Telnet Parser

A Telnet protocol command parser built with Flex (lexical analyzer) and Bison (parser generator). It reads lines of text, identifies Telnet IAC (Interpret As Command) sequences, and classifies each line as either an IAC command or regular body text.

Overview

Telnet uses a command/response protocol where special sequences are prefixed with IAC (Interpret As Command, byte 0xFF). This parser reads textual representations of Telnet commands (e.g., IAC DO ECHO) and parses them into structured output.

Example

Input:

IAC DO ECHO
Hello from Telnet client!
IAC WILL TERMINAL-TYPE

Output:

[IAC] DO ECHO
[Body] Hello from Telnet client!
[IAC] WILL TERMINAL-TYPE

Grammar

The parser recognizes two types of lines:

• IAC lines: IAC <COMMAND> <OPTION>
  • Commands: DO, DONT, WILL, WONT
  • Options: ECHO, SUPPRESS-GO-AHEAD, TERMINAL-TYPE, NAWS, LINEMODE, NEW-ENVIRON, STATUS, BINARY, SGA, or any WORD
• Body lines: Free-form text (one or more words)

Building

Prerequisites

• GCC (or any C99-compatible compiler)
• Flex (lexical analyzer)
• Bison (parser generator)

Build

make

Run

make run
# or
./telnet_parser < input.text

Clean

make clean       # remove binary
make distclean   # remove binary AND generated C files

Files

File	Description
telnet.y	Bison grammar definition (.y file)
telnet.l	Flex lexer definition (.l file)
telnet.tab.c	Generated C parser (from Bison)
telnet.tab.h	Generated parser header (from Bison)
lex.yy.c	Generated C lexer (from Flex)
input.text	Sample input file
Makefile	Build automation

Customization

Adding Telnet Options

Edit telnet.l and add new option tokens following the existing pattern:

"MY-NEW-OPTION"  { yylval.str = strdup("MY-NEW-OPTION"); return OPTION; }

Case Sensitivity

The lexer uses %option case-insensitive, so iac do echo, IAC DO ECHO, and Iac Do Echo are all treated identically.

License

MIT