sftpd: add support for some SSH commands

md5sum, sha1sum are used by rclone.
cd, pwd improve the support for RemoteFiles mobile app.

These commands are all implemented inside SFTPGo so they work even
if the matching system commands are not available, for example on Windows

sftpd/sftpd.go

@@ -22,6 +22,7 @@ import (
 const (
 	logSender         = "sftpd"
 	logSenderSCP      = "scp"
+	logSenderSSH      = "ssh"
 	uploadLogSender   = "Upload"
 	downloadLogSender = "Download"
 	renameLogSender   = "Rename"
@@ -38,6 +39,7 @@ const (
 	operationRename   = "rename"
 	protocolSFTP      = "SFTP"
 	protocolSCP       = "SCP"
+	protocolSSH       = "SSH"
 	handshakeTimeout  = 2 * time.Minute
 )
 
@@ -58,6 +60,9 @@ var (
 	actions              Actions
 	uploadMode           int
 	setstatMode          int
+	supportedSSHCommands = []string{"scp", "md5sum", "sha1sum", "sha256sum", "sha384sum", "sha512sum", "cd", "pwd"}
+	defaultSSHCommands   = []string{"md5sum", "sha1sum", "cd", "pwd"}
+	sshHashCommands      = []string{"md5sum", "sha1sum", "sha256sum", "sha384sum", "sha512sum"}
 )
 
 type connectionTransfer struct {
@@ -101,21 +106,41 @@ type ConnectionStatus struct {
 	ConnectionTime int64 `json:"connection_time"`
 	// Last activity as unix timestamp in milliseconds
 	LastActivity int64 `json:"last_activity"`
-	// Protocol for this connection: SFTP or SCP
+	// Protocol for this connection: SFTP, SCP, SSH
 	Protocol string `json:"protocol"`
 	// active uploads/downloads
 	Transfers []connectionTransfer `json:"active_transfers"`
+	// for protocol SSH this is the issued command
+	SSHCommand string `json:"ssh_command"`
 }
 
 type sshSubsystemExitStatus struct {
 	Status uint32
 }
 
+type sshSubsystemExecMsg struct {
+	Command string
+}
+
 func init() {
 	openConnections = make(map[string]Connection)
 	idleConnectionTicker = time.NewTicker(5 * time.Minute)
 }
 
+// GetDefaultSSHCommands returns the SSH commands enabled as default
+func GetDefaultSSHCommands() []string {
+	result := make([]string, len(defaultSSHCommands))
+	copy(result, defaultSSHCommands)
+	return result
+}
+
+// GetSupportedSSHCommands returns the supported SSH commands
+func GetSupportedSSHCommands() []string {
+	result := make([]string, len(supportedSSHCommands))
+	copy(result, supportedSSHCommands)
+	return result
+}
+
 // GetConnectionDuration returns the connection duration as string
 func (c ConnectionStatus) GetConnectionDuration() string {
 	elapsed := time.Since(utils.GetTimeFromMsecSinceEpoch(c.ConnectionTime))
@@ -123,9 +148,14 @@ func (c ConnectionStatus) GetConnectionDuration() string {
 }
 
 // GetConnectionInfo returns connection info.
-// Protocol,Client Version and RemoteAddress are returned
+// Protocol,Client Version and RemoteAddress are returned.
+// For SSH commands the issued command is returned too.
 func (c ConnectionStatus) GetConnectionInfo() string {
-	return fmt.Sprintf("%v. Client: %#v From: %#v", c.Protocol, c.ClientVersion, c.RemoteAddress)
+	result := fmt.Sprintf("%v. Client: %#v From: %#v", c.Protocol, c.ClientVersion, c.RemoteAddress)
+	if c.Protocol == protocolSSH && len(c.SSHCommand) > 0 {
+		result += fmt.Sprintf(". Command: %#v", c.SSHCommand)
+	}
+	return result
 }
 
 // GetTransfersAsString returns the active transfers as string
@@ -251,6 +281,7 @@ func GetConnectionsStats() []ConnectionStatus {
 			LastActivity:   utils.GetTimeAsMsSinceEpoch(c.lastActivity),
 			Protocol:       c.protocol,
 			Transfers:      []connectionTransfer{},
+			SSHCommand:     c.command,
 		}
 		for _, t := range activeTransfers {
 			if t.connectionID == c.ID {