feat: allow to set a server banner

refs charmbracelet/wish#205

server.go

@@ -37,6 +37,7 @@ type Server struct {
 	Handler     Handler  // handler to invoke, ssh.DefaultHandler if nil
 	HostSigners []Signer // private keys for the host key, must have at least one
 	Version     string   // server version to be sent before the initial handshake
+	Banner      string   // server banner
 
 	KeyboardInteractiveHandler    KeyboardInteractiveHandler    // keyboard-interactive authentication handler
 	PasswordHandler               PasswordHandler               // password authentication handler
@@ -132,6 +133,11 @@ func (srv *Server) config(ctx Context) *gossh.ServerConfig {
 	if srv.Version != "" {
 		config.ServerVersion = "SSH-2.0-" + srv.Version
 	}
+	if srv.Banner != "" {
+		config.BannerCallback = func(conn gossh.ConnMetadata) string {
+			return srv.Banner
+		}
+	}
 	if srv.PasswordHandler != nil {
 		config.PasswordCallback = func(conn gossh.ConnMetadata, password []byte) (*gossh.Permissions, error) {
 			applyConnMetadata(ctx, conn)