[Openvpn-devel,v3] Add per session pseudo-random jitter to --reneg-sec intervals

Message ID 20171111134758.7373-1-steffan@karger.me
State Superseded
Headers show
Series
  • [Openvpn-devel,v3] Add per session pseudo-random jitter to --reneg-sec intervals
Related show

Commit Message

Steffan Karger Nov. 11, 2017, 1:47 p.m.
From: Simon Matter <simon.matter@invoca.ch>

While we were suffering from the "TLS Renegotiation Slowdown" bug here
https://community.openvpn.net/openvpn/ticket/854 we realized that there is
still room for improvement in our use case.

It appears that TLS renegotiation is getting more and more expensive in
terms of CPU cycles with recent changes for more security. To make things
worse, we realized that most renegotiation procedures took place at almost
the same time and increased the CPU load too much during these periods.
That's especially true on large, multi-instance openvpn setups.

I've created attached patch to add a per session pseudo-random component to
the --reneg-sec intervals so that renegotiation is evenly spread over time.
It is configured by simply adding a second value to --reneg-sec as
described in the --help text:

--reneg-sec max [min] : Renegotiate data chan. key after at most max
                  (default=3600) and at least min (default 90% of max on
                  servers and equal to max on clients).

The jitter is only enabled by default on servers, because the actual reneg
time is min(reneg_server, reneg_client).  Introducing jitter at both ends
would bias the actual reneg time to the min value.

Note that the patch also slightly changes the log output to show the sec
value in the same way as the bytes/pkts values:

TLS: soft reset sec=3084/3084 bytes=279897/-1 pkts=1370/0

The idea and first versions of this patch are from Simon Matter.  Steffan
Karger later incorporated the mailing list comments into this patch.  So
credits go to Simon, and all bugs are Steffan's fault ;-)

Signed-off-by: Simon Matter <simon.matter@invoca.ch>
Signed-off-by: Steffan Karger <steffan@karger.me>
---
v3: incorporate comments from openvpn-devel@, don't add jitter by
    default on the client side.

 doc/openvpn.8         | 26 +++++++++++++++++++++-----
 src/openvpn/init.c    | 15 ++++++++++++++-
 src/openvpn/options.c | 11 +++++++++--
 src/openvpn/options.h |  1 +
 src/openvpn/ssl.c     |  6 +++---
 5 files changed, 48 insertions(+), 11 deletions(-)

Comments

Simon Ruderich Nov. 12, 2017, 8:21 p.m. | #1
On Sat, Nov 11, 2017 at 02:47:58PM +0100, Steffan Karger wrote:
> --- a/src/openvpn/init.c
> +++ b/src/openvpn/init.c
> @@ -2693,7 +2693,20 @@ do_init_crypto_tls(struct context *c, const unsigned int flags)
>      to.packet_timeout = options->tls_timeout;
>      to.renegotiate_bytes = options->renegotiate_bytes;
>      to.renegotiate_packets = options->renegotiate_packets;
> -    to.renegotiate_seconds = options->renegotiate_seconds;
> +    if (options->renegotiate_seconds_min < 0)
> +    {
> +        /* Add 10% jitter to the reneg-sec of each connection by default */

Maybe change that to "of each server connection".

> +        int auto_jitter = options->mode != MODE_SERVER ? 0 :
> +                get_random() % max_int(options->renegotiate_seconds / 10, 1);
> +        to.renegotiate_seconds = options->renegotiate_seconds - auto_jitter;
> +    }
> +    else
> +    {
> +        /* Add user-specific jitter to the renge-sec of each connection */

s/renge/reneg/

> +        to.renegotiate_seconds = options->renegotiate_seconds -
> +                (get_random() % max_int(options->renegotiate_seconds
> +                                        - options->renegotiate_seconds_min, 1));
> +    }
>      to.single_session = options->single_session;
>      to.mode = options->mode;
>      to.pull = options->pull;

Regards
Simon
Simon Matter Nov. 13, 2017, 7:03 a.m. | #2
Hi Steffan,

Thanks for taking the time to improve this!

