Wireshark · Wireshark-dev: [Wireshark-dev] Possible bug (Wireshark blocking VPN traffic)

[Binarus <lists@xxxxxxxxxx>]

Dear Wireshark developers,

at first, I'd like to thank you for your great work! Wireshark helped me
analyze and solve problems a zillion of times, and it has never crashed
or was unreliable.

However, I believe that I now have found a bug. If you need more
information than I provide below, I'll happily assist. And please bear
in mind that I am not an expert in this field, so I may have missed
something.

To clarify and to save time, please note the following:

- This is NOT about Wireshark not capturing correctly
- This is NOT about Wireshark crashing or being unstable

This IS solely about Wireshark blocking traffic through a VPN
connection. Having said this:

O/S
---
Windows 10 Enterprise X64 v1909, patched up-to-date

Wireshark version
-----------------
3.2.7 (v3.2.7-0-gfb6522d84a3a)

Compiled (64-bit) with Qt 5.12.9, with WinPcap SDK (WpdPack) 4.1.2, with
GLib 2.52.3, with zlib 1.2.11, with SMI 0.4.8, with c-ares 1.15.0, with
Lua 5.2.4, with GnuTLS 3.6.3 and PKCS #11 support, with Gcrypt 1.8.3,
with MIT Kerberos, with MaxMind DB resolver, with nghttp2 1.39.2, with
brotli, with LZ4, with Zstandard, with Snappy, with libxml2 2.9.9, with
QtMultimedia, with automatic updates using WinSparkle 0.5.7, with
AirPcap, with SpeexDSP (using bundled resampler), with SBC, with
SpanDSP, with bcg729.

Running on 64-bit Windows 10 (1909), build 18363, with Intel(R) Xeon(R)
CPU E3-1230 v5 @ 3.40GHz (with SSE4.2), with 16277 MB of physical
memory, with locale German_Germany.1252, with light display mode,
without HiDPI, with Npcap version 0.9997, based on libpcap version
1.9.1, with GnuTLS 3.6.3, with Gcrypt 1.8.3, with brotli 1.0.2, without
AirPcap, binary plugins supported (19 loaded).

Built using Microsoft Visual Studio 2019 (VC++ 14.27, build 29111).

I have recently downloaded that version (I did not compile myself).

Problem
-------
When I start Wireshark and start capturing at all interfaces (regardless
whether or not there is a capturing filter active and regardless of what
that filter actually is), the traffic through VPN connections is
blocked. As soon as I stop capturing in Wireshark, the VPN tunnel works
again (I don't need to close Wireshark, I just need to stop capturing
for that).

Exact steps to reproduce
------------------------

1) The Windows client shall use a LAN connection with IP 192.168.20.100,
and its subnet shall be 192.168.20/24, and shall be connected to the
internet by a common SOHO router with NAT.

2) Using the Windows UI, create a VPN connection with name "Test", using
the native Windows IPSec client, using IKEv2, using machine
certificates, MOBIKE disabled, encryption level "Require", credential
storing disabled.

3) Fine-tune that VPN connection using the following Powershell commands
as Administrator:

Set-VpnConnection -Name Test -AllUserConnection -ServerName
remote.vpn.gateway -TunnelType Ikev2 -AuthenticationMethod
MachineCertificate -EncryptionLevel Required -SplitTunneling $true

Set-VpnConnectionIPsecConfiguration -ConnectionName Test
-AuthenticationTransformConstants GCMAES256 -CipherTransformConstants
GCMAES256 -EncryptionMethod GCMAES256 -IntegrityCheckMethod SHA384
-PfsGroup PFS2048 -DHGroup Group14 -AllUserConnection

4) Install an appropriate machine certificate and the CA certificate
(which has signed the remote gateway's certificate) in Windows.

5) Configure the remote gateway so that it accepts the connection when
the Windows client establishes the VPN created in the previous steps.
Furthermore, make sure that the following conditions are met:

- The remote gateway (besides its public IP address) has the IP address
192.168.0.250, and it is in the local subnet 192.168.0.0/24.

- Furthermore, it uses IKE-CFG to assign an IP address to the Windows
client when the latter establishes a VPN connection. The IP address it
assigns shall be 192.168.3.1

- The remote gateway routes traffic between 192.168.3.1 and 192.68.0.0,
and the remote gateway's firewall (if any) lets this traffic pass.

6) In Windows, establish the VPN connection, and verify that (after
connecting successfully) there is a new network interface with IP
192.168.3.1 (if it isn't, the VPN is not set up correctly, and this must
be sorted out before proceeding), e.g. by typing ipconfig /all in a
command prompt.

7) To tell Windows how it can reach the subnet behind the remote VPN
gateway, add the appropriate route by giving the following command in a
command prompt with administrative permissions:

route add 192.168.0.0 mask 255.255.255.0 192.168.3.1

Check whether Windows now can reach the remote subnet, e.g. via ping:

ping 192.168.0.10

(provided that 192.168.0.10 is reachable in the remote subnet and does
not block ICMP from other subnets, this should work; if it doesn't, the
VPN is not set up correctly, and this must be sorted out before proceeding).

8) On the Windows client, start Wireshark and let it capture on all
interfaces. Repeat the ping command from above and note that it does not
work any more (it produces timeouts, no reply comes back).

9) Do not close Wireshark, but stop capturing. Repeat the ping command
from above and note that it works again (no more timeouts, reply comes
back as expected).

To confirm that the problem can be reproduced, start and stop capturing
in Wireshark as often as you like, and each time check whether the ping
command from above works. Confirm that it does not work while Wireshark
is capturing, and that it does work while Wireshark is not capturing.


I know that my setup may be somewhat exotic (normally, you let a VPN
gateway assign IP addresses to the client which are from its internal
subnet so that you don't need to add a route on the client). However, I
believe that this should work and that Wireshark blocking traffic
through the VPN channel is a bug.

Could you please let me know what you think about that and whether I can
assist in solving the problem?

Thank you very much, and best regards,

Binarus