HTTP content encoding and caching analysis with tcpdump and wireshark

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Published on June 26th 2015 - Listed in Linux Network

To analyze a http caching problem I needed to grab the network connections and take a look into the http protocol and find possible problematic http requests. I used tcpdump to capture the tcp streams and wireshark to analyze the captured packets.

The following filters came in handy.

Show connections which requested (the main domain) on the webserver

http.request.uri == "/" && == "" && ip.dst ==

Show connections which contain a HTTP 200 response code but don't contain the "Content-Encoding" http header:

http.response.code == 200 && !http.content_encoding

Show http responses where the content was gzip compressed:

http.content_encoding == gzip

Don't show http content, only headers (from

http.response !=0 || http.request.method != "TRACE"

To be able to create filters with custom http headers, I first needed to add them to Wiresharks preferences:
Edit -> Preferences -> Protocols -> HTTP -> Custom HTTP headers fields -> Edit

I added the following additional headers:

  • Age: Age of Cache
  • X-Cache: Cache Type
  • X-Varnish-Hostname: Hostname of Varnish Cache

To use these custom http headers as a filter, you need to use the http.header prefix.

Show http responses which weren't cached, which don't contain the "Content-Encoding" header and which were treated by varnish-3 server:

http.header.X-Cache == MISS && !http.content_encoding && http.header.X-Varnish-Hostname == varnish-3

Show http responses which were cached (HIT) and treated by varnish-4 server:

http.header.X-Cache == HIT && http.header.X-Varnish-Hostname == varnish-4

Show http responses which pass through a varnish server (so the header X-Varnish-Hostname exists), have response code 200 and don't contain the "Content-Encoding" header:

http.header.X-Varnish-Hostname && http.response.code == 200 && !http.content_encoding

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