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The Internet Content Adaptation Protocol, as specified in RFC 3507 and errata (these latter maintained by the Measurement Factory. I've written an introductory guide, "The Working Man's Guide to ICAP". Implementors and users of ICAP technology will likely also require extensive knowledge of encapsulated protocols, most usually HTTP. The book Content Networking: Architecture, Protocols and Practice has pretty good ICAP/HTTP coverage. A closely related protocol is OPES/OCP.

Implementation notes


  • Squid 3.0 was released in December 2007, advertising far better ICAP support than previous releases.
  • Squid 3.0 seems to postpend headers to the HTTP body when a REQMOD is answered with Request-Satisfaction, as well as drop the CRLF between headers and the body. This results in a browser stall (personal observation).
  • Squid 3.0RC6 is reported to break X-TE handling (bug report).


  • The 6.0+ series sends "X-Encapsulated-Protocol", with values of "http", "https", "ftp" or "smtp".
  • The 6.0+ series sends "X-Proxy-Addr", with the numeric IP address of the ICAP client as its value.
  • The 6.0+ series sends "X-Chunk-Extensions", with values including "req-cont" and "req-stop".


  • When previews are enabled, the Proxy SG sends "X-Scan-Progress-Interval".


  • The NetCache C1300 sends the "NetCache-TE" header, as opposed to "X-TE".

Symantec Anti-Virus Engine

  • Sends "X-Infection-Found" and "X-Violations-Found with arbitrary string values

See Also