Tag Archives: tcpdump

Scratch’n’Sniff – An easy tool for remote Packet Captures

Linux, Python, Software, ,

A lesson learned a long time ago in Net Eng, is that packet captures (seldom) lie, and the answers are almost always in the packets.

The issue is just getting those packets.

The Problem

But if you’re anything like me, you’re working on remote systems from your workstation, and trying to see what’s going on.

For me this goes like this:

  1. SSH into machine in question
  2. Start TCPdump
  3. Hope that I have run it for long enough to capture the event of interest
  4. Stop TCPdump
  5. Change permissions on PCAP file created so I can copy it
  6. SFTP into the machine in question
  7. Transfer the PCAP to my local machine
  8. View the PCAP in Wireshark
  9. Discover I had not run the PCAP for long enough and repeat

Being a Mikrotik user I fell in love with the remote packet sniffer functionality built into them, where the switch/router will copy packets matching a filter and just stream them to the IP of my workstation.

If only there was something I could use to get this same functionality on remote machines – without named pipes, X11 forwarding or any of the other “hacky” solutions…

The Solution

Introducing Scratch’n’Sniff, a simple tcpdump front end that encapsulates all the filtered traffic of interest in TZSP the same as Mikrotiks do, and stream it (in real time) to your local machine for real time viewing in Wireshark.

Using it is very simple:

Capture all traffic on port 5060 on interface enp0s25 and send it to 10.0.1.252
python3 scratchnsniff.py --dstip 10.0.1.252 --packetfilter 'port 5060' --interface enp0s25

Capture all sctp and icmp traffic on interface lo and send it to 10.98.1.2:
python3 scratchnsniff.py --dstip 10.98.1.2 --packetfilter 'sctp or icmp' --interface lo

If you’re keen to try it out you can grab it from GitHub – Scratch’n’Sniff and start streaming packets remotely.

Enjoy!

Samsung-Sysdump-IMS-Debug-DM-View_Cropped

VoLTE/IMS Debugging on Samsung Handsets using Sysdump & Samsung IMS Logger

EPC, EUTRAN, IMS / VoLTE, LTE, Mobile Networks, Security, Software, VoIP, , , , , , , ,

Samsung handsets have a feature built in to allow debugging from the handset, called Sysdump.

Entering *#9900# from the Dialing Screen will bring up the Sysdump App, from here you can dump logs from the device, and run a variety of debugging procedures.

Samsung share information about this app publicly on their website,

Sysdump App in Samsung handsets used for debugging the device

But for private LTE operators, the two most interesting options are by far the TCPDUMP START option and IMS Logger, but both are grayed out.

Tapping on them asks for a one-time password and has a challenge key.

OTP Authentication required to unlock IMS Debugging and TCPDUMP on Samsung Sysdump tool

These options are not available in the commercial version of the OS and need to be unlocked with a one time key generated by a tool Samsung for unlocking engineering firmware on handsets.

Luckily this authentication happens client side, which means we can work out the password it’s expecting.

For those interested I’ve done a write up of how I reversed the password validation algorithm to take the key given in the OTP challenge and generate a valid response.

For those who just want to unlock these features you can click here to run the tool that generates the response.

Once you’ve entered the code and successfully unlocked the IMS Debugging tool there’s a few really cool features in the hamburger menu in the top right.

DM View

This shows the SIP / IMS Messaging and the current signal strength parameters (used to determine which RAN type to use (Ie falling back from VoLTE to UMTS / Circuit Switched when the LTE signal strength drops).

Screenshot of Samsung Sysdump tool in the IMS Debug - DM View section

Tapping on the SIP messages expands them and allows you to see the contents of the SIP messages.

Viewing SIP Messaging directly from the handset

Interesting the actual nitty-gritty parameters in the SIP headers are missing, replaced with X for anything “private” or identifiable.

Luckily all this info can be found in the Pcap.

The DM View is great for getting a quick look at what’s going on, on the mobile device itself, without needing a PC.

Logging

The real power comes in the logging functions,

There’s a lot of logging options, including screen recording, TCPdump (as in Packet Captures) and Syslog logging.

From the hamburger menu we can select the logging parameters we want to change.

Settings for Samsung IMS Logger

From the Filter Options menu we can set what info we’re going to log,

Filter options used in Dump output of Samsung IMS Logger application

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.

SNgrep – Command line SIP Debugging

Linux, RFCs & Standards, VoIP, , , ,

If you, like me, spend a lot of time looking at SIP logs, sngrep is an awesome tool for debugging on remote machines. It’s kind of like if VoIP Monitor was ported back to the days of mainframes & minimal remote terminal GUIs.

Installation

It’s in the Repos for Debian and Ubuntu:

apt-get install sngrep

GUI Usage

sngrep can be used to parse packet captures and create packet captures by capturing off an interface, and view them at the same time.

We’ll start by just calling sngrep on a box with some SIP traffic, and waiting to see the dialogs appear.

Here we can see some dialogs, two REGISTERs and 4 INVITEs.

By using the up and down arrow keys we can select a dialog, hitting Enter (Return) will allow us to view that dialog in more detail:

Again we can use the up and down arrow keys to view each of the responses / messages in the dialog.

Hitting Enter again will show you that message in full screen, and hitting Escape will bring you back to the first screen.

From the home screen you can filter with F7, to find the dialog you’re interested in.

Command Line Parameters

One of the best features about sngrep is that you can capture and view at the same time.

As a long time user of TCPdump, I’d been faced with two options, capture the packets, download them, view them and look for what I’m after, or view it live with a pile of chained grep statements and hope to see what I want.

By adding -O filename.pcap to sngrep you can capture to a packet capture and view at the same time.

You can use expression matching to match only specific dialogs.