Message-ID: <20140921004259.GA23120@x201t.vpn.hinterhof.net>
Date:	Sun, 21 Sep 2014 02:42:59 +0200
From:	Max Vozeler <max@...terhof.net>
To:	Maximilian Eschenbacher <maximilian@...henbacher.email>
Cc:	linux-kernel@...r.kernel.org, valentina.manea.m@...il.com,
	shuah.kh@...sung.com, gregkh@...uxfoundation.org,
	Dominik Paulus <dominik.paulus@....de>,
	Fjodor Schelichow <fjodor.schelichow@...mail.com>,
	Johannes Stadlinger <johannes.stadlinger@....de>,
	Tobias Polzer <tobias.polzer@....de>
Subject: Re: [PATCH 02/18] usbip: Add support for client authentication

Hi,

On Tue, Sep 16, 2014 at 11:38:39PM +0000, Maximilian Eschenbacher wrote:
> From: Dominik Paulus <dominik.paulus@....de>
> 
> This patch adds support for authenticating both client and server using
> a pre-shared passphrase using SRP (Secure Remote Password) over TLS (see
> RFC 5054) using GnuTLS. Both usbip and usbipd now accept a shared secret
> as a command line argument.

The command line is not a good fit for passing secrets.

There is no way for the caller to provide the passphrase securely
because anything in the command line can be seen by other local users.

I suggest to read the passphrase from a file instead.

> Currently, the established TLS connection is
> only used to perform a secure handshake and dropped before the socket is
> passed to the kernel. The code may be extended to exchange a session key
> over TLS and pass it to the kernel to perform IPsec.
> 
> Signed-off-by: Maximilian Eschenbacher <maximilian@...henbacher.email>
> Signed-off-by: Fjodor Schelichow <fjodor.schelichow@...mail.com>
> Signed-off-by: Johannes Stadlinger <johannes.stadlinger@....de>
> Signed-off-by: Dominik Paulus <dominik.paulus@....de>
> Signed-off-by: Tobias Polzer <tobias.polzer@....de>
> ---
>  tools/usb/usbip/configure.ac        |  14 +++
>  tools/usb/usbip/doc/usbip.8         |   4 +
>  tools/usb/usbip/doc/usbipd.8        |   5 +
>  tools/usb/usbip/src/usbip.c         |  30 +++++-
>  tools/usb/usbip/src/usbip_attach.c  |   2 +-
>  tools/usb/usbip/src/usbip_list.c    |   2 +-
>  tools/usb/usbip/src/usbip_network.c | 185 ++++++++++++++++++++++++++++++++++++
>  tools/usb/usbip/src/usbip_network.h |  11 ++-
>  tools/usb/usbip/src/usbipd.c        | 115 ++++++++++++++++------
>  9 files changed, 335 insertions(+), 33 deletions(-)
> 
> diff --git a/tools/usb/usbip/configure.ac b/tools/usb/usbip/configure.ac
> index 607d05c..6a8156f 100644
> --- a/tools/usb/usbip/configure.ac
> +++ b/tools/usb/usbip/configure.ac
> @@ -83,6 +83,20 @@ AC_ARG_WITH([tcp-wrappers],
>  		AC_DEFINE([HAVE_LIBWRAP], [1], [use tcp wrapper])],
>  	       [AC_MSG_RESULT([no]); LIBS="$saved_LIBS"])])
>  
> +# Checks for the GnuTLS library
> +AC_ARG_WITH([gnutls],
> +			[AS_HELP_STRING([--with-gnutls],
> +							[use the GnuTLS library for authentication])],
> +			dnl [ACTION-IF-GIVEN]
> +			[if test "$withval" = "yes"; then
> +			 PKG_CHECK_MODULES([GNUTLS], [gnutls])

GnuTLS version 2.12 passes this configure test but then fails during the
build. Please check for the minimum version here:

 +			 PKG_CHECK_MODULES([GNUTLS], [gnutls >= 3.0])

