📬 SMTP Parser

A lightweight SMTP protocol parser built with Lex and Yacc (Flex/Bison). This tool validates the structure of SMTP sessions — checking that commands appear in the correct order and format according to the SMTP protocol.

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📖 Overview

This project implements a lexical analyzer and grammar-based parser for the SMTP protocol. It reads an SMTP session (from a file or stdin) and verifies whether the command sequence is syntactically valid.

The parser recognizes a complete SMTP conversation flow:

HELO → MAIL FROM → RCPT TO → DATA → Subject + Body → (.) → QUIT

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📁 Project Structure

Smtp_parser/
├── lex_smtp.l    # Lexer: tokenizes SMTP commands and message content
├── parser.y      # Parser: validates grammar and command ordering
├── email.txt     # Sample valid SMTP session
└── gmail.txt     # Sample Gmail-style SMTP session

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⚙️ How It Works

Lexer (lex_smtp.l)

Identifies and returns tokens for each recognized SMTP command:

Token	Matches
TOKEN_HELO	HELO <domain>
TOKEN_MAIL_FROM	MAIL FROM:<address>
TOKEN_RCPT_TO	RCPT TO:<address>
TOKEN_DATA	DATA
TOKEN_SUBJECT	Subject: <text>
TOKEN_END_OF_DATA	. (end of message body)
TOKEN_QUIT	QUIT
TOKEN_TEXT	Any other non-empty line

Parser (parser.y)

Validates that the tokens appear in the correct order based on SMTP grammar rules. If any command is out of order or missing, the parser reports a descriptive syntax error and aborts.

Grammar:

smtp    → HELO MAIL_FROM RCPT_TO DATA message QUIT
message → SUBJECT body END_OF_DATA
body    → TEXT
        | body TEXT

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🛠️ Requirements

• flex (Lex)
• bison (Yacc)
• gcc

Install on Ubuntu/Debian:

sudo apt install flex bison gcc

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🚀 Build & Run

Step 1 — Generate the parser and lexer:

bison -d parser.y
flex lex_smtp.l

Step 2 — Compile:

gcc -o smtp_parser lex.yy.c parser.tab.c -lfl

Step 3 — Run with a sample SMTP file:

./smtp_parser < email.txt

Expected output for a valid session:

SMTP syntax is correct.

Example output for an invalid session:

Syntax Error: RCPT TO expected after MAIL FROM.

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📄 Sample Input (email.txt)

HELO example.com
MAIL FROM:<sender@example.com>
RCPT TO:<receiver@example.com>
DATA
Subject: Hello World
This is the body of the email.
.
QUIT

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❌ Error Handling

The parser provides meaningful error messages for common mistakes:

Situation	Error Message
Missing MAIL FROM after HELO	Syntax Error: MAIL FROM expected after HELO.
Missing RCPT TO after MAIL FROM	Syntax Error: RCPT TO expected after MAIL FROM.
Missing DATA after RCPT TO	Syntax Error: DATA expected after RCPT TO.
Missing subject/body after DATA	Syntax Error: Subject and body expected after DATA.
Missing body after Subject	Syntax Error: Body expected after Subject.
Completely invalid structure	Syntax Error: Invalid SMTP command structure.

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📚 Technologies

• Flex (Fast Lexical Analyzer) — tokenizes the input stream
• Bison (GNU Parser Generator) — validates grammar rules
• C — underlying implementation language

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📜 License

This project is open source. Feel free to use, modify, and distribute it.