/errors /nginx/host-not-found-in-upstream

nginx: [emerg] host not found in upstream "backend"

updated 2026-10-20

tool: nginx
exit code: 1
versions: nginx 1.x
os: any

nginx resolves every hostname in proxy_pass and upstream { server … } once, when it loads the configuration. If a name does not resolve at that moment, nginx -t, start and reload all fail with [emerg] host not found in upstream "backend-app". This happens a lot with containers, where the backend's DNS name exists only while that container is running.

Reproduce it

bash

mkdir -p logs temp
cat > nginx.conf <<'EOF'
pid logs/nginx.pid;
events {}
http {
  server {
    listen 127.0.0.1:8080;
    location / {
      proxy_pass http://backend-app:3000;
    }
  }
}
EOF
# -p . uses this directory as the prefix; sed drops the log timestamp and process id
nginx -p . -c nginx.conf -e stderr -t 2>&1 | sed -E 's/^[0-9/]+ [0-9:]+ \[([a-z]+)\] [0-9#]+: /[\1] /'
exit "${PIPESTATUS[0]}"

Output · exit 1

[emerg] host not found in upstream "backend-app" in ./nginx.conf:7
nginx: configuration file ./nginx.conf test failed

Fix: make the name resolvable

bash

getent hosts backend-app          # from inside the nginx container: docker compose exec nginx getent hosts backend-app

compose.yaml

services:
  nginx:
    image: nginx:1.29
    depends_on: [backend-app]
  backend-app:
    image: example/app:1.0

Services in the same Compose project share a default network, where each service name is a DNS name.

Fix: resolve at request time

With a variable in proxy_pass, nginx looks the name up per request through the resolver (Docker's embedded DNS is 127.0.0.11), so it starts even when the backend is down and follows IP changes:

nginx

resolver 127.0.0.11 valid=10s;
location / {
    set $backend http://backend-app:3000;
    proxy_pass $backend;
}

Sources

errors · nginxnginx: "proxy_pass" cannot have URI part in location given by regular expressionnginx rejects proxy_pass http://host/path/ inside a regex location, named location, if or limit_except. Remove the URI part and change the path with rewrite.commands · nginxnginx -t: test the configuration before reloadingnginx -t checks the configuration syntax and the files it references without touching the running server; nginx -T also prints the full merged config.glossary · nginxWhat is a reverse proxy?A reverse proxy is a server in front of your applications that receives client requests and forwards them to backends, adding TLS, caching and balancing.

Reproduce it

Fix: make the name resolvable

Fix: resolve at request time

Sources

Related pages