When you encounter an empty response from curl (error code 52), it typically indicates that the TCP connection was established but the HTTP server didn't send any data before closing the connection. Let's break down what's happening at each layer:

# TCP handshake succeeds (SYN -> SYN-ACK -> ACK)
# HTTP request sent (visible in verbose output)
# Server closes connection without sending response

Before jumping to conclusions, gather these critical data points:

# 1. Full verbose output (already shown in question)
curl -v http://example.com

# 2. TCP dump of the connection
tcpdump -i any host 111.222.159.30 and port 80 -w curl_debug.pcap

# 3. Check if port is actually listening
nc -zv 111.222.159.30 80
telnet 111.222.159.30 80

# 4. Test with different HTTP methods
curl -X HEAD -v http://111.222.159.30
curl -X POST -d "test" -v http://111.222.159.30

From my experience troubleshooting similar issues, these are the most frequent culprits:

• Firewall interference: The connection is allowed but responses are blocked
• Server misconfiguration: Web server isn't properly binding to the port
• Application crashes: The service accepts connections but crashes before responding
• Load balancer timeouts: Intermediate devices terminate idle connections
• MTU issues: Packet fragmentation problems preventing response delivery

When basic checks don't reveal the issue, try these advanced techniques:

# Check for packet loss
ping -M do -s 1472 111.222.159.30  # Test MTU

# Verify routing path
traceroute -n -T -p 80 111.222.159.30

# Test with different TCP window sizes
curl --tcp-nodelay -v http://111.222.159.30

# Check SSL/TLS if applicable
openssl s_client -connect 111.222.159.30:443 -showcerts

I once debugged a case where curl showed empty responses intermittently. Packet capture revealed:

16:32:45.123 IP client.45234 > server.80: Flags [S], seq 12345
16:32:45.124 IP server.80 > client.45234: Flags [S.], seq 67890
16:32:45.124 IP client.45234 > server.80: Flags [.], ack 67891
16:32:45.125 IP client.45234 > server.80: Flags [P.], seq 1:78, HTTP GET
16:32:45.126 IP server.80 > client.45234: Flags [R.], seq 67891

The issue turned out to be a misconfigured load balancer that was resetting connections after 100ms of inactivity, despite the TCP handshake completing successfully.

For production environments, consider this Python script to test connectivity:

import socket
import time

def test_connection(host, port, timeout=5):
    try:
        start = time.time()
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.settimeout(timeout)
        s.connect((host, port))
        s.send(b"GET / HTTP/1.1\r\nHost: " + host.encode() + b"\r\n\r\n")
        data = s.recv(1024)
        duration = time.time() - start
        s.close()
        return {
            'status': 'success' if data else 'empty_response',
            'duration': duration,
            'data': data
        }
    except Exception as e:
        return {
            'status': 'error',
            'error': str(e)
        }

print(test_connection("111.222.159.30", 80))

If you've exhausted all options and still see empty responses, try:

1. Testing from different network locations
2. Checking server-side logs (often reveals connection attempts)
3. Using alternative tools like wget or Postman
4. Contacting your hosting provider with packet captures

When you encounter an empty response from cURL (specifically the "Empty reply from server" message), it typically indicates that:

1. The TCP connection was successfully established (three-way handshake completed)
2. The HTTP request was sent
3. No HTTP response headers or body were received before the connection terminated

Here's your network troubleshooting toolkit:

# Check basic connectivity
ping 111.222.159.30

# Verify port accessibility
telnet 111.222.159.30 80
nc -zv 111.222.159.30 80

# TCP dump analysis
tcpdump -i any host 111.222.159.30 and port 80 -w capture.pcap

# Advanced cURL debugging
curl -vvv --trace-time --trace-ascii debug.log http://111.222.159.30

The most common scenarios when getting empty responses:

Try these cURL variations to gather more information:

# Force HTTP/1.0
curl -v --http1.0 http://111.222.159.30

# Test with different HTTP methods
curl -v -X HEAD http://111.222.159.30
curl -v -X OPTIONS http://111.222.159.30

# Add timeout controls
curl -v --connect-timeout 5 --max-time 10 http://111.222.159.30

When the server is remote, consider these network factors:

• MTU size issues (try adding --interface eth0 -M 1460 to curl)
• TCP window scaling problems
• Path MTU discovery failures

If you have access to the server, run these checks:

# Verify web server is listening
netstat -tulnp | grep :80
ss -tulnp | grep :80

# Check web server error logs
tail -f /var/log/nginx/error.log
journalctl -u apache2 --since "5 minutes ago"

Sample tshark commands to analyze the traffic:

# Basic HTTP analysis
tshark -r capture.pcap -Y "http"

# TCP stream analysis
tshark -r capture.pcap -z follow,tcp,ascii,0

# Detailed timing analysis
tshark -r capture.pcap -Y "tcp.port==80" -T fields -e frame.time_delta

Based on years of troubleshooting experience, most empty response cases fall into these categories:

1. Web server process crashed but port remains open
2. Application firewall silently dropping packets
3. TCP stack issues (especially with IPv6)
4. Application-level timeouts before sending response