[Openvpn-devel,5/6] Change default MTU in server mode to 1420 and push it to client

Message ID 20220621161649.2872985-5-arne@rfc2549.org
State New
Headers show
Series
  • [Openvpn-devel,1/6] Remove leftover frame_set_mtu_dynamic definitions in mtu.h
Related show

Commit Message

Arne Schwabe June 21, 2022, 4:16 p.m.
This changes the default MTU of the tun-mtu to 1420 to avoid MTU related
issues that are even more prominent when DCO server or clients are involved.

To maximise compatibility to lie our MTU in the default OCC message and also
push the real MTU to clients that support pushing the MTU.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 Changes.rst                              |  5 ++++
 doc/man-sections/vpn-network-options.rst | 36 ++++++++++++++++++++----
 src/openvpn/options.c                    | 31 ++++++++++++++++++--
 src/openvpn/options.h                    |  1 +
 src/openvpn/push.c                       | 16 +++++++++++
 5 files changed, 81 insertions(+), 8 deletions(-)

Comments

Frank Lichtenheld June 24, 2022, 8:35 a.m. | #1
Code changes look fine to me, but several documentation suggestions.

On Tue, Jun 21, 2022 at 06:16:48PM +0200, Arne Schwabe wrote:
> This changes the default MTU of the tun-mtu to 1420 to avoid MTU related
> issues that are even more prominent when DCO server or clients are involved.
> 
> To maximise compatibility to lie our MTU in the default OCC message and also
> push the real MTU to clients that support pushing the MTU.
> 
> Signed-off-by: Arne Schwabe <arne@rfc2549.org>
> ---
>  Changes.rst                              |  5 ++++
>  doc/man-sections/vpn-network-options.rst | 36 ++++++++++++++++++++----
>  src/openvpn/options.c                    | 31 ++++++++++++++++++--
>  src/openvpn/options.h                    |  1 +
>  src/openvpn/push.c                       | 16 +++++++++++
>  5 files changed, 81 insertions(+), 8 deletions(-)
> 
> diff --git a/Changes.rst b/Changes.rst
> index 67a23c792..79b79d608 100644
> --- a/Changes.rst
> +++ b/Changes.rst
> @@ -141,6 +141,11 @@ User-visible Changes
>  - Option ``--nobind`` is default when ``--client`` or ``--pull`` is used in the configuration
>  - :code:`link_mtu` parameter is removed from environment or replaced with 0 when scripts are
>    called with parameters. This parameter is unreliable and no longer internally calculated.
> +- the default of ``--tun-mtu`` has been changed to ``--tun 1420 1500`` when

"--tun" -> "--tun-mtu"

> +  running in server mode. This will create an mtu mismatch with older client

"clients"

> +  (newer client allow pushable mtu) but the most common server platforms

"clients"

> +  (Linux and FreeBSD) allow receiving 1500 byte packets even tun-mtu is set
> +  to 1420, still allowing larger packets from client with 1500 byte mtu.

"clients"

>  
>  Overview of changes in 2.5
>  ==========================
> diff --git a/doc/man-sections/vpn-network-options.rst b/doc/man-sections/vpn-network-options.rst
> index 5b2f84707..2e4fff5df 100644
> --- a/doc/man-sections/vpn-network-options.rst
> +++ b/doc/man-sections/vpn-network-options.rst
> @@ -500,21 +500,45 @@ routing.
>    arguments of ``--ifconfig`` to mean "address netmask", no longer "local
>    remote".
>  
> ---tun-mtu n
> -  Take the TUN device MTU to be **n** and derive the link MTU from it
> -  (default :code:`1500`). In most cases, you will probably want to leave
> -  this parameter set to its default value.
> +--tun-mtu args
> +
> +  Valid syntaxes:
> +  ::
> +
> +      tun-mtu tun-mtu
> +      tun-mtu tun-mtu occ-mtu
> +
> +  Take the TUN device MTU to be ``tun-mtu`` and derive the link MTU from it.
> +  In most cases, you will probably want to leave this parameter set to
> +  its default value.
> +
> +  Starting with OpenVPN 2.6 in when running server mode (``--mode server``,

"in when running" -> "when running in"

> +  ``--server`` or ``-server-ipv6`` options present in the configuration),

", or" Oxford comma FTW!

