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Diffstat (limited to 'README_FILES/XFORWARD_README')
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diff --git a/README_FILES/XFORWARD_README b/README_FILES/XFORWARD_README new file mode 100644 index 0000000..84802ed --- /dev/null +++ b/README_FILES/XFORWARD_README @@ -0,0 +1,179 @@ +PPoossttffiixx XXFFOORRWWAARRDD HHoowwttoo + +------------------------------------------------------------------------------- + +PPuurrppoossee ooff tthhee XXFFOORRWWAARRDD eexxtteennssiioonn ttoo SSMMTTPP + +When an SMTP server announces support for the XFORWARD command, an SMTP client +may send information that overrides one or more client-related logging +attributes. The XFORWARD command targets the following problem: + + * Logging after SMTP-based content filter. With the deployment of Internet- + >MTA1->filter->MTA2 style content filter applications, the logging of + client and message identifying information changes when MTA1 gives the mail + to the content filter. To simplify the interpretation of MTA2 logging, it + would help if MTA1 could forward remote client and/or message identifying + information through the content filter to MTA2, so that the information + could be logged as part of mail handling transactions. + +This extension is implemented as a separate ESMTP command, and can be used to +transmit client or message attributes incrementally. It is not implemented by +passing additional parameters via the MAIL FROM command, because doing so would +require extending the MAIL FROM command length limit by another 600 or more +characters beyond the space that is already needed to support other extensions +such as AUTH and DSN. + +XXFFOORRWWAARRDD CCoommmmaanndd ssyynnttaaxx + +An example of a client-server conversation is given at the end of this +document. + +In SMTP server EHLO replies, the keyword associated with this extension is +XFORWARD. The keyword is followed by the names of the attributes that the +XFORWARD implementation supports. + +After receiving the server's announcement for XFORWARD support, the client may +send XFORWARD requests at any time except in the middle of a mail delivery +transaction (i.e. between MAIL and RSET or DOT). The command may be pipelined +when the server supports ESMTP command pipelining. + +The syntax of XFORWARD requests is described below. Upper case and quoted +strings specify terminals, lowercase strings specify meta terminals, and SP is +whitespace. Although command and attribute names are shown in upper case, they +are in fact case insensitive. + + xforward-command = XFORWARD 1*( SP attribute-name"="attribute-value ) + + attribute-name = ( NAME | ADDR | PORT | PROTO | HELO | IDENT | SOURCE ) + + attribute-value = xtext + + * Attribute values are xtext encoded as per RFC 1891. + + * The NAME attribute specifies the up-stream hostname, or [UNAVAILABLE] when + the information is unavailable. The hostname may be a non-DNS hostname. + + * The ADDR attribute specifies the up-stream network address: a numerical + IPv4 network address, an IPv6 address prefixed with IPV6:, or [UNAVAILABLE] + when the address information is unavailable. Address information is not + enclosed with []. + + * The PORT attribute specifies an up-stream client TCP port number in + decimal, or [UNAVAILABLE] when the information is unavailable. + + * The PROTO attribute specifies the mail protocol for receiving mail from the + up-stream host. This may be an SMTP or non-SMTP protocol name of up to 64 + characters, or [UNAVAILABLE] when the information is unavailable. + + * The HELO attribute specifies the hostname that the up-stream host announced + itself with (not necessarily via the SMTP HELO command), or [UNAVAILABLE] + when the information is unavailable. The hostname may be a non-DNS + hostname. + + * The IDENT attribute specifies a local message identifier on the up-stream + host, or [UNAVAILABLE] when the information is unavailable. The down-stream + MTA may log this information together with its own local message identifier + to facilitate message tracking across MTAs. + + * The SOURCE attribute specifies LOCAL when the message was received from a + source that is local with respect to the up-stream host (for example, the + message originated from the up-stream host itself), REMOTE for all other + mail, or [UNAVAILABLE] when the information is unavailable. The down-stream + MTA may decide to enable features such as header munging or address + qualification with mail from local sources but not other sources. + +Note 1: an attribute-value element must not be longer than 255 characters +(specific attributes may impose shorter lengths). After xtext decoding, +attribute values must not contain control characters, non-ASCII characters, +whitespace, or other characters that are special in message headers. + +Note 2: DNS hostnames can be up to 255 characters long. The XFORWARD client +implementation must not send XFORWARD commands that exceed the 512 character +limit for SMTP commands. + +Note 3: [UNAVAILABLE] may be specified in upper case, lower case or mixed case. + +Note 4: Postfix implementations prior to version 2.3 do not xtext encode +attribute values. Servers that wish to interoperate with these older +implementations should be prepared to receive unencoded information. + +XXFFOORRWWAARRDD SSeerrvveerr ooppeerraattiioonn + +The server maintains a set of XFORWARD attributes with forwarded information, +in addition the current SMTP session attributes. Normally, all XFORWARD +attributes are in the undefined state, and the server uses the current SMTP +session attributes for logging purposes. + +Upon receipt of an initial XFORWARD command, the SMTP server initializes all +XFORWARD attributes to [UNAVAILABLE]. With each valid XFORWARD command, the +server updates XFORWARD attributes with the specified values. + +The server must not mix client attributes from XFORWARD with client attributes +from the current SMTP session. + +At the end of each MAIL FROM transaction (i.e. RSET or DOT), the server resets +all XFORWARD attributes to the undefined state, and is ready to receive another +initial XFORWARD command. + +XXFFOORRWWAARRDD SSeerrvveerr rreeppllyy ccooddeess + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + |CCooddee|MMeeaanniinngg | + |_ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |250 |success | + |_ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |421 |unable to proceed, disconnecting| + |_ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |501 |bad command parameter syntax | + |_ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |503 |mail transaction in progress | + |_ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |550 |insufficient authorization | + |_ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + +XXFFOORRWWAARRDD EExxaammppllee + +In the following example, information sent by the client is shown in bold font. + + 220 server.example.com ESMTP Postfix + EEHHLLOO cclliieenntt..eexxaammppllee..ccoomm + 250-server.example.com + 250-PIPELINING + 250-SIZE 10240000 + 250-VRFY + 250-ETRN + 250-XFORWARD NAME ADDR PROTO HELO + 250 8BITMIME + XXFFOORRWWAARRDD NNAAMMEE==ssppiikkee..ppoorrccuuppiinnee..oorrgg AADDDDRR==116688..110000..118899..22 PPRROOTTOO==EESSMMTTPP + 250 Ok + XXFFOORRWWAARRDD HHEELLOO==ssppiikkee..ppoorrccuuppiinnee..oorrgg + 250 Ok + MMAAIILL FFRROOMM::<<wwiieettssee@@ppoorrccuuppiinnee..oorrgg>> + 250 Ok + RRCCPPTT TTOO::<<uusseerr@@eexxaammppllee..ccoomm>> + 250 Ok + DDAATTAA + 354 End data with <CR><LF>.<CR><LF> + .. .. ..mmeessssaaggee ccoonntteenntt.. .. .. + .. + 250 Ok: queued as 3CF6B2AAE8 + QQUUIITT + 221 Bye + +SSeeccuurriittyy + +The XFORWARD command changes audit trails. Use of this command must be +restricted to authorized clients. + +SSMMTTPP ccoonnnneeccttiioonn ccaacchhiinngg + +SMTP connection caching makes it possible to deliver multiple messages within +the same SMTP session. The XFORWARD attributes are reset after the MAIL FROM +transaction completes (after RSET or DOT), so there is no risk of information +leakage. + +RReeffeerreenncceess + +Moore, K, "SMTP Service Extension for Delivery Status Notifications", RFC 1891, +January 1996. + |