Support multiple private (host) keys

With this patch, one can configure one or more private (host) keys in
the configuration file.

I made it a nested struct, so we can add more options later. Eg. host
certificates might be a useful addition if I can figure out how this is
done in golang's crypto/ssh...

Signed-off-by: Jo Vandeginste <Jo.Vandeginste@kuleuven.be>

README.md

@@ -49,7 +49,7 @@ Alternately you can use distro packages:
 
 The `sftpgo` executable supports the following command line flags:
 
-- `-config-dir` string. Location of the config dir. This directory should contain the `sftpgo.conf` configuration file, the private key for the SFTP server (`id_rsa` file) and the SQLite database if you use SQLite as data provider. The server private key will be autogenerated if the user that executes SFTPGo has write access to the config-dir. The default value is "."
+- `-config-dir` string. Location of the config dir. This directory should contain the `sftpgo.conf` configuration file and is used as the base for files with a relative path (eg. the private keys for the SFTP server, the SQLite database if you use SQLite as data provider). The default value is "."
 - `-log-file-path` string. Location for the log file, default "sftpgo.log"
 - `-log-max-size` int. Maximum size in megabytes of the log file before it gets rotated. Default 10
 - `-log-max-backups` int. Maximum number of old log files to retain. Default 5
@@ -57,6 +57,8 @@ The `sftpgo` executable supports the following command line flags:
 - `-log-compress` boolean. Determine if the rotated log files should be compressed using gzip
 - `-log-verbose` boolean. Enable verbose logs. Default `true`
 
+If you don't configure any private host keys, the daemon will use `id_rsa` in the configuration directory. If that file doesn't exist, the daemon will attempt to autogenerate it (if the user that executes SFTPGo has write access to the config-dir). The server supports any private key format supported by [`crypto/ssh`](https://github.com/golang/crypto/blob/master/ssh/keys.go#L32).
+
 Before starting `sftpgo` a dataprovider must be configured.
 
 Sample SQL scripts to create the required database structure can be found insite the source tree [sql](https://github.com/drakkan/sftpgo/tree/master/sql "sql") directory. The SQL scripts filename's is, by convention, the date as `YYYYMMDD` and the suffix `.sql`. You need to apply all the SQL scripts for your database ordered by name, for example `20190706.sql` must be applied before `20190728.sql` and so on.
@@ -82,6 +84,8 @@ The `sftpgo.conf` configuration file contains the following sections:
             - `username`
             - `path`
             - `target_path`, added for `rename` action only
+    - `keys`, struct array. It contains the daemon's private keys
+        - `private_key`, path to the private key file
 - **"data_provider"**, the configuration for the data provider
     - `driver`, string. Supported drivers are `sqlite`, `mysql`, `postgresql`
     - `name`, string. Database name. For driver `sqlite` this can be the database name relative to the config dir or the absolute path to the SQLite database.
@@ -116,7 +120,15 @@ Here is a full example showing the default config:
       "execute_on": [],
       "command": "",
       "http_notification_url": ""
+    },
+    "keys": [
+      {
+        "private_key": "id_rsa"
+      },
+      {
+        "private_key": "id_ecdsa"
       }
+    ]
   },
   "data_provider": {
     "driver": "sqlite",

main.go

@@ -36,9 +36,8 @@ func main() {
 		logCompress   bool
 		logVerbose    bool
 	)
-	flag.StringVar(&configDir, "config-dir", ".", "Location for SFTPGo config dir. It must contain sftpgo.conf, "+
-		"the private key for the SFTP server (id_rsa file) and the SQLite database if you use SQLite as data provider. "+
-		"The server private key will be autogenerated if the user that executes SFTPGo has write access to the config-dir")
+	flag.StringVar(&configDir, "config-dir", ".", "Location for SFTPGo config dir. It must contain sftpgo.conf "+
+		"and is used as the base for files with a relative path (eg. the private keys for the SFTP server, the SQLite database if you use SQLite as data provider).")
 	flag.StringVar(&logFilePath, "log-file-path", "sftpgo.log", "Location for the log file")
 	flag.IntVar(&logMaxSize, "log-max-size", 10, "Maximum size in megabytes of the log file before it gets rotated.")
 	flag.IntVar(&logMaxBackups, "log-max-backups", 5, "Maximum number of old log files to retain")

sftpd/server.go

@@ -25,6 +25,8 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
+const defaultPrivateKeyName = "id_rsa"
+
 // Configuration for the SFTP server
 type Configuration struct {
 	// Identification string used by the server
@@ -43,6 +45,14 @@ type Configuration struct {
 	Umask string `json:"umask"`
 	// Actions to execute on SFTP create, download, delete and rename
 	Actions Actions `json:"actions"`
+	// Keys are a list of host keys
+	Keys []Key `json:"keys"`
+}
+
+// Struct containing information about host keys
+type Key struct {
+	// The private key
+	PrivateKey string `json:"private_key"`
 }
 
 // Initialize the SFTP server and add a persistent listener to handle inbound SFTP connections.
@@ -76,17 +86,29 @@ func (c Configuration) Initialize(configDir string) error {
 		ServerVersion: "SSH-2.0-" + c.Banner,
 	}
 
-	if _, err := os.Stat(filepath.Join(configDir, "id_rsa")); os.IsNotExist(err) {
-		logger.Info(logSender, "creating new private key for server")
-		logger.InfoToConsole("id_rsa does not exist, creating new private key for server")
-		if err := c.generatePrivateKey(configDir); err != nil {
+	if len(c.Keys) == 0 {
+		autoFile := filepath.Join(configDir, defaultPrivateKeyName)
+		if _, err := os.Stat(autoFile); os.IsNotExist(err) {
+			logger.Info(logSender, "No host keys configured and %s does not exist; creating new private key for server", autoFile)
+			logger.InfoToConsole("No host keys configured and %s does not exist; creating new private key for server", autoFile)
+			if err := c.generatePrivateKey(autoFile); err != nil {
 				return err
 			}
 		} else if err != nil {
 			return err
 		}
 
-	privateBytes, err := ioutil.ReadFile(filepath.Join(configDir, "id_rsa"))
+		c.Keys = append(c.Keys, Key{PrivateKey: defaultPrivateKeyName})
+	}
+
+	for _, k := range c.Keys {
+		privateFile := k.PrivateKey
+		if !filepath.IsAbs(privateFile) {
+			privateFile = filepath.Join(configDir, privateFile)
+		}
+		logger.Info(logSender, "Loading private key: %s", privateFile)
+
+		privateBytes, err := ioutil.ReadFile(privateFile)
 		if err != nil {
 			return err
 		}
@@ -98,6 +120,7 @@ func (c Configuration) Initialize(configDir string) error {
 
 		// Add our private key to the server configuration.
 		serverConfig.AddHostKey(private)
+	}
 
 	listener, err := net.Listen("tcp", fmt.Sprintf("%s:%d", c.BindAddress, c.BindPort))
 	if err != nil {
@@ -273,13 +296,13 @@ func (c Configuration) validatePasswordCredentials(conn ssh.ConnMetadata, pass [
 }
 
 // Generates a private key that will be used by the SFTP server.
-func (c Configuration) generatePrivateKey(configDir string) error {
+func (c Configuration) generatePrivateKey(file string) error {
 	key, err := rsa.GenerateKey(rand.Reader, 4096)
 	if err != nil {
 		return err
 	}
 
-	o, err := os.OpenFile(filepath.Join(configDir, "id_rsa"), os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
+	o, err := os.OpenFile(file, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
 	if err != nil {
 		return err
 	}