Multi-Tap Network Packet Capturing

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This section will demonstrate the use of Multi-Tap Network Packet Capture with NST. The NST WUI implementation supports simultaneous Packet Capture on up to 4 network interfaces (Quad Tap) per multi-tap capture session. Multi-segment network packet capture and decode analysis can be performed with the NST Multi-Tap Network Packet Capture implementation. NST uses the Wireshark network protocol analyzer suite for network packet capture and decode.

Theory Of Operation

Prior to commencing a multi-tap capture session, each enabled "Tap" interface must be associated with a registered network interface on the NST probe. Up to 4 "Tap" interfaces can be enabled. A "Tap" interface is independent of any registered network interface on the NST probe. By assigning a separate network interface to each "Tap", it is possible to perform a network capture as packets flow across a networking device (e.g., router or firewall). One can also associate the same network interface to multiple "Tap" interfaces. This allows one to apply a different "Capture Filter" for the same network interface during packet capture.

A "Capture Data Directory" must also be selected for the resultant merged multi-tap capture file, multi-tap log file and if chosen, the individual tap member capture files. One should consider using a "RAM-based" file system for packet storage while capturing data packets on high traffic network segments. The Linux "RAMFS" or "TMPFS" file systems make excellent choices. The NST WUI provides convenient access for the creation of either one of these file systems during multi-tap capture setup.

The Wireshark light-weight network packet capture tool: "dumpcap" is used as the capture engine. Each enabled "Tap" interface will run a separate "dumpcap" process when the multi-tap capture session is started. Each "dumpcap" process can be configured to have a separate "Capture Filter" and other associated options.

A "Multi-Tap Merge Manager" process is also started when the multi-tap capture session is commenced. The "Multi-Tap Merge Manager" is responsible for monitoring each separate "dumpcap" process during the course of the multi-tap capture session. If any one multi-tap member "dumpcap" process terminates, the "Multi-Tap Merge Manager" will terminate all other "dumpcap" processes associated with the multi-tap capture session. Typically, a termination threshold (i.e. Duration, File Size or Packet Count) that has been satisfied is the usual reason for a "dumpcap" process to terminate. Once all "dumpcap" processes have been terminated, all "Tap" member capture files will then be merged using the Wireshark "mergecap" utility resulting with a multi-tap capture file. Each individual "Tap" member capture file will normally be deleted after the merge by the "Multi-Tap Merge Manager" unless an option was selected to disable removal of these files. A multi-tap capture log file will also be produced by merging all multi-tap member log files. The multi-tap capture log file contains relevant "Tap" member information and identity for historical multi-tap decode analysis.

The NST WUI is session state aware for each attached browser client. This means that each user or multiple users can in many areas of the NST WUI have its own separate configuration and operation. The NST WUI Multi-Tap Network Packet Capture implementation is fully session state capable. Based on this, multiple instances of a multi-tap capture session can occur simultaneously. This type of operation is typically more suited to an enterprise deployment of NST on a systems configured with 8 or more network interfaces.

Capture Taps

When capturing packets at Gigabit Ethernet rates and one needs total visibility on the link, then a passive tap is required. Net Optics, a global leader in passive monitoring, makes an excellent 10/100/1000BaseT Tap (TP-CU3) for passively allowing access to monitor GigaBit traffic.

The TP-CU3 tap also allows for full-duplex traffic as if it were in-line, including Layer 1 and Layer 2 errors to be presented to the NST multi-tap capture interface. Typically a switch mirroring port may drop layer 1 and select layer 2 errors depending on what has been deemed as high priority for that particular switch.

Configuration 1: Multi-Tap Network Packet Capture Across A Firewall - NAT/PAT Traffic

The diagram depicted below shows an example Multi-Tap Capture Setup for monitoring GigaBit traffic across a firewall boundary. One may need to explore the capturing of packets as they transverse the firewall and undergo both Network and Port Address Translation (NAT/PAT).

Nst quad tap networking.png

Configuration 2: Multi-Tap Network Packet Capture - Traffic Between Gigabit Switches

The diagram displayed below shows an example Dual-Tap Capture Setup for monitoring network traffic between 2 Gigabit switches. In this case a generic notebook computer was used and configured with 3 network interfaces (A built-in Gigabit LAN adapter, a Gigabit LAN adapter PC-Card and a built-in 802.11g/n wireless adapter for secure remote access and control of NST).

Nst dual tap networking.png

Example 1: Multi-Tap Network Packet Capture Session Step-By-Step

This section will demonstrate a Multi-Tap Network Packet Capture session using 2 network adapters (eth1 and eth2) as the target capture interfaces. We will concentrate on email traffic (POP and SMTP) occurring on network: "". It is assumed that the NST probe has sufficient network interface adapters (at least 2 in this case) to perform this capture and that these interfaces are attached to a network tap similar to what is shown in the diagrams above or some type of port mirroring (e.g., SPAN - Switched Port Analyzer) configuration.

Step: 1 NST WUI Multi-Tap Network Packet Capture Page

From the NST WUI Start page go to the NST WUI Multi-Tap Network Packet Capture page. This is accomplished by using the NST WUI Menu. Go to "Networking" then "Protocol Analyzers" and then go to the "Multi-Tap Network Packet Capture" page as shown below.

Navigate To The Multi-Tap Network Packet Page

Step: 2 Multi-Tap Interfaces & Data Directory Selection

Use the navigational aid link: [Select Tap Interfaces/Directory] to go to the "Multi-Tap Interfaces & Data Directory Selection" section. This is where you select the target capture interfaces and data directory on the NST probe when you will store your multi-tap capture.