> +			 AC_DEFINE([HAVE_GNUTLS], [1], [use gnutls])
> +			 CFLAGS="$CFLAGS $GNUTLS_CFLAGS"
> +			 LDFLAGS="$LDFLAGS $GNUTLS_LIBS"
> +			 fi
> +			],
> +			)
> +
>  # Sets directory containing usb.ids.
>  AC_ARG_WITH([usbids-dir],
>  	    [AS_HELP_STRING([--with-usbids-dir=DIR],
> diff --git a/tools/usb/usbip/doc/usbip.8 b/tools/usb/usbip/doc/usbip.8
> index a6097be..847aa40 100644
> --- a/tools/usb/usbip/doc/usbip.8
> +++ b/tools/usb/usbip/doc/usbip.8
> @@ -27,6 +27,10 @@ Log to syslog.
>  \fB\-\-tcp-port PORT\fR
>  .IP
>  Connect to PORT on remote host (used for attach and list --remote).
> +
> +\fB\-\-auth\fR
> +.IP
> +Set the password to be used for client authentication. See usbipd(8) for more information.
>  .PP
>  
>  .SH COMMANDS
> diff --git a/tools/usb/usbip/doc/usbipd.8 b/tools/usb/usbip/doc/usbipd.8
> index ac4635d..8beb95a 100644
> --- a/tools/usb/usbip/doc/usbipd.8
> +++ b/tools/usb/usbip/doc/usbipd.8
> @@ -52,6 +52,11 @@ If no FILE specified, use /var/run/usbipd.pid
>  \fB\-tPORT\fR, \fB\-\-tcp\-port PORT\fR
>  .IP
>  Listen on TCP/IP port PORT.
> +
> +.HP
> +\fB\-s\fR, \fB\-\-auth\fR
> +.IP
> +Sets the password to be used for client authentication. If -a is used, the server will only accept connections from authenticated clients. Note: USB traffic will still be unencrypted, this currently only serves for authentication.
>  .PP
>  
>  \fB\-h\fR, \fB\-\-help\fR
> diff --git a/tools/usb/usbip/src/usbip.c b/tools/usb/usbip/src/usbip.c
> index d7599d9..cd7e420 100644
> --- a/tools/usb/usbip/src/usbip.c
> +++ b/tools/usb/usbip/src/usbip.c
> @@ -25,6 +25,12 @@
>  #include <getopt.h>
>  #include <syslog.h>
>  
> +#include "../config.h"
> +
> +#ifdef HAVE_GNUTLS
> +#include <gnutls/gnutls.h>
> +#endif
> +
>  #include "usbip_common.h"
>  #include "usbip_network.h"
>  #include "usbip.h"
> @@ -35,8 +41,12 @@ static int usbip_version(int argc, char *argv[]);
>  static const char usbip_version_string[] = PACKAGE_STRING;
>  
>  static const char usbip_usage_string[] =
> -	"usbip [--debug] [--log] [--tcp-port PORT] [version]\n"
> -	"             [help] <command> <args>\n";
> +	"usbip "
> +#ifdef HAVE_GNUTLS
> +	"[--auth PASSWORD] "
> +#endif
> +	"[--debug] [--log] [--tcp-port PORT]\n"
> +	"             [version] [help] <command> <args>\n";
>  
>  static void usbip_usage(void)
>  {
> @@ -148,6 +158,7 @@ int main(int argc, char *argv[])
>  		{ "debug",    no_argument,       NULL, 'd' },
>  		{ "log",      no_argument,       NULL, 'l' },
>  		{ "tcp-port", required_argument, NULL, 't' },
> +		{ "auth",     required_argument, NULL, 's' },
>  		{ NULL,       0,                 NULL,  0  }
>  	};
>  
> @@ -158,12 +169,25 @@ int main(int argc, char *argv[])
>  	usbip_use_stderr = 1;
>  	opterr = 0;
>  	for (;;) {
> -		opt = getopt_long(argc, argv, "+dlt:", opts, NULL);
> +		opt = getopt_long(argc, argv, "+dls:t:", opts, NULL);
>  
>  		if (opt == -1)
>  			break;
>  
>  		switch (opt) {
> +		case 's':
> +#ifdef HAVE_GNUTLS
> +			usbip_srp_password = optarg;
> +			rc = gnutls_global_init();
> +			if (rc < 0) {
> +				err("Unable to initialize GnuTLS library: %s",
> +				    gnutls_strerror(rc));
> +				return EXIT_FAILURE;
> +			}

The initialization may be better placed outside the getopt loop or it
could be called multiple times.

