My OVH load balancer shows Error 503: Backend unavailable

Error 503: Backend unavailable


This type of error usually results of an unavailability of servers behind IP Load Balancing.





here is nmap -Pn on the backend server:
Result of the nmap test on the Backend: (nmap -Pn "ip_du_backend"):
Starting Nmap 7.94 ( https://nmap.org ) at 2023-09-16 13:33 Greenwich Standard Time
Nmap scan report for vps-1cacc0d9.vps.ovh.net (141.94.68.128)
Host is up (0.13s latency).
Not shown: 930 filtered tcp ports (no-response), 58 closed tcp ports (reset)
PORT STATE SERVICE
21/tcp open ftp
25/tcp open smtp
53/tcp open domain
80/tcp open http
110/tcp open pop3
143/tcp open imap
443/tcp open https
465/tcp open smtps
587/tcp open submission
993/tcp open imaps
995/tcp open pop3s
3306/tcp open mysql

Nmap done: 1 IP address (1 host up) scanned in 6.84 seconds

Result of the iptable command on the backend: (iptables -L):


Error message:
login as: ubuntu
ubuntu@141.94.68.128's password:
login as: ubuntu
ubuntu@141.94.68.128's password:
Welcome to Ubuntu 20.04.6 LTS (GNU/Linux 5.4.0-162-generic x86_64)

* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/advantage

System information as of Sat Sep 16 11:07:54 UTC 2023

System load: 0.05 Processes: 291
Usage of /: 17.8% of 154.88GB Users logged in: 0
Memory usage: 5% IPv4 address for ens3: 141.94.68.128
Swap usage: 0%

* Strictly confined Kubernetes makes edge and IoT secure. Learn how MicroK8s
just raised the bar for easy, resilient and secure K8s cluster deployment.

https://ubuntu.com/engage/secure-kubernetes-at-the-edge

Expanded Security Maintenance for Applications is not enabled.

26 updates can be applied immediately.
To see these additional updates run: apt list --upgradable

46 additional security updates can be applied with ESM Apps.
Learn more about enabling ESM Apps service at https://ubuntu.com/esm


Last login: Fri Sep 15 10:16:03 2023 from 197.239.80.129
ubuntu@141-94-68-128:~$ sodo -i
WARNING:root:could not open file '/etc/apt/sources.list.d/cpanel-plugins.list'


Command 'sodo' not found, did you mean:

command 'nodo' from snap nodo (master)
command 'sudo' from deb sudo (1.8.31-1ubuntu1.5)
command 'sudo' from deb sudo-ldap (1.8.31-1ubuntu1.5)
command 'todo' from deb devtodo (0.1.20-7build1)

See 'snap info ' for additional versions.