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The following ASCII drawing describes my problem:

    +------------+
    |  Internet  |
    +-----^------+
          |
          |
          |
    +-----+------+
    |   Router   <------------------------+
    +-----^------+                        |
          |                               |
          |                               |
          |                               |
          |                               |
          |                               |
+---------+-------------+       +---------+--------------------+
| ==SERVER (Ubuntu)==   |       |       == CLIENT ==           |
|                       |       |                              |
|SoftEther VPN Server   |       |L2TP VPN Client               |
|Full speed to internet |       |Speed without VPN: 50down 10up|
|        50down 10up    |       |Speed with    VPN: 1down 8up  |
+-----------------------+       +------------------------------+
  • The issue: Very slow download, but fast upload when using the VPN.
  • The connection is made using local network connections only
  • Some investigation has shown that this issue is related to the Linux machine
    • Using a Windows machine yields full speed
  • No advanced network configurations
  • The VPN server uses a local bridge to eth0 (a cat5e Ethernet connection).
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1 Answer 1

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The issue was that I had a tun device, and deleting it turns out to be the solution.

First, run ifconfig:

eth0      Link encap:Ethernet  HWaddr ae:50:74:cc:73:ed
          inet addr:192.168.2.200  Bcast:192.168.2.255  Mask:255.255.255.0
          inet6 addr: fe80::ac50:74ff:fecc:73ed/64 Scope:Link
          inet6 addr: 2003:72:2d65:e401:ac50:74ff:fecc:73ed/64 Scope:Global
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3560109 errors:0 dropped:3 overruns:0 frame:0
          TX packets:4436401 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1195365679 (1.1 GB)  TX bytes:5572562837 (5.5 GB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:3616 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3616 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:480408 (480.4 KB)  TX bytes:480408 (480.4 KB)

tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:10.8.0.1  P-t-P:10.8.0.2  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

wlp7s0    Link encap:Ethernet  HWaddr a4:17:31:ec:29:e7
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

Locate the tun device and delete it (in this case, tun0).

sudo ip link delete tun0

Now I got my full speed back.

Note: The network interface will reappear after a restart, but you do not need to delete it again. Just deleting it once will resolve it, and it's perfectly fine if the recreated interface stays.

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  • I have this exact same problem with the exact same set-up, but without a tun device anywhere on my machine. Just ens3 and lo
    – Dwebtron
    Apr 24, 2019 at 19:19

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