> +  the default will be 1420 for the tun mtu size and 1500 for the ``occ-mtu``.
> +
> +  The OCC MTU can be used to avoid warnings about mismatched MTU from
> +  clients. If :code:`occ-mtu` is not specified, it will to default to the

Maybe explicitely "older clients"?

> +  tun-mtu

Missing full stop.

>  
>    The MTU (Maximum Transmission Units) is the maximum datagram size in
>    bytes that can be sent unfragmented over a particular network path.
>    OpenVPN requires that packets on the control and data channels be sent
>    unfragmented.
>  
> +  It is generally advisable to set the tun MTU low enough that with the
> +  encapsulation overhead is lower than the MTU of the network that is used
> +  to transport the VPN packets.

I know what you want to say ;) But as written it makes no sense. Maybe
trying another way:

Note that the tun MTU must be lower than the MTU of the actual network that is
used to transport the VPN packets. That is due to the additional encapsulation
applied as part of the OpenVPN protocol. The default is chosen to be safe
with a network MTU of 1500 independently of the used OpenVPN options (which
lead to slightly different encapsulation overhead).

> +
>    MTU problems often manifest themselves as connections which hang during
>    periods of active usage.
>  
> -  It's best to use the ``--fragment`` and/or ``--mssfix`` options to deal
> -  with MTU sizing issues.
> +  If lowering the tun MTU to avoid MTU related problems (e.g. when tap is used
> +  and an MTU of 1500 is rdquired), the ``--fragment`` and/or ``--mssfix``

"required"

> +  options can be also used to deal with MTU sizing issues.
> +
> +  Note: Depending on the platform, the operating system allows to receive
> +  packets larger than ``tun-mtu`` (e.g. Linux and FreeBSD) but other platforms
> +  (like macOS) limit received packets to the same size as the MTU.
>  
>  --tun-mtu-extra n
>    Assume that the TUN/TAP device might return as many as ``n`` bytes more
> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index 9a0634a5e..69c839fb6 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -814,6 +814,7 @@ init_options(struct options *o, const bool init_gc)
>      o->status_file_version = 1;
>      o->ce.bind_local = true;
>      o->ce.tun_mtu = TUN_MTU_DEFAULT;
> +    o->ce.occ_mtu = 0;
>      o->ce.link_mtu = LINK_MTU_DEFAULT;
>      o->ce.mtu_discover_type = -1;
>      o->ce.mssfix = 0;
> @@ -3031,6 +3032,16 @@ options_postprocess_mutate_ce(struct options *o, struct connection_entry *ce)
>          if (!ce->tun_mtu_defined && !ce->link_mtu_defined)
>          {
>              ce->tun_mtu_defined = true;
> +            if (o->mode == MODE_SERVER)
> +            {
> +                /* If we are running in P2MP mode we default to a MTU
