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Back to Basics: Packet Capture & Analysis

November 8, 2018

packet capture analysis garland technology

Network monitoring and analysis tools aren’t new topics by any means, but as with everything, sometimes it’s great to go back to the basics and get a refresher.  While security tools and trends tend to get all the attention, it’s important not to forget how important packet capture is to long-term business success. Packet capture is the ultimate troubleshooting tool for your network.  After all, if you really want to know what’s going on in the network, you need to capture traffic.

By definition, packet capture refers to the real-time interception of a data packet that crosses or moves over a specific portion of a computer network. Once that packet is captured, it is then stored temporarily to be analyzed by either another tool or the network administrator.

It’s something so simple, but capturing packets from key points in the network can help diagnose and solve any network problems, as well as determining if security protocols are being followed correctly. By comparing captured packets from the same point in a network overtime, you can determine and observe normal behavior, making it easier to identify deviations from the baseline.

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The are numerous tools and techniques to use for packet capture, so the network and business requirements will really drive what tool(s) are used. Depending on the use case, capturing just a portion of a packet may be enough, which other times you’ll need to capture the complete packets, which include both the header and payload, so you can access the metadata for source and destination addresses.

Being a free and open source packet analyzer, and who doesn’t like free, Wireshark is commonly used by most network technicians as a preliminary tool for network troubleshooting and analysis. However for most networks, Wireshark will not be enough, so companies will rely on a more advanced tool like the ones from Corelight for a more in depth analysis of the network traffic. To ensure that your packet capture tool gets 100% of the network traffic, industry best practice states to use a network TAP rather than rely on SPAN ports as the access method. SPAN ports often experience packet dropping when ports become oversubscribed and can have degraded performance due to the use of production switches and routers. Network TAPs on the other hand enable packet capture that doesn’t distort or drop data regardless of bandwidth or physical layer errors.

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Heartbeats Packets Inside the Bypass TAP

If the inline security tool goes off-line, the TAP will bypass the tool and automatically keep the link flowing. The Bypass TAP does this by sending heartbeat packets to the inline security tool. As long as the inline security tool is on-line, the heartbeat packets will be returned to the TAP, and the link traffic will continue to flow through the inline security tool.

If the heartbeat packets are not returned to the TAP (indicating that the inline security tool has gone off-line), the TAP will automatically 'bypass' the inline security tool and keep the link traffic flowing. The TAP also removes the heartbeat packets before sending the network traffic back onto the critical link.

While the TAP is in bypass mode, it continues to send heartbeat packets out to the inline security tool so that once the tool is back on-line, it will begin returning the heartbeat packets back to the TAP indicating that the tool is ready to go back to work. The TAP will then direct the network traffic back through the inline security tool along with the heartbeat packets placing the tool back inline.

Some of you may have noticed a flaw in the logic behind this solution!  You say, “What if the TAP should fail because it is also in-line? Then the link will also fail!” The TAP would now be considered a point of failure. That is a good catch – but in our blog on Bypass vs. Failsafe, I explained that if a TAP were to fail or lose power, it must provide failsafe protection to the link it is attached to. So our network TAP will go into Failsafe mode keeping the link flowing.

Glossary

  1. Single point of failure: a risk to an IT network if one part of the system brings down a larger part of the entire system.

  2. Heartbeat packet: a soft detection technology that monitors the health of inline appliances. Read the heartbeat packet blog here.

  3. Critical link: the connection between two or more network devices or appliances that if the connection fails then the network is disrupted.

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