diff options
Diffstat (limited to 'doc/doc-txt/NewStuff')
| -rw-r--r-- | doc/doc-txt/NewStuff | 116 |
1 files changed, 115 insertions, 1 deletions
diff --git a/doc/doc-txt/NewStuff b/doc/doc-txt/NewStuff index ba1c5afc4..e0d87c44a 100644 --- a/doc/doc-txt/NewStuff +++ b/doc/doc-txt/NewStuff @@ -1,4 +1,4 @@ -$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 -------------------- @@ -121,6 +121,120 @@ TF/03 The control = fakereject ACL modifier now has a fakedefer counterpart, 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 ------------ |