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 <a...@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
+ 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;
}
--
2.32.1 (Apple Git-133)
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