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

Thiago Figueiro TFigueiro at
Wed Feb 9 22:55:19 CET 2011

(sorry for top-posting)


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.  



From: varnish-misc-bounces at [mailto:varnish-misc-bounces at] On Behalf Of Stefan Pommerening
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 director?(Inline-C)


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 here.

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;

This works quite well, the failed (503 delivering) request is forwarded to the next backend in the director and is successfully served then.

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 <http://%25s%25s> ", 
   VRT_backend_string(sp), VRT_r_server_hostname(sp), VRT_IP_string(sp,VRT_r_server_ip(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,


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


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