How to find out a single failing backend from a round-robin director?(Inline-C)

Stefan Pommerening pom at
Thu Feb 10 10:33:39 CET 2011

Hi Thiago,

Am 09.02.2011 22:55, schrieb Thiago Figueiro:

 > I believe varnishlog has all you need:
 >    54 BackendOpen  b backend_name source_ip source_port dest_ip dest_port
 > Just track the id on the left (54 in this case) for the request and 
response and you should be able to find the culprit.

That's correct - in my case the following (example):

   417 VCL_call     c recv
   417 VCL_return   c lookup
   417 VCL_call     c hash
   417 VCL_return   c hash
   417 VCL_call     c miss
   417 VCL_return   c fetch
    31 BackendClose - cmsweb5f3cae1
    31 BackendOpen  b cmsweb5f3cae1 10.40.XXX.YYY 37607 10.40.XXX.ZZZ 80
   417 Backend      c 31 group5cae1 cmsweb5f3cae1
    31 TxRequest    b GET

Unfortunately the inline-C expression VRT_backend_string(sp) gives me 
only the director name, which is 'group5cae1'.

What I need to get in my inline C is the specific backend choosen from 
the director, 'cmsweb5f3cae1' in this example.
How to get this? Any VRT_something for this?

-- stefan

 > From: varnish-misc-bounces at 
[mailto:varnish-misc-bounces at] On Behalf Of Stefan 
 > Sent: Wednesday, 9 February 2011 9:47 PM
 > To: varnish-misc at
 > Subject: How to find out a single failing backend from a round-robin 
 > Hi all,
 > I'm working with a set of varnish-caches (2.1.3) in front of some 
round-robin director configured backends.
 > Sometimes I experience a 503 error due to some unforseeable 
circumstances like network hiccups, content server problems and other 
magic...  ;-)
 > Nevertheless all backends are healthy regarding the large majority of 
other requests (and watchdog of course), we deal with sporadic failures 
 > Obviously I don't want those 503's to go through to the user.
 > I inserted the following code in my vcl_fetch:
 > if (beresp.status == 503) {
 >    set beresp.saintmode = 2s;
 >    restart;
 > }
 > This works quite well, the failed (503 delivering) request is 
forwarded to the next backend in the director and is successfully served 
 > What I am missing is the exact backend information which backend did 
not serve the request or better said: where did the 503 come from?
 > I already included some inline-C in order to send info to syslog, but 
I am missing some VRT_(r_)something to find out the failing backend 
without searching the backend's logfiles directly.
 > My current syslog()-msg is:
 > syslog (LOG_LOCAL7 | LOG_INFO, "VarnishLog(vcl_fetch): %s/%s[%s] 
[REST=%02d] %s http://%s%s",
 >    VRT_backend_string(sp), VRT_r_server_hostname(sp), 
 >    VRT_r_req_restarts(sp), VRT_r_req_request(sp), VRT_GetHdr(sp, 
HDR_REQ, "\005host:"), VRT_r_req_url(sp));
 > I managed to get the backend definition, but not the specific backend 
machine name (or IP) from the backend director.
 > ... and: is there some sort of overview on which VRT_magic fields 
(for use with inline-C) exists?
 > Thanks in advance,
 > Stefan


Dipl.-Inform. Stefan Pommerening
Informatik-Büro: IT-Dienste & Projekte, Consulting & Coaching

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