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Re: [Openvpn-users] Incorrect tcp and udp checksums on packets from gateway to vpn clients (solved)


  • Subject: Re: [Openvpn-users] Incorrect tcp and udp checksums on packets from gateway to vpn clients (solved)
  • From: Georg Graf <g.graf@xxxxxxxx>
  • Date: Tue, 03 Apr 2007 12:06:03 +0200

Hi!

Giampaolo Carmagnani wrote:

> --ip-win32 method
>     When using --ifconfig on Windows, set the TAP-Win32 adapter IP
> address and netmask using method. Don't use this option unless you are
> also using --ifconfig.

Giampaolo, I'm afraid you misunderstood the cause of my posting. I am 
not having any fragmentation problems.

The problem was: The pyhsical Network Card attached to the server bridge 
was by default doing TCP checksums in hardware (Intel Gigabit Ethernet 
Adapter, em in FreeBSD Terms), to take this workload off the kernel. So 
  the packets generated by the server host had no valid TCP checksum on 
them, entered the bridge without seeing the Intel Card and arrived at my 
VPN Clients with no valid checksum on them.

"ifconfig em0 -txcsum" solved the problem. Now the kernel generates the 
checksums for all packets, they are valid when they enter the bridge and 
the communication works fine.

I'll report this issue to the FreeBSD People.

   George



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