1<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN" 2 "http://www.w3.org/TR/html4/loose.dtd"> 3 4<html> 5 6<head> 7 8<title>Postfix XCLIENT Howto</title> 9 10<meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> 11 12</head> 13 14<body> 15 16<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix XCLIENT Howto</h1> 17 18<hr> 19 20<h2>Purpose of the XCLIENT extension to SMTP</h2> 21 22<p> When an SMTP server announces support for the XCLIENT command, 23an SMTP client may send information that overrides one or more 24client-related session attributes. The XCLIENT command targets the 25following problems: </p> 26 27<ol> 28 29 <li> <p> Access control tests. SMTP server access rules are 30 difficult to verify when decisions can be triggered only by 31 remote clients. In order to facilitate access rule testing, 32 an authorized SMTP client test program needs the ability to 33 override the SMTP server's idea of the SMTP client hostname, 34 network address, and other client information, for the entire 35 duration of an SMTP session. </p> 36 37 <li> <p> Client software that downloads mail from an up-stream 38 mail server and injects it into a local MTA via SMTP. In order 39 to take advantage of the local MTA's SMTP server access rules, 40 the client software needs the ability to override the SMTP 41 server's idea of the remote client name, client address and 42 other information. Such information can typically be extracted 43 from the up-stream mail server's Received: message header. </p> 44 45 <li> <p> Post-filter access control and logging. With 46 Internet->filter->MTA style content filter applications, 47 the filter can be simplified if it can delegate decisions 48 concerning mail relay and other access control to the MTA. This 49 is especially useful when the filter acts as a transparent 50 proxy for SMTP commands. This requires that the filter can 51 override the MTA's idea of the SMTP client hostname, network 52 address, and other information. </p> 53 54</ol> 55 56<h2>XCLIENT Command syntax</h2> 57 58<p> An example client-server conversation is given at the end 59of this document. </p> 60 61<p> In SMTP server EHLO replies, the keyword associated with this 62extension is XCLIENT. It is followed by the names of the attributes 63that the XCLIENT implementation supports. </p> 64 65<p> The XCLIENT command may be sent at any time, except in the 66middle of a mail delivery transaction (i.e. between MAIL and DOT, 67or MAIL and RSET). The XCLIENT command may be pipelined when the 68server supports ESMTP command pipelining. To avoid triggering 69spamware detectors, the command should be sent at the end of a 70command group. </p> 71 72<p> The syntax of XCLIENT requests is described below. Upper case 73and quoted strings specify terminals, lowercase strings specify 74meta terminals, and SP is whitespace. Although command and attribute 75names are shown in upper case, they are in fact case insensitive. 76</p> 77 78<blockquote> 79<p> 80 xclient-command = XCLIENT 1*( SP attribute-name"="attribute-value ) 81</p> 82<p> 83 attribute-name = ( NAME | ADDR | PORT | PROTO | HELO ) 84</p> 85<p> 86 attribute-value = xtext 87</p> 88</blockquote> 89 90<ul> 91 92 <li> <p> Attribute values are xtext encoded as per <a href="http://tools.ietf.org/html/rfc1891">RFC 1891</a>. 93 </p> 94 95 <li> <p> The NAME attribute specifies an SMTP client hostname 96 (not an SMTP client address), [UNAVAILABLE] when client hostname 97 lookup failed due to a permanent error, or [TEMPUNAVAIL] when 98 the lookup error condition was transient. </p> 99 100 <li> <p> The ADDR attribute specifies an SMTP client numerical 101 IPv4 network address, an IPv6 address prefixed with IPV6:, or 102 [UNAVAILABLE] when the address information is unavailable. 103 Address information is not enclosed with []. </p> 104 105 <li> <p> The PORT attribute specifies the SMTP client TCP port 106 number as a decimal number, or [UNAVAILABLE] when the information 107 is unavailable. </p> 108 109 <li> <p> The PROTO attribute specifies either SMTP or ESMTP. 