Regards,
Simon

> From: Simon Matter <simon.matter@invoca.ch>
>
> While we were suffering from the "TLS Renegotiation Slowdown" bug here
> https://community.openvpn.net/openvpn/ticket/854 we realized that there is
> still room for improvement in our use case.
>
> It appears that TLS renegotiation is getting more and more expensive in
> terms of CPU cycles with recent changes for more security. To make things
> worse, we realized that most renegotiation procedures took place at almost
> the same time and increased the CPU load too much during these periods.
> That's especially true on large, multi-instance openvpn setups.
>
> I've created attached patch to add a per session pseudo-random component
> to
> the --reneg-sec intervals so that renegotiation is evenly spread over
> time.
> It is configured by simply adding a second value to --reneg-sec as
> described in the --help text:
>
> --reneg-sec max [min] : Renegotiate data chan. key after at most max
>                   (default=3600) and at least min (default 90% of max on
>                   servers and equal to max on clients).
>
> The jitter is only enabled by default on servers, because the actual reneg
> time is min(reneg_server, reneg_client).  Introducing jitter at both ends
> would bias the actual reneg time to the min value.
>
> Note that the patch also slightly changes the log output to show the sec
> value in the same way as the bytes/pkts values:
>
> TLS: soft reset sec=3084/3084 bytes=279897/-1 pkts=1370/0
>
> The idea and first versions of this patch are from Simon Matter.  Steffan
> Karger later incorporated the mailing list comments into this patch.  So
> credits go to Simon, and all bugs are Steffan's fault ;-)
>
> Signed-off-by: Simon Matter <simon.matter@invoca.ch>
> Signed-off-by: Steffan Karger <steffan@karger.me>
> ---
> v3: incorporate comments from openvpn-devel@, don't add jitter by
>     default on the client side.
>
>  doc/openvpn.8         | 26 +++++++++++++++++++++-----
>  src/openvpn/init.c    | 15 ++++++++++++++-
>  src/openvpn/options.c | 11 +++++++++--
>  src/openvpn/options.h |  1 +
>  src/openvpn/ssl.c     |  6 +++---
>  5 files changed, 48 insertions(+), 11 deletions(-)
>
> diff --git a/doc/openvpn.8 b/doc/openvpn.8
> index a4189ac2..eb5258f9 100644
> --- a/doc/openvpn.8
> +++ b/doc/openvpn.8
> @@ -33,7 +33,7 @@
>  .\" .ft -- normal face
>  .\" .in +|-{n} -- indent
>  .\"
> -.TH openvpn 8 "25 August 2016"
> +.TH openvpn 8 "04 April 2017"
>  .\"*********************************************************
>  .SH NAME
>  openvpn \- secure IP tunnel daemon.
> @@ -4957,10 +4957,26 @@ Renegotiate data channel key after
>  packets sent and received (disabled by default).
>  .\"*********************************************************
>  .TP
> -.B \-\-reneg\-sec n
> -Renegotiate data channel key after
> -.B n
> -seconds (default=3600).
> +.B \-\-reneg\-sec max [min]
> +Renegotiate data channel key after at most
> +.B max
> +seconds (default=3600) and at least
> +.B min
> +seconds (default is 90% of
> +.B max
> +for servers, and equal to
> +.B max
> +for clients).
> +
> +The effective
> +.B reneg\-sec
> +value used is per session pseudo-uniform-randomized between
> +.B min
> +and
> +.B max\fR.
> +
> +With the default value of 3600 this results in an effective per session
> value
> +in the range of 3240..3600 seconds for servers, or just 3600 for clients.
>
>  When using dual\-factor authentication, note that this default value may
>  cause the end user to be challenged to reauthorize once per hour.
> diff --git a/src/openvpn/init.c b/src/openvpn/init.c
> index 1ed2c55e..0b64930e 100644
> --- a/src/openvpn/init.c
> +++ b/src/openvpn/init.c
> @@ -2693,7 +2693,20 @@ do_init_crypto_tls(struct context *c, const
> unsigned int flags)
>      to.packet_timeout = options->tls_timeout;
>      to.renegotiate_bytes = options->renegotiate_bytes;
>      to.renegotiate_packets = options->renegotiate_packets;
> -    to.renegotiate_seconds = options->renegotiate_seconds;
> +    if (options->renegotiate_seconds_min < 0)
> +    {
> +        /* Add 10% jitter to the reneg-sec of each connection by default
> */
> +        int auto_jitter = options->mode != MODE_SERVER ? 0 :
> +                get_random() % max_int(options->renegotiate_seconds / 10,
> 1);
> +        to.renegotiate_seconds = options->renegotiate_seconds -
> auto_jitter;
> +    }
> +    else
> +    {
> +        /* Add user-specific jitter to the renge-sec of each connection
> */
> +        to.renegotiate_seconds = options->renegotiate_seconds -
> +                (get_random() % max_int(options->renegotiate_seconds
> +                                        -
> options->renegotiate_seconds_min, 1));
> +    }
>      to.single_session = options->single_session;
>      to.mode = options->mode;
>      to.pull = options->pull;
> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index 641a26e2..7fe22064 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -603,7 +603,9 @@ static const char usage_message[] =
>      "                  if no ACK from remote within n seconds
> (default=%d).\n"
>      "--reneg-bytes n : Renegotiate data chan. key after n bytes sent and
> recvd.\n"
>      "--reneg-pkts n  : Renegotiate data chan. key after n packets sent
> and recvd.\n"
> -    "--reneg-sec n   : Renegotiate data chan. key after n seconds
> (default=%d).\n"
> +    "--reneg-sec max [min] : Renegotiate data chan. key after at most max
> (default=%d)\n"
> +    "                  and at least min (defaults to 90%% of max on
> servers and equal\n"
> +    "                  to max on clients).\n"
>      "--hand-window n : Data channel key exchange must finalize within n
> seconds\n"
>      "                  of handshake initiation by any peer
> (default=%d).\n"
>      "--tran-window n : Transition window -- old key can live this many
> seconds\n"
> @@ -870,6 +872,7 @@ init_options(struct options *o, const bool init_gc)
>      o->tls_timeout = 2;
>      o->renegotiate_bytes = -1;
>      o->renegotiate_seconds = 3600;
> +    o->renegotiate_seconds_min = -1;
>      o->handshake_window = 60;
>      o->transition_window = 3600;
>      o->ecdh_curve = NULL;
> @@ -7999,10 +8002,14 @@ add_option(struct options *options,
>          VERIFY_PERMISSION(OPT_P_TLS_PARMS);
>          options->renegotiate_packets = positive_atoi(p[1]);
>      }
> -    else if (streq(p[0], "reneg-sec") && p[1] && !p[2])
> +    else if (streq(p[0], "reneg-sec") && p[1] && !p[3])
>      {
>          VERIFY_PERMISSION(OPT_P_TLS_PARMS);
>          options->renegotiate_seconds = positive_atoi(p[1]);
> +        if (p[2])
> +        {
> +            options->renegotiate_seconds_min = positive_atoi(p[2]);
> +        }
>      }
>      else if (streq(p[0], "hand-window") && p[1] && !p[2])
>      {
> diff --git a/src/openvpn/options.h b/src/openvpn/options.h
> index 496c1143..a74dc94d 100644
> --- a/src/openvpn/options.h
> +++ b/src/openvpn/options.h
> @@ -548,6 +548,7 @@ struct options
>      int renegotiate_bytes;
>      int renegotiate_packets;
>      int renegotiate_seconds;
> +    int renegotiate_seconds_min;
>
>      /* Data channel key handshake must finalize
>       * within n seconds of handshake initiation. */
> diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
> index cb94229a..174f13cb 100644
> --- a/src/openvpn/ssl.c
> +++ b/src/openvpn/ssl.c
> @@ -2732,9 +2732,9 @@ tls_process(struct tls_multi *multi,
>                  && ks->n_packets >= session->opt->renegotiate_packets)
>              ||
> (packet_id_close_to_wrapping(&ks->crypto_options.packet_id.send))))
>      {
> -        msg(D_TLS_DEBUG_LOW,
> -            "TLS: soft reset sec=%d bytes=" counter_format "/%d pkts="
> counter_format "/%d",
> -            (int)(ks->established + session->opt->renegotiate_seconds -
> now),
> +        msg(D_TLS_DEBUG_LOW, "TLS: soft reset sec=%lld/%d bytes="
> counter_format
> +            "/%d pkts=" counter_format "/%d",
> +            (now - ks->established), session->opt->renegotiate_seconds,
>              ks->n_bytes, session->opt->renegotiate_bytes,
>              ks->n_packets, session->opt->renegotiate_packets);
>          key_state_soft_reset(session);
> --
> 2.14.1
>



