smtpd(3tcl)

📦
version 2.0 · 2 versions
, in debian trixie, ubuntu questing, ubuntu resolute, ubuntu stonking

smtpd 💌✨

okay bestie, this is the manpage for smtpd, a.k.a. the Tcl SMTP Server Package from the tcllib fam. if you've ever wanted to run a whole mail server out of your Tcl script, buckle up, because this is literally that. 🎀

NAME 🦋

smtpd — Tcl SMTP server implementation. yes that's the whole name section, she's concise. 💅

SYNOPSIS ✨

package require Tcl 8.5 9

package require smtpd ?1.6?

::smtpd::start ?myaddr? ?port?

::smtpd::stop

::smptd::configure ?option value? ?option value ...?

::smptd::cget ?option?
not me noticing that it's ::smptd:: with the letters swapped on those last two 😭 that's literally what the page says, so we're keeping it, no cap.

DESCRIPTION 💖

so the smtpd package gives you a simple, Tcl-only server library for the Simple Mail Transfer Protocol, exactly as described in RFC 821 (http://www.rfc-editor.org/rfc/rfc821.txt) and RFC 2821 (http://www.rfc-editor.org/rfc/rfc2821.txt). by default the server will bind to the default network address and the standard SMTP port, which is 25. classic. 🫶

this package was designed to permit testing of Mail User Agent code from a developer's workstation. it does not attempt to deliver mail to your mailbox — she is not a mail delivery girlie, she's a test harness. instead, users of this package are expected to write a procedure that will be called when mail arrives. once that procedure returns, the server has nothing further to do with the mail. done. gone. periodt. 💀

SECURITY 🚨

okay this is the serious section, bestie, pay attention. 🙈

on Unix platforms, binding to the SMTP port requires root privileges. the author would not recommend running any script-based server as root unless there is some method for dropping root privileges immediately after the socket is bound. under Windows platforms, it is not necessary to have root or administrator privileges to bind low numbered sockets — however, security on these platforms is weak anyway. 💀

in short: this code should probably not be used as a permanently running Mail Transfer Agent on an Internet connected server, even though we are careful not to evaluate remote user input. there are many other well tested and security audited programs that can be used as mail servers for internet connected hosts. so like, use those for prod. this one's for the vibes. 🌸

TLS SECURITY CONSIDERATIONS 🔐

this package uses the TLS package to handle the security for https urls and other socket connections.

policy decisions like the set of protocols to support and what ciphers to use are not the responsibility of TLS, nor of this package itself however. such decisions are the responsibility of whichever application is using the package, and are likely influenced by the set of servers the application will talk to as well. iykyk. 💅

for example, in light of the recent POODLE attack [http://googleonlinesecurity.blogspot.co.uk/2014/10/this-poodle-bites-exploiting-ssl-30.html] discovered by Google, many servers will disable support for the SSLv3 protocol. to handle this change the applications using TLS must be patched — and not this package, nor TLS itself. such a patch may be as simple as generally activating tls1 support, as shown in the example below. 👇

package require tls
tls::init -tls1 1 ;# forcibly activate support for the TLS1 protocol

... your own application code ...

COMMANDS 🛠️

::smtpd::start ?myaddr? ?port? 🚀

start the service listening on port, or the default port 25. if myaddr is given as a domain-style name or numerical dotted-quad IP address then the server socket will be bound to that network interface. by default the server is bound to all network interfaces. for example:

set sock [::smtpd::start [info hostname] 0]

will bind to the host's internet interface on the first available port. slay. ✨

at present the package only supports a single instance of a SMTP server. this could be changed if required at the cost of making the package a little more complicated to read. if there is a good reason for running multiple SMTP services then it will only be necessary to fix the options array and the ::smtpd::stopped variable usage. noted. 📝

as the server code uses fileevent(3tcl) handlers to process the input on sockets, you will need to run the event loop. this means either you should be running from within wish(1) or you should vwait(3tcl) on the ::smtpd::stopped variable, which is set when the server is stopped. no event loop = no mail, bestie. 😭

::smtpd::stop 🛑

halt the server and release the listening socket. if the server has not been started then this command does nothing. the ::smtpd::stopped variable is set for use with vwait(3tcl).

it should be noted that stopping the server does not disconnect any currently active sessions, as these are operating over an independent channel. only explicitly tracking and closing these sessions, or exiting the server process, will close down all the running sessions. this is similar to the usual unix daemon practice where the server performs a fork(2) and the client session continues on the child process. 💀

::smptd::configure ?option value? ?option value ...? ⚙️

set configuration options for the SMTP server. most values are the name of a callback procedure to be called at various points in the SMTP protocol. see the CALLBACKS section for details of the procedures. 👑

  • -banner text — text of a custom banner message. the default banner is "tcllib smtpd 1.6". note that changing the banner does not affect the bracketing text in the full greeting, printing status 220, server-address, and timestamp. 🎀
  • -validate_host proc — callback to authenticate new connections based on the ip-address of the client.
  • -validate_sender proc — callback to authenticate new connections based on the sender's email address.
  • -validate_recipient proc — callback to validate and authorize a recipient email address.
  • -deliverMIME proc — callback used to deliver mail as a mime token created by the tcllib mime package.
  • -deliver proc — callback used to deliver email. this option has no effect if the -deliverMIME option has been set.

