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
- 1 How to connect to the web service
- 2 Test connectivity for the local web service
- 3 Test connectivity from LAN to the server
- 4 Test connectivity from WAN to the remote server
- 5 Tools
- 6 References
How to connect to the web service
If the service is the web server:
- (1) Open the browser, (2) Type the address: or
- telnet command
- e.g. or
- (after telnet-ed the web 80 port, key in)  +
- (optional if curl command was not installed) Learn how to install & run cURL on Windows/MacOSX/Linux | Ubidots Help Center
- commands e.g.
- (1) Write output to <file> instead of stdout using
- (2) Write output to stdout using or
- (3) Write output to <file> with the remote file name using
- If failed. It will show the message: "failed: Connection refused"
- Linux wget software e.g.
- use the third-party website monitoring the services: Web Ping
If the service is not the web server, use telnet instead
- (optional) Install Telnet Client | Microsoft Docs on Win . Bring telnet back on macOS high Sierra – Ayuth Mangmesap (blackSource) – Medium on Mac
- If failed to connect the web service. It will show the message: "telnet: Unable to connect to remote host: Connection refused"
Using alternative web services if the Python was installed
Test connectivity for the local web service
WAN ○ -------> ○ LAN -------> ● server
Check if your computer/server Is being assigned the IP Address
- Linux : (1) Check the IP Address. . (2) Re-assign the IP Address and restart the networking service. for CentOS
- Win : Check the IP Address.
- More on Network problem
Is the web service alive
- Apache  on CentOS7
- nginx on CentOS7
- Docker container  to check if the container is running & expose the port
Check if a port was occupied by other process?
Check the port if used by other protocol
Example output of netstat command
$ netstat -tulpn | grep LISTEN tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN - // means the 80 port service is running
Check if a port is blocked by browser
Check if a port is blocked by browser
- Chrome net/base/port_util.cc - chromium/src.git - Git at Google
- Fetch Standard 2.9. Port blocking
Is the port opened for external users?
Check the OS firewall or protected by antivirus software
- iptables: (1) to list all rules (2) rule with smaller line-number will override the rule with larger line-number
- FirewallD rules: (1) return 'public' (2)  (3) Allow port with FirewallD
- SELinux Policy Management: (1) After installed , input 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:
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 Linux for
- double check the server/service logs
Test connectivity 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.*) and may not have the firewall limit. Private network / Private IP address
- Is the service alive?
- Connect to the internal IP eg: Private IP Network Numbers (2) where 23 is the port number where the service running (1) IP start with 172.18... is
- If the local service is web server, you can also try to connect the internal IP e.g. Linux for
- Double check the server/service logs
- Is the internal IP correct?
Test connectivity from WAN to the remote 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?
- 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?
- 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!
- CurrPorts: Monitoring TCP/IP network connections on Windows
- PFPortCheck Program: check TCP/UDP port from localhost(where you installed the program) to portforward.com
- TCPView for Windows
- Check server: Check host - online website monitoring, Check IP "checks the possibility of a TCP connection to host's specified port from different locations around the world." quoted from website. [Last visited: 2015-08-02]
- Install and Use Lynx Browser on Ubuntu
- Lynx Users Guide v2.8.7
- HTTP Check Port 80 with the Telnet Command
- curl - How To Use
- 20.19. SimpleHTTPServer — Simple HTTP request handler — Python 2.7.17 documentation
- http.server — HTTP servers — Python 3.8.1 documentation
- How to restart the networking service? - Ask Ubuntu
- How to check running status of LAMP stack | E2E Networks Knowledgebase
- docker ps | Docker Documentation
- 7 Linux Grep OR, Grep AND, Grep NOT Operator Examples
- How to solve WAMP and Skype conflict on Windows 7? - Stack Overflow
- 開發與部署網站時需注意不要使用到 ERR_UNSAFE_PORT 不安全的埠號 | The Will Will Web
- HowTos/Network/IPTables - CentOS Wiki
- How to edit iptables rules - FedoraProject
- How To Set Up a Firewall Using FirewallD on CentOS 7 | DigitalOcean
- semanage command not found on CentOS 7 and RHEL 7
- networking - Command for determining my public IP? - Ask Ubuntu