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HTTP sniffing and low-level visualisation
JavaScript CSS
Tag: v0.1.3

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htracr is a packet sniffer and visualisation tool for HTTP. It does not give you a score, grade, or hold your hand when you're crying because your site is so damn slow, but it will give you unparalleled insight into what's actually happening on the wire between your browser and the Web.


htracr can be installed with npm like this:

npm install htracr

which will install dependencies automatically. See for more information about npm.

Under the covers, htracr relies upon:

You'll also need a modern Web browser (known to work: Safari 5, FF 4, Chrome)

Using htracr

htracer is designed for use on the same machine your web browser or other client runs on; while it's possible to run it on a server, it'll be difficult to make sense of all of the traffic coming to a normal server.

To use htracr, start it up like this:

htracr [listen-port]

where listen_port is the port you'd like htracr to be available on. Then, point your browser at it; e.g.:

htracr 8000

means you should point at:


On some operating systems, you may need to specify the interface to listen on. For example:

htracr 8000 eth0

and in some cases, you may need permission to listen to the device, making the appopriate command line something like:

sudo htracr 8000 eth0

Then, press 'start' to start capturing HTTP traffic, and 'stop' to show it. Currently, htracr only captures traffic on port 80.

The slider will adjust the time scale.


Mark Nottingham

Obligatory Screenshot

htracr screenshot


TCP visualisation

  • Make connection states visible:
    • handshaking
    • connected
    • half-connected
    • idle
    • disconnecting
    • buffer-full?
  • per-connection stats:
    • total packets (graph of types?)
    • congestion window (over time?)
    • receive window (over time?)
    • rtt over time?
  • handle tcp-retransmit / tcp-reset
  • show ack relationships?

HTTP visualisation

  • Highlight message headers that are split across packets
  • Highlight pipelining
  • message stats
    • message delay
    • number of round trips
  • click on response to open window with it
  • click on request to re-make request
  • server stall time (based upon rtt / packet sizes / psh)
  • content types - message colours?

Misc. Features

  • configurable sniff port(s) in-browser
  • printing
  • trace DNS
  • per-server stats
  • allow removing connections / servers
  • keyboard controls
  • dump / load pcap sessions
  • magnifier, because packets get lost between the pixels


  • needs one mother of a refactoring
  • header names are case-sensitive here
  • prettify packet display
  • need HTTP status phrase
  • some requests not drawn in firefox 3
  • connection: close on requests - do we get a http-res-end?
  • pipelining
  • improve RTT calculation
  • keep centre on zoom
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