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Push server mtu to client when supported and support occ mtu

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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>
This commit is contained in:
Arne Schwabe 2022-11-09 16:48:10 +01:00 committed by Gert Doering
parent 01aed6a5df
commit 761575cb7b
4 changed files with 57 additions and 6 deletions
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25
doc/man-sections/vpn-network-options.rst
View File

@ -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.

21
src/openvpn/options.c
View File

@ -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])
{

1
src/openvpn/options.h
View File

@ -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 */

16
src/openvpn/push.c
View File

@ -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;
}