We ran into this on a recent project, and I've seen similar issues in the queue, so here's a block that prints helpful server info for domain loading.

Comments

Status:Active» Patch (to be ported)
StatusFileSize
new6.88 KB

And the patch. Committed to 7.x.3. Needs backport.

StatusFileSize
new6.9 KB

Patch committed for 7.x.2.

Version:7.x-3.x-dev» 6.x-2.x-dev

Moving versions.

Documentation, which needs to be added to the d.o. docs.

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5.3 Block -- Domain Access Server Information
Provides a block of information related to the current page request. Use this
block to help determine how a server request is being handled by Domain Access.
If you are having trouble with domains not resolving correctly, this block can
help you pinpoint the problem. In particular, note the first two lines of the
output:
  HTTP_HOST request     example.com
  Domain match          TRUE
The first line tells you what HTTP_HOST (i.e. the domain string) was passed by
the server to Drupal. The second line indicates how Domain Access reads that
value.
Possible replies for the 'Domain match' value are:
  TRUE
    Indicates a matching domain record.
  FALSE: Using default domain
    Indicates a non-matching domain, and that Domain Access is using the
    default domain as a fallback.
  ALIAS: Active id #
    Used when Domain Alias is installed. Indicates that the request matched a
    known alias of the active domain.
Below the Domain match line, you will see additional information about the
current $_domain global object.
NOTE: By design, this block is viewable by all users.  However, its content
should only be shown to site developers or during debugging.  You should use
the normal block visiblity settings as appropriate to your site.

Component:User interface» Documentation
Status:Patch (to be ported)» Needs work
Issue tags:+handbook
StatusFileSize
new3.96 KB

The patch for D6, also committed.

Title:Provide server debugging infoDocument server debugging info block
StatusFileSize
new451 bytes

Forgot to disable block cache for that. Follow-up for D6, also committed.

Status:Needs work» Fixed

Automatically closed -- issue fixed for 2 weeks with no activity.