> +#else
> +			err("usbip has been compiled without GnuTLS support");
> +#endif
> +			break;
>  		case 'd':
>  			usbip_use_debug = 1;
>  			break;
> diff --git a/tools/usb/usbip/src/usbip_attach.c b/tools/usb/usbip/src/usbip_attach.c
> index d58a14d..4421ebc 100644
> --- a/tools/usb/usbip/src/usbip_attach.c
> +++ b/tools/usb/usbip/src/usbip_attach.c
> @@ -175,7 +175,7 @@ static int attach_device(char *host, char *busid)
>  	int rc;
>  	int rhport;
>  
> -	sockfd = usbip_net_tcp_connect(host, usbip_port_string);
> +	sockfd = usbip_net_connect(host);
>  	if (sockfd < 0) {
>  		err("tcp connect");
>  		return -1;
> diff --git a/tools/usb/usbip/src/usbip_list.c b/tools/usb/usbip/src/usbip_list.c
> index d5ce34a..6042b7e 100644
> --- a/tools/usb/usbip/src/usbip_list.c
> +++ b/tools/usb/usbip/src/usbip_list.c
> @@ -132,7 +132,7 @@ static int list_exported_devices(char *host)
>  	int rc;
>  	int sockfd;
>  
> -	sockfd = usbip_net_tcp_connect(host, usbip_port_string);
> +	sockfd = usbip_net_connect(host);
>  	if (sockfd < 0) {
>  		err("could not connect to %s:%s: %s", host,
>  		    usbip_port_string, gai_strerror(sockfd));
> diff --git a/tools/usb/usbip/src/usbip_network.c b/tools/usb/usbip/src/usbip_network.c
> index b4c37e7..6b8f949 100644
> --- a/tools/usb/usbip/src/usbip_network.c
> +++ b/tools/usb/usbip/src/usbip_network.c
> @@ -29,11 +29,20 @@
>  #include <tcpd.h>
>  #endif
>  
> +#include "../config.h"
> +#ifdef HAVE_GNUTLS
> +#include <gnutls/gnutls.h>
> +#include <gnutls/crypto.h>
> +#endif
> +
>  #include "usbip_common.h"
>  #include "usbip_network.h"
>  
>  int usbip_port = 3240;
>  char *usbip_port_string = "3240";
> +#ifdef HAVE_GNUTLS
> +char *usbip_srp_password;
> +#endif
>  
>  void usbip_setup_port_number(char *arg)
>  {
> @@ -301,3 +310,179 @@ int usbip_net_tcp_connect(char *hostname, char *service)
>  
>  	return sockfd;
>  }
> +
> +#ifdef HAVE_GNUTLS
> +static gnutls_datum_t usbip_net_srp_salt, usbip_net_srp_verifier;
> +static gnutls_srp_server_credentials_t usbip_net_srp_cred;
> +
> +#define SRP_GROUP gnutls_srp_2048_group_generator
> +#define SRP_PRIME gnutls_srp_2048_group_prime
> +
> +int usbip_net_srp_client_handshake(int sockfd)
> +{
> +	int ret;
> +	gnutls_session_t session;
> +	gnutls_srp_client_credentials_t usbip_net_srp_cred;
> +
> +	ret = gnutls_srp_allocate_client_credentials(&usbip_net_srp_cred);
> +	if (ret < 0)
> +		return ret;
> +
> +	gnutls_srp_set_client_credentials(usbip_net_srp_cred, "dummyuser",
> +		usbip_srp_password);
>
> +	ret = gnutls_init(&session, GNUTLS_CLIENT);
> +	if (ret < 0) {
> +		gnutls_srp_free_client_credentials(usbip_net_srp_cred);
> +		return ret;
> +	}
> +
> +	gnutls_priority_set_direct(session, "NORMAL:+SRP", NULL);
> +
> +	gnutls_credentials_set(session, GNUTLS_CRD_SRP, usbip_net_srp_cred);
> +	gnutls_transport_set_int (session, sockfd);
> +
> +	do {
> +		ret = gnutls_handshake(session);
> +	} while (ret < 0 && !gnutls_error_is_fatal(ret));
> +
> +	gnutls_bye(session, GNUTLS_SHUT_RDWR);
> +
> +	gnutls_deinit(session);
> +	gnutls_srp_free_client_credentials(usbip_net_srp_cred);
> +
> +	return ret;
> +}
> +
> +int net_srp_server_handshake(int connfd)
> +{
> +	int ret;
> +	gnutls_session_t session;
> +
> +	if (gnutls_init(&session, GNUTLS_SERVER) != 0)
> +		return -1;
> +	gnutls_priority_set_direct(session, "NORMAL:-KX-ALL:+SRP", NULL);
> +	ret = gnutls_credentials_set(session, GNUTLS_CRD_SRP,
> +				     usbip_net_srp_cred);
> +	if (!ret)
> +		return -1;
> +
> +	gnutls_transport_set_int(session, connfd);
> +
> +	do {
> +		ret = gnutls_handshake(session);
> +	} while (ret < 0 && gnutls_error_is_fatal(ret) == 0);
> +
> +	if (ret < 0)
> +		err("GnuTLS handshake failed (%s)", gnutls_strerror(ret));
> +	else
> +		info("GnuTLS handshake completed");
> +
> +	if (gnutls_bye(session, GNUTLS_SHUT_RDWR) != 0)
> +		err("Unable to shutdown TLS connection.");
> +	gnutls_deinit(session);
> +
> +	return ret;
> +}
> +
> +static int net_srp_callback(gnutls_session_t sess, const char *username,
> +	gnutls_datum_t *nsalt, gnutls_datum_t *nverifier, gnutls_datum_t *g,
> +	gnutls_datum_t *n)
> +{
> +	/*
> +	 * GnuTLS expects us to allocate all data returned from callbacks
> +	 * using gnutls_malloc(), thus, we have to create a fresh copy of
> +	 * our static credentials for every connection.
> +	 */
> +	nsalt->data = gnutls_malloc(usbip_net_srp_salt.size);
> +	nverifier->data = gnutls_malloc(usbip_net_srp_verifier.size);
> +	if (nsalt->data == NULL || nverifier->data == NULL) {

