TCP Port Check
Check if TCP ports are open, closed, or filtered on any host
This page provides original, human-reviewed information about TCP port checking, explaining connection states, common service ports, and how to distinguish closed ports from firewall-filtered ones.
TCP port scanning is a network diagnostic technique used to determine whether specific TCP ports on a target host are open, closed, or filtered. This technology is crucial for network management, security auditing, and troubleshooting.
Main Uses:
- Network Service Discovery:Identify network services running on target hosts
- Security Auditing:Check for unnecessary open ports and assess security risks
- Troubleshooting:Diagnose network connectivity issues and service availability
- Firewall Testing:Verify that firewall rules are correctly configured
- Compliance Checking:Ensure systems meet security policy requirements
TCP Port States:
Open
Port accepts connections, service is listening on this port
Closed
Port does not accept connections, no service listening
Filtered
Firewall or other network device blocks the connection
Timeout
Connection request timed out, may be dropped or delayed
Response Time
Time from sending connection request to receiving response, measured in milliseconds. Lower response times indicate better network performance.
- < 50ms: Excellent
- 50-100ms: Good
- 100-200ms: Average
- > 200ms: Slow
Service Detection
Identify possible service types based on port numbers. Common services include:
Version Detection
Attempt to identify service versions running on open ports. This helps with:
- Security vulnerability assessment
- Compatibility checking
- System inventory management
Service Banner
Identification information returned by services when establishing connections, containing service names, versions, and other detailed information.Banner information is very useful for security analysis and service identification.
Common Ports Reference
Comprehensive list of commonly used ports, with special focus on email services:
| Port | Service | Protocol | Description | Security Notes |
|---|---|---|---|---|
| 25 | SMTP | TCP | Simple Mail Transfer Protocol - Email sending | Plain text transmission, vulnerable to eavesdropping |
| 110 | POP3 | TCP | Post Office Protocol v3 - Email retrieval | Plain text transmission, SSL version recommended |
| 143 | IMAP | TCP | Internet Message Access Protocol - Email access | Plain text transmission, SSL version recommended |
| 465 | SMTPS | TCP | SMTP over SSL - Secure email sending | Encrypted transmission, recommended |
| 587 | SMTP-MSA | TCP | Mail Submission Agent - Email submission | Supports STARTTLS, modern standard |
| 993 | IMAPS | TCP | IMAP over SSL - Secure email access | Encrypted transmission, recommended |
| 995 | POP3S | TCP | POP3 over SSL - Secure email retrieval | Encrypted transmission, recommended |
| 21 | FTP | TCP | File Transfer Protocol - File transfer | Plain text transmission, security risks |
| 22 | SSH | TCP | Secure Shell - Secure remote login | Encrypted transmission, relatively secure |
| 23 | Telnet | TCP | Telnet Protocol - Remote login | Plain text transmission, extremely insecure |
| 53 | DNS | TCP/UDP | Domain Name System - Domain resolution | Critical service, DDoS protection needed |
| 80 | HTTP | TCP | Hypertext Transfer Protocol - Web service | Plain text transmission, HTTPS recommended |
| 443 | HTTPS | TCP | HTTP Secure - Secure web service | Encrypted transmission, modern standard |
| 3389 | RDP | TCP | Remote Desktop Protocol - Remote desktop | Frequently attacked, strong authentication required |
Email Service Security Recommendations: Prioritize encrypted ports (465, 587, 993, 995) and avoid plain text ports (25, 110, 143).Configure firewall to restrict access sources, enable strong password policies and multi-factor authentication.
All ports show as filtered
This usually indicates that the target host has a firewall blocking connections, or the host does not exist.
Solution: Check if the target host is online and verify firewall settings.
Connection timeout
High network latency or slow target host response causes connection timeout.
Solution: Increase timeout duration, check network connection quality.
DNS resolution failure
Unable to resolve domain name to IP address, possibly due to DNS server issues.
Solution: Test directly with IP address, or change DNS server.
Slow scanning speed
Large-scale port scanning may take considerable time, especially for remote hosts.
Solution: Use common port scanning, or perform scanning in batches.
High-Risk Ports
The following ports may pose security risks if unnecessarily open:
Protection Measures
- Close unnecessary services and ports
- Use firewalls to restrict access sources
- Enable strong authentication mechanisms
- Regularly update systems and services
- Monitor abnormal connection activities
- Use VPN to protect remote access
Security Scanning Recommendations
- Regularly perform internal network scanning
- Test firewall effectiveness from external perspective
- Record and analyze scanning results
- Establish baselines and monitor changes
- Use in combination with vulnerability scanning tools
Compliance Requirements
Many security standards require regular port scanning:
- PCI DSS - Payment Card Industry Standards
- ISO 27001 - Information Security Management
- NIST - Cybersecurity Framework
- SOX - Sarbanes-Oxley Act Compliance
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