smtp

Documentation

Overview

smtp package implements a rather forgiving TCP server that carries on and decodes an SMTP conversation. I would like to express my gratitude to Chris Siebenmann for his inspiring pioneer work on an implementation of SMTP server written in go.

Index

• Constants
• type Command
• type Config
• type Connection
  • func NewConnection(conn net.Conn, cfg Config, logger *lalog.Logger) *Connection
  • func (conn *Connection) AnswerNegative()
  • func (conn *Connection) AnswerRateLimited()
  • func (conn *Connection) CarryOn() Command
• type ConversationState
• type ProtocolVerb
  • func (verb ProtocolVerb) String() string

Constants

const (
	StageGreeting      commandStage = iota
	StageAfterGreeting commandStage = 1 << iota
	StageHello
	StateMailAddress
	StageRecipient
	StageMessageData
	StageAfterMessageData
	StageQuit
	StageAbort
)

const MaxCommandLength = 4096

MaxCommandLength is the maximum acceptable length of a command in the middle of an ongoing SMTP conversation. The maximum length does not apply to mail message and attachments.

Types

type Command

type Command struct {
	State     ConversationState
	Verb      ProtocolVerb
	Parameter string
}

Command comes from the result of interpretation of latest protocol command in an ongoing conversation. In addition to stating whether the SMTP conversation is carrying on, it also records the latest protocol command verb and parameter value.

type Config

type Config struct {
	// TLSConfig grants SMTP server StartTLS capability.
	TLSConfig *tls.Config
	// IOTimeout governs the timeout of each read and write operation.
	IOTimeout time.Duration
	/*
		MaxMessageLength is the maximum size (in bytes) of a mail message (and attachment) during an SMTP conversation.
		An ordinary protocol command that does not carry mail message nor attachment uses MaxCommandLength constant
		instead.
	*/
	MaxMessageLength int64
	/*
		MaxConsecutiveUnrecognisedCommands is the maximum number of unknown protocol commands to tolerate before giving
		up on the connection, which will result in the connection being closed.
	*/
	MaxConsecutiveUnrecognisedCommands int
	/*
		ServerName is the complete Internet host name of the mail server, it is used to greet mail clients. Some clients
		use the greeting to further establish authenticity of the mail server.
	*/
	ServerName string
}

Config provides behaviour and fault tolerance tuning to SMTP conversation connection.

type Connection

type Connection struct {
	// Config is supplied by caller.
	Config Config

	// TLSAttempted is an indication flag to inform caller that StartTLS has been attempted.
	TLSAttempted bool
	// TLSState helps caller to debug TLS connection issue.
	TLSState tls.ConnectionState
	// TLSHelp contains a text description that explains the latest TLS error from SMTP conversation's perspective.
	TLSHelp string
	// contains filtered or unexported fields
}

Connection is the server side of an SMTP connection and it offers functions for caller to interact with an SMTP conversation, and eventually acquire the complete mail message prior to the conclusion of the connection.

func NewConnection

func NewConnection(conn net.Conn, cfg Config, logger *lalog.Logger) *Connection

func (*Connection) AnswerNegative

func (conn *Connection) AnswerNegative()

AnswerNegative produces a negative reply appropriate for the stage of SMTP conversation.

func (*Connection) AnswerRateLimited

func (conn *Connection) AnswerRateLimited()

AnswerRateLimited produces a negative answer to the SMTP conversation to inform SMTP client that it has been rate limited. The connection is closed afterwards.

func (*Connection) CarryOn

func (conn *Connection) CarryOn() Command

CarryOn continues the SMTP conversation until the next stage is reached, at which point the latest command (such as mail address or mail data) is returned to caller.

type ConversationState

type ConversationState int

ConversationState represents the latest state of the ongoing SMTP conversation, determined by the latest series of protocol commands. Caller may use the state to determine whether the conversation is carrying on, and whether mail message has been received completely.

const (
	ConvReceivedCommand ConversationState = iota
	ConvReceivedData
	ConvCompleted
	ConvAborted
)

type ProtocolVerb

type ProtocolVerb int

ProtocolVerb is an enumeration of SMTP verbs supported by this server.

const (
	VerbAbsent  ProtocolVerb = iota
	VerbUnknown ProtocolVerb = iota
	VerbHELO
	VerbEHLO
	VerbSTARTTLS
	VerbVRFY
	VerbMAILFROM
	VerbRCPTTO
	VerbDATA
	VerbQUIT
	VerbRSET
	VerbNOOP
)

func (ProtocolVerb) String

func (verb ProtocolVerb) String() string

String returns a descriptive string representation of an SMTP Verb.