------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
Simon Matter Nov. 15, 2017, 6:50 a.m. | #3
Hi Steffan,

While running your v3 version of the patch I found an issue with the
modified logging. It gives the following error while building

gcc -DHAVE_CONFIG_H -I. -I../.. -I../../include  -I../../include
-I../../src/compat       -DPLUGIN_LIBDIR=\"/usr/lib64/openvpn/plugins\" 
-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions
-fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches  
-m64 -mtune=generic -std=c99 -c -o ssl.o ssl.c
ssl.c: In function 'tls_process':
ssl.c:2735:9: warning: format '%lld' expects argument of type 'long long
int', but argument 3 has type 'time_t' [-Wformat=]
         msg(D_TLS_DEBUG_LOW, "TLS: soft reset sec=%lld/%d bytes="
counter_format
         ^
and the log output is also corrupted

TLS: soft reset sec=14774687501680/336605974
bytes=18446744069414584320/581335 pkts=0/0
TLS: soft reset sec=14787572403571/77375 bytes=18446744069414584320/885
pkts=0/-1080308296
TLS: soft reset sec=14164802145506/6746662
bytes=18446744069414584320/86778 pkts=0/0
TLS: soft reset sec=15264313773538/186529 bytes=18446744069414584320/1108
pkts=0/13935244

Please have a look at the attached v4 patch which fixes it for me. The
output is now

TLS: soft reset sec=3474/3474 bytes=8470454/-1 pkts=108673/0
TLS: soft reset sec=3489/3489 bytes=429623769/-1 pkts=656033/0
TLS: soft reset sec=3517/3517 bytes=89365/-1 pkts=877/0
TLS: soft reset sec=3537/3537 bytes=206142/-1 pkts=1123/0

which seems reasonable to me.
In the patch I've modified the lines below:

         msg(D_TLS_DEBUG_LOW,
-            "TLS: soft reset sec=%d bytes=" counter_format "/%d pkts="
counter_format "/%d",
-            (int)(ks->established + session->opt->renegotiate_seconds -
now),
+            "TLS: soft reset sec=%d/%d bytes=" counter_format "/%d pkts="
counter_format "/%d",
+            (int)(now - ks->established), session->opt->renegotiate_seconds,
             ks->n_bytes, session->opt->renegotiate_bytes,

