add configurable smtp greeter

config.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"reflect"
 	"strings"
+	"text/template"
 	"time"
 
 	"github.com/phires/go-guerrilla/backends"
@@ -60,6 +61,9 @@ type ServerConfig struct {
 	// XClientOn when using a proxy such as Nginx, XCLIENT command is used to pass the
 	// original client's IP address & client's HELO
 	XClientOn bool `json:"xclient_on,omitempty"`
+	// GreeterTemplate will be used to greet the client.
+	// Defaults to defaultGreeterString
+	GreeterTemplate *template.Template `json:"greeter_template"`
 }
 
 type ServerTLSConfig struct {
@@ -151,10 +155,13 @@ var TLSClientAuthTypes = map[string]tls.ClientAuthType{
 	"RequireAndVerifyClientCert": tls.RequireAndVerifyClientCert,
 }
 
-const defaultMaxClients = 100
-const defaultTimeout = 30
-const defaultInterface = "127.0.0.1:2525"
-const defaultMaxSize = int64(10 << 20) // 10 Mebibytes
+const (
+	defaultMaxClients    = 100
+	defaultTimeout       = 30
+	defaultInterface     = "127.0.0.1:2525"
+	defaultMaxSize       = int64(10 << 20) // 10 Mebibytes
+	defaultGreeterString = "220 {{ .Hostname }} SMTP Guerrilla({{ .Version }}) #{{ .ClientID }} ({{ .ActiveClientsCount }}) {{ .Time }}"
+)
 
 // Unmarshalls json data into AppConfig struct and any other initialization of the struct
 // also does validation, returns error if validation failed or something went wrong
@@ -279,6 +286,10 @@ func (c *AppConfig) setDefaults() error {
 	if err != nil {
 		return err
 	}
+	defaultGreeterTemplate, err := template.New("greeter").Parse(defaultGreeterString)
+	if err != nil {
+		return fmt.Errorf("unable to parse template: %v", err)
+	}
 	if len(c.Servers) == 0 {
 		sc := ServerConfig{}
 		sc.LogFile = c.LogFile
@@ -288,6 +299,7 @@ func (c *AppConfig) setDefaults() error {
 		sc.MaxClients = defaultMaxClients
 		sc.Timeout = defaultTimeout
 		sc.MaxSize = defaultMaxSize
+		sc.GreeterTemplate = defaultGreeterTemplate
 		c.Servers = append(c.Servers, sc)
 	} else {
 		// make sure each server has defaults correctly configured
@@ -304,6 +316,9 @@ func (c *AppConfig) setDefaults() error {
 			if c.Servers[i].MaxSize == 0 {
 				c.Servers[i].MaxSize = defaultMaxSize // 10 Mebibytes
 			}
+			if c.Servers[i].GreeterTemplate == nil {
+				c.Servers[i].GreeterTemplate = defaultGreeterTemplate
+			}
 			if c.Servers[i].ListenInterface == "" {
 				return fmt.Errorf("listen interface not specified for server at index %d", i)
 			}

server.go

@@ -67,6 +67,14 @@ type allowedHosts struct {
 	sync.Mutex                 // guard access to the map
 }
 
+type greeterData struct {
+	Hostname           string
+	Version            string
+	ClientID           uint64
+	ActiveClientsCount int
+	Time               string
+}
+
 type command []byte
 
 var (
@@ -368,9 +376,16 @@ func (s *server) handleClient(client *client) {
 	s.log().Infof("Handle client [%s], id: %d", client.RemoteIP, client.ID)
 
 	// Initial greeting
-	greeting := fmt.Sprintf("220 %s SMTP Guerrilla(%s) #%d (%d) %s",
-		sc.Hostname, Version, client.ID,
-		s.clientPool.GetActiveClientsCount(), time.Now().Format(time.RFC3339))
+	greeterData := greeterData{
+		Hostname:           sc.Hostname,
+		Version:            Version,
+		ClientID:           client.ID,
+		ActiveClientsCount: s.clientPool.GetActiveClientsCount(),
+		Time:               time.Now().Format(time.RFC3339),
+	}
+
+	var greeting bytes.Buffer
+	sc.GreeterTemplate.Execute(&greeting, greeterData)
 
 	helo := fmt.Sprintf("250 %s Hello", sc.Hostname)
 	// ehlo is a multi-line reply and need additional \r\n at the end
@@ -405,7 +420,7 @@ func (s *server) handleClient(client *client) {
 	for client.isAlive() {
 		switch client.state {
 		case ClientGreeting:
-			client.sendResponse(greeting)
+			client.sendResponse(greeting.String())
 			client.state = ClientCmd
 		case ClientCmd:
 			client.bufin.setLimit(CommandLineMaxLength)