Multi-Tap Interfaces & Data Directory Selection

Select interface: "eth1" for Tap0 and select interface: "eth2" for Tap1. For now use the default capture data directory: "/var/nst/wuiout/wireshark". Your configuration should look similar to the NST WUI image caption above. Next select the "Select Multi-Tap Interfaces/Capture Data Directory" button to make your configuration known to NST.

One can also click on a "NIC Adapter" icon to view and control a specific network interface adapter. The caption below shows information for NIC Adapter: "eth2" and results from running the "ethtool eth2" command. Even though the interface state is: "Down", a link has been established and detected on this interface. This is valuable information to know prior to capturing on a particular network interface.

NIC Adapter Information And Control

Both interfaces are initially shown in the "Down" state indicated by the red down arrow superimposed on the "NIC Adapter" icons. The network interface will automatically be put in the "Up" state prior to starting a capture session.

Step: 3 Start Multi-Tap Capture Form

After you have selected your "Tap Interfaces" and "Capture Data Directory" next go to the "Start Multi-Tap Capture" section. For this demonstration we will use a "dumpcap" capture filter expression for "POP" and "SMTP" traffic on network: "". Since we selected to use only 2 Taps (Tap0 and Tap1), then only 2 Taps will be dynamically available for configuration out of a possible 4 Taps in the "Start Multi-Tap Capture" form as shown below.

Multi-Tap Start Capture Form

Fill in the form similar to what is shown above. Optionally use the ['dumpcap' Capture Filter Expressions] navigation aid link to help fill in the "dumpcap" capture filter expression field for each Tap. Each Tap can have a unique "dumpcap" capture filter expression. Use: "Monitor Start" as one of the "WUI Startup Options". Adjust your capture termination thresholds accordingly. In this case, the capture will terminate automatically when either of the following thresholds have been exceeded: a "10 seconds" capture time has elapsed, a file size of: "4MB" has been reached or "100" network packets have been collected.

Step: 4 Multi-Tap Network Packet Capture Start

Once you are satisfied with values in the "Start Multi-Tap Network Packet Capture" form we will now commence a "Multi-Tap Network Packet Capture" session. Click on the "Multi-Tap Network Packet Capture Start" button to begin the capture session. An automatic sequence of NST WUI Multi-Tap Network Packet Capture pages will now appear until the "Pending Multi-Tap Capture" page that is shown below is reached.

Multi-Tap Network Packet Capture Pending

Near real-time dynamic updates using AJAX will occur until one of the termination thresholds is reached. Once termination has been reached, another automatic sequence of NST WUI Multi-Tap Network Packet Capture pages will now appear until the "Multi-Tap Capture Summary" section is now displayed as show below:

Multi-Tap Network Packet Capture Summary

One can see from the "capinfos" summary output that the packet count termination threshold of: "100 Packets" caused the capture session to end.

Step: 5 Multi-Tap Network Packet Capture Decode Analysis

At this point we are now ready to perform a decode of the packet capture. Use the [New Decode] navigation aid link to go to the "Multi-Tap Text-Based Protocol Analyzer Decode" section.

Example 2: Multi-Tap Network Packet Capture Across A Firewall Boundary - NAT/PAT Traffic

This example will explore network packet traffic as it flows across a firewall device. We will use a remote "SSH" client request for this demonstration and explore the NAT/PAT translation that occurs by decoding the capture. The following diagram shows the network environment for this multi-tap capture. The "SSH" client is located remotely on the Internet using IP Address: and the "SSH" server is NAT/PAT translated using IP Address: its real internal IP Address is:

Nst multi tap networking firewall.png

We will be using 3 tap interfaces (Tap0 - 2) for this multi-tap capture. The Hub works in "Half-Duplex" mode and presents both the Transmit and Receive network traffic from the "Dirty" (unfiltered) side of the Firewall device to: "Tap0: eth2". Taps "1" and "2" will be connected to a Net Optics Gigabit copper tap: TP-CU3 for capturing network traffic between the Firewall device and the Gigabit Switch.

Multi-Tap Interfaces & Data Directory Selection Form: NAT/PAT SSH Traffic

Multi-Tap Interfaces & Data Directory Selection Form: NAT/PAT SSH Traffic

Multi-Tap Network Interface Information (eth2): NAT/PAT SSH Traffic

Multi-Tap Network Interface Information (eth2): NAT/PAT SSH Traffic

Multi-Tap Start Capture Form: NAT/PAT SSH Traffic

Multi-Tap Start Capture Form: NAT/PAT SSH Traffic

Multi-Tap Pending Capture: NAT/PAT SSH Traffic

Multi-Tap Pending Capture: NAT/PAT SSH Traffic

Multi-Tap Capture ToolTip Summary: NAT/PAT SSH Traffic

Multi-Tap Capture Summary Status ToolTip

Multi-Tap Capture Decode Form: NAT/PAT SSH Traffic

Multi-Tap Capture Decode Form

Multi-Tap Capture Decode Summary: NAT/PAT SSH Traffic

Nst multi tap networking firewall decode.png

Multi-Tap Capture PDML TCP/IP Decode: NAT/PAT SSH Traffic

Multi-Tap Capture PDML TCP/IP Decode

Multi-Tap Capture PDML Network Packet Details Decode: NAT/PAT SSH Traffic

Multi-Tap Capture PDML Network Packet Details Decode

Multi-Tap Capture PDF Decode: NAT/PAT SSH Traffic

Multi-Tap Capture PDF Decode