I'm not a developer and not a git user, so please accept my hand crafted
patch work :-)

Thanks and regards,
Simon

> From: Simon Matter <simon.matter@invoca.ch>
>
> While we were suffering from the "TLS Renegotiation Slowdown" bug here
> https://community.openvpn.net/openvpn/ticket/854 we realized that there is
> still room for improvement in our use case.
>
> It appears that TLS renegotiation is getting more and more expensive in
> terms of CPU cycles with recent changes for more security. To make things
> worse, we realized that most renegotiation procedures took place at almost
> the same time and increased the CPU load too much during these periods.
> That's especially true on large, multi-instance openvpn setups.
>
> I've created attached patch to add a per session pseudo-random component
> to
> the --reneg-sec intervals so that renegotiation is evenly spread over
> time.
> It is configured by simply adding a second value to --reneg-sec as
> described in the --help text:
>
> --reneg-sec max [min] : Renegotiate data chan. key after at most max
>                   (default=3600) and at least min (default 90% of max on
>                   servers and equal to max on clients).
>
> The jitter is only enabled by default on servers, because the actual reneg
> time is min(reneg_server, reneg_client).  Introducing jitter at both ends
> would bias the actual reneg time to the min value.
>
> Note that the patch also slightly changes the log output to show the sec
> value in the same way as the bytes/pkts values:
>
> TLS: soft reset sec=3084/3084 bytes=279897/-1 pkts=1370/0
>
> The idea and first versions of this patch are from Simon Matter.  Steffan
> Karger later incorporated the mailing list comments into this patch.  So
> credits go to Simon, and all bugs are Steffan's fault ;-)
>
> Signed-off-by: Simon Matter <simon.matter@invoca.ch>
> Signed-off-by: Steffan Karger <steffan@karger.me>
> ---
> v3: incorporate comments from openvpn-devel@, don't add jitter by
>     default on the client side.
>
>  doc/openvpn.8         | 26 +++++++++++++++++++++-----
>  src/openvpn/init.c    | 15 ++++++++++++++-
>  src/openvpn/options.c | 11 +++++++++--
>  src/openvpn/options.h |  1 +
>  src/openvpn/ssl.c     |  6 +++---
>  5 files changed, 48 insertions(+), 11 deletions(-)
>
> diff --git a/doc/openvpn.8 b/doc/openvpn.8
> index a4189ac2..eb5258f9 100644
> --- a/doc/openvpn.8
> +++ b/doc/openvpn.8
> @@ -33,7 +33,7 @@
>  .\" .ft -- normal face
>  .\" .in +|-{n} -- indent
>  .\"
> -.TH openvpn 8 "25 August 2016"
> +.TH openvpn 8 "04 April 2017"
>  .\"*********************************************************
>  .SH NAME
>  openvpn \- secure IP tunnel daemon.
> @@ -4957,10 +4957,26 @@ Renegotiate data channel key after
>  packets sent and received (disabled by default).
>  .\"*********************************************************
>  .TP
> -.B \-\-reneg\-sec n
> -Renegotiate data channel key after
> -.B n
> -seconds (default=3600).
> +.B \-\-reneg\-sec max [min]
> +Renegotiate data channel key after at most
> +.B max
> +seconds (default=3600) and at least
> +.B min
> +seconds (default is 90% of
> +.B max
> +for servers, and equal to
> +.B max
> +for clients).
> +
> +The effective
> +.B reneg\-sec
> +value used is per session pseudo-uniform-randomized between
> +.B min
> +and
> +.B max\fR.
> +
> +With the default value of 3600 this results in an effective per session
> value
> +in the range of 3240..3600 seconds for servers, or just 3600 for clients.
>
>  When using dual\-factor authentication, note that this default value may
>  cause the end user to be challenged to reauthorize once per hour.
> diff --git a/src/openvpn/init.c b/src/openvpn/init.c
> index 1ed2c55e..0b64930e 100644
> --- a/src/openvpn/init.c
> +++ b/src/openvpn/init.c
> @@ -2693,7 +2693,20 @@ do_init_crypto_tls(struct context *c, const
> unsigned int flags)
>      to.packet_timeout = options->tls_timeout;
>      to.renegotiate_bytes = options->renegotiate_bytes;
>      to.renegotiate_packets = options->renegotiate_packets;
> -    to.renegotiate_seconds = options->renegotiate_seconds;
> +    if (options->renegotiate_seconds_min < 0)
> +    {
> +        /* Add 10% jitter to the reneg-sec of each connection by default
> */
> +        int auto_jitter = options->mode != MODE_SERVER ? 0 :
> +                get_random() % max_int(options->renegotiate_seconds / 10,
> 1);
> +        to.renegotiate_seconds = options->renegotiate_seconds -
> auto_jitter;
> +    }
> +    else
> +    {
> +        /* Add user-specific jitter to the renge-sec of each connection
> */
> +        to.renegotiate_seconds = options->renegotiate_seconds -
> +                (get_random() % max_int(options->renegotiate_seconds
