Tag: wireshark

Network based forensics of P2P traffic has many applications, particularly in law enforcement. Recently, I added preliminary support for Gnutella in Unsniff Network Analyzer as a proof of concept. Wireshark has a rather rudimentary support for this protocol.

We would like to :

1. Monitor what various subjects are searching for on the P2P network
2. Monitor query responses including the filenames and their respective SHA1 hash
3. Identify the file actually downloaded
4. Reconstruct the file actually downloaded

This blog post is a summary of what I found and how you can put this feature to use.

Setup

This was the test setup.

• Limewire 5.5.9 which connects using the Gnutella Protocol 0.6
• Connected to the Gnutella network using the default setttings
• Searched for “flotilla” videos
• Downloaded one of them

All traffic from startup to shutdown of Limewire was captured using Unsniff.  You can convert these files to libpcap format by opening them in Unsniff and selecting File > Export > TCPDUMP

Limewire_Complete.usnf	Contains complete traffic dump including bootstrapping, punching hole via UPnP, handshakes, downloading the file, and shutdown
Limewire_Handshake.usnf	A single TCP session which contains a Gnutella handshake. Recommend you start here first to understand how the compression works

Notes on protocol

A quick note about the P2P protocol elements viewed from a forensics viewpoint.

Bootstrapping

When Limewire first starts up, it uses plain HTTP to download a list of peers from one of many well known hosts. Next it attempts to punch a hole in your firewall using UPnP.  It then selects a suitable peer based on your Geo location and starts a handshake process with it.More details about the bootstrapping process are available at the Gnutella Development Forum. You can observe the bootstrapping process in sessions 1,2,3 in the capture file Limewire_Complete.usnf .

Use of compression

The handshake takes place over a single TCP connection. The initial part of the handshake is in plaintext, but if both sides signal support for compression, the same TCP connection switches over to stream compression. Unsniff creates a “synthetic” uncompressed TCP stream corresponding to each compressed TCP handshake stream. You can view the uncompressed stream by opening the second capture file Limewire_Handshake.usnf .

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Use of encryption

Many messages including the actual file transfers are encrypted using AES-128 whose keys are exchanged using Anonymous-Diffie Hellman. This is bad news because our forensics ability effectively ends here. But traffic analysis can give us the some clues about what might have been transferred across the encrypted session.

Here is the complete stream that was used to transfer the selected file (click for bigger picture). You can view the TLS handshake settling upon the Anon-DH key exchange protocol and AES128 encryption.

Messages

Gnutella defines a handful of messages and extensions. The relevant protocol messages for this exercise are QUERY and QUERY HIT.  To view these messages,

• switch to the PDU sheet (we are far removed from link layer packets)
• click on a message to view its breakout

Analyzing Gnutella with Unsniff

Viewing Gnutella messages

1. With USNF files, you just have to open the capture file.

2. With PCAP files or live captures, you need to set up Unsniff to recognize Gnutella.

Since Gnutella uses random TCP ports, just assign Port 65535  to Gnutella. This is the special ‘catch all’ port in Unsniff.  All unrecognized TCP traffic will be treated as Gnutella. To do this :  Select Manage > Access Points > Click on TCP > Add Access Point > Enter 65535 + Gnutella.

3. Import the PCAP file or start a live capture

4. Look at the PDU sheet, it contains all the Gnutella messages.

Scripting to automate this task

To really use this in forensics applications you need to be able to script this stuff. Luckily Unsniff’s scripting API allows us to ditch the GUI altogether and run scripts to extract just what we want.

Here is a useful sample script in Ruby.

• This prints a list of  peers, the filenames, and the SHA1 hash.

Running this code as

Produces this report containing the peer, files available, and their SHA-1 hash.

The SHA1 hash is in Base32.

Conclusions

It is possible to conduct network based forensics on P2P traffic, but it requires tools with advanced reconstruction abilities.

• You CAN conduct surveillance of location of content
• You CAN conduct surveillance of content being searched for
• You CANNOT reconstruct the file actually transferred due to encryption and use of Anon-Diffie-Hellman
• You CAN however get a fair idea, not of forensic evidence quality about what was transferred

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