The ‘Flows’ entry in the top toolbar can be selected to visualize realtime traffic information on the currently active flows. A flow can be thought of as a logical, bi-directional communication channel between two hosts [1]. Multiple simultaneous flows can exist between the same pair of hosts.

Active Flows

Active Flows Page

Flows are uniquely identified via a 5-tuple composed of:

  • Source and destination IP address
  • Source and destination port
  • Layer-4 protocol

Each flow is shown as a row entry in the flows table. Flows are sortable by application using the rightmost dropdown menu at the top right edge of the table. Similarly, the other dropdown menu enables the user to choose the number of flows displayed on each page.

Flows have multiple information fields, namely, Application, Layer-4 Protocol, Client and Server hosts, Duration, Client and Server Breakdown, Current Throughput, Total Bytes, and Additional Information. Information fields are briefly discussed below.


Application is the Layer-7 program which is exchanging data through the flow. This is the piece of software that lays closest to the end user. Examples of Applications are Skype, Redis, HTTP, and Bit Torrent. Layer-7 applications are detected by the NTOP open source Deep Packet Inspection (DPI) engine named nDPI [2]. In case application detection fails, ntopng marks the flow as ‘Unknown’. If the detection succeeds, the application name and an informative icon are shown.

Here is a list of possible informative icons:

  • The lock icon tells that the protocol carries information in a secure way (e.g. via SSL)
  • The thumb-up icons tells that the protocol is generally not harmful for network performance
  • The thumb-down icons tells that the protocol is generally harmful for network performance
  • The smile face tells that the protocol is generally used for user entertainment
  • The yellow triangle indicates a possible problem on the flow. By clicking a flow details it’s possible to see what the reported problem is (e.g. flow with low goodput)

The application name can be clicked to see all hosts generating traffic for the application.

Layer-4 Protocol (L4 Proto)

The layer-4 protocol is the one used at the transport level. Most common transport protocol are the reliable Transmission Control Protocol (TCP) and the best-effort User Datagram Protocol (UDP).


This field contains host and port information regarding the client endpoint of the flow. An host is considered a client if it is the initiator of the flow. Information is shown as host:port and both information are clickable. If the host has a public IP address, ntopng also shows the country flag for that client [3] . A blue flag is drawn when the host is the ntopng host.


Similarly to the client, this field contains information regarding the server endpoint of the flow. An host is considered a server if it is not the initiator of the flow. We refer the reader to the previous paragraph for a detailed description.


This is the amount of time that has elapsed since the flow was opened by the client.


Flows are bi-directional, in the sense that traffic flows both from the server to the client and from the client to the server. This coloured bar gives and indication on the amount of traffic exchanged in each of the two directions. Client to server traffic in shown in orange, while server to client in blue.

Actual Throughput

The throughput is computed periodically (the refresh time is a few seconds).

Total Bytes

The amount of traffic exchanged thought the flow. This total value is the sum of traffic exchanged in each of the two directions (client to server and server to client).

TLS Information

ntopng provides detailed information on TLS flows:

TLS information
  • The TLS certificate requested by the client and the server names returned by the server
  • The TLS certificate validity time frame
  • Client and server JA3 signatures, which represent a fingerprint of the most relevant information in the TLS handshake. By clicking on the signature it is possible to manually check if the signature corresponds to a known malware into the abuse.ch database.
  • The Client TLS ALPN, which contains the list of application protocols provided by the client during the TLS handshake
  • The supported TLS protocols of the client

This information is very valuable in identifying potential threats on the encrypted traffic, which include but are not limited to:

  • Malicious clients/server (by matching the JA3 signature)
  • Expired TLS certificates
  • Unsecure TLS protocols in use
  • MITM TLS attacks

ntopng already reports such events with specialized alerts (the available alerts depend on the ntopng version used). For an in depth discussion on the challenges with encrypted traffic and how the metadata can help in identifying network threats, check out the related ntop blog post and related posts.

SSH Signature

In a similar way to the JA3 TLS signature, HASSH is a fingerprint on the SSH handshake. ntopng generates the HASSH fingerprint of both the client and the server hosts of the flow. ntopng also extracts and visualizes the SSH application banner which usually reports the name and version of the SSH client/server application used.

This information can be used to identify outdated and vulnerable programs, which undermine the hosts security. Moreover, the HASSH fingerprint can be matched against known malware signatures to identify known threats.


Extra information nDPI is able to extract from the detected flow is made available in this field. This field may include URLs, traffic profiles (in the Professional Version), contents of DNS requests, and so on.

[1]Actually, flows may also exist between a host and a multicast group, as well as a broadcast domain.
[3]These data are based on MaxMind databases.