D-Link NetDefend DFL-210 User Manual page 353

9.4.5. Troubleshooting with ikesnoop
The ikesnoop command can be entered via a CLI console or directly via the RS232 Console.
To begin monitoring the full command is:
gw-world:/> ikesnoop -on -verbose
This means that ikesnoop output will be sent to the console for every VPN tunnel IKE negotiation.
The output can be overwhelming so to limit the output to a single IP address, for example the IP
address 10.1.1.10, the command would be:
gw-world:/> ikesnoop -on 10.1.1.10 -verbose
The IP address used is the IP address of the VPN tunnel's remote endpoint (either the IP of the
remote endpoint or the client IP). To turn off monitoring, the command is:
gw-world:/> ikesnoop -off
The output from verbose option can be troublesome to interpret by an administrator seeing it for the
first time. Presented below is some typical ikesnoop output with annotations to explain it. The tunnel
negotiation considered is based on Pre-shared Keys. A negotiation based on certificates is not
discussed here but the principles are similar.
Complete ikesnoop command options can be found in the CLI Reference Guide.
The Client and the Server
The two parties involved in the tunnel negotiation are referred to in this section as the client and
server. In this context, the word client is used to refer to the device which is the initiator of the
negotiation and the server refers to the device which is the responder.
Step 1. Client Initiates Exchange by Sending a Supported Algorithm List
The verbose option output initially shows the proposed list of algorithms that the client first sends to
the server. This list details the protocols and encryption methods it can support. The purpose of the
algorithm list is that the client is trying to find a matching set of protocols/methods supported by the
server. The server examines the list and attempts to find a combination of the protocols/methods
sent by the client which it can support. This matching process is one of the key purposes of the IKE
exchange.
IkeSnoop: Received IKE packet from 192.168.0.10:500 Exchange type :
Identity Protection (main mode) ISAKMP Version : 1.0
Flags
Cookies
Message ID
Packet length
# payloads
Payloads:
SA (Security Association)
Payload data length : 152 bytes
DOI : 1 (IPsec DOI)
Proposal 1/1
Protocol 1/1
:
: 0x6098238b67d97ea6 -> 0x00000000
: 0x00000000
: 324 bytes
: 8
Protocol ID
SPI Size
Transform 1/4
Transform ID
Encryption algorithm
Key length
Hash algorithm
: ISAKMP
: 0
: IKE
: Rijndael-cbc (aes)
: 128
: MD5
353
Chapter 9. VPN