Windows to Linux Network TCP Connection Troubleshooting Guide

Problem: Windows Client Cannot Connect to Linux Server on Specific Ports

Symptoms

• Test-NetConnection from Windows fails with connection timeout or refused
• ping works successfully between Windows and Linux
• Service appears to be running on Linux server
• Connection works locally on Linux server

Common Scenario

# This works:
ping 192.168.1.100

# This fails:
Test-NetConnection 192.168.1.100 -Port 1502

Step-by-Step Troubleshooting

1. Verify Basic Network Connectivity

# From Windows - test basic connectivity
ping [LINUX_SERVER_IP]

If ping fails, check network configuration, cables, and switches.

2. Check Service Status on Linux Server

# Verify the service is actually listening
sudo netstat -tlnp | grep :[PORT]
# or
sudo ss -tlnp | grep :[PORT]

Key points to check:

• Service shows as LISTEN
• Binding address should be 0.0.0.0:[PORT] (all interfaces)
• NOT 127.0.0.1:[PORT] (localhost only)
• NOT :::[PORT] (IPv6 only)

3. Test Local Connection on Linux

# Test locally first
telnet 127.0.0.1 [PORT]
# or
nc -zv 127.0.0.1 [PORT]

If this fails, the service isn’t properly configured or running.

4. Check Linux Firewall (Most Common Issue)

Ubuntu/Debian (UFW):

# Check status
sudo ufw status

# If active, temporarily disable for testing
sudo ufw disable

# Re-enable after testing
sudo ufw enable

# Or allow specific port
sudo ufw allow [PORT]

CentOS/RHEL/Fedora (firewalld):

# Check status
sudo systemctl status firewalld
sudo firewall-cmd --list-all

# Temporarily disable
sudo systemctl stop firewalld

# Or allow specific port
sudo firewall-cmd --add-port=[PORT]/tcp --permanent
sudo firewall-cmd --reload

Direct iptables:

# Check current rules
sudo iptables -L -n

# Temporarily flush all rules (removes ALL firewall protection)
sudo iptables -F
sudo iptables -P INPUT ACCEPT

5. Test from Windows After Firewall Changes

Test-NetConnection [LINUX_SERVER_IP] -Port [PORT]

6. Service Configuration Issues

If the service binds to wrong interface:

Check binding configuration:

# Find the service process
ps aux | grep [SERVICE_NAME]

# Check process details
sudo lsof -p [PID]

Common configuration fixes:

• Change bind = 127.0.0.1 to bind = 0.0.0.0
• Change host = localhost to host = 0.0.0.0
• Change listen = ::1 to listen = 0.0.0.0

Common Solutions Summary

Prevention Tips

1. Always test locally first on the Linux server
2. Use 0.0.0.0 binding for services that need external access
3. Document firewall rules when opening ports
4. Use netstat/ss to verify service binding before testing remotely
5. Test one service at a time to isolate issues

Quick Diagnostic Commands

Linux Server:

# Show all listening services
sudo netstat -tlnp

# Check specific port
sudo netstat -tlnp | grep :[PORT]

# Check firewall
sudo ufw status
sudo iptables -L -n

Windows Client:

# Test connectivity
Test-NetConnection [IP] -Port [PORT]

# Alternative with telnet
telnet [IP] [PORT]

# Test multiple ports
80,443,22,8080 | ForEach-Object { Test-NetConnection [IP] -Port $_ }

Remember

Document changes: Keep track of firewall and service modifications

Security first: Re-enable firewalls after testing

Specific rules: Allow only necessary ports, not all traffic