tailfeather 0.1.0

Allocation-free, transport-agnostic building blocks for a Tailscale-compatible mesh node
Documentation 
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//! Minimal allocation-free STUN binding requests and IPv4 responses.

pub const TRANSACTION_ID_LEN: usize = 12;
pub const REQUEST_LEN: usize = 40;
const HEADER_LEN: usize = 20;
const COOKIE: u32 = 0x2112_a442;
const BINDING_REQUEST: u16 = 0x0001;
const BINDING_SUCCESS: u16 = 0x0101;
const SOFTWARE: u16 = 0x8022;
const XOR_MAPPED_ADDRESS: u16 = 0x0020;
const FINGERPRINT: u16 = 0x8028;
const FINGERPRINT_XOR: u32 = 0x5354_554e;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Error {
    Length,
    Type,
    Cookie,
    Transaction,
    Attribute,
    Fingerprint,
    MissingAddress,
    AddressFamily,
}

/// Encode a binding request with Tailscale's `tailnode` software marker.
pub fn request(transaction: [u8; TRANSACTION_ID_LEN]) -> [u8; REQUEST_LEN] {
    let mut packet = [0; REQUEST_LEN];
    packet[..2].copy_from_slice(&BINDING_REQUEST.to_be_bytes());
    packet[2..4].copy_from_slice(&20u16.to_be_bytes());
    packet[4..8].copy_from_slice(&COOKIE.to_be_bytes());
    packet[8..20].copy_from_slice(&transaction);
    packet[20..22].copy_from_slice(&SOFTWARE.to_be_bytes());
    packet[22..24].copy_from_slice(&8u16.to_be_bytes());
    packet[24..32].copy_from_slice(b"tailnode");
    packet[32..34].copy_from_slice(&FINGERPRINT.to_be_bytes());
    packet[34..36].copy_from_slice(&4u16.to_be_bytes());
    let fingerprint = crc32(&packet[..32]) ^ FINGERPRINT_XOR;
    packet[36..40].copy_from_slice(&fingerprint.to_be_bytes());
    packet
}

/// Validate a binding-success response and return the reflexive IPv4 endpoint.
pub fn response(
    packet: &[u8],
    transaction: [u8; TRANSACTION_ID_LEN],
) -> Result<([u8; 4], u16), Error> {
    if packet.len() < HEADER_LEN {
        return Err(Error::Length);
    }
    if u16::from_be_bytes([packet[0], packet[1]]) != BINDING_SUCCESS {
        return Err(Error::Type);
    }
    let declared = usize::from(u16::from_be_bytes([packet[2], packet[3]]));
    if declared % 4 != 0 || HEADER_LEN + declared != packet.len() {
        return Err(Error::Length);
    }
    if u32::from_be_bytes(packet[4..8].try_into().unwrap()) != COOKIE {
        return Err(Error::Cookie);
    }
    if packet[8..20] != transaction {
        return Err(Error::Transaction);
    }

    let cookie = COOKIE.to_be_bytes();
    let mut mapped = None;
    let mut offset = HEADER_LEN;
    while offset < packet.len() {
        if packet.len() - offset < 4 {
            return Err(Error::Attribute);
        }
        let kind = u16::from_be_bytes([packet[offset], packet[offset + 1]]);
        let len = usize::from(u16::from_be_bytes([packet[offset + 2], packet[offset + 3]]));
        let value_start = offset + 4;
        let value_end = value_start.checked_add(len).ok_or(Error::Attribute)?;
        let padded_end = value_start
            .checked_add(len.div_ceil(4) * 4)
            .ok_or(Error::Attribute)?;
        if value_end > packet.len() || padded_end > packet.len() {
            return Err(Error::Attribute);
        }
        match kind {
            XOR_MAPPED_ADDRESS => {
                if len != 8 {
                    return Err(Error::Attribute);
                }
                if packet[value_start + 1] != 1 {
                    return Err(Error::AddressFamily);
                }
                let port = u16::from_be_bytes([
                    packet[value_start + 2] ^ cookie[0],
                    packet[value_start + 3] ^ cookie[1],
                ]);
                let address = [
                    packet[value_start + 4] ^ cookie[0],
                    packet[value_start + 5] ^ cookie[1],
                    packet[value_start + 6] ^ cookie[2],
                    packet[value_start + 7] ^ cookie[3],
                ];
                mapped = Some((address, port));
            }
            FINGERPRINT => {
                if len != 4 || padded_end != packet.len() {
                    return Err(Error::Fingerprint);
                }
                let actual = u32::from_be_bytes(packet[value_start..value_end].try_into().unwrap());
                let expected = crc32(&packet[..offset]) ^ FINGERPRINT_XOR;
                if actual != expected {
                    return Err(Error::Fingerprint);
                }
            }
            _ => {}
        }
        offset = padded_end;
    }
    mapped.ok_or(Error::MissingAddress)
}

