Network support for the Lua language

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## SMTP

The `smtp` namespace provides functionality to send e-mail messages. The high-level API consists of two functions: one to define an e-mail message, and another to actually send the message. Although almost all users will find that these functions provide more than enough functionality, the underlying implementation allows for even more control (if you bother to read the code).

The implementation conforms to the Simple Mail Transfer Protocol, RFC 2821. Another RFC of interest is RFC 2822, which governs the Internet Message Format. Multipart messages (those that contain attachments) are part of the MIME standard, but described mainly in RFC 2046

In the description below, good understanding of LTN012, Filters sources and sinks and the MIME module is assumed. In fact, the SMTP module was the main reason for their creation.

To obtain the `smtp` namespace, run:

    -- loads the SMTP module and everything it requires
    local smtp = require("socket.smtp")

MIME headers are represented as a Lua table in the form:

> `headers = { field-1-name = field-1-value, field-2-name = field-2-value, field-3-name = field-3-value, ...
> field-n-name = field-n-value
> }`

Field names are case insensitive (as specified by the standard) and all functions work with lowercase field names. Field values are left unmodified.

Note: MIME headers are independent of order. Therefore, there is no problem in representing them in a Lua table.

The following constants can be set to control the default behavior of the SMTP module:

- `DOMAIN`: domain used to greet the server;
- `PORT`: default port used for the connection;
- `SERVER`: default server used for the connection;
- `TIMEOUT`: default timeout for all I/O operations;
- `ZONE`: default time zone.

smtp.send{
from = string,
rcpt = string or string-table,
source = LTN12 source,
[user = string,]
[password = string,]
[server = string,]
[port = number,]
[domain = string,]
[step = LTN12 pump step,]
[create = function]
}

Sends a message to a recipient list. Since sending messages is not as simple as downloading an URL from a FTP or HTTP server, this function doesn't have a simple interface. However, see the `message` source factory for a very powerful way to define the message contents.

The sender is given by the e-mail address in the `from` field. `Rcpt` is a Lua table with one entry for each recipient e-mail address, or a string in case there is just one recipient. The contents of the message are given by a simple LTN12 `source`. Several arguments are optional:

- `user`, `password`: User and password for authentication. The function will attempt LOGIN and PLAIN authentication methods if supported by the server (both are unsafe);
- `server`: Server to connect to. Defaults to "localhost";
- `port`: Port to connect to. Defaults to 25;
- `domain`: Domain name used to greet the server; Defaults to the local machine host name;
- `step`: LTN12 pump step function used to pass data from the source to the server. Defaults to the LTN12 `pump.step` function;
- `create`: An optional function to be used instead of `socket.tcp` when the communications socket is created.

If successful, the function returns 1. Otherwise, the function returns `nil` followed by an error message.

Note: SMTP servers can be very picky with the format of e-mail addresses. To be safe, use only addresses of the form "`<fulano@example.com>`" in the `from` and `rcpt` arguments to the `send` function. In headers, e-mail addresses can take whatever form you like.

Big note: There is a good deal of misconception with the use of the destination address field headers, i.e., the '`To`', '`Cc`', and, more importantly, the '`Bcc`' headers. Do not add a '`Bcc`' header to your messages because it will probably do the exact opposite of what you expect.

Only recipients specified in the `rcpt` list will receive a copy of the message. Each recipient of an SMTP mail message receives a copy of the message body along with the headers, and nothing more. The headers are part of the message and should be produced by the LTN12 `source` function. The `rcpt` list is not part of the message and will not be sent to anyone.

RFC 2822 has two important and short sections, "3.6.3. Destination address fields" and "5. Security considerations", explaining the proper use of these headers. Here is a summary of what it says:

- `To`: contains the address(es) of the primary recipient(s) of the message;
- `Cc`: (where the "Cc" means "Carbon Copy" in the sense of making a copy on a typewriter using carbon paper) contains the addresses of others who are to receive the message, though the content of the message may not be directed at them;
- `Bcc`: (where the "Bcc" means "Blind Carbon Copy") contains addresses of recipients of the message whose addresses are not to be revealed to other recipients of the message.

