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diff --git a/README_FILES/XCLIENT_README b/README_FILES/XCLIENT_README new file mode 100644 index 0000000..89b11bf --- /dev/null +++ b/README_FILES/XCLIENT_README @@ -0,0 +1,199 @@ +PPoossttffiixx XXCCLLIIEENNTT HHoowwttoo + +------------------------------------------------------------------------------- + +PPuurrppoossee ooff tthhee XXCCLLIIEENNTT eexxtteennssiioonn ttoo SSMMTTPP + +When an SMTP server announces support for the XCLIENT command, an SMTP client +may send information that overrides one or more client-related session +attributes. The XCLIENT command targets the following problems: + + 1. Access control tests. SMTP server access rules are difficult to verify when + decisions can be triggered only by remote clients. In order to facilitate + access rule testing, an authorized SMTP client test program needs the + ability to override the SMTP server's idea of the SMTP client hostname, + network address, and other client information, for the entire duration of + an SMTP session. + + 2. Client software that downloads mail from an up-stream mail server and + injects it into a local MTA via SMTP. In order to take advantage of the + local MTA's SMTP server access rules, the client software needs the ability + to override the SMTP server's idea of the remote client name, client + address and other information. Such information can typically be extracted + from the up-stream mail server's Received: message header. + + 3. Post-filter access control and logging. With Internet->filter->MTA style + content filter applications, the filter can be simplified if it can + delegate decisions concerning mail relay and other access control to the + MTA. This is especially useful when the filter acts as a transparent proxy + for SMTP commands. This requires that the filter can override the MTA's + idea of the SMTP client hostname, network address, and other information. + +XXCCLLIIEENNTT CCoommmmaanndd ssyynnttaaxx + +An example client-server conversation is given at the end of this document. + +In SMTP server EHLO replies, the keyword associated with this extension is +XCLIENT. It is followed by the names of the attributes that the XCLIENT +implementation supports. + +The XCLIENT command may be sent at any time, except in the middle of a mail +delivery transaction (i.e. between MAIL and DOT, or MAIL and RSET). The XCLIENT +command may be pipelined when the server supports ESMTP command pipelining. To +avoid triggering spamware detectors, the command should be sent at the end of a +command group. + +The syntax of XCLIENT 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. + + xclient-command = XCLIENT 1*( SP attribute-name"="attribute-value ) + + attribute-name = ( NAME | ADDR | PORT | PROTO | HELO | LOGIN | DESTADDR | + DESTPORT ) + + attribute-value = xtext + + * Attribute values are xtext encoded as per RFC 1891. + + * The NAME attribute specifies a remote SMTP client hostname (not an SMTP + client address), [UNAVAILABLE] when client hostname lookup failed due to a + permanent error, or [TEMPUNAVAIL] when the lookup error condition was + transient. + + * The ADDR attribute specifies a remote SMTP client 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 a remote SMTP client TCP port number as a + decimal number, or [UNAVAILABLE] when the information is unavailable. + + * The PROTO attribute specifies either SMTP or ESMTP. + + * The DESTADDR attribute specifies a local SMTP server 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 DESTPORT attribute specifies a local SMTP server TCP port number as a + decimal number, or [UNAVAILABLE] when the information is unavailable. + + * The HELO attribute specifies an SMTP HELO parameter value, or the value + [UNAVAILABLE] when the information is unavailable. + + * The LOGIN attribute specifies a SASL login name, or the value [UNAVAILABLE] + when the information is unavailable. + +Note 1: syntactically valid NAME and HELO attribute-value elements can be up to +255 characters long. The client must not send XCLIENT commands that exceed the +512 character limit for SMTP commands. To avoid exceeding the limit the client +should send the information in multiple XCLIENT commands; for example, send +NAME and ADDR last, after HELO and PROTO. Once ADDR is sent, the client is +usually no longer authorized to send XCLIENT commands. + +Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6: may be specified in upper case, +lower case or mixed case. + +Note 3: 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. + +Note 4: Some Postfix implementations do not implement the PORT or LOGIN +attributes. + +XXCCLLIIEENNTT SSeerrvveerr rreessppoonnssee + +Upon receipt of a correctly formatted XCLIENT command, the server resets state +to the initial SMTP greeting protocol stage. Depending on the outcome of +optional access decisions, the server responds with 220 or with a suitable +rejection code. + +For practical reasons it is not always possible to reset the complete server +state to the initial SMTP greeting protocol stage: + + * TLS session information may not be reset, because turning off TLS leaves + the connection in an undefined state. Consequently, the server may not + announce STARTTLS when TLS is already active, and access decisions may be + influenced by client certificate information that was received prior to the + XCLIENT command. + + * The SMTP server must not reset attributes that were received with the last + XCLIENT command. This includes HELO or PROTO attributes. + +NOTE: Postfix implementations prior to version 2.3 do not jump back to the +initial SMTP greeting protocol stage. These older implementations will not +correctly simulate connection-level access decisions under some conditions. + +XXCCLLIIEENNTT sseerrvveerr rreeppllyy ccooddeess + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + |CCooddee |MMeeaanniinngg | + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |220 |success | + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |421 |unable to proceed, disconnecting | + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |501 |bad command parameter syntax | + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |503 |mail transaction in progress | + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |550 |insufficient authorization | + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + |other|connection rejected by connection-level access decision| + |_ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ | + +XXCCLLIIEENNTT EExxaammppllee + +In the example, the client impersonates a mail originating system by passing +all SMTP client information via the XCLIENT command. 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-XCLIENT NAME ADDR PROTO HELO + 250 8BITMIME + XXCCLLIIEENNTT NNAAMMEE==ssppiikkee..ppoorrccuuppiinnee..oorrgg AADDDDRR==116688..110000..118899..22 + 220 server.example.com ESMTP Postfix + EEHHLLOO ssppiikkee..ppoorrccuuppiinnee..oorrgg + 250-server.example.com + 250-PIPELINING + 250-SIZE 10240000 + 250-VRFY + 250-ETRN + 250-XCLIENT NAME ADDR PROTO HELO + 250 8BITMIME + 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 763402AAE6 + QQUUIITT + 221 Bye + +SSeeccuurriittyy + +The XCLIENT command changes audit trails and/or SMTP client access permissions. +Use of this command must be restricted to authorized SMTP clients. + +SSMMTTPP ccoonnnneeccttiioonn ccaacchhiinngg + +XCLIENT attributes persist until the end of an SMTP session. If one session is +used to deliver mail on behalf of different SMTP clients, the XCLIENT +attributes need to be reset as appropriate before each MAIL FROM command. + +RReeffeerreenncceess + +Moore, K, "SMTP Service Extension for Delivery Status Notifications", RFC 1891, +January 1996. + |