> +                 * that is low enough by default to fit into a 1492
> +                 * MTU UDP IPv6 packet:
> +                 *
> +                 */
> +                ce->tun_mtu = frame_calculate_default_mtu(o);
> +                ce->occ_mtu = TUN_MTU_DEFAULT;
> +            }
>          }
>          if ((dev == DEV_TYPE_TAP) && !ce->tun_mtu_extra_defined)
>          {
> @@ -4018,7 +4029,15 @@ options_string(const struct options *o,
>      buf_printf(&out, ",link-mtu %u",
>                 (unsigned int) calc_options_string_link_mtu(o, frame));
>  
> -    buf_printf(&out, ",tun-mtu %d", frame->tun_mtu);
> +    if (o->ce.occ_mtu != 0)
> +    {
> +        buf_printf(&out, ",tun-mtu %d", o->ce.occ_mtu);
> +    }
> +    else
> +    {
> +        buf_printf(&out, ",tun-mtu %d", frame->tun_mtu);
> +    }
> +
>      buf_printf(&out, ",proto %s",  proto_remote(o->ce.proto, remote));
>  
>      bool p2p_nopull = o->mode == MODE_POINT_TO_POINT && !PULL_DEFINED(o);
> @@ -6262,11 +6281,19 @@ add_option(struct options *options,
>          options->ce.link_mtu = positive_atoi(p[1]);
>          options->ce.link_mtu_defined = true;
>      }
> -    else if (streq(p[0], "tun-mtu") && p[1] && !p[2])
> +    else if (streq(p[0], "tun-mtu") && p[1] && !p[3])
>      {
>          VERIFY_PERMISSION(OPT_P_MTU|OPT_P_CONNECTION);
>          options->ce.tun_mtu = positive_atoi(p[1]);
>          options->ce.tun_mtu_defined = true;
> +        if (p[2])
> +        {
> +            options->ce.occ_mtu = positive_atoi(p[2]);
> +        }
> +        else
> +        {
> +            options->ce.occ_mtu = 0;
> +        }
>      }
>      else if (streq(p[0], "tun-mtu-extra") && p[1] && !p[2])
>      {
> diff --git a/src/openvpn/options.h b/src/openvpn/options.h
> index c2937dc37..1085a462a 100644
> --- a/src/openvpn/options.h
> +++ b/src/openvpn/options.h
> @@ -118,6 +118,7 @@ struct connection_entry
>      const char *socks_proxy_authfile;
>  
>      int tun_mtu;         /* MTU of tun device */
> +    int occ_mtu;         /* if non-null, this is the MTU we announce to peers in OCC */
>      bool tun_mtu_defined; /* true if user overriding parm with command line option */
>      int tun_mtu_extra;
>      bool tun_mtu_extra_defined;
> diff --git a/src/openvpn/push.c b/src/openvpn/push.c
> index 63257348a..463957a82 100644
> --- a/src/openvpn/push.c
> +++ b/src/openvpn/push.c
> @@ -603,6 +603,22 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
>      {
>          push_option_fmt(gc, push_list, M_USAGE, "key-derivation tls-ekm");
>      }
> +
> +    /* Push our mtu to the peer if it supports pushable MTUs */
> +    int client_max_mtu = 0;
> +    const char *iv_mtu = extract_var_peer_info(tls_multi->peer_info, "IV_MTU=", gc);
> +
> +    if (iv_mtu &&  sscanf(iv_mtu, "%d", &client_max_mtu) == 1)

