What is TCP ping?

While "ping" commonly refers to using the ICMP (Internet Control Message Protocol) echo request to test network connectivity, "TCP ping" refers to a method of probing a TCP port on a remote host to check if the host is reachable and listening on that port. It doesn't use the standard ping command, which relies on ICMP. Instead, TCP ping attempts to establish a TCP connection to the specified port.

Here's a breakdown:

• ICMP Ping (Standard Ping): Uses ICMP echo requests and replies to determine network reachability and round-trip time. This relies on the ping command being allowed by firewalls and network policies. Many networks block ICMP pings for security reasons.

• TCP Ping (Connection-Based Ping): Attempts to establish a TCP connection to a specific port on a remote host. If a connection is established (or refused), it indicates that the host is reachable and the port is either open or closed, respectively. If the connection times out, it usually means the host is unreachable or a firewall is blocking the connection attempt.

Why Use TCP Ping?

• Bypassing ICMP Blocking: When ICMP ping is blocked by a firewall, TCP ping can be used as an alternative connectivity test. This is because firewalls often allow TCP traffic on specific ports (e.g., port 80 for HTTP, port 443 for HTTPS).
• Verifying Service Availability: You can use TCP ping to check if a specific service (e.g., a web server) is running and listening on its designated port.
• Firewall Testing: TCP ping can help determine if a firewall is blocking specific ports.

How TCP Ping Works

TCP ping tools typically work by:

1. Attempting to establish a TCP connection to a specified port on a target host.
2. Analyzing the response:
   • Connection Established: The port is open and accepting connections.
   • Connection Refused: The host is reachable, but the port is closed. This generally means a service isn't listening on that port.
   • Timeout: The host is unreachable or a firewall is blocking the connection attempt.

Examples of TCP Ping Tools

Several tools can perform TCP ping:

• nc (Netcat): A versatile network utility that can be used to create TCP connections. Example: nc -vz <host> <port> ( -v for verbose, -z for zero-I/O mode, meaning it just scans for open ports).
• nmap: A powerful network scanner that can perform TCP connect scans. Example: nmap -p <port> <host>
• hping3: A packet crafting tool that can send custom TCP packets.

Important Considerations:

• Firewall Rules: Network firewalls and host-based firewalls can significantly impact the results of TCP ping tests.
• Permissions: Performing TCP ping usually requires appropriate user privileges.

In summary, TCP ping is a technique that leverages TCP connections to test network connectivity and service availability, offering an alternative to ICMP ping, particularly when ICMP is blocked.