web3270

Browser-based TN3270 / TN3270E terminal that talks to real mainframes. The data stream is parsed in C# (.NET 10), pushed to the browser through SignalR as a structured snapshot, and rendered on an HTML5 canvas.

[browser canvas] <—SignalR (JSON)—> [ASP.NET Core hub] <—TCP/Telnet—> [mainframe]

Screenshots

What's implemented

Telnet / TN3270E layer (RFC 854 / 1205 / 1041 / 2355)

• IAC negotiation: BINARY, EOR, TERMINAL-TYPE, SUPPRESS-GO-AHEAD, 3270-Regime.
• Full TN3270E sub-negotiation: DEVICE_TYPE REQUEST/IS, FUNCTIONS REQUEST/IS, CONNECT-LU support, REJECT handling.
• 5-byte TN3270E command header on every record once the session is in TN3270E mode (data type, request flag, response flag, sequence).
• Automatic POSITIVE_RESPONSE (DEVICE_END) when the host requests ALWAYS_RESPONSE — VTAM expects this acknowledgement.
• IAC 0xFF doubling on outbound, IAC EOR record framing on both sides.

3270 data stream parser

• Commands: W, EW, EWA, EAU, RB, RM, RMA, WSF.
• Orders: SF, SFE, SBA, SA, MF, IC, PT, RA, EUA, GE.
• Format Control Orders rendered as spaces: NUL, FF, CR, NL, EM, DUP, FM, SUB, 8-ones (matches dm3270's FormatControlOrder.process).
• 12-bit and 14-bit buffer addressing.
• WCC bit decoding: ResetMDT, KeyboardRestore, SoundAlarm, StartPrinter, PrinterFormat.
• SF / SFE attribute byte: protected, numeric, intensity, MDT, hidden.
• Per-character extended attributes via SA (foreground / background / highlight) — running state across the write so a single field can carry multi-coloured text (TK5 banner style).
• RA edge case: when current address equals the target, fills the entire buffer instead of being a no-op.
• EBCDIC CP037 codepage with bidirectional Unicode translation.

Structured Fields

• Inbound dispatch: ReadPartition (Query / QueryList), EraseReset, SetReplyMode, Outbound 3270DS (recursive embedded write).
• Query Reply response generated automatically and sent back on Read Partition Query: Summary, UsableArea, CharacterSets, Color (16 pairs), Highlight (Default / Blink / Reverse / Underscore / Intensify), ReplyModes (Field / Extended Field / Character), ImplicitPartition. This is what allows TSO, CICS, IMS etc. to hand-shake capabilities and proceed past BIND.

Outbound (terminal → host)

• Read-Modified packing with run-based emission: only contiguous cells the user actually typed (cell.Modified) are sent, with one SBA per run. Avoids echoing back host pre-fill spaces — without this real hosts (TSO on TK5/TK4-) reject every Enter with IKT00405I screen-erasure recovery.
• Cursor address resolved to the first user-modified cell, mirroring x3270 / dm3270 / Soldier-of-Fortran tn3270lib behaviour.
• AID short-reads (PA1-PA3, Clear, SysReq) emit just the AID byte.

Screen buffer

• Linear cell store with row-major addressing.
• Cyclic field discovery — the field that contains position 0 is the one whose attribute byte is the last one in the buffer; the snapshot factory resolves this wrap-around so cells before the first SF render correctly.
• 3279 base palette (protected/unprotected × normal/high) used when no extended colour was negotiated.
• Buffer reset to 0x00 on Erase Write (NULL, not space) so Read-Modified can suppress them per RFC 1205.