> +                                        -
> options->renegotiate_seconds_min, 1));
> +    }
>      to.single_session = options->single_session;
>      to.mode = options->mode;
>      to.pull = options->pull;
> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index 641a26e2..7fe22064 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -603,7 +603,9 @@ static const char usage_message[] =
>      "                  if no ACK from remote within n seconds
> (default=%d).\n"
>      "--reneg-bytes n : Renegotiate data chan. key after n bytes sent and
> recvd.\n"
>      "--reneg-pkts n  : Renegotiate data chan. key after n packets sent
> and recvd.\n"
> -    "--reneg-sec n   : Renegotiate data chan. key after n seconds
> (default=%d).\n"
> +    "--reneg-sec max [min] : Renegotiate data chan. key after at most max
> (default=%d)\n"
> +    "                  and at least min (defaults to 90%% of max on
> servers and equal\n"
> +    "                  to max on clients).\n"
>      "--hand-window n : Data channel key exchange must finalize within n
> seconds\n"
>      "                  of handshake initiation by any peer
> (default=%d).\n"
>      "--tran-window n : Transition window -- old key can live this many
> seconds\n"
> @@ -870,6 +872,7 @@ init_options(struct options *o, const bool init_gc)
>      o->tls_timeout = 2;
>      o->renegotiate_bytes = -1;
>      o->renegotiate_seconds = 3600;
> +    o->renegotiate_seconds_min = -1;
>      o->handshake_window = 60;
>      o->transition_window = 3600;
>      o->ecdh_curve = NULL;
> @@ -7999,10 +8002,14 @@ add_option(struct options *options,
>          VERIFY_PERMISSION(OPT_P_TLS_PARMS);
>          options->renegotiate_packets = positive_atoi(p[1]);
>      }
> -    else if (streq(p[0], "reneg-sec") && p[1] && !p[2])
> +    else if (streq(p[0], "reneg-sec") && p[1] && !p[3])
>      {
>          VERIFY_PERMISSION(OPT_P_TLS_PARMS);
>          options->renegotiate_seconds = positive_atoi(p[1]);
> +        if (p[2])
> +        {
> +            options->renegotiate_seconds_min = positive_atoi(p[2]);
> +        }
>      }
>      else if (streq(p[0], "hand-window") && p[1] && !p[2])
>      {
> diff --git a/src/openvpn/options.h b/src/openvpn/options.h
> index 496c1143..a74dc94d 100644
> --- a/src/openvpn/options.h
> +++ b/src/openvpn/options.h
> @@ -548,6 +548,7 @@ struct options
>      int renegotiate_bytes;
>      int renegotiate_packets;
>      int renegotiate_seconds;
> +    int renegotiate_seconds_min;
>
>      /* Data channel key handshake must finalize
>       * within n seconds of handshake initiation. */
> diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
> index cb94229a..174f13cb 100644
> --- a/src/openvpn/ssl.c
> +++ b/src/openvpn/ssl.c
> @@ -2732,9 +2732,9 @@ tls_process(struct tls_multi *multi,
>                  && ks->n_packets >= session->opt->renegotiate_packets)
>              ||
> (packet_id_close_to_wrapping(&ks->crypto_options.packet_id.send))))
>      {
> -        msg(D_TLS_DEBUG_LOW,
> -            "TLS: soft reset sec=%d bytes=" counter_format "/%d pkts="
> counter_format "/%d",
> -            (int)(ks->established + session->opt->renegotiate_seconds -
> now),
> +        msg(D_TLS_DEBUG_LOW, "TLS: soft reset sec=%lld/%d bytes="
> counter_format
> +            "/%d pkts=" counter_format "/%d",
> +            (now - ks->established), session->opt->renegotiate_seconds,
>              ks->n_bytes, session->opt->renegotiate_bytes,
>              ks->n_packets, session->opt->renegotiate_packets);
>          key_state_soft_reset(session);
> --
> 2.14.1
>
diff -Naur openvpn-2.4.4.orig/doc/openvpn.8 openvpn-2.4.4/doc/openvpn.8
--- openvpn-2.4.4.orig/doc/openvpn.8	2017-09-26 11:27:37.000000000 +0200
+++ openvpn-2.4.4/doc/openvpn.8	2017-11-14 09:10:35.000000000 +0100
@@ -33,7 +33,7 @@
 .\" .ft -- normal face
 .\" .in +|-{n} -- indent
 .\"
-.TH openvpn 8 "25 August 2016"
+.TH openvpn 8 "04 April 2017"
 .\"*********************************************************
 .SH NAME
 openvpn \- secure IP tunnel daemon.
@@ -4958,10 +4958,26 @@
 packets sent and received (disabled by default).
 .\"*********************************************************
 .TP
-.B \-\-reneg\-sec n
-Renegotiate data channel key after
-.B n
-seconds (default=3600).
+.B \-\-reneg\-sec max [min]
+Renegotiate data channel key after at most
+.B max
+seconds (default=3600) and at least
+.B min
+seconds (default is 90% of
+.B max
+for servers, and equal to
+.B max
+for clients).
+
+The effective
+.B reneg\-sec
+value used is per session pseudo-uniform-randomized between
+.B min
+and
+.B max\fR.
+
+With the default value of 3600 this results in an effective per session value
+in the range of 3240..3600 seconds for servers, or just 3600 for clients.
 
