api platform: Error (null) Backend fetch failed - rest

Question

We use symfony with the platform api in docker and we have a problem with varnish.
For local development varnish works with the default.vcl configuration file (https://github.com/api-platform/api-platform/blob/master/api/docker/varnish/conf/default.vcl). When uploading to the server, varnish provides the error "Error (null) Backend fetch failed"
When disabling varnish and redirecting to nginx with php-fpm, api-platform works properly.
I increased the http_resp_hdr_len parameters to 131072 bytes (128K) and http_rasp_size to 10485760 bytes (10Mb), but it doesn't help, and the error remains.
docker command to start varnish:
CMD ["varnishd", "-F", "-f", "/etc/varnish/default.vcl", "-p", "http_resp_hdr_len=131072", "-p", "http_resp_size=10485760"]
Also the parameter - .first_byte_timeout = 600s; was added to default.vcl varnish
default.vcl
vcl 4.0;
import std;
backend default {
.host = "api";
.port = "80";
.first_byte_timeout = 600s;
# Health check
.probe = {
.url = "/";
.timeout = 5s;
.interval = 50s;
.window = 5;
.threshold = 3;
}
}
# Hosts allowed to send BAN requests
acl invalidators {
"localhost";
"php";
# local Kubernetes network
"10.0.0.0"/8;
"172.16.0.0"/12;
"192.168.0.0"/16;
}
sub vcl_recv {
if (req.restarts > 0) {
set req.hash_always_miss = true;
}
# Remove the "Forwarded" HTTP header if exists (security)
unset req.http.forwarded;
# To allow API Platform to ban by cache tags
if (req.method == "BAN") {
if (client.ip !~ invalidators) {
return (synth(405, "Not allowed"));
}
if (req.http.ApiPlatform-Ban-Regex) {
ban("obj.http.Cache-Tags ~ " + req.http.ApiPlatform-Ban-Regex);
return (synth(200, "Ban added"));
}
return (synth(400, "ApiPlatform-Ban-Regex HTTP header must be set."));
}
# For health checks
if (req.method == "GET" && req.url == "/healthz") {
return (synth(200, "OK"));
}
}
sub vcl_hit {
if (obj.ttl >= 0s) {
# A pure unadulterated hit, deliver it
return (deliver);
}
if (std.healthy(req.backend_hint)) {
# The backend is healthy
# Fetch the object from the backend
return (restart);
}
# No fresh object and the backend is not healthy
if (obj.ttl + obj.grace > 0s) {
# Deliver graced object
# Automatically triggers a background fetch
return (deliver);
}
# No valid object to deliver
# No healthy backend to handle request
# Return error
return (synth(503, "API is down"));
}
sub vcl_deliver {
# Don't send cache tags related headers to the client
unset resp.http.url;
# Comment the following line to send the "Cache-Tags" header to the client (e.g. to use CloudFlare cache tags)
unset resp.http.Cache-Tags;
}
sub vcl_backend_response {
# Ban lurker friendly header
set beresp.http.url = bereq.url;
# Add a grace in case the backend is down
set beresp.grace = 1h;
}
Please advise what might be a problem with varnish and how to make it work correctly?

Answer 1

The problem was solved as follows - I took the parameters for running varnish from thomasmoreaumaster (https://github.com/api-platform/api-platform/issues/1367).
CMD ["varnishd", "-F", "-f", "/etc/varnish/default.vcl", "-p", "http_resp_hdr_len=128k", "-p", "http_resp_size=128k", "-p", "http_req_hdr_len=64k", "-p", "workspace_backend=256k", "-p", "workspace_client=256k", "-p", "http_max_hdr=256"]
Also in the proxying nginx api-platform removed directory binding with ssl and formed an nginx-proxy with ssl enabled via volume.
Now varnish works well.
Thanks for your help and support.