Wireshark-users: Re: [Wireshark-users] L2TP-over-IPsec (may be off topic)
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From: Sake Blok <sake@xxxxxxxxxx>
Date: Tue, 14 Sep 2010 19:59:24 +0200
On 14 sep 2010, at 19:01, Kok-Yong Tan wrote:

> However, I have a physically separate hardware firewall in between  
> the endpoints (a L2TP-over-IPsec client and a L2TP-over-IPsec server)  
> and I've discovered that the L2TP-over-IPsec VPN will only  
> successfully connect if UDP port 1701 is open on the firewall.

What do you mean by successfully connect? If that means the L2TP-over-IPsec client and the L2TP-over-IPsec server can communicate with each other? Did you check whether there is actually a tunnel formed? If not, it's just a L2TP connection and that will work, but it will not be encrypted.

It seems like the L2TP tunnel just does not trigger the IPsec encapsulation to kick in. What does a network trace say? Only traffic on UDP port 1701, no UDP-500, no ip proto 50 and no UDP port 4500? That would be in sync with the above.

What type of L2TP-over-IPsec client and L2TP-over-IPsec server are involved?

Cheers,
Sake