How can I customize the message shown to a user who reaches a Guru Meditation error when making a request thru my Varnish server? - varnish

Question

I am trying to customize what appears on the screen for a user when he reaches a Guru Meditation error screen, upon making an erring request to my backend, which has a Varnish reverse proxy in front. I have tried placing log.info(client.ip) in the different subroutines of default.vcl, only to run into compilation errors when trying to start the varnish service. I am using a Linux virtual machine.

Answer 1

You certainly can. Have a look at the following tutorial I created: https://www.varnish-software.com/developers/tutorials/vcl-synthetic-output-template-file/.
Modifying the synthetic output templates
Here's the VCL code you need to extend the regular vcl_synth subroutine in case you call return(synth()) from your VCL code, as well as the code you need to extend vcl_backend_response in case of a backend fetch error:
vcl 4.1;
import std;
sub vcl_synth {
set resp.http.Content-Type = "text/html; charset=utf-8";
set resp.http.Retry-After = "5";
set resp.body = regsuball(std.fileread("/etc/varnish/synth.html"),"<<REASON>>",resp.reason);
return (deliver);
}
sub vcl_backend_error {
set beresp.http.Content-Type = "text/html; charset=utf-8";
set beresp.http.Retry-After = "5";
set beresp.body = regsuball(std.fileread("/etc/varnish/synth.html"),"<<REASON>>",beresp.reason);
return (deliver);
}
As explained in the tutorial, you can store the HTML code you want to display in an HTML and load this into your VCL output.
The trick is to put a <<REASON>> placeholder in your HTML where the actual error message gets parsed into.
Adding custom logging to your VCL
If you want to add custom logging that gets sent to VSL, you can use the std.log() function that is part of vmod_std.
Here's some example VCL code that uses this function:
vcl 4.1;
import std;
sub vcl_recv {
std.log("Client IP: " + client.ip);
}
The log will be displayed through a VCL_Log tag in your VSL output.
If you want to filter out VCL_Log tags, you can use the following command:
varnishlog -g request -i VCL_Log
This is the output you may receive:
* << Request >> 32770
- VCL_Log Client IP: 127.0.0.1
** << BeReq >> 32771
If you're not filtering the VCL_Log tag, you'll see it appear in your VSL output if your run varnishlog -g request.
Tip: if you want to see the full log transaction but only for a specific URL, just run varnishlog -g request -q "ReqUrl eq '/'". This will only display the logs for the homepage.
Update: displaying the client IP in the synthetic output
The VCL code below injects the X-Forwarded-For header into the output by concatenating to the reason phrase:
vcl 4.1;
import std;
sub vcl_synth {
set resp.http.Content-Type = "text/html; charset=utf-8";
set resp.http.Retry-After = "5";
set resp.body = regsuball(std.fileread("/etc/varnish/synth.html"),"<<REASON>>",resp.reason + " (" + req.http.X-Forwarded-For + ")");
return (deliver);
}
sub vcl_backend_error {
set beresp.http.Content-Type = "text/html; charset=utf-8";
set beresp.http.Retry-After = "5";
set beresp.body = regsuball(std.fileread("/etc/varnish/synth.html"),"<<REASON>>",beresp.reason + " (" + bereq.http.X-Forwarded-For + ")");
return (deliver);
}
It's also possible to provide a second placeholder in the template and perform an extra regsuball() call. But for the sake of simplicity, the X-Forwarded-For header is just attached to the reason string.