random space before sscanf

> +    {
> +        push_option_fmt(gc, push_list, M_USAGE, "tun-mtu %d", o->ce.tun_mtu);
> +        if (client_max_mtu < o->ce.tun_mtu)
> +        {
> +            msg(M_WARN, "Warning reported maximum MTU from client (%d) is lower "

Missing colon after "Warning".

> +                "than MTU used on the server (%d). Add tun-max-mtu %d "
> +                "to client configuration.", client_max_mtu,
> +                o->ce.tun_mtu, o->ce.tun_mtu);
> +        }
> +    }
>      return true;
>  }
Frank Lichtenheld June 24, 2022, 8:38 a.m. | #2
On Fri, Jun 24, 2022 at 10:35:24AM +0200, Frank Lichtenheld wrote:
> Code changes look fine to me, but several documentation suggestions.

Hmm, after reading the next patch might I suggest to move the
change to push.c actually to the next patch? Both references
to IV_MTU and tun-max-mtu only make sense then, right?
 
> On Tue, Jun 21, 2022 at 06:16:48PM +0200, Arne Schwabe wrote:
> > diff --git a/src/openvpn/push.c b/src/openvpn/push.c
> > index 63257348a..463957a82 100644
> > --- a/src/openvpn/push.c
> > +++ b/src/openvpn/push.c
> > @@ -603,6 +603,22 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
> >      {
> >          push_option_fmt(gc, push_list, M_USAGE, "key-derivation tls-ekm");
> >      }
> > +
> > +    /* Push our mtu to the peer if it supports pushable MTUs */
> > +    int client_max_mtu = 0;
> > +    const char *iv_mtu = extract_var_peer_info(tls_multi->peer_info, "IV_MTU=", gc);
> > +
> > +    if (iv_mtu &&  sscanf(iv_mtu, "%d", &client_max_mtu) == 1)
> 
> random space before sscanf
> 
> > +    {
> > +        push_option_fmt(gc, push_list, M_USAGE, "tun-mtu %d", o->ce.tun_mtu);
> > +        if (client_max_mtu < o->ce.tun_mtu)
> > +        {
> > +            msg(M_WARN, "Warning reported maximum MTU from client (%d) is lower "
> 
> Missing colon after "Warning".
> 
> > +                "than MTU used on the server (%d). Add tun-max-mtu %d "
> > +                "to client configuration.", client_max_mtu,
> > +                o->ce.tun_mtu, o->ce.tun_mtu);
> > +        }
> > +    }
> >      return true;
> >  }
> 
> 
> _______________________________________________
> Openvpn-devel mailing list
> Openvpn-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/openvpn-devel
Arne Schwabe June 24, 2022, 10:25 a.m. | #3
Am 24.06.22 um 10:38 schrieb Frank Lichtenheld:
> On Fri, Jun 24, 2022 at 10:35:24AM +0200, Frank Lichtenheld wrote:
>> Code changes look fine to me, but several documentation suggestions.
> 
> Hmm, after reading the next patch might I suggest to move the
> change to push.c actually to the next patch? Both references
> to IV_MTU and tun-max-mtu only make sense then, right?

Well basically this implements the server side functionality while the 
next patch implements the client side. And the changes to push.c are 
server side. I could also reorder these two to first introduce the 
client side support and then make use of it.

Arne
Gert Doering June 24, 2022, 10:35 a.m. | #4
Hi,

On Tue, Jun 21, 2022 at 06:16:48PM +0200, Arne Schwabe wrote:
> This changes the default MTU of the tun-mtu to 1420 to avoid MTU related
> issues that are even more prominent when DCO server or clients are involved.

I'm not convinced that this change "by default" is a desirable change.

Being able to do that (pushable MTU) is good, but changing the default is
fairly fundamental, and it will break use cases that rely on 1500 inner
MTU (and outside fragmentation). 

Originally, it was said that "with DCO, outside fragmentation is not
possible", but if I remember right, this turned out to be incorrect,
and implementations handle outside fragmentation just fine.  So there
seems to be no hard pressing need to change the default (I am willing
to be corrected if I misremember something).

Not saying that outside fragmentation is *desirable*, so for TCP, 
"mssfix 1492 mtu" will (already) ensure we have no fragmentation
issues - whatever tun-mtu we have.

gert
Arne Schwabe June 24, 2022, 10:52 a.m. | #5
Am 24.06.22 um 12:35 schrieb Gert Doering:
> Hi,
> 
> On Tue, Jun 21, 2022 at 06:16:48PM +0200, Arne Schwabe wrote:
>> This changes the default MTU of the tun-mtu to 1420 to avoid MTU related
>> issues that are even more prominent when DCO server or clients are involved.
> 
> I'm not convinced that this change "by default" is a desirable change.
> 
> Being able to do that (pushable MTU) is good, but changing the default is
> fairly fundamental, and it will break use cases that rely on 1500 inner
> MTU (and outside fragmentation).
> 
> Originally, it was said that "with DCO, outside fragmentation is not
> possible", but if I remember right, this turned out to be incorrect,
> and implementations handle outside fragmentation just fine.  So there
> seems to be no hard pressing need to change the default (I am willing
> to be corrected if I misremember something).
> 
> Not saying that outside fragmentation is *desirable*, so for TCP,
> "mssfix 1492 mtu" will (already) ensure we have no fragmentation
> issues - whatever tun-mtu we have.

I understand that this is quite fundamental change but I think 
alternative of always using a wrong MTU and hoping that mssfix and 
outside fragmentation are going to save the day what we currently do, is 
lying to ourselves.

I still think this is a reasonable change. Yes, it might break in some 
very obscure setups but for those setups, people can still set the MTU 
back to 1500. Tap still uses the 1500 default anyway.

Arne
Gert Doering June 24, 2022, 11:16 a.m. | #6
Hi,

On Fri, Jun 24, 2022 at 12:52:23PM +0200, Arne Schwabe wrote:
> I still think this is a reasonable change. Yes, it might break in some 
> very obscure setups but for those setups, people can still set the MTU 
> back to 1500. Tap still uses the 1500 default anyway.

It will break all setups that have openvpn "in the path" and something
else that throws away ICMP fragmentation required messages.

