NETRESEC Network Security Blog - Tag : flow

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Don't Delete PCAP Files - Trim Them!

We are happy to release TrimPCAP today! TrimPCAP is a free open source tool that reduces the size of capture files in an intelligent way.

TrimPCAP

The retention period of a packet capture solution is typically limited by either legal requirements or available disk space. In the latter case the oldest capture files are simply removed when the storage starts getting full. This means that if there is a long ongoing session, such as a download of a large ISO file, streamed video or a reverse shell backdoor, then the start of this session will likely be removed.

I know from experience that it’s painful to analyze network traffic where the start of a session is missing. The most important and interesting stuff generally happens in the beginning of each session, such as the HTTP GET request for an ISO file. As an analyst you don’t need to look at all the other packets in that ISO download (unless you believe the ISO contains malware), it’s enough to see that there is a GET request for the file and the server responds with a “200 OK”.

CapLoader Transcript of ISO download

Image: CapLoader transcript of an ISO download

If that download had been truncated, so that only the last few packets were remaining, then it would be really difficult to know what was being downloaded. The same is true also for other protocols, including proprietary C2 protocols used by botnets and other types of malware.


  TrimPCAP  

TrimPCAP is designed to overcome the issue with truncated sessions by removing data from the end of sessions rather than from the beginning. This also comes with a great bonus when it comes to saving on disk usage, since the majority of the bytes transferred across the Internet are made up of big sessions (a.k.a “Elephant Flows”). Thus, by trimming a PCAP file so that it only contains the first 100kB of each TCP and UDP session it’s possible to significantly reduce required storage for that data.

The following command reduces the PCAP dataset used in our Network Forensics Training from 2.25 GB to just 223 MB:

user@so$ python trimpcap.py 102400 /nsm/sensor_data/so-eth1/dailylogs/*/*
Trimming capture files to max 102400 bytes per flow.
Dataset reduced by 90.30% = 2181478771 bytes
user@so$

A maximum session size (or "flow cutoff") of 100kB enables trimpcap.py to reduce the required storage for that dataset to about 10% of its original size, which will significantly extend the maximum retention period.


Putting TrimPCAP Into Practice

Let’s assume that your organization currently has a maximum full content PCAP retention period of 10 days and that trimming sessions to 100kB reduces the required storage to 10%. TrimPCAP will then enable you to store 5 days with full session data plus 50 additional days with trimmed sessions using the same disk space as before!

TrimPCAP retention period extension

A slightly more advanced scheme would be to have multiple trim limits, such as trimming to 1MB after 3 days, 100kB after 6 days and 10kB after 30 days. Such a setup would probably extend your total retention period from 10 days to over 100 days.

An even more advanced trimming scheme is implemented in our packet capture agent PacketCache. PacketCache constantly trims its PCAP dataset because it is designed to use only 1 percent of a PC’s RAM to store observed packets, in case they are needed later on for incident response. PacketCache uses a trim limit which declines individually for each observed TCP and UDP session depending on when they were last observed.


Downloading TrimPCAP

TrimPCAP is open source software and is released under the GNU General Public License version 2 (GPLv2). You can download your own copy of TrimPCAP from the official TrimPCAP page:
https://www.netresec.com/?page=TrimPCAP

Happy trimming!

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Posted by Erik Hjelmvik on Tuesday, 05 December 2017 12:40:00 (UTC/GMT)

Tags: #PCAP#python#flow

Short URL: https://netresec.com/?b=17Cf81f


CapLoader 1.5 Released

CapLoader 1.5 Logo

We are today happy to announce the release of CapLoader 1.5. This new version of CapLoader parses pcap and pcap-ng files even faster than before and comes with new features, such as a built-in TCP stream reassembly engine, as well as support for Linux and macOS.

Support for ICMP Flows

CapLoader is designed to group packets together that belong to the same bi-directional flow, i.e. all UDP, TCP and SCTP packets with the same 5-tuple (regardless of direction) are considered being part of the same flow.

5-tuple
/fʌɪv ˈtjuːp(ə)l/
noun

A combination of source IP, destination IP, source port, destination port and transport protocol (TCP/UDP/SCTP) used to uniquely identify a flow or layer 4 session in computer networking.

The flow concept in CapLoader 1.5 has been extended to also include ICMP. Since there are no port numbers in the ICMP protocol CapLoader sets the source and destination port of ICMP flows to 0. The addition of ICMP in CapLoader also allows input filters and display filters like “icmp” to be leveraged.

Flows tab in CapLoader 1.5 with display filter BPF 'icmp'
Image: CapLoader 1.5 showing only ICMP flows due to display filter 'icmp'.

