Test connectivity for the web service

find the problem occurred: (1) local service at the server (localhost) (2) from LAN to server (3) from WAN to server

Connecting from the local machine to web service at the same server

WAN ○ -------> ○ LAN -------> ● server

Testing on localhost if you have the administrative permission to manage the server.

Check if your computer/server Is being assigned the IP Address

• • Linux : (1) Check the IP Address. ifconfig. (2) Re-assign the IP Address and restart the networking service. service network restart for CentOS
  • Win : Check the IP Address. ifconfig/all
  • More on Network problem

Is the service alive or the port was occupied by other process?

• • Linux
    • nmap "Network exploration tool and security / port scanner." e.g. nmap -p 80,443 ip
    • ss "utility to investigate sockets" e.g. ss -tnlp | grep 80 or using OR operator ss -tnlp | grep '80\|443'
    • ps List the current processes. Input ps -aux | grep "PORT_NUMBER" e.g. ps -aux | grep 443
    • netstat Command netstat -tulpn | grep LISTEN e.g.
  • Win Using the software CurrPorts to check the process and port. e.g. Apache will not running if the 80 port was occupied by skype.

$ netstat -tulpn | grep LISTEN

tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      - 

// means the 80 port service is running

• more on Find process running on port

Is the port opened for external users?

Check the OS firewall or protected by antivirus software

• • Linux
    • iptables: (1) iptables -L to list all rules (2) rule with smaller line-number will override the rule with larger line-number
    • FirewallD rules: (1) sudo firewall-cmd --get-default-zone return 'public' (2) sudo firewall-cmd --list-all --zone=public (3) Allow port with FirewallD
    • SELinux Policy Management: (1) After installed policycoreutils-python, input semanage port -l to list open ports. (2) Allow port with semanage.
  • Win Windows Firewall with Advanced Security -> configure firewall rules
  • protected by antivirus software?
• connect to localhost ex: telnet 127.0.0.1 23 where 23 is the port number where the service running
  • Install Telnet Client on Win
  • if the local service is web server, you can also try lynx http://127.0.0.1 for Linux
• double check the server/service logs

Connecting from LAN to the server

WAN ○ -------> ● LAN -------> ● server

Testing on LAN (local network, subnet scope): The different servers between LAN or with the same ip class (ex: 1.2.3.*) may not have the firewall limit.

• Is the service alive?
• Connect to the internal IP eg: telnet 172.18.0.1 23 (1) IP start with 172.18... is Private IP Network Numbers (2) where 23 is the port number where the service running
• If the local service is web server, you can also try to connect the internal IP e.g. lynx http://172.18.0.1 for Linux
• Double check the server/service logs

Connecting from WAN to the server

WAN ● -------> ● LAN -------> ● server

We met the trouble on WAN (wide area network). If we do no have the administrative permission, the tests we can do as follows:

• Down for everyone or just me? more on Web Ping
• Switch to different ISP? Is hardware firewall enabled?
• Is gateway alive?
• Is DNS alive?
• Is the external IP correct?
  • If the firewall rule is IP specified, verify the IP if correct from What Is My IP Address? or web access log.
  • Query the IP Address Details - ipinfo.io. e.g. Input the command curl ipinfo.io/ip for macOS & Linux
• Is the port was opened for external users? Open Port Check Tool - Port Forwarding Port Check Tool
• Is institutional/ISP firewall enabled? Contact your MIS. (Management information system) of your company or ISP?

other issue:

• bandwidth cost too much by certain users
• exceed the request limit of (hardware) firewall
• the external connection of updated Windows was down until reboot Win
• Amazon EC2 security group
• refresh browser cache: Refresh your cache - When hitting F5 just isn't enough!