Network Communication Scripts

       

A simple multi-client TCP chat app in Python, built for learning and practical local/LAN testing.

This project gives you:

• a server (host.py)
• a client (clint.py)
• optional secure mode (TLS + password-protected encrypted messages)
• real user names (clients now choose their own names)
• unit tests + GitHub Actions CI

Note: the client file is intentionally named clint.py.

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Table of contents

• What this project does
• Features
• How secure mode works
• Quick start (interactive)
• TLS with self-signed certificate (recommended local flow)
• Command-line reference
• Example terminal output
• Tests
• GitHub Actions (green tick guide)
• Project structure
• Troubleshooting

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What this project does

• host.py starts a chat server.
• clint.py connects clients to that server.
• Every connected client can send messages to everyone.
• You can run in:
  • PLAINTEXT mode (simple TCP)
  • ENCRYPTED mode (TLS transport + password-authenticated encrypted payloads)

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Features

• Multi-client chat room
• Broadcast messaging
• Join/leave system announcements
• Live connected-client counter
• Optional TLS transport
• Optional password-protected message encryption
• Interactive prompts for host, port, password, and user name
• Client certificate controls:
  • strict verify (default)
  • trust custom cert via --cert
  • bypass verify via --tls-insecure (local testing only)

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How secure mode works

When secure mode is ON:

1. Server enforces TLS.
2. Client performs TLS handshake.
3. Client sends chosen user name + password.
4. Server validates password.
5. Messages are encrypted at the application layer before being broadcast.

When secure mode is OFF:

• no password is required
• messages are plain text at app layer
• connection is normal TCP unless TLS is explicitly used

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Quick start (Without TLS with self-signed certificate)

Open one terminal for server and one or more for clients.

1) Start server

python -u host.py

2) Start client

python -u clint.py

The client will prompt for:

• server IP/host
• server port
• user name
• password (only if server is in secure mode)

3) Start more clients

Run python -u clint.py again in additional terminals and use different names.

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TLS with self-signed certificate (recommended local flow)

Generate cert (OpenSSL with addext; recent OpenSSL versions):

openssl req -x509 -newkey rsa:2048 -nodes -keyout key.pem -out cert.pem -days 365 `
  -subj "/C=BD/ST=Dhaka/L=Dhaka/O=MultiHAT/OU=RedHAT/CN=localhost" `
  -addext "subjectAltName=DNS:localhost,IP:127.0.0.1"

Start the host:

python -u host.py --port 12345 --secure --password mypass --tls --cert cert.pem --key key.pem

Connect the client (trust the cert):

python -u clint.py --host 127.0.0.1 --port 12345 --tls --cert cert.pem

If you only want quick local testing and do not want certificate verification:

python -u clint.py --host 127.0.0.1 --port 12345 --tls --tls-insecure

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Command-line reference

Server (host.py)

python -u host.py --bind 0.0.0.0 --port 12345 --timeout 0

Options:

• --bind bind address (default: all interfaces)
• --port TCP port
• --timeout socket timeout in seconds (0 = blocking)
• --secure enable secure mode without prompt
• --password set secure password non-interactively
• --tls enable TLS
• --cert certificate PEM file
• --key private key PEM file

Client (clint.py)

python -u clint.py --host 127.0.0.1 --port 12345

Options:

• --host server host/IP
• --port server port
• --timeout socket timeout
• --name client display name (if omitted, prompt asks)
• --password secure-mode password (if omitted, prompt asks)
• --tls force TLS first
• --cert certificate/CA PEM to trust during TLS verification
• --tls-insecure disable certificate verification (testing only)

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Example terminal output

Server:

Listening on 127.0.0.1:12345... (Ctrl-C to stop)
Security mode: ENCRYPTED
Connection from 127.0.0.1:56582
Alice joined from 127.0.0.1:56582
+ Total connected clints: [1]
..:: Alice[56582]: Hello everyone

Client:

--> Enter the IP address of the server: 127.0.0.1
--> Enter the port you want to connect to: 12345
--> Enter your user name: Alice
--> Enter password:
Authenticated as Alice
Connected to 127.0.0.1:12345

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Tests

Install test dependency:

pip install -r requirements.txt

Run tests:

pytest -q

Current tests cover shared utilities in net_utils.py:

• host/port validation
• timeout validation
• name validation
• encryption/decryption behavior
• packet framing and JSON parsing

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GitHub Actions (green tick guide)

Workflow file:

• .github/workflows/python-ci.yml

It runs on push and pull request against Python:

• 3.9
• 3.10
• 3.11
• 3.12

To maximize chance of green tick in Get started with GitHub Actions:

1. Run tests locally first (pytest -q).
2. Keep .github/workflows/python-ci.yml in place.
3. Push your commit.
4. Confirm all matrix jobs pass in the Actions tab.

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Project structure

host.py
clint.py
net_utils.py
tests/
.github/workflows/python-ci.yml

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Troubleshooting

• TLS certificate verification failed on client
  • Use --cert cert.pem, or for local tests only use --tls-insecure.
• tlsv1 alert unknown ca / bad certificate on server logs
  • Usually means client rejected the cert or hostname/SAN does not match.
• Authentication failed
  • Password mismatch in secure mode.
• Connection refused
  • Host not running, wrong host/port, or firewall issue.

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Notes

This project is intentionally lightweight and easy to read. If you want, the next natural improvements are:

• private/direct messages
• persisted chat history
• richer client commands (/help, /users, /rename)
• stronger cryptography primitives and formal protocol versioning