What Is a TCP Connectivity Test?

What Is a TCP Connectivity Test?

A TCP connectivity test is a network diagnostic tool that tests whether a TCP connection can be established to a specific host and port. TCP connectivity tests verify network connectivity, port availability, and service accessibility by attempting to establish TCP connections.

TCP connectivity tests work by: sending TCP SYN packets to target host and port, waiting for TCP SYN-ACK response, completing TCP handshake, and reporting connection status (success or failure).

TCP connectivity tests are used for: verifying service availability (checking if services are online), port connectivity testing (testing if ports are open), network troubleshooting (identifying connectivity issues), and firewall testing (verifying firewall rules). TCP connectivity tests are essential for network diagnostics and service availability verification.

TCP Test Purpose

1. Service Availability

TCP connectivity tests verify service availability by checking if services are online and accepting connections.

2. Port Connectivity

TCP tests verify port connectivity by testing if specific ports are open and accepting connections.

3. Network Troubleshooting

TCP tests help troubleshoot network connectivity issues by identifying connection failures and network problems.

4. Firewall Testing

TCP tests verify firewall rules by testing if ports are accessible through firewalls.

5. Service Verification

TCP tests verify services are running and accessible, ensuring service availability and functionality.

How TCP Test Works

1. TCP SYN Packet

TCP connectivity test sends TCP SYN (synchronize) packet to target host and port, initiating connection request.

2. TCP SYN-ACK Response

If port is open and service is available, target host responds with TCP SYN-ACK (synchronize-acknowledge) packet.

3. TCP Handshake Completion

TCP handshake completes with ACK (acknowledge) packet, establishing TCP connection.

4. Connection Status

TCP test reports connection status: success (connection established) or failure (connection refused or timeout).

5. Connection Closure

TCP connection is closed after test completes, releasing network resources.

TCP Handshake

What Is TCP Handshake?

TCP handshake is the three-way process of establishing TCP connections: SYN, SYN-ACK, ACK.

Handshake Steps

1. SYN: Client sends SYN packet to server
2. SYN-ACK: Server responds with SYN-ACK packet
3. ACK: Client sends ACK packet, completing handshake

Connection Establishment

TCP handshake establishes TCP connection, enabling data transmission between client and server.

Handshake Failure

If TCP handshake fails (no SYN-ACK response, connection refused, timeout), TCP connectivity test fails.

Handshake Success

Successful TCP handshake indicates: port is open, service is available, and connection can be established.

Using TCP Tests

1. Test Service Availability

Use TCP connectivity tests to verify services (web servers, mail servers, databases) are online and accepting connections.

2. Test Port Connectivity

Test specific ports (80 for HTTP, 443 for HTTPS, 25 for SMTP, 587 for SMTP submission) to verify port accessibility.

3. Network Troubleshooting

Use TCP tests to troubleshoot network connectivity issues, identify connection failures, and verify network configuration.

4. Firewall Verification

Test TCP connectivity through firewalls to verify firewall rules allow connections to specific ports.

5. Service Monitoring

Use TCP connectivity tests for service monitoring, ensuring services remain available and accessible.

6. Common Tools

Common TCP connectivity test tools: telnet, nc (netcat), nmap, and online TCP port checkers.