Frontend (canvas + SignalR)

• Click-and-drag selection with translucent overlay.
• ⌘C / Ctrl+C copies the selection to the clipboard. Hidden cells (password fields) are emitted as spaces — credentials are never leaked.
• ⌘V / Ctrl+V pastes the clipboard into the cursor field as Type events (newlines/tabs collapsed to spaces).
• ⌘A / Ctrl+A selects the whole screen.
• Plain click moves the terminal cursor (without dragging).
• Cursor position display in the toolbar (1-indexed row / col).
• Rule toggle — thin cross-hair lines through the cursor row and column, x3270-style.
• Edge-triggered alarm: WCC SoundAlarm beeps once per pulse (server consumes the flag after every snapshot, client beeps only on false → true transitions).
• Single shared AudioContext for beeps to avoid resource exhaustion.
• AID buttons in the toolbar: Enter, Clear, PA1-PA3, PF1-PF12.
• Keyboard mappings:
  • F1-F12 → PF1-PF12, Shift+F1-F12 → PF13-PF24
  • Esc → Clear
  • PageUp / PageDown → PF7 / PF8 (ISPF scroll convention)
  • Tab → next unprotected field
  • Home → first unprotected field
  • End → last typed position in the current field
  • Backspace → erase previous char (NUL fill, MDT set)
  • Delete → delete char at cursor and pull the rest of the field left
  • Arrow keys → move the cursor cell-by-cell (wraps at edges)
• Per-session stream recorder writes a hex+ASCII dump of every TN3270 record (in/out) to traces/<timestamp>_<connectionId>.log when enabled. Useful for offline parser debugging.

Building and running

dotnet build
dotnet run --project src/Web3270.Server

Open http://localhost:5001, choose the terminal model, set host/port, click Connect.

Configuration (appsettings.json)

Logging is wired through Serilog and read from configuration:

"Serilog": {
  "MinimumLevel": {
    "Default": "Information",
    "Override": {
      "Microsoft.AspNetCore": "Warning",
      "Web3270.Tn3270": "Information",
      "Web3270.Server":  "Information"
    }
  }
}

Lower a category to Debug or Trace to surface hex dumps of every record. The [LoggerMessage] source-generated methods automatically gate on IsEnabled, so the cost is zero when a level is disabled.

Per-session stream capture is opt-in:

"Tn3270": {
  "StreamCapture": {
    "Enabled": false,        // true = write traces/<id>.log per session
    "Directory": "traces"    // relative to ContentRoot, or absolute
  }
}

It is off by default in production. appsettings.Development.json enables it. Override at runtime with the standard ASP.NET Core env var: Tn3270__StreamCapture__Enabled=true.

Public TN3270 hosts for testing

Host	Port	Notes
mainframe.hercules-390.org	3270	Hercules + MVS 3.8j
tn3270.themainframe.org	23	z/OS
pub400.com	23	It works, but with some caveats!!!

Wire format (browser ↔ server)

ScreenUpdate (server → client)

{
  "rows": 24,
  "columns": 80,
  "cursor": 12,
  "keyboardLocked": false,
  "alarm": false,
  "cells": [
    { "glyph": "H", "protected": false, "hidden": false, "highlight": false,
      "modified": false, "foreground": 240, "background": 0, "highlightAttr": 244 }
    /* ... rows*columns entries, row-major ... */
  ],
  "fields": [
    { "start": 1, "length": 40, "protected": true, "numeric": false, "hidden": false }
  ]
}

SendKey (client → server)

{ "kind": "Aid",       "value": "Enter|PF1..PF24|PA1..PA3|Clear|SysReq" }
{ "kind": "Type",      "value": "single character or string" }
{ "kind": "Cursor",    "address": 123 }
{ "kind": "Tab" }
{ "kind": "Backspace" }
{ "kind": "Delete" }

Trace files

When Tn3270:StreamCapture:Enabled=true, two endpoints are exposed for grabbing per-session captures:

• GET /traces — list captured sessions (newest first).
• GET /traces/<filename> — download a specific log.

Each line in a capture is timestamped and prefixed with direction (<< inbound, >> outbound) plus a hexdump-style payload, matching what the parser saw on the wire after IAC framing was stripped.

Known limitations

• IND$FILE file transfer is not implemented (the Structured Field identifier 0xD0 is recognised but no file transfer state machine).
• Only EBCDIC code page CP037 (US English / Latin-1). Other code pages require additional translation tables.
• Field validation, field outlining and programmed symbols (loadable character sets) are parsed but not enforced.
• Insert mode (ESC + Insert) is not implemented — typing always overwrites.

References

• RFC 854 — Telnet protocol.
• RFC 1205 — TN3270 (plain).
• RFC 2355 — TN3270E.
• IBM 3270 Information Display System Data Stream Programmer's Reference.