 When using dual\-factor authentication, note that this default value may
 cause the end user to be challenged to reauthorize once per hour.
diff -Naur openvpn-2.4.4.orig/src/openvpn/init.c openvpn-2.4.4/src/openvpn/init.c
--- openvpn-2.4.4.orig/src/openvpn/init.c	2017-09-26 11:27:37.000000000 +0200
+++ openvpn-2.4.4/src/openvpn/init.c	2017-11-14 09:10:35.000000000 +0100
@@ -2706,7 +2706,20 @@
     to.packet_timeout = options->tls_timeout;
     to.renegotiate_bytes = options->renegotiate_bytes;
     to.renegotiate_packets = options->renegotiate_packets;
-    to.renegotiate_seconds = options->renegotiate_seconds;
+    if (options->renegotiate_seconds_min < 0)
+    {
+        /* Add 10% jitter to the reneg-sec of each server connection by default */
+        int auto_jitter = options->mode != MODE_SERVER ? 0 :
+                get_random() % max_int(options->renegotiate_seconds / 10, 1);
+        to.renegotiate_seconds = options->renegotiate_seconds - auto_jitter;
+    }
+    else
+    {
+        /* Add user-specific jitter to the reneg-sec of each connection */
+        to.renegotiate_seconds = options->renegotiate_seconds -
+                (get_random() % max_int(options->renegotiate_seconds
+                                        - options->renegotiate_seconds_min, 1));
+    }
     to.single_session = options->single_session;
     to.mode = options->mode;
     to.pull = options->pull;
diff -Naur openvpn-2.4.4.orig/src/openvpn/options.c openvpn-2.4.4/src/openvpn/options.c
--- openvpn-2.4.4.orig/src/openvpn/options.c	2017-09-26 11:27:37.000000000 +0200
+++ openvpn-2.4.4/src/openvpn/options.c	2017-11-14 09:10:35.000000000 +0100
@@ -604,7 +604,9 @@
     "                  if no ACK from remote within n seconds (default=%d).\n"
     "--reneg-bytes n : Renegotiate data chan. key after n bytes sent and recvd.\n"
     "--reneg-pkts n  : Renegotiate data chan. key after n packets sent and recvd.\n"
-    "--reneg-sec n   : Renegotiate data chan. key after n seconds (default=%d).\n"
+    "--reneg-sec max [min] : Renegotiate data chan. key after at most max (default=%d)\n"
+    "                  and at least min (defaults to 90%% of max on servers and equal\n"
+    "                  to max on clients).\n"
     "--hand-window n : Data channel key exchange must finalize within n seconds\n"
     "                  of handshake initiation by any peer (default=%d).\n"
     "--tran-window n : Transition window -- old key can live this many seconds\n"
@@ -872,6 +874,7 @@
     o->tls_timeout = 2;
     o->renegotiate_bytes = -1;
     o->renegotiate_seconds = 3600;
+    o->renegotiate_seconds_min = -1;
     o->handshake_window = 60;
     o->transition_window = 3600;
     o->ecdh_curve = NULL;
@@ -8018,10 +8021,14 @@
         VERIFY_PERMISSION(OPT_P_TLS_PARMS);
         options->renegotiate_packets = positive_atoi(p[1]);
     }
-    else if (streq(p[0], "reneg-sec") && p[1] && !p[2])
+    else if (streq(p[0], "reneg-sec") && p[1] && !p[3])
     {
         VERIFY_PERMISSION(OPT_P_TLS_PARMS);
         options->renegotiate_seconds = positive_atoi(p[1]);
+        if (p[2])
+        {
+            options->renegotiate_seconds_min = positive_atoi(p[2]);
+        }
     }
     else if (streq(p[0], "hand-window") && p[1] && !p[2])
     {
diff -Naur openvpn-2.4.4.orig/src/openvpn/options.h openvpn-2.4.4/src/openvpn/options.h
--- openvpn-2.4.4.orig/src/openvpn/options.h	2017-09-26 11:27:37.000000000 +0200
+++ openvpn-2.4.4/src/openvpn/options.h	2017-11-14 09:10:35.000000000 +0100
@@ -549,6 +549,7 @@
     int renegotiate_bytes;
     int renegotiate_packets;
     int renegotiate_seconds;
+    int renegotiate_seconds_min;
 
