cURL Errors and Reasons - Complete Troubleshooting Guide

cURL errors can be frustrating when making HTTP requests. Understanding error codes, their causes, and solutions is essential for debugging API calls, web scraping, and HTTP operations.

This comprehensive guide covers all common cURL errors with explanations and solutions.

Introduction to cURL Errors

cURL (Client URL) is a command-line tool and library for transferring data. Errors occur due to network issues, SSL problems, timeouts, or server responses.

Error Format:

• Error codes: Numeric codes (e.g., 6, 7, 28)
• Error messages: Descriptive text
• HTTP status codes: Server response codes

Common Scenarios:

• API requests failing
• SSL certificate issues
• Network connectivity problems
• Timeout errors
• Authentication failures

Common cURL Error Codes

Error Code 6: Couldn’t resolve host

Meaning: DNS resolution failed

Causes:

• Invalid domain name
• DNS server issues
• Network connectivity problems

Solutions:

# Check DNS
nslookup example.com

# Use IP instead
curl http://192.168.1.1

# Specify DNS server
curl --dns-servers 8.8.8.8 http://example.com

Error Code 7: Failed to connect to host

Meaning: Couldn’t connect to server

Causes:

• Server is down
• Firewall blocking
• Wrong port
• Network unreachable

Solutions:

# Check if server is up
ping example.com

# Try different port
curl http://example.com:8080

# Check firewall
# Allow connection through firewall

Error Code 28: Operation timeout

Meaning: Request timed out

Causes:

• Server too slow
• Network latency
• Timeout too short

Solutions:

# Increase timeout
curl --max-time 60 http://example.com

# Or
curl --connect-timeout 30 http://example.com

Connection Errors

Error Code 35: SSL connect error

Meaning: SSL/TLS handshake failed

Solutions:

# Ignore SSL verification (development only)
curl -k https://example.com

# Use specific TLS version
curl --tlsv1.2 https://example.com

# Check certificate
openssl s_client -connect example.com:443

Error Code 51: SSL peer certificate

Meaning: SSL certificate verification failed

Solutions:

# Skip certificate verification (not recommended)
curl -k https://example.com

# Update CA certificates
# Linux: Update ca-certificates package
# macOS: Update via system
# Windows: Update certificate store

Error Code 60: SSL certificate problem

Meaning: Self-signed or invalid certificate

Solutions:

# Accept certificate (development)
curl -k https://example.com

# Add certificate to trust store
# Or fix server certificate

SSL/TLS Errors

Certificate Verification

Disable verification (development only):

curl -k https://example.com

Use specific CA bundle:

curl --cacert /path/to/ca-bundle.crt https://example.com

TLS Version Issues

Specify TLS version:

curl --tlsv1.2 https://example.com
curl --tlsv1.3 https://example.com

Certificate Chain Issues

View certificate:

curl -v https://example.com

Check certificate chain:

openssl s_client -showcerts -connect example.com:443

Timeout Errors

Connection Timeout

Set connection timeout:

curl --connect-timeout 10 http://example.com

Total Timeout

Set maximum time:

curl --max-time 30 http://example.com

Retry on Timeout

curl --retry 3 --retry-delay 5 http://example.com

HTTP Status Errors

400 Bad Request

Meaning: Invalid request

Check:

• Request format
• Headers
• Body content

401 Unauthorized

Meaning: Authentication required

Solution:

curl -u username:password http://example.com
# Or
curl -H "Authorization: Bearer token" http://example.com

403 Forbidden

Meaning: Access denied

Possible causes:

• Insufficient permissions
• IP blocked
• Rate limiting

404 Not Found

Meaning: Resource doesn’t exist

Check:

• URL correctness
• Resource availability

500 Internal Server Error

Meaning: Server error

Action: Contact server administrator

Troubleshooting Guide

Enable Verbose Mode

curl -v http://example.com

Shows detailed connection information.

Check Response Headers

curl -I http://example.com

Save Output for Analysis

curl -v http://example.com -o output.txt 2>&1

Test Connectivity

# Test DNS
nslookup example.com

# Test connectivity
ping example.com

# Test port
telnet example.com 80

Best Practices

1. Always Check Error Codes

curl -f http://example.com || echo "Request failed"

2. Use Appropriate Timeouts

curl --connect-timeout 10 --max-time 30 http://example.com

3. Handle SSL Properly

• Verify certificates in production
• Use proper CA bundles
• Don’t disable verification in production

4. Log Errors

curl http://example.com 2>&1 | tee curl.log

5. Retry Logic

curl --retry 3 --retry-delay 5 http://example.com

Common Error Reference

• 6: Couldn’t resolve host
• 7: Failed to connect
• 28: Timeout
• 35: SSL connect error
• 51: SSL peer certificate
• 60: SSL certificate problem

Conclusion

cURL errors indicate:

• Network issues - Connection problems
• SSL problems - Certificate issues
• Server errors - HTTP status codes
• Configuration - Timeout, retry settings

Key Points:

• Use verbose mode (-v) for debugging
• Check error codes for specific issues
• Handle SSL properly
• Set appropriate timeouts
• Implement retry logic

Understanding cURL errors helps debug HTTP requests and API integrations effectively.