Swatantra-66 opened a new pull request, #20586:
URL: https://github.com/apache/nuttx/pull/20586
## Summary
This PR replaces the 31-line placeholder stub in
`Documentation/applications/netutils/telnetd/index.rst` with a comprehensive,
authoritative documentation guide for the `telnetd` (Telnet daemon) network
utility.
Part of the ongoing `netutils` documentation enhancement series (alongside
#20472, #20493, #20513, #20514, and #20578).
### Contents Added:
1. **Protocol Specifications & Standards**:
- Covers RFC 854 (Core Telnet Specification), RFC 855 (Option
Specifications), and RFC 1073 (Window Size - NAWS).
- Security considerations addressing unencrypted plaintext transmission,
management network isolation, and Dropbear SSH alternatives.
2. **Operational Architecture & Lifecycle**:
- Detailed breakdown and ASCII workflow of the modular split between
`apps/netutils/telnetd` (user-space daemon) and `drivers/net/telnet.c` (kernel
character driver factory).
- Dynamic `/dev/telnet%d` character driver allocation, IAC command state
machine, and `dup2()` STDIO virtualization.
- Line-ending handling: RFC 854 transmit formatting (CR LF / deferred CR
NUL) and transparent receive passthrough to shell line discipline.
3. **Comprehensive Configuration Options**:
- Configuration tables for daemon settings, driver ring buffers
(`CONFIG_TELNET_RXBUFFER_SIZE`, `CONFIG_TELNET_TXBUFFER_SIZE`,
`CONFIG_TELNET_MAXLCLIENTS`), and NSH Telnet console (`CONFIG_NSH_TELNET`,
ports, daemon/client stack & priorities).
- STDIO packet buffering optimization guidelines
(`CONFIG_STDIO_LINEBUFFER`, `CONFIG_STDIO_BUFFER_SIZE >= 64`).
4. **Public C API Reference**:
- Complete field-level documentation for `struct telnetd_config_s` and
`telnetd_daemon()`.
5. **Complete Application Example**:
- Runnable C implementation demonstrating background listener task
creation and interactive shell callbacks for both flat/protected (`t_entry`)
and kernel (`t_path`) builds.
6. **Client Connection Walkthroughs**:
- Terminal connection guides for Linux, macOS (including Homebrew
installation prerequisite and netcat alternative), and Windows (PuTTY).
7. **Troubleshooting & Best Practices Table**:
- Addressing character latency, connection refused, task stack sizing,
client limits, and escape sequence commands.
## Impact
- Documentation only. No runtime, API, or binary code changes.
## Testing
- Verified with `tools/checkpatch.sh -g HEAD` (0 errors, 0 warnings).
- Built and validated with Sphinx and Docutils rST parser (0 syntax or
indentation errors).
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