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Re: [Openvpn-users] icmp works, tcp not, tcp packets have incorrect checksums

  • Subject: Re: [Openvpn-users] icmp works, tcp not, tcp packets have incorrect checksums
  • From: Jon Bendtsen <jon.bendtsen@xxxxxxxxxx>
  • Date: Wed, 8 Mar 2006 17:28:44 +0100

Den onsdag 8.mar kl. 16:53 skrev harald.lampesberger@xxxxxxx:


maybe someone can help me with my problem:


i have following setup:


server-config of openvpn server, running on VPN-Box:
proto udp
port 1194
dev tun
push "route"
ifconfig-pool-persist ipp.txt
dh /etc/openvpn/keys/dh1024.pem
ca /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/idsserver.crt
key /etc/openvpn/keys/idsserver.key
keepalive 10 60
verb 1

remote <VPN Box ip>
proto udp
port 1194
dev tun
user nobody
group nogroup
ca /etc/openvpn/keys/ca.crt
key /etc/openvpn/keys/sensor.key
cert /etc/openvpn/keys/sensor.crt
keepalive 10 60
verb 1

The VPN-Box has following routes and ip's: Internal ip:, net.ipv4.ip_forward=1, no iptables rules * UH 0 0 0 tun0 * U 0 0 0 eth0 UG 0 0 0 tun0
default UG 0 0 0 eth0

The Target has following routes and ip's: IP:, no iptables rules * U 0 0 0 eth0 UG 0 0 0 eth0
default UG 0 0 0 eth0

And here is my problem:
I am able to establish a tunnel from the client to the VPN-Box. The client
gets a 192.168.1.x ip and the, which is pushed by the
server, is set.

From the client, I can ping the VPN-Box through and,
i can also ping the Target with can ping the client and
all interfaces from the VPN-Box.

How big ping packets have you tried with?

But i cannot send data through an established tcp-session (no firewalls)
from Target to client. Packets from Client to Target arive without
problems, but the other direction makes problems. TCP Communication
between VPN-Box and Target works flawlessly without checksum problems.

TCP-Packets monitored with tcpdump at eth0 of the VPN-Box: Client ---> Target OK Target ---> Client Data-Packets have incorrect checksums

check for bad memory/netcards/cables somewhere.

Check if using a TCP tunnel works better than UDP.


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