fn crc32(bytes: &[u8]) -> u32 {
    let mut crc = u32::MAX;
    for byte in bytes {
        crc ^= u32::from(*byte);
        for _ in 0..8 {
            let mask = 0u32.wrapping_sub(crc & 1);
            crc = (crc >> 1) ^ (0xedb8_8320 & mask);
        }
    }
    !crc
}

#[cfg(test)]
mod tests {
    use super::*;

    fn success(transaction: [u8; 12], address: [u8; 4], port: u16) -> [u8; 40] {
        let cookie = COOKIE.to_be_bytes();
        let mut packet = [0; 40];
        packet[..2].copy_from_slice(&BINDING_SUCCESS.to_be_bytes());
        packet[2..4].copy_from_slice(&20u16.to_be_bytes());
        packet[4..8].copy_from_slice(&cookie);
        packet[8..20].copy_from_slice(&transaction);
        packet[20..22].copy_from_slice(&XOR_MAPPED_ADDRESS.to_be_bytes());
        packet[22..24].copy_from_slice(&8u16.to_be_bytes());
        packet[25] = 1;
        let encoded_port = port ^ (COOKIE >> 16) as u16;
        packet[26..28].copy_from_slice(&encoded_port.to_be_bytes());
        for index in 0..4 {
            packet[28 + index] = address[index] ^ cookie[index];
        }
        packet[32..34].copy_from_slice(&FINGERPRINT.to_be_bytes());
        packet[34..36].copy_from_slice(&4u16.to_be_bytes());
        let fingerprint = crc32(&packet[..32]) ^ FINGERPRINT_XOR;
        packet[36..40].copy_from_slice(&fingerprint.to_be_bytes());
        packet
    }

    #[test]
    fn request_has_tailnode_marker_and_a_valid_fingerprint() {
        let packet = request([3; 12]);
        assert_eq!(&packet[24..32], b"tailnode");
        assert_eq!(
            u32::from_be_bytes(packet[36..40].try_into().unwrap()),
            crc32(&packet[..32]) ^ FINGERPRINT_XOR
        );
    }

    #[test]
    fn xor_mapped_ipv4_round_trips() {
        let transaction = [9; 12];
        assert_eq!(
            response(&success(transaction, [203, 0, 113, 7], 41_641), transaction),
            Ok(([203, 0, 113, 7], 41_641))
        );
    }

    #[test]
    fn response_without_optional_fingerprint_is_accepted() {
        let transaction = [9; 12];
        let mut packet = success(transaction, [203, 0, 113, 7], 41_641);
        packet[2..4].copy_from_slice(&12u16.to_be_bytes());
        assert_eq!(
            response(&packet[..32], transaction),
            Ok(([203, 0, 113, 7], 41_641))
        );
    }

    #[test]
    fn wrong_transaction_and_fingerprint_are_rejected() {
        let transaction = [9; 12];
        let mut packet = success(transaction, [203, 0, 113, 7], 41_641);
        assert_eq!(response(&packet, [8; 12]), Err(Error::Transaction));
        packet[39] ^= 1;
        assert_eq!(response(&packet, transaction), Err(Error::Fingerprint));
    }
}