The LuaSocket `send` function does not care or interpret the headers you send, but it gives you full control over what is sent and to whom it is sent:

- If someone is to receive the message, the e-mail address has to be in the recipient list. This is the only parameter that controls who gets a copy of the message;
- If there are multiple recipients, none of them will automatically know that someone else got that message. That is, the default behavior is similar to the `Bcc` field of popular e-mail clients;
- It is up to you to add the `To` header with the list of primary recipients so that other recipients can see it;
- It is also up to you to add the `Cc` header with the list of additional recipients so that everyone else sees it;
- Adding a header `Bcc` is nonsense, unless it is empty. Otherwise, everyone receiving the message will see it and that is exactly what you don't want to happen!

I hope this clarifies the issue. Otherwise, please refer to RFC 2821 and RFC 2822.

    -- load the smtp support
    local smtp = require("socket.smtp")

    -- Connects to server "localhost" and sends a message to users
    -- "fulano@example.com",  "beltrano@example.com",
    -- and "sicrano@example.com".
    -- Note that "fulano" is the primary recipient, "beltrano" receives a
    -- carbon copy and neither of them knows that "sicrano" received a blind
    -- carbon copy of the message.
    from = "<luasocket@example.com>"

    rcpt = {
      "<fulano@example.com>",
      "<beltrano@example.com>",
      "<sicrano@example.com>"
    }

    mesgt = {
      headers = {
        to = "Fulano da Silva <fulano@example.com>",
        cc = '"Beltrano F. Nunes" <beltrano@example.com>',
        subject = "My first message"
      },
      body = "I hope this works. If it does, I can send you another 1000 copies."
    }

    r, e = smtp.send{
      from = from,
      rcpt = rcpt,
      source = smtp.message(mesgt)
    }

smtp.message(mesgt)

Returns a simple LTN12 source that sends an SMTP message body, possibly multipart (arbitrarily deep).

The only parameter of the function is a table describing the message. `Mesgt` has the following form (notice the recursive structure):

> `mesgt = { headers = header-table, body = LTN12 source or string or multipart-mesgt } multipart-mesgt = {
> [preamble = string,]
> [1] = mesgt,
> [2] = mesgt,
> ...
> [n] = mesgt,
> [epilogue = string,]
> }
> `

For a simple message, all that is needed is a set of `headers` and the `body`. The message `body` can be given as a string or as a simple LTN12 source. For multipart messages, the body is a table that recursively defines each part as an independent message, plus an optional `preamble` and `epilogue`.

The function returns a simple LTN12 source that produces the message contents as defined by `mesgt`, chunk by chunk. Hopefully, the following example will make things clear. When in doubt, refer to the appropriate RFC as listed in the introduction.

    -- load the smtp support and its friends
    local smtp = require("socket.smtp")
    local mime = require("mime")
    local ltn12 = require("ltn12")

    -- creates a source to send a message with two parts. The first part is
    -- plain text, the second part is a PNG image, encoded as base64.
    source = smtp.message{
      headers = {
         -- Remember that headers are *ignored* by smtp.send.
         from = "Sicrano de Oliveira <sicrano@example.com>",
         to = "Fulano da Silva <fulano@example.com>",
         subject = "Here is a message with attachments"
      },
      body = {
        preamble = "If your client doesn't understand attachments, \r\n" ..
                   "it will still display the preamble and the epilogue.\r\n" ..
                   "Preamble will probably appear even in a MIME enabled client.",
        -- first part: no headers means plain text, us-ascii.
        -- The mime.eol low-level filter normalizes end-of-line markers.
        [1] = {
          body = mime.eol(0, [[
            Lines in a message body should always end with CRLF.
            The smtp module will *NOT* perform translation. However, the
            send function *DOES* perform SMTP stuffing, whereas the message
            function does *NOT*.
          ]])
        },
        -- second part: headers describe content to be a png image,
        -- sent under the base64 transfer content encoding.
        -- notice that nothing happens until the message is actually sent.
        -- small chunks are loaded into memory right before transmission and
        -- translation happens on the fly.
        [2] = {
          headers = {
            ["content-type"] = 'image/png; name="image.png"',
            ["content-disposition"] = 'attachment; filename="image.png"',
            ["content-description"] = 'a beautiful image',
            ["content-transfer-encoding"] = "BASE64"
          },
          body = ltn12.source.chain(
            ltn12.source.file(io.open("image.png", "rb")),
            ltn12.filter.chain(
              mime.encode("base64"),
              mime.wrap()
            )
          )
        },
        epilogue = "This might also show up, but after the attachments"
      }
    }

    -- finally send it
    r, e = smtp.send{
        from = "<sicrano@example.com>",
        rcpt = "<fulano@example.com>",
        source = source,
    }