TFTP readWritePacket

Read Requests (RRQ, opcode 1) and Write Requests (WRQ, opcode 2) share the same packet structure. They are the first message a client sends to initiate a file transfer.

Source file: tftp.c
Hammer concepts: h_int_range · h_ch_range · h_token · h_many1 · h_sequence

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RRQ/WRQ Packet Layout

 2 bytes     string    1 byte    string   1 byte
+---------+-----------+------+-----------+------+
| Opcode  | Filename  |  0   |   Mode    |  0   |
+---------+-----------+------+-----------+------+

According to the RFC, a read or write request consists of:

• Opcode - 2 bytes, value 1 (RRQ) or 2 (WRQ)
• Filename - Variable-length string in netascii, terminated by a zero byte (\x00)
• Mode - One of "netascii", "octet", or "mail", terminated by a zero byte (\x00)

All three sections must be valid for the packet to pass.

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Step 1: Opcode with h_int_range

The opcode must be either 1 or 2:

H_RULE(opc, h_int_range(h_uint16(), 1, 2));

This reads 16 bits and rejects any value outside the range [1, 2].

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Step 2: Zero Byte Terminator

Both the filename and mode are terminated by a zero byte:

H_RULE(zbyt, h_int_range(h_uint8(), 0, 0));

This reads 8 bits and only accepts a value of 0.

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Step 3: Filename with h_ch_range and h_many1

A TFTP filename is a sequence of printable netascii characters. We define parsers for each valid character class:

H_RULE(lcAlph, h_ch_range('a', 'z'));
H_RULE(ucAlph, h_ch_range('A', 'Z'));
H_RULE(numbers, h_ch_range('0', '9'));
H_RULE(spclChars, h_choice(h_ch('.'), h_ch('_'), h_ch('/'),
                            h_ch('-'), h_ch(' '), NULL));

Then combine them into the filename parser using h_many1 (one or more characters) followed by the zero byte terminator:

H_RULE(filename, h_sequence(h_many1(h_choice(lcAlph, ucAlph,
    numbers, spclChars, NULL)), zbyt, NULL));

h_many1 requires at least one character, so empty filenames are rejected.

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Step 4: Mode String with h_token

The mode field must be exactly one of three strings. h_token matches a specific byte sequence:

H_RULE(netascii, h_token((const uint8_t *)"netascii", 8));
H_RULE(octet, h_token((const uint8_t *)"octet", 5));
H_RULE(mail, h_token((const uint8_t *)"mail", 4));
H_RULE(mode, h_sequence(h_choice(netascii, octet, mail, NULL),
    zbyt, NULL));

h_token(string, length) succeeds only if the next length bytes match string exactly. The mode is followed by a zero byte terminator.

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Step 5: Assemble the RRQ/WRQ Parser

H_RULE(rrqwrq, h_sequence(opc, filename, mode, NULL));

When rrqwrq runs, it parses the opcode, filename (with terminator), and mode (with terminator) in order. If any part fails, the entire parse fails.

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Summary

Concept	Hammer Function	Why It Matters in TFTP
Constrain values	h_int_range	Opcode must be 1 or 2
Match characters	h_ch_range	Filename characters must be printable ASCII
Match exact strings	h_token	Mode must be "netascii", "octet", or "mail"
One or more	h_many1	Filenames must have at least one character
Parse in order	h_sequence	Fields appear consecutively

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