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Debug calls with SNGREP

SNGREP: what it is and why to use it​

Sngrep is a command-line tool that captures all VoIP SIP traffic passing through the network interfaces and displays it grouped by conversation for better readability.

It is especially useful for:

  • Viewing what happens at the SIP level.
  • Understanding what NethVoice sends and receives.
  • Reducing the time needed to identify configuration errors.
  • Saving captures to send directly to Nethesis support, speeding up troubleshooting.
  • Capturing sporadic issues that cannot be easily reproduced or monitored in real time.
note

Sngrep only interprets plain SIP traffic. It does not capture:

  • Traffic from phones configured with TLS
  • Traffic between the proxy app (Flexysip) and applications
  • WebRTC CTI traffic (from Janus to browser clients)

To analyze these cases as well, you need to enable TLS support in Sngrep as described below.

Installation​

If sngrep is not already installed on your NethServer, you can install it easily. Execute the following command as root from the cluster:

dnf -y install http://repo.okay.com.mx/centos/9/x86_64/release/sngrep-1.6.0-1.el9.x86_64.rpm

Launching Sngrep​

The quickest way to start a live capture is to run (as root):

sngrep

Useful options​

OptionDescription
-rCaptures RTP traffic as well, useful if you want to save audio streams.
-I [file]Reads a saved capture file instead of capturing live traffic.
-O [file]Saves the current capture directly to a file.
-cCaptures only INVITE messages (incoming/outgoing calls).
-d [device]Limits the capture to a specific network interface (default: any).

Using the interface​

Once started, Sngrep will begin showing all SIP dialogs in real time. At the top, you can see the number of dialogs captured (default limit: 20,000).

Main controls​

  • Arrow keys: scroll through dialogs.
  • Enter: view SIP messages of the selected dialog.
  • Spacebar: select or deselect a message or dialog.
  • F2: save a .pcap capture file (includes audio if started with -r).
  • F3: view RTP streams, packets, and codec information.
  • F7: set filters (useful for removing irrelevant dialogs like OPTIONS or SUBSCRIBE).
  • F10: add or remove columns (e.g. “Time”).
  • ESC: exit Sngrep.

Enable TLS in SNGREP​

To visualize TLS traffic in SNGREP for NethVoice, follow these steps.

1. Enable siptrace in Kamailio​

runagent -m nethvoice-proxy1 kamcmd siptrace.status on

2. Create the configuration file​

Create /root/.sngrephep2rc with the following content:

cat > /root/.sngrephep2rc <<'EOF'
set capture.device lo
set eep.listen on
set eep.listen.version 2
set eep.listen.address 127.0.0.1
set eep.listen.port 5065
EOF

3. Start SNGREP​

Run:

sngrep -f ~/.sngrephep2rc -d any

Note: Only one user at a time can run Sngrep using this configuration. Multiple simultaneous sessions are not supported.

4. (Optional) Enable Kamailio Debug Logging​

To enable debug logging:

runagent -m nethvoice-proxy1 kamcmd pv.shvSet debug int 1

Disable it when finished​

runagent -m nethvoice-proxy1 kamcmd pv.shvSet debug int 0

Check current debug status​

runagent -m nethvoice-proxy1 kamcmd pv.shvGet debug

Example screens​

  • Main screen: shows captured SIP dialogs and their states.

    Main screen

  • Incoming call example: Screenshot of an incoming call (INVITE) from the trunk that was answered by internal extension 201. You can see the two call legs: the one between the trunk and NethVoice, and the one from NethVoice to internal extension 200.

    Incoming call example

  • Multiple dialog view: allows comparing SIP messages exchanged in parallel between call legs.

    Multiple dialog view

Summary​

With this configuration, SNGREP can capture and visualize both SIP and TLS traffic on NethVoice NS8. It is an essential tool for:

  • Call analysis
  • Quick configuration troubleshooting
  • Advanced technical support

Always remember to disable Kamailio debug logging after finishing your analysis.

NethVoice 8.0