Scapy扫描目标主机80端口显示RA标记，端口明明开启问题排查

Hey there! Let's work through why your Scapy port scan is showing port 80 as closed (returning RA flags) even though you have Chrome open on the target host. First, let's clear up the flag confusion, then dive into possible missteps.

First: Confirming SA vs RA Flag Meanings #

You didn't mix these up—your understanding is correct:

• SA (SYN-ACK): Sent by the target when a port is open and listening. It's the "yes, let's start a connection" response to your SYN packet.
• RA (RST-ACK): Sent when the port is closed, no service is listening, or the connection is blocked. It's the "stop, this port isn't accepting connections" response.

So the RA flag your script is seeing does mean the scan thinks the port is closed—let's figure out why that's happening despite Chrome being open.

Possible Reasons for the Discrepancy #

1. Chrome Isn't Listening on Port 80 #

This is the most likely culprit. Chrome acts as a client when accessing websites (it connects to other servers' port 80), not a server. It doesn't open or listen on port 80 by default. For port 80 to be open, you need an actual HTTP server running on the target—like Apache, Nginx, or a local dev server (e.g., Python's http.server).

To verify, run this on the target host to check if any process is listening on port 80:

# Linux/macOS
ss -tulpn | grep :80

# Windows (Command Prompt)
netstat -an | findstr ":80"

If nothing shows up, port 80 really isn't open—Chrome being open doesn't change that.

2. Firewall Blocking the Scan Traffic #

Most modern OSes have firewalls enabled by default (Windows Defender Firewall, Linux iptables, etc.). These firewalls might:

• Block incoming SYN packets to port 80
• Block outgoing SYN-ACK responses from the target
• Drop traffic that looks like a port scan

Try temporarily disabling the target's firewall and re-running your scan to rule this out. If it works with the firewall off, you'll need to add an inbound rule allowing port 80 traffic.

3. Scapy Needs Elevated Permissions #

Scapy sends raw network packets, which requires admin/root privileges:

• On Linux/macOS: Run your script with sudo (e.g., sudo python3 your_scan_script.py). Without this, Scapy might not send packets correctly, leading to incorrect RA responses.
• On Windows: Run your terminal/IDE as Administrator, and make sure you have Npcap/WinPcap installed (required for Scapy to work properly).

4. Script Logic Errors #

Double-check your Scapy code to make sure you're constructing and interpreting packets correctly. Here's a minimal, correct SYN scan snippet for reference:

from scapy.all import IP, TCP, sr1

target_ip = "192.168.1.100"  # Replace with your target's IP
target_port = 80

# Build SYN packet
syn_packet = IP(dst=target_ip)/TCP(dport=target_port, flags="S")
# Send packet and wait for response (timeout after 2s)
response = sr1(syn_packet, timeout=2, verbose=0)

if response:
    if response.haslayer(TCP):
        if response[TCP].flags == "SA":
            print(f"Port {target_port} is open")
        elif response[TCP].flags == "RA":
            print(f"Port {target_port} is closed")
else:
    print(f"No response from port {target_port} (might be filtered)")

Make sure you're not misspelling flags, using the wrong target IP, or missing timeout handling.

5. Network Routing/NAT Issues #

If the target is on a different network, NAT (network address translation) or routing problems might prevent your SYN packet from reaching the target, or the response from coming back. First confirm basic connectivity: ping the target to ensure it's reachable. If ping works but the scan doesn't, check for network-level filters (like a router firewall).

Final Quick Checks #

1. Verify a server is actually listening on port 80 of the target (use netstat/ss).
2. Run your Scapy script with admin/root privileges.
3. Test with the target's firewall temporarily disabled.
4. Confirm the target IP in your script is correct.

内容的提问来源于stack exchange，提问作者Lior shem