3270 TN3270 Server

A Python implementation of a TN3270 server that presents an authentic IBM mainframe experience — complete TSO/E logon panel, RACF authentication, and the ISPF Primary Option Menu — over a standard TCP connection. Connect any real 3270 emulator and it just works.

What you can do

1. Connect any TN3270 emulator (wc3270, x3270, Vista TN3270, etc.) to localhost:2323
2. Log in using a RACF userid and password — the server validates credentials and shows proper error messages for bad passwords or missing userids
3. Navigate the ISPF Primary Option Menu — the keyboard is fully live; type an option number and press Enter

Logon panel

The server presents an authentic z/OS V2R5.0 TSO/E LOGON screen. Fill in your userid and password and press Enter.

Built-in credentials

Userid	Password
IBMUSER	SYS1
TESTUSER	RACF

The server follows z/OS RACF conventions: userids and passwords are case-insensitive (uppercased before comparison). A wrong password returns IKJ56425I PASSWORD NOT CORRECT FOR <userid>. A missing userid returns IKJ56700I USERID MUST BE SPECIFIED.

ISPF Primary Option Menu

After a successful login you land on the ISPF Primary Option Menu. The keyboard is unlocked — you can type option numbers and press Enter. Entering X or pressing PF3 logs you off and returns to the TSO/E logon panel.

The full z/OS ISPF 7.1.0 menu is rendered, including the user ID, system ID (SY1), and current time in the status block. Options 0–13 and X are listed. (Sub-menus are not yet implemented — selecting any option returns you to the main menu with a short message.)

Quick start

Prerequisites

• Python 3.8+
• A TN3270 emulator — wc3270 (Windows) or x3270 (Linux/macOS) are free and work out of the box

Run the server

python server.py

The server listens on port 2323 by default (no root/administrator required, unlike port 23).

Connect with wc3270 (Windows)

wc3270 localhost:2323

Connect with x3270 (Linux / macOS)

x3270 localhost:2323

Connect with any emulator

Point your emulator at localhost, port 2323. The server negotiates TN3270E binary mode, EOR, and terminal-type options automatically and supports IBM-3278 model 2–5 terminals.

How it works

TN3270 protocol

TN3270 is Telnet extended with IBM 3270 data-stream framing. The server performs the full Telnet option negotiation (BINARY, EOR, TERMINAL-TYPE) before sending any screen data.

3270 data stream

Screens are built with authentic 3270 orders:

Order	Hex	Purpose
ERASE_WRITE	0xF5	Clear screen and write new data
SBA	0x11	Set Buffer Address — position the write cursor
SF	0x1D	Start Field — define a protected or unprotected input field
IC	0x13	Insert Cursor — place the cursor in an input field

The Write Control Character (WCC) sent after ERASE_WRITE uses 0x43 — the correct x3270/wc3270 bit layout (WCC_RESET_BIT | WCC_KEYBOARD_RESTORE_BIT | WCC_RESET_MDT_BIT) — so the keyboard unlocks immediately after every screen update.

Field parsing

When the user presses Enter or a PF key, the emulator sends an AID byte followed by the cursor address and the contents of all modified fields. The server decodes the 12-bit packed buffer addresses and reads each field's EBCDIC text, then strips whitespace and uppercases credential fields before comparing.

Project structure

server.py   — the entire TN3270 server (single file)

Key functions:

Function	What it does
tn3270_negotiate	Performs Telnet option handshake
send_tso_logon	Builds and sends the TSO/E logon panel
send_ispf_menu	Builds and sends the ISPF Primary Option Menu
read_client_input	Reads and parses an AID response from the client
write_control_character	Encodes a WCC byte with correct x3270 bit positions
encode_pack_addr	Converts (row, col) to a 12-bit 3270 buffer address
handle_client	Main session loop: logon → ISPF → logoff

Extending

To add real sub-menus, replace the short_msg response in handle_client's ISPF loop with a call to a new send_ispf_option_N function that builds and sends the appropriate screen, then reads the user's response.

To add more users, extend the _CREDENTIALS dict at the top of server.py.

References

• RFC 2355 — TN3270E
• IBM 3270 Data Stream Programming Reference
• x3270 / wc3270 emulator
• pmattes/x3270 source (3270ds.h) — canonical WCC and field-attribute bit definitions

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For educational and prototyping purposes.