[squid-users] ICAP and Allow 204 Header

[Alex Rousskov]

On 01/25/2016 10:28 AM, Gilles Bardouillet wrote:

> I'm using SQUID with CAS ICAP Server but I have one issue :
> 
>  * for some images, squid receive icap error as ICAP_ERR_OTHER

It may be useful to know more details about that ICAP error. What ICAP
response, if any, does Squid receive when it generates ICAP_ERR_OTHER?


>  * I noticed that for all these errors, Squid dont send the HTTP header
>    Allows 204

Allow:204 is not an HTTP header field. It is an ICAP header field.


>  * I read the code and find the Allow 204 header _is only set when
>    preview is enabled_.

Are you sure? Several factors affect ICAP Allow:204 request header
presence. Preview availability should not be one of them because
Allow:204 is about 204 responses _outside_ of Preview. See RFC 3507
Section 4.6.


> My icap conf activated preview and preview size as follow :
> icap_preview_enable on
> icap_preview_size 1024

IIRC, Squid ignores icap_preview_size in squid.conf (a bug). The ICAP
service OPTIONS response determines the Preview size (subject to an
internal limit of 64KB).


> I read that the preview size value can be overwritten by OPTIONS
> requests, so can give me some details, hints in order to find why some
> pictures dont offer preview and then fails ?

See RFC 3507 Section 4.5 for details on how Preview is negotiated. If
you think Squid violates the ICAP protocol, please file a bug report
with the corresponding capture of ICAP messages (from and to Squid).

As for ICAP 204 outside of Preview, I believe Squid can offer to support
that ICAP response if all of the checks below are successful:

  * the origin server OPTIONS response includes Allow:204;
  * the message content length is known at the ICAP request time; and
  * the message content length does not exceed 64KB.

If you prefer to analyze the code, see
Adaptation::Icap::ModXact::shouldAllow204() and
Adaptation::Icap::ModXact::canBackupEverything().


HTH,

Alex.