	if (!nsalt->data || !nverifier->data) {

> +		gnutls_free(nsalt->data);
> +		gnutls_free(nverifier->data);
> +		return -1;
> +	}
> +	nsalt->size = usbip_net_srp_salt.size;
> +	nverifier->size = usbip_net_srp_verifier.size;
> +	memcpy(nverifier->data, usbip_net_srp_verifier.data,
> +	       usbip_net_srp_verifier.size);
> +	memcpy(nsalt->data, usbip_net_srp_salt.data, usbip_net_srp_salt.size);
> +
> +	*g = SRP_GROUP;
> +	*n = SRP_PRIME;
> +
> +	/* We only have a single session, thus, ignore it */
> +	(void) sess;
> +
> +	if (strcmp(username, "dummyuser"))
> +		/* User invalid, stored dummy data in g and n. */
> +		return 1;

Could you describe the role of "dummyuser" in a comment? It seems to be
a placeholder for GnuTLS SRP requiring a username.

> +	return 0;
> +}
> +
> +int usbip_net_init_gnutls(void)
> +{
> +	int ret;
> +
> +	gnutls_global_init();
> +
> +	usbip_net_srp_salt.data = gnutls_malloc(16);

This should be freed somewhere.

> +	if (!usbip_net_srp_salt.data)
> +		return GNUTLS_E_MEMORY_ERROR;
> +
> +	ret = gnutls_rnd(GNUTLS_RND_NONCE, usbip_net_srp_salt.data, 16);
> +	if (ret < 0)
> +		return ret;
> +
> +	usbip_net_srp_salt.size = 16;
> +
> +	ret = gnutls_srp_allocate_server_credentials(&usbip_net_srp_cred);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = gnutls_srp_verifier("dummyuser", optarg, &usbip_net_srp_salt,
> +				  &SRP_GROUP, &SRP_PRIME,
> +				  &usbip_net_srp_verifier);
> +	if (ret < 0)
> +		return ret;
> +
> +	gnutls_srp_set_server_credentials_function(usbip_net_srp_cred,
> +						   net_srp_callback);
> +
> +	return GNUTLS_E_SUCCESS;
> +}
> +#endif
> +
> +/*
> + * Connect to the server. Performs the TCP connection attempt
> + * and - if necessary - the TLS handshake used for authentication.
> + */
> +int usbip_net_connect(char *hostname)
> +{
> +	int sockfd;
> +
> +	sockfd = usbip_net_tcp_connect(hostname, usbip_port_string);
> +	if (sockfd < 0)
> +		return sockfd;
> +
> +#ifdef HAVE_GNUTLS
> +	if (usbip_srp_password) {
> +		int rc;
> +
> +		rc = usbip_net_send_op_common(sockfd, OP_REQ_STARTTLS, 0);
> +		if (rc < 0) {
> +			err("usbip_net_send_op_common failed");
> +			return EAI_SYSTEM;
> +		}
> +
> +		rc = usbip_net_srp_client_handshake(sockfd);
> +		if (rc < 0) {
> +			err("Unable to perform TLS handshake (wrong password?): %s",
> +			    gnutls_strerror(rc));
> +			close(sockfd);
> +			return EAI_SYSTEM;
> +		}
> +	}
> +#endif
> +
> +	return sockfd;
> +}
> diff --git a/tools/usb/usbip/src/usbip_network.h b/tools/usb/usbip/src/usbip_network.h
> index c1e875c..292d584 100644
> --- a/tools/usb/usbip/src/usbip_network.h
> +++ b/tools/usb/usbip/src/usbip_network.h
> @@ -15,6 +15,7 @@
>  
>  extern int usbip_port;
>  extern char *usbip_port_string;
> +extern char *usbip_srp_password;
>  void usbip_setup_port_number(char *arg);
>  
>  /* ---------------------------------------------------------------------- */
> @@ -167,6 +168,12 @@ struct op_devlist_reply_extra {
>  	usbip_net_pack_uint32_t(pack, &(reply)->ndev);\
>  } while (0)
>  
> +/* ---------------------------------------------------------------------- */
> +/* Initiate encrypted connection. */
> +#define OP_STARTTLS 0x06
> +#define OP_REQ_STARTTLS   (OP_REQUEST | OP_STARTTLS)
> +#define OP_REP_STARTTLS   (OP_REPLY   | OP_STARTTLS)
> +
>  void usbip_net_pack_uint32_t(int pack, uint32_t *num);
