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I'm currently at my wits end trying to configure an L2TP over IPSec VPN connection on my Debian using openswan and xl2tp box connecting to a server of unknown configuration.

I've managed to successfully establish the connection and everything appears to be working well until I attempt to set the VPN connection as my default route, at which point I see a massive flood of packets simultaneously being transmitted (on the tune of ~1.5 GB in about 2min) until the server drops my connection. Prior to this network traffic on all my interfaces is minimal.

According to iftop the majority of this traffic appears to be coming out of port 12, although I can't seem to figure out how to finger a specific process. If I instead just route traffic destined for through it I'm able to access Google's servers through the VPN without issue.

My xl2tp.conf file is:

[lac vpn-nl]
lns =
name = myusername
pppoptfile = /etc/ppp/options.l2tpd.client

My options.l2tpd.client file is:

idle 1800
mtu 1410
mru 1410
name myusername
password mypassword
connect-delay 5000

And my routing table looks like:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface        *      UH    0      0        0 ppp0       *        U     0      0        0 eth0       *          U     0      0        0 eth0        *          U     0      0        0 eth0     *          U     0      0        0 eth0
loopback        *            U     0      0        0 lo
default         *              U     0      0        0 ppp0

I'm seeing absolutely nothing in auth.log and syslog during this time and can't seem to find any other log files it might be writing to.

Any suggestions would be appreciated!

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up vote 2 down vote accepted

Alright. I figured it out. The problem was with my ipsec.conf file (not shown) in which I had the lines:

left= # my local IP address

The problem appeared to be after I set my VPN to the default route it would attempt to send packets destined for the VPN server through the vpn causing a nice feedback loop. Modifying these lines to read

left=          # my local IP address
leftnexthop=    # my local gateway

and adding static routes to the routing table pointing traffic to the VPN server through eth0 instead of ppp0 appears to have solved the problem.

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