Like

  Host A ---(1500)--> OpenVPN --(1400)--> Host B

if Host A sends a 1500 byte packet with DF bit (or IPv6), the network
stack on the OpenVPN server needs to return an ICMP "packet too big"
message.  If that gets lost - and there are too many folks that do throw
away ICMP packets - we have a black hole.


We see this in commercial VPN setups as well - having a large-enough
MTU for the occasional UDP (or whatever else) packet, and ensuring TCP
packets are of the desirable packet size by means of MSS manipulation
is more robust.

So, still NAK on changing the default.  Turning your argument around: 
whoever things they need this can enable this on their own.

gert

Patch

diff --git a/Changes.rst b/Changes.rst
index 67a23c792..79b79d608 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -141,6 +141,11 @@  User-visible Changes
 - Option ``--nobind`` is default when ``--client`` or ``--pull`` is used in the configuration
 - :code:`link_mtu` parameter is removed from environment or replaced with 0 when scripts are
   called with parameters. This parameter is unreliable and no longer internally calculated.
+- the default of ``--tun-mtu`` has been changed to ``--tun 1420 1500`` when
+  running in server mode. This will create an mtu mismatch with older client
+  (newer client allow pushable mtu) but the most common server platforms
+  (Linux and FreeBSD) allow receiving 1500 byte packets even tun-mtu is set
+  to 1420, still allowing larger packets from client with 1500 byte mtu.
 
 Overview of changes in 2.5
 ==========================
diff --git a/doc/man-sections/vpn-network-options.rst b/doc/man-sections/vpn-network-options.rst
index 5b2f84707..2e4fff5df 100644
--- a/doc/man-sections/vpn-network-options.rst
+++ b/doc/man-sections/vpn-network-options.rst
@@ -500,21 +500,45 @@  routing.
   arguments of ``--ifconfig`` to mean "address netmask", no longer "local
   remote".
 
---tun-mtu n
-  Take the TUN device MTU to be **n** and derive the link MTU from it
-  (default :code:`1500`). In most cases, you will probably want to leave
-  this parameter set to its default value.
+--tun-mtu args
+
+  Valid syntaxes:
+  ::
+
+      tun-mtu tun-mtu
+      tun-mtu tun-mtu occ-mtu
+
+  Take the TUN device MTU to be ``tun-mtu`` and derive the link MTU from it.
+  In most cases, you will probably want to leave this parameter set to
+  its default value.
+
+  Starting with OpenVPN 2.6 in when running server mode (``--mode server``,
+  ``--server`` or ``-server-ipv6`` options present in the configuration),
+  the default will be 1420 for the tun mtu size and 1500 for the ``occ-mtu``.
+
+  The OCC MTU can be used to avoid warnings about mismatched MTU from
+  clients. If :code:`occ-mtu` is not specified, it will to default to the
+  tun-mtu
 
   The MTU (Maximum Transmission Units) is the maximum datagram size in
   bytes that can be sent unfragmented over a particular network path.
   OpenVPN requires that packets on the control and data channels be sent
   unfragmented.
 
+  It is generally advisable to set the tun MTU low enough that with the
+  encapsulation overhead is lower than the MTU of the network that is used
+  to transport the VPN packets.
+
   MTU problems often manifest themselves as connections which hang during
   periods of active usage.
 
-  It's best to use the ``--fragment`` and/or ``--mssfix`` options to deal
-  with MTU sizing issues.
+  If lowering the tun MTU to avoid MTU related problems (e.g. when tap is used
+  and an MTU of 1500 is rdquired), the ``--fragment`` and/or ``--mssfix``
+  options can be also used to deal with MTU sizing issues.
+
+  Note: Depending on the platform, the operating system allows to receive
+  packets larger than ``tun-mtu`` (e.g. Linux and FreeBSD) but other platforms
+  (like macOS) limit received packets to the same size as the MTU.
 