>  void usbip_net_pack_uint16_t(int pack, uint16_t *num);
>  void usbip_net_pack_usb_device(int pack, struct usbip_usb_device *udev);
> @@ -180,6 +187,8 @@ int usbip_net_set_reuseaddr(int sockfd);
>  int usbip_net_set_nodelay(int sockfd);
>  int usbip_net_set_keepalive(int sockfd);
>  int usbip_net_set_v6only(int sockfd);
> -int usbip_net_tcp_connect(char *hostname, char *port);
> +int usbip_net_connect(char *hostname);
> +int net_srp_server_handshake(int connfd);
> +int usbip_net_init_gnutls(void);
>  
>  #endif /* __USBIP_NETWORK_H */
> diff --git a/tools/usb/usbip/src/usbipd.c b/tools/usb/usbip/src/usbipd.c
> index 2f87f2d..23c15d7 100644
> --- a/tools/usb/usbip/src/usbipd.c
> +++ b/tools/usb/usbip/src/usbipd.c
> @@ -36,6 +36,11 @@
>  #include <tcpd.h>
>  #endif
>  
> +#ifdef HAVE_GNUTLS
> +#include <gnutls/gnutls.h>
> +#include <gnutls/crypto.h>
> +#endif
> +
>  #include <getopt.h>
>  #include <signal.h>
>  #include <poll.h>
> @@ -64,6 +69,11 @@ static const char usbipd_help_string[] =
>  	"	-6, --ipv6\n"
>  	"		Bind to IPv6. Default is both.\n"
>  	"\n"
> +#ifdef HAVE_GNUTLS
> +	"	-sPASSWORD, --auth PASSWORD\n"
> +	"		Set PASSWORD as key used for authentication.\n"
> +	"\n"
> +#endif
>  	"	-D, --daemon\n"
>  	"		Run as a daemon process.\n"
>  	"\n"
> @@ -83,6 +93,8 @@ static const char usbipd_help_string[] =
>  	"	-v, --version\n"
>  	"		Show version.\n";
>  
> +static int need_auth;
> +
>  static void usbipd_help(void)
>  {
>  	printf("%s\n", usbipd_help_string);
> @@ -239,14 +251,7 @@ static int recv_request_devlist(int connfd)
>  
>  static int recv_pdu(int connfd)
>  {
> -	uint16_t code = OP_UNSPEC;
> -	int ret;
> -
> -	ret = usbip_net_recv_op_common(connfd, &code);
> -	if (ret < 0) {
> -		dbg("could not receive opcode: %#0x", code);
> -		return -1;
> -	}
> +	int auth = !need_auth, cont = 1, ret;
>  	ret = usbip_host_refresh_device_list();
>  	if (ret < 0) {
> @@ -254,25 +259,63 @@ static int recv_pdu(int connfd)
>  		return -1;
>  	}
>  
> -	info("received request: %#0x(%d)", code, connfd);
> -	switch (code) {
> -	case OP_REQ_DEVLIST:
> -		ret = recv_request_devlist(connfd);
> -		break;
> -	case OP_REQ_IMPORT:
> -		ret = recv_request_import(connfd);
> -		break;
> -	case OP_REQ_DEVINFO:
> -	case OP_REQ_CRYPKEY:
> -	default:
> -		err("received an unknown opcode: %#0x", code);
> -		ret = -1;
> -	}
> +	/*
> +	 * Process opcodes. We might receive more than one, as the
> +	 * client might send STARTTLS first
> +	 */
> +	while (cont) {
> +		uint16_t code = OP_UNSPEC;
>  
> -	if (ret == 0)
> -		info("request %#0x(%d): complete", code, connfd);
> -	else
> -		info("request %#0x(%d): failed", code, connfd);
> +		ret = usbip_net_recv_op_common(connfd, &code);
> +		if (ret < 0) {
> +			dbg("could not receive opcode: %#0x", code);
> +			return -1;
> +		}
> +
> +		info("received request: %#0x(%d)", code, connfd);
> +
> +		/* We require an authenticated encryption */
> +		if (!auth && code != OP_REQ_STARTTLS) {
> +			usbip_net_send_op_common(connfd, OP_REPLY, ST_NA);
> +			return -1;
> +		}
> +
> +		switch (code) {
> +#ifdef HAVE_GNUTLS
> +		case OP_REQ_STARTTLS:
> +			if (!need_auth) {
> +				ret = -1;
> +				err("Unexpected TLS handshake attempt (client uses password, server doesn't)");
> +			} else {
> +				ret = net_srp_server_handshake(connfd);
> +				if (ret != 0)
> +					err("TLS handshake failed");
> +				auth = 1;

