X-Git-Url: https://git.exim.org/users/jgh/exim.git/blobdiff_plain/2e2a30b495b1ef8052259093f9422f57903b1717..c206415fdbb29ff2ed8fe1036f3876173a7ccd3d:/doc/doc-txt/NewStuff diff --git a/doc/doc-txt/NewStuff b/doc/doc-txt/NewStuff index f0abee45a..143fb6cb0 100644 --- a/doc/doc-txt/NewStuff +++ b/doc/doc-txt/NewStuff @@ -1,4 +1,4 @@ -$Cambridge: exim/doc/doc-txt/NewStuff,v 1.39 2005/05/03 14:20:00 ph10 Exp $ +$Cambridge: exim/doc/doc-txt/NewStuff,v 1.46 2005/05/25 09:58:16 fanf2 Exp $ New Features in Exim -------------------- @@ -9,6 +9,233 @@ updated when there is a relatively large batch of changes). The doc/ChangeLog file contains a listing of all changes, including bug fixes. +Exim version 4.52 +----------------- + +TF/01 Support for checking Client SMTP Authorization has been added. CSA is a + system which allows a site to advertise which machines are and are not + permitted to send email. This is done by placing special SRV records in + the DNS, which are looked up using the client's HELO domain. At this + time CSA is still an Internet-Draft. + + Client SMTP Authorization checks are performed by the ACL condition + verify=csa. This will fail if the client is not authorized. If there is + a DNS problem, or if no valid CSA SRV record is found, or if the client + is authorized, the condition succeeds. These three cases can be + distinguished using the expansion variable $csa_status, which can take + one of the values "fail", "defer", "unknown", or "ok". The condition + does not itself defer because that would be likely to cause problems + for legitimate email. + + The error messages produced by the CSA code include slightly more + detail. If $csa_status is "defer" this may be because of problems + looking up the CSA SRV record, or problems looking up the CSA target + address record. There are four reasons for $csa_status being "fail": + the client's host name is explicitly not authorized; the client's IP + address does not match any of the CSA target IP addresses; the client's + host name is authorized but it has no valid target IP addresses (e.g. + the target's addresses are IPv6 and the client is using IPv4); or the + client's host name has no CSA SRV record but a parent domain has + asserted that all subdomains must be explicitly authorized. + + The verify=csa condition can take an argument which is the domain to + use for the DNS query. The default is verify=csa/$sender_helo_name. + + This implementation includes an extension to CSA. If the query domain + is an address literal such as [192.0.2.95], or if it is a bare IP + address, Exim will search for CSA SRV records in the reverse DNS as if + the HELO domain was e.g. 95.2.0.192.in-addr.arpa. Therefore it is + meaningful to say, for example, verify=csa/$sender_host_address - in + fact, this is the check that Exim performs if the client does not say + HELO. This extension can be turned off by setting the main + configuration option dns_csa_use_reverse = false. + + If a CSA SRV record is not found for the domain itself, then a search + is performed through its parent domains for a record which might be + making assertions about subdomains. The maximum depth of this search is + limited using the main configuration option dns_csa_search_limit, which + takes the value 5 by default. Exim does not look for CSA SRV records in + a top level domain, so the default settings handle HELO domains as long + as seven (hostname.five.four.three.two.one.com) which encompasses the + vast majority of legitimate HELO domains. + + The dnsdb lookup also has support for CSA. Although dnsdb already + supports SRV lookups, this is not sufficient because of the extra + parent domain search behaviour of CSA, and (as with PTR lookups) + dnsdb also turns IP addresses into lookups in the reverse DNS space. + The result of ${lookup dnsdb {csa=$sender_helo_name} } has two + space-separated fields: an authorization code and a target host name. + The authorization code can be "Y" for yes, "N" for no, "X" for explicit + authorization required but absent, or "?" for unknown. + +PH/01 The amount of output produced by the "make" process has been reduced, + because the compile lines are often rather long, making it all pretty + unreadable. The new style is along the lines of the 2.6 Linux kernel: + just a short line for each module that is being compiled or linked. + However, it is still possible to get the full output, by calling "make" + like this: + + FULLECHO='' make -e + + The value of FULLECHO defaults to "@", the flag character that suppresses + command reflection in "make". When you ask for the full output, it is + given in addition to the the short output. + +TF/02 There have been two changes concerned with submission mode: + + Until now submission mode always left the return path alone, whereas + locally-submitted messages from untrusted users have the return path + fixed to the user's email address. Submission mode now fixes the return + path to the same address as is used to create the Sender: header. If + /sender_retain is specified then both the Sender: header and the return + path are left alone. + + Note that the changes caused by submission mode take effect after the + predata ACL. This means that any sender checks performed before the + fix-ups will use the untrusted sender address specified by the user, not + the trusted sender address specified by submission mode. Although this + might be slightly unexpected, it does mean that you can configure ACL + checks to spot that a user is trying to spoof another's address, for + example. + + There is also a new /name= option for submission mode which allows you + to specify the user's full name to be included in the Sender: header. + For example: + + accept authenticated = * + control = submission/name=${lookup {$authenticated_id} \ + lsearch {/etc/exim/namelist} } + + The namelist file contains entries like + + fanf: Tony Finch + + And the resulting Sender: header looks like + + Sender: Tony Finch + +TF/03 The control = fakereject ACL modifier now has a fakedefer counterpart, + which works in exactly the same way except it causes a fake SMTP 450 + response after the message data instead of a fake SMTP 550 response. + You must take care when using fakedefer because it will cause messages + 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 = /

/ / + + If the average client sending rate is less than m messages per time + period p then the condition is false, otherwise it is true. + + 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 ------------