message = Sender rate $sender_rate exceeds \
+ $sender_rate_limit messages per $sender_rate_period
+ ratelimit = ${lookup {$sender_host_address} \
+ cdb {DB/ratelimits.cdb} \
+ {$value} {RATELIMIT} }
+
Version 4.51
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git://git.exim.org / exim.git / blobdiff