Pass connection information to backend

Closes #15.

backend.go

@@ -14,11 +14,11 @@ var (
 type Backend interface {
 	// Authenticate a user. Return smtp.ErrAuthUnsupported if you don't want to
 	// support this.
-	Login(username, password string) (User, error)
+	Login(state *ConnectionState, username, password string) (User, error)
 
 	// Called if the client attempts to send mail without logging in first.
 	// Return smtp.ErrAuthRequired if you don't want to support this.
-	AnonymousLogin() (User, error)
+	AnonymousLogin(state *ConnectionState) (User, error)
 }
 
 // An authenticated user.

backendutil/transform.go

@@ -8,21 +8,21 @@ import (
 
 // TransformBackend is a backend that transforms messages.
 type TransformBackend struct {
-	Backend smtp.Backend
+	Backend   smtp.Backend
 	Transform func(from string, to []string, r io.Reader) (string, []string, io.Reader, error)
 }
 
 // Login implements the smtp.Backend interface.
-func (be *TransformBackend) Login(username, password string) (smtp.User, error) {
-	u, err := be.Backend.Login(username, password)
+func (be *TransformBackend) Login(state *smtp.ConnectionState, username, password string) (smtp.User, error) {
+	u, err := be.Backend.Login(state, username, password)
 	if err != nil {
 		return nil, err
 	}
 	return &transformUser{u, be}, nil
 }
 
-func (be *TransformBackend) AnonymousLogin() (smtp.User, error) {
-	u, err := be.Backend.AnonymousLogin()
+func (be *TransformBackend) AnonymousLogin(state *smtp.ConnectionState) (smtp.User, error) {
+	u, err := be.Backend.AnonymousLogin(state)
 	if err != nil {
 		return nil, err
 	}

conn.go

@@ -25,6 +25,12 @@ type message struct {
 	To []string
 }
 
+type ConnectionState struct {
+	Hostname   string
+	RemoteAddr net.Addr
+	TLS        tls.ConnectionState
+}
+
 type Conn struct {
 	conn      net.Conn
 	text      *textproto.Conn
@@ -160,6 +166,19 @@ func (c *Conn) TLSConnectionState() (state tls.ConnectionState, ok bool) {
 	return tc.ConnectionState(), true
 }
 
+func (c *Conn) State() ConnectionState {
+	state := ConnectionState{}
+	tlsState, ok := c.TLSConnectionState()
+	if ok {
+		state.TLS = tlsState
+	}
+
+	state.Hostname = c.helo
+	state.RemoteAddr = c.conn.RemoteAddr()
+
+	return state
+}
+
 func (c *Conn) authAllowed() bool {
 	_, isTLS := c.TLSConnectionState()
 	return !c.server.AuthDisabled && (isTLS || c.server.AllowInsecureAuth)
@@ -216,7 +235,8 @@ func (c *Conn) handleMail(arg string) {
 	}
 
 	if c.User() == nil {
-		user, err := c.server.Backend.AnonymousLogin()
+		state := c.State()
+		user, err := c.server.Backend.AnonymousLogin(&state)
 		if err != nil {
 			c.WriteResponse(502, err.Error())
 			return

server.go

@@ -60,7 +60,8 @@ func NewServer(be Backend) *Server {
 						return errors.New("Identities not supported")
 					}
 
-					user, err := be.Login(username, password)
+					state := conn.State()
+					user, err := be.Login(&state, username, password)
 					if err != nil {
 						return err
 					}

server_test.go

@@ -25,7 +25,7 @@ type backend struct {
 	userErr error
 }
 
-func (be *backend) Login(username, password string) (smtp.User, error) {
+func (be *backend) Login(_ *smtp.ConnectionState, username, password string) (smtp.User, error) {
 	if be.userErr != nil {
 		return &user{}, be.userErr
 	}
@@ -36,7 +36,7 @@ func (be *backend) Login(username, password string) (smtp.User, error) {
 	return &user{backend: be}, nil
 }
 
-func (be *backend) AnonymousLogin() (smtp.User, error) {
+func (be *backend) AnonymousLogin(_ *smtp.ConnectionState) (smtp.User, error) {
 	if be.userErr != nil {
 		return &user{}, be.userErr
 	}