Hey there! Let's fix your Telnet password validation and retry logic issue. Since you mentioned using read_until(b'P...') to judge the result, we can build around that to create a reliable flow.

Core Idea #

Telnet sessions typically give clear cues after password attempts:

• Success: You'll get a command line prompt (e.g., root@vm:~# )
• Failure: You'll see the password prompt again (like Password: ) or an error message (e.g., Login incorrect)

We can use these cues with telnetlib's read_until to verify success, and wrap the password input in a loop for retries.

Working Code Example #

Here's a practical implementation using Python's telnetlib and getpass for secure input:

import getpass
import telnetlib

# Configure your VM details
VM_HOST = "127.0.0.1"  # Replace with your VM's IP/hostname
VM_USERNAME = "your_username"  # Replace with your VM username
MAX_RETRY_ATTEMPTS = 3

def establish_telnet_connection():
    # Initialize Telnet connection
    tn = telnetlib.Telnet(VM_HOST)
    
    # Wait for login prompt and send username
    tn.read_until(b"Login: ")
    tn.write(f"{VM_USERNAME}\r\n".encode("ascii"))
    
    # Retry loop for password attempts
    for attempt in range(MAX_RETRY_ATTEMPTS):
        # Wait for password prompt (matches your b'P...' plan)
        tn.read_until(b"Password: ")
        
        # Securely get password from user input
        user_password = getpass.getpass(f"Enter password (attempt {attempt+1}/{MAX_RETRY_ATTEMPTS}): ")
        tn.write(f"{user_password}\r\n".encode("ascii"))
        
        # Check if the attempt succeeded
        # Adjust the timeout and target strings to match your VM's actual output
        response = tn.read_until(b"Password: ", timeout=2)
        
        if b"Password: " not in response:
            # Success! Proceed with your commands
            print("✅ Password verified successfully!")
            tn.write(b"uname -a\r\n")  # Example command
            print(tn.read_all().decode("ascii"))
            tn.close()
            return
        else:
            print(f"❌ Attempt {attempt+1} failed. Wrong password.")
    
    # All retries exhausted
    print(f"❌ All {MAX_RETRY_ATTEMPTS} attempts failed. Closing connection.")
    tn.close()

if __name__ == "__main__":
    establish_telnet_connection()

Key Adjustments for Your VM #

• Update prompt strings: Replace b"Login: " and b"Password: " with the exact byte strings your VM outputs. You can get these from your terminal test (copy the exact prompt text).
• Add error checks: If your VM outputs Login incorrect on failure, add that to the failure condition:

  if b"Password: " in response or b"Login incorrect" in response:
      # Handle failure
  

• Adjust timeout: The timeout=2 value ensures we don't wait indefinitely—tweak it based on your VM's response speed.

Why This Works #

• getpass.getpass() hides the password input for security, unlike plain input().
• The loop limits retries to avoid infinite loops.
• read_until lets us reliably detect the VM's response to our password attempt.

内容的提问来源于stack exchange，提问作者madcrazydrumma