Connection Errors Troubleshooting

This guide helps you diagnose and fix connection-related errors in CurlDotNet.

Quick Diagnosis

// Test basic connectivity first
var result = await Curl.ExecuteAsync("curl -v https://www.google.com");
if (!result.IsSuccess)
{
    Console.WriteLine($"Basic connectivity test failed: {result.Error}");
}

Common Connection Errors

CurlCouldntConnectException

Error: “Failed to connect to host”

Causes:

Solutions:

  1. Check if server is running: 
    # Test with ping (ICMP)
ping api.example.com

# Test specific port with telnet
telnet api.example.com 443
  2. Check firewall settings: 
    // Try different ports
var result = await Curl.ExecuteAsync("curl http://api.example.com:80");  // HTTP
var result = await Curl.ExecuteAsync("curl https://api.example.com:443"); // HTTPS
  3. Use IP address directly: 
    // Bypass DNS to test connectivity
var result = await Curl.ExecuteAsync("curl https://93.184.216.34");

CurlCouldntResolveHostException

Error: “Could not resolve host”

Causes:

Solutions:

  1. Verify hostname: 
    # Check DNS resolution
nslookup api.example.com
dig api.example.com
host api.example.com
  2. Use different DNS: 
    // Specify DNS server
var result = await Curl.ExecuteAsync("curl --dns-servers 8.8.8.8 https://api.example.com");
  3. Clear DNS cache: 
    # Windows
ipconfig /flushdns

# macOS
sudo dscacheutil -flushcache

# Linux
sudo systemd-resolve --flush-caches

CurlCouldntResolveProxyException

Error: “Could not resolve proxy”

Causes:

Solutions:

  1. Check proxy settings: 
    // Test without proxy
var result = await Curl.ExecuteAsync("curl --noproxy '*' https://api.example.com");

// Use specific proxy
var result = await Curl.ExecuteAsync("curl -x http://proxy.company.com:8080 https://api.example.com");
  2. Proxy with authentication: 
    var result = await Curl.ExecuteAsync(@"
    curl -x http://proxy.company.com:8080 \
         -U username:password \
         https://api.example.com
");

CurlOperationTimeoutException

Error: “Connection timed out”

Causes:

Solutions:

  1. Increase timeouts: 
    // Separate connection and total timeout
var result = await Curl.ExecuteAsync(@"
    curl --connect-timeout 30 \
         --max-time 120 \
         https://slow-api.example.com
");
  2. Test with shorter timeouts to isolate issue: 
    // Quick timeout to test if server responds at all
var result = await Curl.ExecuteAsync("curl --connect-timeout 5 https://api.example.com");

Network Diagnostics

1. Test Network Path

// Trace route to server
var result = await Curl.ExecuteAsync("curl --trace-time -v https://api.example.com");

2. Check MTU Issues

// Reduce packet size for problematic networks
var result = await Curl.ExecuteAsync("curl --tcp-nodelay https://api.example.com");

3. IPv4 vs IPv6

// Force IPv4
var result = await Curl.ExecuteAsync("curl -4 https://api.example.com");

// Force IPv6
var result = await Curl.ExecuteAsync("curl -6 https://api.example.com");

Proxy Configuration

Corporate Proxy

// Auto-detect proxy from environment
var result = await Curl.ExecuteAsync("curl https://api.example.com");

// Manual proxy configuration
var result = await Curl.ExecuteAsync(@"
    curl -x http://proxy.company.com:8080 \
         -U domain\\username:password \
         https://external-api.com
");

SOCKS Proxy

// SOCKS5 proxy
var result = await Curl.ExecuteAsync("curl --socks5 localhost:1080 https://api.example.com");

// SOCKS5 with authentication
var result = await Curl.ExecuteAsync("curl --socks5 user:pass@localhost:1080 https://api.example.com");

Firewall Issues

Test Different Protocols

// Try HTTP instead of HTTPS
var result = await Curl.ExecuteAsync("curl http://api.example.com");

// Try different ports
var result = await Curl.ExecuteAsync("curl https://api.example.com:8443");

Bypass SSL/TLS Issues

// Test without SSL (development only!)
var result = await Curl.ExecuteAsync("curl -k https://api.example.com");

// Use specific TLS version
var result = await Curl.ExecuteAsync("curl --tlsv1.2 https://api.example.com");

Advanced Debugging

Enable All Verbose Output

var result = await Curl.ExecuteAsync("curl -vvv --trace-ascii debug.txt https://api.example.com");
Console.WriteLine("Check debug.txt for full trace");

Network Interface Selection

// Use specific network interface
var result = await Curl.ExecuteAsync("curl --interface eth0 https://api.example.com");

// Bind to specific local IP
var result = await Curl.ExecuteAsync("curl --interface 192.168.1.100 https://api.example.com");

Keep-Alive Settings

// Disable keep-alive
var result = await Curl.ExecuteAsync("curl --no-keepalive https://api.example.com");

// Set keep-alive time
var result = await Curl.ExecuteAsync("curl --keepalive-time 60 https://api.example.com");

Platform-Specific Issues

Windows Firewall

# Check Windows Firewall rules
netsh advfirewall firewall show rule name=all | findstr "YourApp"

Linux iptables

# Check iptables rules
sudo iptables -L -n

Docker Networking

# Ensure proper network mode
docker run --network host myapp

# Or use bridge with port mapping
docker run -p 8080:8080 myapp

Connection Pooling

// Reuse connections for better performance
var curl = new LibCurl();
curl.ConfigureConnection(options =>
{
    options.MaxConnections = 10;
    options.ConnectionTimeout = TimeSpan.FromSeconds(30);
});

// Make multiple requests with same connection
for (int i = 0; i < 100; i++)
{
    var result = await curl.GetAsync($"https://api.example.com/item/{i}");
}

Retry Logic

// Implement retry for transient failures
int maxRetries = 3;
for (int i = 0; i < maxRetries; i++)
{
    try
    {
        var result = await Curl.ExecuteAsync("curl https://api.example.com");
        if (result.IsSuccess) break;
    }
    catch (CurlConnectionException ex) when (i < maxRetries - 1)
    {
        await Task.Delay(TimeSpan.FromSeconds(Math.Pow(2, i))); // Exponential backoff
    }
}

For persistent connection issues, check your network administrator or ISP.