This marks the connection as authenticated even if the handshake has
failed, which doesn't seem right.

The err() used here is a macro that merely logs the error and moves
on. It does not exit() like the err.h err() functions.

So this should probably be

> +				if (ret != 0)
> +					err("TLS handshake failed");
				else
> +					auth = 1;

instead?

> +			}
> +			break;
> +#endif
> +		case OP_REQ_DEVLIST:
> +			ret = recv_request_devlist(connfd);
> +			cont = 0;
> +			break;
> +		case OP_REQ_IMPORT:
> +			ret = recv_request_import(connfd);
> +			cont = 0;
> +			break;
> +		case OP_REQ_DEVINFO:
> +		case OP_REQ_CRYPKEY:
> +		default:
> +			err("received an unknown opcode: %#0x", code);
> +			ret = -1;
> +		}
> +
> +		if (ret == 0)
> +			info("request %#0x(%d): complete", code, connfd);
> +		else {
> +			info("request %#0x(%d): failed", code, connfd);
> +			break;
> +		}
> +	}
>  
>  	return ret;
>  }
> @@ -593,6 +636,7 @@ int main(int argc, char *argv[])
>  		{ "tcp-port", required_argument, NULL, 't' },
>  		{ "help",     no_argument,       NULL, 'h' },
>  		{ "version",  no_argument,       NULL, 'v' },
> +		{ "auth",     required_argument, NULL, 's' },
>  		{ NULL,	      0,                 NULL,  0  }
>  	};
>  
> @@ -605,6 +649,9 @@ int main(int argc, char *argv[])
>  	int daemonize = 0;
>  	int ipv4 = 0, ipv6 = 0;
>  	int opt, rc = -1;
> +#ifdef HAVE_GNUTLS
> +	int ret;
> +#endif
>  
>  	pid_file = NULL;
>  
> @@ -616,7 +663,7 @@ int main(int argc, char *argv[])
>  
>  	cmd = cmd_standalone_mode;
>  	for (;;) {
> -		opt = getopt_long(argc, argv, "46DdP::t:hv", longopts, NULL);
> +		opt = getopt_long(argc, argv, "46s:DdP::t:hv", longopts, NULL);
>  
>  		if (opt == -1)
>  			break;
> @@ -628,6 +675,20 @@ int main(int argc, char *argv[])
>  		case '6':
>  			ipv6 = 1;
>  			break;
> +		case 's':
> +#ifdef HAVE_GNUTLS
> +			need_auth = 1;
> +			ret = usbip_net_init_gnutls();
> +			if (ret < 0) {
> +				err("Unable to initialize GnuTLS: %s",
> +				    gnutls_strerror(ret));
> +				return EXIT_FAILURE;
> +			}
> +			break;

Same comment as above (move outside getopt loop).

	Max
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