     /* Data channel key handshake must finalize
      * within n seconds of handshake initiation. */
diff -Naur openvpn-2.4.4.orig/src/openvpn/ssl.c openvpn-2.4.4/src/openvpn/ssl.c
--- openvpn-2.4.4.orig/src/openvpn/ssl.c	2017-09-26 11:27:37.000000000 +0200
+++ openvpn-2.4.4/src/openvpn/ssl.c	2017-11-14 09:19:03.000000000 +0100
@@ -2733,8 +2733,8 @@
             || (packet_id_close_to_wrapping(&ks->crypto_options.packet_id.send))))
     {
         msg(D_TLS_DEBUG_LOW,
-            "TLS: soft reset sec=%d bytes=" counter_format "/%d pkts=" counter_format "/%d",
-            (int)(ks->established + session->opt->renegotiate_seconds - now),
+            "TLS: soft reset sec=%d/%d bytes=" counter_format "/%d pkts=" counter_format "/%d",
+            (int)(now - ks->established), session->opt->renegotiate_seconds,
             ks->n_bytes, session->opt->renegotiate_bytes,
             ks->n_packets, session->opt->renegotiate_packets);
         key_state_soft_reset(session);
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
Steffan Karger Nov. 15, 2017, 9:13 a.m. | #4
Hi,

On 15 November 2017 at 07:50, Simon Matter <simon.matter@invoca.ch> wrote:
> Hi Steffan,
>
> While running your v3 version of the patch I found an issue with the
> modified logging. It gives the following error while building
>
> gcc -DHAVE_CONFIG_H -I. -I../.. -I../../include  -I../../include
> -I../../src/compat       -DPLUGIN_LIBDIR=\"/usr/lib64/openvpn/plugins\"
> -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions
> -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches
> -m64 -mtune=generic -std=c99 -c -o ssl.o ssl.c
> ssl.c: In function 'tls_process':
> ssl.c:2735:9: warning: format '%lld' expects argument of type 'long long
> int', but argument 3 has type 'time_t' [-Wformat=]
>          msg(D_TLS_DEBUG_LOW, "TLS: soft reset sec=%lld/%d bytes="
> counter_format
>          ^
> and the log output is also corrupted
>
> TLS: soft reset sec=14774687501680/336605974
> bytes=18446744069414584320/581335 pkts=0/0
> TLS: soft reset sec=14787572403571/77375 bytes=18446744069414584320/885
> pkts=0/-1080308296
> TLS: soft reset sec=14164802145506/6746662
> bytes=18446744069414584320/86778 pkts=0/0
> TLS: soft reset sec=15264313773538/186529 bytes=18446744069414584320/1108
> pkts=0/13935244
>
> Please have a look at the attached v4 patch which fixes it for me. The
> output is now
>
> TLS: soft reset sec=3474/3474 bytes=8470454/-1 pkts=108673/0
> TLS: soft reset sec=3489/3489 bytes=429623769/-1 pkts=656033/0
> TLS: soft reset sec=3517/3517 bytes=89365/-1 pkts=877/0
> TLS: soft reset sec=3537/3537 bytes=206142/-1 pkts=1123/0
>
> which seems reasonable to me.
> In the patch I've modified the lines below:
>
>          msg(D_TLS_DEBUG_LOW,
> -            "TLS: soft reset sec=%d bytes=" counter_format "/%d pkts="
> counter_format "/%d",
> -            (int)(ks->established + session->opt->renegotiate_seconds -
> now),
> +            "TLS: soft reset sec=%d/%d bytes=" counter_format "/%d pkts="
> counter_format "/%d",
> +            (int)(now - ks->established), session->opt->renegotiate_seconds,
>              ks->n_bytes, session->opt->renegotiate_bytes,
>
> I'm not a developer and not a git user, so please accept my hand crafted
> patch work :-)

Uch.  Good find.  Since we recently decided that the right way to
print a time_t is to cast to long long and use the %lld format
specifier, I wanted to do that here too. I did the latter, but forgot
the first and didn't notice because on my x64 system time_t actually
*is* a long long (so no compiler warnings).

I'll send a follow-up version that incorporates the comments of both
Simon's, probably tomorrow.

-Steffan

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot

Patch

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index a4189ac2..eb5258f9 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -33,7 +33,7 @@ 
 .\" .ft -- normal face
 .\" .in +|-{n} -- indent
 .\"
-.TH openvpn 8 "25 August 2016"
+.TH openvpn 8 "04 April 2017"
 .\"*********************************************************
 .SH NAME
 openvpn \- secure IP tunnel daemon.
@@ -4957,10 +4957,26 @@  Renegotiate data channel key after
 packets sent and received (disabled by default).
 .\"*********************************************************
 .TP
-.B \-\-reneg\-sec n
-Renegotiate data channel key after
-.B n
-seconds (default=3600).
+.B \-\-reneg\-sec max [min]
+Renegotiate data channel key after at most
+.B max
+seconds (default=3600) and at least
+.B min
+seconds (default is 90% of
+.B max
+for servers, and equal to
+.B max
+for clients).
+
+The effective
+.B reneg\-sec
+value used is per session pseudo-uniform-randomized between
+.B min
+and
+.B max\fR.
+
+With the default value of 3600 this results in an effective per session value
+in the range of 3240..3600 seconds for servers, or just 3600 for clients.
 