110 </p> 111 112 <li> <p> The HELO attribute specifies an SMTP HELO parameter 113 value, or the value [UNAVAILABLE] when the information is 114 unavailable. </p> 115 116</ul> 117 118<p> Note 1: syntactically valid NAME and HELO attribute-value 119elements can be up to 255 characters long. The client must not send 120XCLIENT commands that exceed the 512 character limit for SMTP 121commands. To avoid exceeding the limit the client should send the 122information in multiple XCLIENT commands; for example, send NAME 123and ADDR first, then HELO and PROTO. </p> 124 125<p> Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6: may be specified 126in upper case, lower case or mixed case. </p> 127 128<p> Note 3: Postfix implementations prior to version 2.3 do not 129xtext encode attribute values. Servers that wish to interoperate 130with these older implementations should be prepared to receive 131unencoded information. </p> 132 133<p> Note 4: Postfix implementations prior to version 2.5 do not 134implement the PORT attribute. </p> 135 136<h2>XCLIENT Server response</h2> 137 138<p> Upon receipt of a correctly formatted XCLIENT command, the 139server resets state to the initial SMTP greeting protocol stage. 140Depending on the outcome of optional access decisions, the server 141responds with 220 or with a suitable rejection code. 142 143<p> For practical reasons it is not always possible to reset the 144complete server state to the initial SMTP greeting protocol stage: 145</p> 146 147<ul> 148 149<li> <p> TLS session information may not be reset, because turning off 150TLS leaves the connection in an undefined state. Consequently, the 151server may not announce STARTTLS when TLS is already active, and 152access decisions may be influenced by client certificate information 153that was received prior to the XCLIENT command. </p> 154 155<li> <p> The SMTP server must not reset attributes that were received 156with the last XCLIENT command. This includes HELO or PROTO attributes. 157</p> 158 159</ul> 160 161<p> NOTE: Postfix implementations prior to version 2.3 do not jump 162back to the initial SMTP greeting protocol stage. These older 163implementations will not correctly simulate connection-level access 164decisions under some conditions. </p> 165 166<h2> XCLIENT server reply codes </h2> 167 168<blockquote> 169 170<table border="1" bgcolor="#f0f0ff"> 171 172<tr> <th> Code </th> <th> Meaning </th> </tr> 173 174<tr> <td> 220 </td> <td> success </td> </tr> 175 176<tr> <td> 421 </td> <td> unable to proceed, disconnecting </td> </tr> 177 178<tr> <td> 501 </td> <td> bad command parameter syntax </td> </tr> 179 180<tr> <td> 503 </td> <td> mail transaction in progress </td> </tr> 181 182<tr> <td> 550 </td> <td> insufficient authorization </td> </tr> 183 184<tr> <td> other </td> <td> connection rejected by connection-level 185access decision </td> </tr> 186 187</table> 188 189</blockquote> 190 191<h2>XCLIENT Example</h2> 192 193<p> In the example, the client impersonates a mail originating 194system by passing all SMTP client information via the XCLIENT 195command. Information sent by the client is shown in bold font. 196</p> 197 198<blockquote> 199<pre> 200220 server.example.com ESMTP Postfix 201<b>EHLO client.example.com</b> 202250-server.example.com 203250-PIPELINING 204250-SIZE 10240000 205250-VRFY 206250-ETRN 207250-XCLIENT NAME ADDR PROTO HELO 208250 8BITMIME 209<b>XCLIENT NAME=spike.porcupine.org ADDR=168.100.189.2</b> 210220 server.example.com ESMTP Postfix 211<b>EHLO spike.porcupine.org</b> 212250-server.example.com 213250-PIPELINING 214250-SIZE 10240000 215250-VRFY 216250-ETRN 217250-XCLIENT NAME ADDR PROTO HELO 218250 8BITMIME 219<b>MAIL FROM:<wietse@porcupine.org></b> 220250 Ok 221<b>RCPT TO:<user@example.com></b> 222250 Ok 223<b>DATA</b> 224354 End data with <CR><LF>.<CR><LF> 225<b>. . .<i>message content</i>. . .</b> 226<b>.</b> 227250 Ok: queued as 763402AAE6 228<b>QUIT</b> 229221 Bye 230</pre> 231</blockquote> 232 233<h2>Security</h2> 234 235<p> The XCLIENT command changes audit trails and/or SMTP client 236access permissions. Use of this command must be restricted to 237authorized SMTP clients. </p> 238 239<h2>SMTP connection caching</h2> 240 241<p> XCLIENT attributes persist until the end of an SMTP session. 242If one session is used to deliver mail on behalf of different SMTP 243clients, the XCLIENT attributes need to be reset as appropriate 244before each MAIL FROM command. </p> 245 246<h2> References </h2> 247 248<p> Moore, K, "SMTP Service Extension for Delivery Status Notifications", 249<a href="http://tools.ietf.org/html/rfc1891">RFC 1891</a>, January 1996. </p> 250 251</body> 252 253</html> 254
