TCPLab

Overview

TCPLab is a web-based network diagnostic toolkit for developers and operators, offering common checks such as IP, DNS, HTTP, and TCP ports. All tests are initiated from real servers rather than running solely in the browser, giving you an external view of network conditions. ## Key Capabilities - **IP Tools**: View your current public IPv4/IPv6 address along with geolocation and ISP details; query single or batch (up to 50) IPs; compare mainland China and global egress IPs. - **DNS Lookup**: Query A, AAAA, CNAME, MX, TXT, and NS records, choosing public resolvers like Cloudflare, Google, or AliDNS to compare results. - **HTTP Check**: Send GET requests and return status codes, response headers, and redirect targets—fetching only headers without loading the full page. - **TCP Port Check**: Perform real handshake tests on specified ports of any host or IP, reporting open, closed, or no response status. - **GFW Check**: Probe targets simultaneously from mainland China and overseas nodes, comparing reachability to determine if a site is blocked by the firewall. Supports domain, IP, and IP+port modes. - **Other Tools**: Includes Domain Health Check (comprehensive WHOIS, DNS, email security, HTTPS/TLS inspection with scoring), SSL certificate check, and WHOIS lookup. ## Use Cases When a website fails to load, use HTTP Check to compare server-side and local responses. To verify whether a proxy or VPN is working, check your egress IP. If you suspect a port is blocked, use Port Check to distinguish between a service not listening and firewall packet drops. To confirm whether a site is accessible in mainland China, use the GFW Check tool. No registration or installation is required—use it directly in your browser. Queries are never stored.

Q&A

What is a network diagnostic tool?AI-generated

It is a set of online tools that answer common questions about network connectivity, such as your public IP, domain resolution results, server reachability, and open ports. TCPLab runs checks from real servers with no software installation required.

How do I check my public IP address?AI-generated

Open the My IP tool. It reads the address your browser uses to connect and displays your public IPv4 and IPv6 (if available) along with location and ISP details. No data is stored.

What's the difference between DNS lookup and nslookup?AI-generated

Both query the same types of records. nslookup is a command-line client built into the operating system, while TCPLab's DNS lookup is a browser-based version that lets you choose record types and public resolvers (such as Cloudflare or Google) and shows TTL and response times.

Why does a port show as closed or no response?AI-generated

Closed means the host actively refused the connection (RST) and no service is listening. No response means no reply was received before timeout, which could indicate firewall filtering or that the host is offline. Remote probing sometimes cannot distinguish between these two cases.

How can I tell if a website is down or just unavailable to me?AI-generated

Use TCPLab's server to check HTTP status and response headers. If the server responds normally but your local request fails, the issue is local (DNS, network, or regional restrictions). If the server also fails, the website itself may be down.

How do I check if a website is blocked in China?AI-generated

Use the GFW Check tool, which probes the target simultaneously from mainland China and overseas nodes. If it's unreachable from China but reachable overseas, it's likely blocked.

Does TCPLab log my queries?AI-generated

No. Queries run against TCPLab's own offline databases, and query content is never stored.