 When using dual\-factor authentication, note that this default value may
 cause the end user to be challenged to reauthorize once per hour.
diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index 1ed2c55e..0b64930e 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -2693,7 +2693,20 @@  do_init_crypto_tls(struct context *c, const unsigned int flags)
     to.packet_timeout = options->tls_timeout;
     to.renegotiate_bytes = options->renegotiate_bytes;
     to.renegotiate_packets = options->renegotiate_packets;
-    to.renegotiate_seconds = options->renegotiate_seconds;
+    if (options->renegotiate_seconds_min < 0)
+    {
+        /* Add 10% jitter to the reneg-sec of each connection by default */
+        int auto_jitter = options->mode != MODE_SERVER ? 0 :
+                get_random() % max_int(options->renegotiate_seconds / 10, 1);
+        to.renegotiate_seconds = options->renegotiate_seconds - auto_jitter;
+    }
+    else
+    {
+        /* Add user-specific jitter to the renge-sec of each connection */
+        to.renegotiate_seconds = options->renegotiate_seconds -
+                (get_random() % max_int(options->renegotiate_seconds
+                                        - options->renegotiate_seconds_min, 1));
+    }
     to.single_session = options->single_session;
     to.mode = options->mode;
     to.pull = options->pull;
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 641a26e2..7fe22064 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -603,7 +603,9 @@  static const char usage_message[] =
     "                  if no ACK from remote within n seconds (default=%d).\n"
     "--reneg-bytes n : Renegotiate data chan. key after n bytes sent and recvd.\n"
     "--reneg-pkts n  : Renegotiate data chan. key after n packets sent and recvd.\n"
-    "--reneg-sec n   : Renegotiate data chan. key after n seconds (default=%d).\n"
+    "--reneg-sec max [min] : Renegotiate data chan. key after at most max (default=%d)\n"
+    "                  and at least min (defaults to 90%% of max on servers and equal\n"
+    "                  to max on clients).\n"
     "--hand-window n : Data channel key exchange must finalize within n seconds\n"
     "                  of handshake initiation by any peer (default=%d).\n"
     "--tran-window n : Transition window -- old key can live this many seconds\n"
@@ -870,6 +872,7 @@  init_options(struct options *o, const bool init_gc)
     o->tls_timeout = 2;
     o->renegotiate_bytes = -1;
     o->renegotiate_seconds = 3600;
+    o->renegotiate_seconds_min = -1;
     o->handshake_window = 60;
     o->transition_window = 3600;
     o->ecdh_curve = NULL;
@@ -7999,10 +8002,14 @@  add_option(struct options *options,
         VERIFY_PERMISSION(OPT_P_TLS_PARMS);
         options->renegotiate_packets = positive_atoi(p[1]);
     }
-    else if (streq(p[0], "reneg-sec") && p[1] && !p[2])
+    else if (streq(p[0], "reneg-sec") && p[1] && !p[3])
     {
         VERIFY_PERMISSION(OPT_P_TLS_PARMS);
         options->renegotiate_seconds = positive_atoi(p[1]);
+        if (p[2])
+        {
+            options->renegotiate_seconds_min = positive_atoi(p[2]);
+        }
     }
     else if (streq(p[0], "hand-window") && p[1] && !p[2])
     {
diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index 496c1143..a74dc94d 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -548,6 +548,7 @@  struct options
     int renegotiate_bytes;
     int renegotiate_packets;
     int renegotiate_seconds;
+    int renegotiate_seconds_min;
 
     /* Data channel key handshake must finalize
      * within n seconds of handshake initiation. */
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index cb94229a..174f13cb 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -2732,9 +2732,9 @@  tls_process(struct tls_multi *multi,
                 && ks->n_packets >= session->opt->renegotiate_packets)
             || (packet_id_close_to_wrapping(&ks->crypto_options.packet_id.send))))
     {
-        msg(D_TLS_DEBUG_LOW,
-            "TLS: soft reset sec=%d bytes=" counter_format "/%d pkts=" counter_format "/%d",
-            (int)(ks->established + session->opt->renegotiate_seconds - now),
+        msg(D_TLS_DEBUG_LOW, "TLS: soft reset sec=%lld/%d bytes=" counter_format
+            "/%d pkts=" counter_format "/%d",
+            (now - ks->established), session->opt->renegotiate_seconds,
             ks->n_bytes, session->opt->renegotiate_bytes,
             ks->n_packets, session->opt->renegotiate_packets);
         key_state_soft_reset(session);