 --tun-mtu-extra n
   Assume that the TUN/TAP device might return as many as ``n`` bytes more
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 9a0634a5e..69c839fb6 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -814,6 +814,7 @@  init_options(struct options *o, const bool init_gc)
     o->status_file_version = 1;
     o->ce.bind_local = true;
     o->ce.tun_mtu = TUN_MTU_DEFAULT;
+    o->ce.occ_mtu = 0;
     o->ce.link_mtu = LINK_MTU_DEFAULT;
     o->ce.mtu_discover_type = -1;
     o->ce.mssfix = 0;
@@ -3031,6 +3032,16 @@  options_postprocess_mutate_ce(struct options *o, struct connection_entry *ce)
         if (!ce->tun_mtu_defined && !ce->link_mtu_defined)
         {
             ce->tun_mtu_defined = true;
+            if (o->mode == MODE_SERVER)
+            {
+                /* If we are running in P2MP mode we default to a MTU
+                 * that is low enough by default to fit into a 1492
+                 * MTU UDP IPv6 packet:
+                 *
+                 */
+                ce->tun_mtu = frame_calculate_default_mtu(o);
+                ce->occ_mtu = TUN_MTU_DEFAULT;
+            }
         }
         if ((dev == DEV_TYPE_TAP) && !ce->tun_mtu_extra_defined)
         {
@@ -4018,7 +4029,15 @@  options_string(const struct options *o,
     buf_printf(&out, ",link-mtu %u",
                (unsigned int) calc_options_string_link_mtu(o, frame));
 
-    buf_printf(&out, ",tun-mtu %d", frame->tun_mtu);
+    if (o->ce.occ_mtu != 0)
+    {
+        buf_printf(&out, ",tun-mtu %d", o->ce.occ_mtu);
+    }
+    else
+    {
+        buf_printf(&out, ",tun-mtu %d", frame->tun_mtu);
+    }
+
     buf_printf(&out, ",proto %s",  proto_remote(o->ce.proto, remote));
 
     bool p2p_nopull = o->mode == MODE_POINT_TO_POINT && !PULL_DEFINED(o);
@@ -6262,11 +6281,19 @@  add_option(struct options *options,
         options->ce.link_mtu = positive_atoi(p[1]);
         options->ce.link_mtu_defined = true;
     }
-    else if (streq(p[0], "tun-mtu") && p[1] && !p[2])
+    else if (streq(p[0], "tun-mtu") && p[1] && !p[3])
     {
         VERIFY_PERMISSION(OPT_P_MTU|OPT_P_CONNECTION);
         options->ce.tun_mtu = positive_atoi(p[1]);
         options->ce.tun_mtu_defined = true;
+        if (p[2])
+        {
+            options->ce.occ_mtu = positive_atoi(p[2]);
+        }
+        else
+        {
+            options->ce.occ_mtu = 0;
+        }
     }
     else if (streq(p[0], "tun-mtu-extra") && p[1] && !p[2])
     {
diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index c2937dc37..1085a462a 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -118,6 +118,7 @@  struct connection_entry
     const char *socks_proxy_authfile;
 
     int tun_mtu;         /* MTU of tun device */
+    int occ_mtu;         /* if non-null, this is the MTU we announce to peers in OCC */
     bool tun_mtu_defined; /* true if user overriding parm with command line option */
     int tun_mtu_extra;
     bool tun_mtu_extra_defined;
diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index 63257348a..463957a82 100644
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -603,6 +603,22 @@  prepare_push_reply(struct context *c, struct gc_arena *gc,
     {
         push_option_fmt(gc, push_list, M_USAGE, "key-derivation tls-ekm");
     }
+
+    /* Push our mtu to the peer if it supports pushable MTUs */
+    int client_max_mtu = 0;
+    const char *iv_mtu = extract_var_peer_info(tls_multi->peer_info, "IV_MTU=", gc);
+
+    if (iv_mtu &&  sscanf(iv_mtu, "%d", &client_max_mtu) == 1)
+    {
+        push_option_fmt(gc, push_list, M_USAGE, "tun-mtu %d", o->ce.tun_mtu);
+        if (client_max_mtu < o->ce.tun_mtu)
+        {
+            msg(M_WARN, "Warning reported maximum MTU from client (%d) is lower "
+                "than MTU used on the server (%d). Add tun-max-mtu %d "
+                "to client configuration.", client_max_mtu,
+                o->ce.tun_mtu, o->ce.tun_mtu);
+        }
+    }
     return true;
 }