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diff --git a/README_FILES/ETRN_README b/README_FILES/ETRN_README new file mode 100644 index 0000000..76bc8de --- /dev/null +++ b/README_FILES/ETRN_README @@ -0,0 +1,250 @@ +PPoossttffiixx EETTRRNN HHoowwttoo + +------------------------------------------------------------------------------- + +PPuurrppoossee ooff tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee + +The SMTP ETRN command was designed for sites that have intermittent Internet +connectivity. With ETRN, a site can tell the mail server of its provider to +"Please deliver all my mail now". The SMTP server searches the queue for mail +to the customer, and delivers that mail bbyy ccoonnnneeccttiinngg ttoo tthhee ccuussttoommeerr''ss SSMMTTPP +sseerrvveerr. The mail is not delivered via the connection that was used for sending +ETRN. + +As of version 1.0, Postfix has a fast ETRN implementation that does not require +Postfix to examine every queue file. Instead, Postfix maintains a record of +what queue files contain mail for destinations that are configured for ETRN +service. ETRN service is no longer available for domains that aren't configured +for the service. + +This document provides information on the following topics: + + * Using the Postfix fast ETRN service + * How Postfix fast ETRN works + * Postfix fast ETRN service limitations + * Configuring the Postfix fast ETRN service + * Configuring a domain for ETRN service only + * Testing the Postfix fast ETRN service + +Other documents with information on this subject: + + * flush(8), flush service implementation + +UUssiinngg tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee + +The following is an example SMTP session that shows how an SMTP client requests +the ETRN service. Client commands are shown in bold font. + + 220 my.server.tld ESMTP Postfix + HHEELLOO mmyy..cclliieenntt..ttlldd + 250 Ok + EETTRRNN ssoommee..ccuussttoommeerr..ddoommaaiinn + 250 Queuing started + QQUUIITT + 221 Bye + +As mentioned in the introduction, the mail is delivered by connecting to the +customer's SMTP server; it is not sent over the connection that was used to +send the ETRN command. + +The Postfix operator can request delivery for a specific customer by using the +command "sendmail -qRdestination" and, with Postfix version 1.1 and later, +"postqueue -sdestination". Access to this feature is controlled with the +authorized_flush_users configuration parameter (Postfix version 2.2 and later). + +HHooww PPoossttffiixx ffaasstt EETTRRNN wwoorrkkss + +When a Postfix delivery agent decides that mail must be delivered later, it +sends the destination domain name and the queue file name to the flush(8) +daemon which maintains per-destination logfiles with file names of queued mail. +These logfiles are kept below $queue_directory/flush. Per-destination logfiles +are maintained only for destinations that are listed with the +$fast_flush_domains parameter and that have syntactically valid domain names. + + Postfix Postfix One logfile + delivery -(domain, queue ID)-> flush -(queue ID)-> per eligible + agent daemon domain + +When Postfix receives a request to "deliver mail for a domain now", the flush +(8) daemon moves all deferred queue files that are listed for that domain to +the incoming queue, and requests that the queue manager deliver them. In order +to force delivery, the queue manager temporarily ignores the lists of +undeliverable destinations: the volatile in-memory list of dead domains, and +the list of message delivery transports specified with the defer_transports +configuration parameter. + +PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee lliimmiittaattiioonnss + +The design of the flush(8) server and of the flush queue introduce a few +limitations that should not be an issue unless you want to turn on fast ETRN +service for every possible destination. + + * The flush(8) daemon maintains per-destination logfiles with queue file + names. When a request to "deliver mail now" arrives, Postfix will attempt + to deliver all recipients in the queue files that have mail for the + destination in question. This does not perform well with queue files that + have recipients in many different domains, such as queue files with + outbound mailing list traffic. + + * The flush(8) daemon maintains per-destination logfiles only for + destinations listed with $fast_flush_domains. With other destinations you + cannot request delivery with "sendmail -qRdestination" or, with Postfix + version 1.1 and later, "postqueue -sdestination". + + * Up to and including early versions of Postfix version 2.1, the "fast flush" + service may not deliver some messages if the request to "deliver mail now" + is received while a deferred queue scan is already in progress. The reason + is that the queue manager does not ignore the volatile in-memory list of + dead domains, and the list of message delivery transports specified with + the defer_transports configuration parameter. + + * Up to and including Postfix version 2.3, the "fast flush" service may not + deliver some messages if the request to "deliver mail now" arrives while an + incoming queue scan is already in progress. + +CCoonnffiigguurriinngg tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee + +The behavior of the flush(8) daemon is controlled by parameters in the main.cf +configuration file. + +By default, Postfix "fast ETRN" service is available only for destinations that +Postfix is willing to relay mail to: + + /etc/postfix/main.cf: + fast_flush_domains = $relay_domains + smtpd_etrn_restrictions = permit_mynetworks, reject + +Notes: + + * The relay_domains parameter specifies what destinations Postfix will relay + to. For destinations that are not eligible for the "fast ETRN" service, + Postfix replies with an error message. + + * The smtpd_etrn_restrictions parameter limits what clients may execute the + ETRN command. By default, any client has permission. + +To enable "fast ETRN" for some other destination, specify: + + /etc/postfix/main.cf: + fast_flush_domains = $relay_domains, some.other.domain + +To disable "fast ETRN", so that Postfix rejects all ETRN requests and so that +it maintains no per-destination logfiles, specify: + + /etc/postfix/main.cf: + fast_flush_domains = + +CCoonnffiigguurriinngg aa ddoommaaiinn ffoorr EETTRRNN sseerrvviiccee oonnllyy + +While an "ETRN" customer is off-line, Postfix will make spontaneous attempts to +deliver mail to it. These attempts are separated in time by increasing time +intervals, ranging from $minimal_backoff_time to $maximal_backoff_time, and +should not be a problem unless a lot of mail is queued. + +To prevent Postfix from making spontaneous delivery attempts you can configure +Postfix to always defer mail for the "ETRN" customer. Mail is delivered only +after the ETRN command or with "sendmail -q", with "sendmail -qRdomain", or +with "postqueue -sdomain"(Postfix version 1.1 and later only), + +In the example below we configure an "etrn-only" delivery transport which is +simply a duplicate of the "smtp" and "relay" mail delivery transports. The only +difference is that mail destined for this delivery transport is deferred as +soon as it arrives. + + 1 /etc/postfix/master.cf: + 2 # ============================================================= + 3 # service type private unpriv chroot wakeup maxproc command + 4 # (yes) (yes) (yes) (never) (100) + 5 # ============================================================= + 6 smtp unix - - n - - smtp + 7 relay unix - - n - - smtp + 8 etrn-only unix - - n - - smtp + 9 + 10 /etc/postfix/main.cf: + 11 relay_domains = customer.tld ...other domains... + 12 defer_transports = etrn-only + 13 transport_maps = hash:/etc/postfix/transport + 14 + 15 /etc/postfix/transport: + 16 customer.tld etrn-only:[mailhost.customer.tld] + +Translation: + + * Line 8: The "etrn-only" mail delivery service is a copy of the "smtp" and + "relay" service. + + * Line 11: Don't forget to authorize relaying for this customer, either via + relay_domains or with the permit_mx_backup feature. + + * Line 12: The "etrn-only" mail delivery service is configured so that + spontaneous mail delivery is disabled. + + * Lines 13-16: Mail for the customer is given to the "etrn-only" mail + delivery service. + + * Line 16: The "[mailhost.customer.tld]" turns off MX record lookups; you + must specify this if your Postfix server is the primary MX host for the + customer's domain. + +TTeessttiinngg tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee + +By default, "fast ETRN" service is enabled for all domains that match +$relay_domains. If you run Postfix with "fast ETRN" service for the very first +time, you need to run "sendmail -q" once in order to populate the per-site +deferred mail logfiles. If you omit this step, no harm is done. The logfiles +will eventually become populated as Postfix routinely attempts to deliver +delayed mail, but that will take a couple hours. After the "sendmail -q" +command has completed all delivery attempts (this can take a while), you're +ready to test the "fast ETRN" service. + +To test the "fast ETRN" service, telnet to the Postfix SMTP server from a +client that is allowed to execute ETRN commands (by default, that's every +client), and type the commands shown in boldface: + + 220 my.server.tld ESMTP Postfix + HHEELLOO mmyy..cclliieenntt..ttlldd + 250 Ok + EETTRRNN ssoommee..ccuussttoommeerr..ddoommaaiinn + 250 Queuing started + +where "some.customer.domain" is the name of a domain that has a non-empty +logfile somewhere under $queue_directory/flush. + +In the maillog file, you should immediately see a couple of logfile records, as +evidence that the queue manager has opened queue files: + + Oct 2 10:51:19 myhostname postfix/qmgr[51999]: 682E8440A4: + from=<whatever>, size=12345, nrcpt=1 (queue active) + Oct 2 10:51:19 myhostname postfix/qmgr[51999]: 02249440B7: + from=<whatever>, size=4711, nrcpt=1 (queue active) + +What happens next depends on whether the destination is reachable. If it's not +reachable, the mail queue IDs will be added back to the some.customer.domain +logfile under $queue_directory/flush. + +Repeat the exercise with some other destination that your server is willing to +relay to (any domain listed in $relay_domains), but that has no mail queued. +The text in bold face stands for the commands that you type: + + 220 my.server.tld ESMTP Postfix + HHEELLOO mmyy..cclliieenntt..ttlldd + 250 Ok + EETTRRNN ssoommee..ootthheerr..ccuussttoommeerr..ddoommaaiinn + 250 Queuing started + +This time, the "ETRN"" command should trigger NO mail deliveries at all. If +this triggers delivery of all mail, then you used the wrong domain name, or +"fast ETRN" service is turned off. + +Finally, repeat the exercise with a destination that your mail server is not +willing to relay to. It does not matter if your server has mail queued for that +destination. + + 220 my.server.tld ESMTP Postfix + HHEELLOO mmyy..cclliieenntt..ttlldd + 250 Ok + EETTRRNN nnoott..aa..ccuussttoommeerr..ddoommaaiinn + 459 <not.a.customer.domain>: service unavailable + +In this case, Postfix should reject the request as shown above. + |