132 lines
4.5 KiB
Markdown
132 lines
4.5 KiB
Markdown
# SecLink Proxy
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Linux-hosted proxy that bridges an 86Box emulated serial port to a
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remote telnet BBS. The 86Box side communicates using the secLink
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protocol (packet framing, DH key exchange, XTEA encryption). The BBS
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side is plain telnet over TCP.
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## Architecture
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```
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86Box (DOS terminal) Remote BBS
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emulated modem telnet:23
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TCP:2323 TCP:23
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+--- secproxy ----------------------------------+
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secLink ←→ plaintext
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(encrypted, reliable)
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```
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The proxy accepts a single TCP connection from 86Box, performs the
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secLink handshake (DH key exchange), then connects to the BBS. All
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traffic between 86Box and the proxy is encrypted via XTEA-CTR on
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channel 0. Traffic between the proxy and the BBS is unencrypted
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telnet.
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## Usage
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```
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secproxy [listen_port] [bbs_host] [bbs_port]
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```
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| Argument | Default | Description |
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|---------------|------------------------|---------------------------------|
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| `listen_port` | 2323 | TCP port for 86Box connection |
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| `bbs_host` | bbs.duensing.digital | BBS hostname |
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| `bbs_port` | 23 | BBS TCP port |
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```
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secproxy # all defaults
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secproxy 5000 # listen on port 5000
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secproxy 2323 bbs.example.com 23 # different BBS
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secproxy --help # show usage
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```
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## Startup Sequence
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1. Listen on the configured TCP port
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2. Wait for 86Box to connect (blocks on accept)
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3. Connect to the remote BBS
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4. Seed the RNG from `/dev/urandom`
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5. Open secLink and perform the DH handshake (blocks until the DOS
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side completes its handshake)
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6. Enter the proxy loop
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## Proxy Loop
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The main loop uses `poll()` with a 10ms timeout to multiplex between
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the two TCP connections:
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- **86Box → BBS**: `secLinkPoll()` reads from the 86Box socket via
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the socket shim, decrypts incoming packets, and the receive callback
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writes plaintext to the BBS socket.
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- **BBS → 86Box**: `read()` from the BBS socket, then
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`secLinkSend()` encrypts and sends to 86Box via the socket shim.
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- **Maintenance**: `secLinkPoll()` also handles packet-layer retransmit
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timers on every iteration.
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The proxy exits cleanly on Ctrl+C (SIGINT), SIGTERM, or when either
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side disconnects.
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## 86Box Configuration
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Configure the 86Box serial port to use a telnet connection:
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1. In 86Box settings, set a COM port to "TCP (server)" or
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"TCP (client)" mode pointing at the proxy's listen port
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2. Enable "No telnet negotiation" to send raw bytes
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3. The DOS terminal application running inside 86Box uses secLink
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over this serial port
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## Socket Shim
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The proxy reuses the same packet, security, and secLink source code
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as the DOS build. A socket shim (`sockShim.h`/`sockShim.c`) provides
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rs232-compatible `rs232Read()`/`rs232Write()` functions backed by TCP
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sockets instead of UART hardware:
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| rs232 function | Socket shim behavior |
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|----------------|-----------------------------------------------|
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| `rs232Open()` | No-op (socket already connected) |
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| `rs232Close()` | Marks port closed (socket managed by caller) |
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| `rs232Read()` | Non-blocking `recv()` with `MSG_DONTWAIT` |
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| `rs232Write()` | Blocking `send()` loop with `MSG_NOSIGNAL` |
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The shim maps COM port indices (0-3) to socket file descriptors via
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`sockShimSetFd()`, which must be called before opening the secLink
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layer.
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DOS-specific headers (`<pc.h>`, `<go32.h>`, `<sys/farptr.h>`) are
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replaced by minimal stubs in `stubs/` that provide no-op
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implementations. The security library's hardware entropy function
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returns zeros on Linux, which is harmless since the proxy seeds the
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RNG from `/dev/urandom` before the handshake.
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## Building
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```
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make # builds ../bin/secproxy
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make clean # removes objects and binary
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```
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Objects are placed in `../obj/proxy/`, the binary in `../bin/`.
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Requires only a standard Linux C toolchain (gcc, libc). No external
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dependencies.
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## Files
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```
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proxy/
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proxy.c main proxy program
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sockShim.h rs232-compatible socket API (header)
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sockShim.c socket shim implementation
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Makefile Linux build
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stubs/
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pc.h stub for DJGPP <pc.h>
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go32.h stub for DJGPP <go32.h>
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sys/
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farptr.h stub for DJGPP <sys/farptr.h>
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```
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