Push server mtu to client when supported and support occ mtu

To maximise compatibility allow to lie our MTU in the default OCC
message.

Patch v2: improve documentation
Patch v3: split changing default MTU into its own patch
Patch v5: remove leftover mentions to default MTU

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221109154810.1268403-2-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25499.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/doc/man-sections/vpn-network-options.rst b/doc/man-sections/vpn-network-options.rst
index 4b2c4a7beb698021eb3344964b245cd3ca928545..8e3c92ee1f40914b45e05418ceaec5a955ccd1e4 100644 (file)
--- a/doc/man-sections/vpn-network-options.rst
+++ b/doc/man-sections/vpn-network-options.rst
@@ -524,10 +524,23 @@ 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.
+
+  The default for :code:`tun-mtu` is 1500.
+
+  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.
@@ -540,6 +553,10 @@ routing.
   It's best to use the ``--fragment`` and/or ``--mssfix`` options 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-max-mtu maxmtu
   This configures the maximum MTU size that a server can push to ``maxmtu``,
   by configuring the internal buffers to allow at least this packet size.

diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 334fc41185787c4ee472a603c4c3c2a13d87432f..b7b34c9cc1079e6717e17bc089188c649b83d75d 100644 (file)
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -825,6 +825,7 @@ 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.tls_mtu = TLS_MTU_DEFAULT;
     o->ce.mtu_discover_type = -1;
@@ -4193,7 +4194,15 @@ 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);
@@ -6447,11 +6456,19 @@ 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_PUSH_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-max") && p[1] && !p[3])
     {

diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index a2bc13a1c76687a333282ccb095f25e2d253ca0e..68ad0cacb72381047c1e7d485f356a286b3ff563 100644 (file)
--- 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 */
     int tun_mtu_max;     /* maximum MTU that can be pushed */
 
     bool tun_mtu_defined; /* true if user overriding parm with command line option */

diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index 1bad00605a35531ed431570de4373861f8812632..92a0bc2d934410bcd9cf6739d645de01611f7d8e 100644 (file)
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -673,6 +673,22 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
         push_option_fmt(gc, push_list, M_USAGE, "protocol-flags%s", buf_str(&proto_flags));
     }
 
+    /* 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;
 }