TCP Stream Reassembly

One of the foundations for making CapLoader a super fast tool for reading and filtering PCAP files is that it doesn’t attempt to reassemble TCP streams. This means that CapLoader’s Transcript view will show out-of-order segments in the order they were received and retransmitted segments will be displayed twice.

The basic concept has been to let other tools do the TCP reassembly, for example by exporting a PCAP for a flow from CapLoader to Wireshark or NetworkMiner.

The steps required to reassemble a TCP stream to disk with Wireshark are:

  1. Right-click a TCP packet in the TCP session of interest.
  2. Select “Follow > TCP Stream”.
  3. Choose direction in the first drop-down-list (client-to-server or server-to-client).
  4. Change format from “ASCII” to “Raw” in the next drop-down-menu.
  5. Press the “Save as...” button to save the reassembled TCP stream to disk.

Follow TCP Stream window in Wireshark

Unfortunately Wireshark fails to properly reassemble some TCP streams. As an example the current stable release of Wireshark (version 2.2.5) shows duplicate data in “Follow TCP Stream” when there are retransmissions with partially overlapping segments. We have also noticed some additional  bugs related to TCP stream reassembly in other recent releases of Wireshark. However, we’d like to stress that Wireshark does perform a correct reassembly of most TCP streams; it is only in some specific situations that Wireshark produces a broken reassembly. Unfortunately a minor bug like this can cause serious consequences, for example when the TCP stream is analyzed as part of a digital forensics investigation or when the extracted data is being used as input for further processing. We have therefore decided to include a TCP stream reassembly engine in CapLoader 1.5. The steps required to reassemble a TCP stream in CapLoader are:

  1. Double click a TCP flow of interest in the “Flows” tab to open a flow transcript.
  2. Click the “Save Client Byte Stream” or “Save Server Byte Stream” button to save the data stream for the desired direction to disk.

Transcript window in CapLoader 1-5

Extracting TCP streams from PCAP files this way not only ensures that the data stream is correctly reassembled, it is also both faster and simpler than having to pivot through Wireshark’s Follow TCP Stream feature.

PCAP Icon Context Menu

CapLoader 1.5 PCAP icon Save As...

The PCAP icon in CapLoader is designed to allow easy drag-and-drop operations in order to open a set of selected flows in an external packet analysis tool, such as Wireshark or NetworkMiner. Right-clicking this PCAP icon will bring up a context menu, which can be used to open a PCAP with the selected flows in an external tool or copy the PCAP to the clipboard. This context menu has been extended in CapLoader 1.5 to also include a “Save As” option. Previous versions of CapLoader required the user to drag-and-drop from the PCAP icon to a folder in order to save filtered PCAP data to disk.

Faster Parsing with Protocol Identification

CapLoader can identify over 100 different application layer protocols, including HTTP, SSL, SSH, RTP, RTCP and SOCKS, without relying on port numbers. The protocol identification has previously slowed down the analysis quite a bit, which has caused many users to disable this powerful feature. This new release of of CapLoader comes with an improved implementation of the port-independent protocol identification feature, which enables PCAP files to be loaded twice as fast as before with the “Identify protocols” feature enabled.

Works in Linux and macOS

One major improvement in CapLoader 1.5 is that this release is compatible with the Mono framework, which makes CapLoader platform independent. This means that you can now run CapLoader on your Mac or Linux machine if you have Mono installed. Please refer to our previous blog posts about how to run NetworkMiner in various flavors of Linux and macOS to find out how to install Mono on your computer. You will, however, notice a performance hit when running CapLoader under Mono instead of using Windows since the Mono framework isn't yet as fast as Microsoft's .NET Framework.

CapLoader 1.5 running in Linux with Mono
Image: CapLoader 1.5 running in Linux (Xubuntu).

Credits

We’d like to thank Sooraj for reporting a bug in the “Open With” context menu of CapLoader’s PCAP icon. This bug has been fixed in CapLoader 1.5 and Sooraj has been awarded an official “PCAP or it didn’t happen” t-shirt for reporting the bug.

PCAP or it didn't happen t-shirt
Image: PCAP or it didn't happen t-shirt

Have a look at our Bug Bounty Program if you also wanna get a PCAP t-shirt!

Downloading CapLoader 1.5

Everything mentioned in this blog post, except for the protocol identification feature, is available in our free trial version of CapLoader. To try it out, simply grab a copy here:
https://www.netresec.com/?page=CapLoader#trial (no registration needed)

All paying customers with an older version of CapLoader can download a free update to version 1.5 from our customer portal.

Posted by Erik Hjelmvik on Tuesday, 07 March 2017 09:11:00 (UTC/GMT)

Tags: #CapLoader#PCAP#pcap-ng#Follow TCP Stream#Wireshark#Linux#Mac#macOS#RTP#SOCKS#Mono#Flow

Short URL: https://netresec.com/?b=1738eba

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