Having multiple different vhosts in the same Varnish is a very typical use-case, and from Varnish 5.0 it is possible to have a separate VCL files for separate vhosts or any other distinct subset of requests.
Assume that we want to handle varnish.org with one VCL file and varnish-cache.org with another VCL file.
First load the two VCL files:
vcl.load vo_1 /somewhere/vo.vcl
vcl.load vc_1 /somewhere/vc.vcl
These are 100% normal VCL files, as they would look if you ran only that single domain on your varnish instance.
Next we need to point VCL labels to them:
vcl.label l_vo vo_1
vcl.label l_vc vc_1
Next we write the top-level VCL program, which branches out to the other two, depending on the Host: header in the request:
/* We have to have a backend, even if we do not use it */
backend default { .host = "127.0.0.1"; }
sub vcl_recv {
if (req.http.host ~ "varnish.org$") {
return (vcl(l_vo));
}
if (req.http.host ~ "varnish-cache.org$") {
return (vcl(l_vc));
}
return (synth(302, "http://varnish-cache.org"));
}
sub vcl_synth {
if (resp.status == 301 || resp.status == 302) {
set resp.http.location = resp.reason;
set resp.reason = "Moved";
return (deliver);
}
}
Finally, we load the top level VCL and make it the active VCL:
vcl.load top_1 /somewhere/top.vcl
vcl.use top_1
If you want to update one of the separated VCLs, you load the new one and change the label to point to it:
vcl.load vo_2 /somewhere/vo.vcl
vcl.label l_vo vo_2
If you want to change the top level VCL, do as you always did:
vcl.load top_2 /somewhere/top.vcl
vcl.use top_2