::smtpd::cget ?option? 🔍

if no option is specified, the command will return a list of all options and their current values. if an option is specified, it will return the value of that option. she's giving inventory management. 💅

CALLBACKS 📞

validate_host callback 🏠

this procedure is called with the client's ip address as soon as a connection request has been accepted and before any protocol commands are processed. if you wish to deny access to a specific host then an error should be returned by this callback. for example:

proc validate_host {ipnum} {
   if {[string match "192.168.1.*" $ipnum]} {
      error "go away!"
   }
}

if access is denied, the client will receive a standard message that includes the text of your error, such as:

550 Access denied: I hate you.

😭 iconic. as per the SMTP protocol, the connection is not closed — instead we wait for the client to send a QUIT command. any other commands cause a 503 Bad Sequence error. rude? maybe. protocol-correct? absolutely. ✨

validate_sender callback 📤

the validate_sender callback is called with the sender's mail address during processing of a MAIL command, to allow you to accept or reject mail based upon the declared sender. to reject mail you should throw an error. for example, to reject mail from user "denied":

proc validate_sender {address} {
   eval array set addr [mime::parseaddress $address]
   if {[string match "denied" $addr(local)]} {
        error "mailbox $addr(local) denied"
   }
   return
}

the content of any error message will not be passed back to the client. so you can be as dramatic as you want in there, she won't tell. 🙈

validate_recipient callback 📥

the validate_recipient callback is similar to the validate_sender callback and permits you to verify a local mailbox and accept mail for a local user address during RCPT command handling. to reject mail, throw an error as above. the error message is ignored. 💀

deliverMIME callback 📦

the deliverMIME callback is called once a mail message has been successfully passed to the server. a mime token is constructed from the sender, recipients and data, and the user's procedure is called with this single argument. when the call returns, the mime token is cleaned up — so if the user wishes to preserve the data she must make a copy. do not skip the copy, bestie, i'm begging. 😭

proc deliverMIME {token} {
    set sender [lindex [mime::getheader $token From] 0]
    set recipients [lindex [mime::getheader $token To] 0]
    set mail "From $sender [clock format [clock seconds]]"
    append mail "\n" [mime::buildmessage $token]
    puts $mail
}

deliver callback 📬

the deliver callback is called once a mail message has been successfully passed to the server and there is no -deliverMIME option set. the procedure is called with the sender, a list of recipients and the text of the mail as a list of lines. for example:

proc deliver {sender recipients data} {
   set mail "From $sender  [clock format [clock seconds]]"
   append mail "\n" [join $data "\n"]
   puts "$mail"
}
note that the DATA command will return an error if no sender or recipient has yet been defined. no skipping steps, she checks. 💅

VARIABLES 🫙

  • ::smtpd::stopped — this variable is set to true during the ::smtpd::stop command to permit the use of the vwait(3tcl) command. she's the little flag that lets your event loop know it's time to go home. 🌙

AUTHOR 👑

written by Pat Thoyts mailto:patthoyts@users.sourceforge.net. thank you legend 🫶

LICENSE 📜

this software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. see the file "license.terms" for more details. (yes the all-caps is the original, and yes it's still shouting.) 📣

BUGS, IDEAS, FEEDBACK 🐛💡

this document, and the package it describes, will undoubtedly contain bugs and other problems. please report such in the category smtpd of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. please also report any ideas for enhancements you may have for either package and/or documentation. we love a feedback loop. 🌈

when proposing code changes, please provide unified diffs, i.e the output of diff -u.

note further that attachments are strongly preferred over inlined patches. attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar. lowkey a whole scavenger hunt but okay. 🗺️

KEYWORDS 🔖

rfc 2821, rfc 821, services, smtp, smtpd, socket, vwait

CATEGORY 🗂️

Networking

Copyright (c) Pat Thoyts <patthoyts@users.sourceforge.net>

okay bestie that's the whole page, we ate and left no crumbs 💅✨ this was tcllib version 1.6, the smtpd(3tcl) manual page — go forth and send some test mail, mwah 🎀🦋💖

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