Changing the URL that gets logged?

Short version: Can you change req.url in vcl_log before actually doing any of the logging? I thought you could, and I don't get an error when I try, but it doesn't seem to have any effect either.

Background: For complicated historical reasons, our logic flow for 404s is like this:

Original request comes in
Request gets sent to origin.
404 is returned from origin.
req.url is rewritten to be like req.url = "/my404page"
restart; is called.
The request, now with req.url = "/my404page", goes to the origin, appropriate content is returned, and the content is served to the user with status 404.
vcl_log is called and logging is done.

But because we rewrote req.url in step 4, what gets logged is a request for "/my404page", which is not useful because now every single request that 404s looks exactly the same. I would like to see the original URL.

I thought I could do something like "3.5 set req.http.x-original-url = req.url", and then "6.5 set req.url = req.http.x-original-url", but it does not seem to have any effect at all, as though req.url were immutable in vcl_log. Any suggestions?

Ilya Kaplan

February 24, 2020 11:14

What happens is that when the VCL is run after the restart req.url is new, so `req.http.x-original-url = req.url` saves the new url as `x-original-url`.

I suggest you do:

if (!req.http.x-original-url) { set req.http.x-original-url = req.url; }

as early as possible in vcl_recv, and in the vcl_log change %{req.url}V to `%{ if(req.http.x-original-url, req.http.x-original-url, req.url) }V`

JD Weiner

February 24, 2020 19:30

Oh yeah, now that you explain it, it makes sense. :^) Thanks!

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