sagittarius 0.1.0

//! DNS wire-format codec.
//!
//! A custom, *lazy* parser and serializer over [`bytes::Bytes`].  The design
//! deliberately avoids a full up-front parse: outer tower middleware layers
//! route on a **shallow parse** (12-byte header + the single question only)
//! and can short-circuit without touching the answer or additional sections.
//!
//! Key properties (see SPEC §2.1 for the full rationale):
//! - The question name is never compressed in a well-formed packet, so the
//!   shallow parser needs no name-decompression logic at all.
//! - The original datagram is carried as a refcounted `Bytes` through the
//!   pipeline, avoiding re-serialization for forwarded/cached responses.
//! - The codec defensively rejects packets with `QDCOUNT != 1` and any
//!   compression pointer appearing in the question section.
//!
//! # Module layout
//!
//! | Submodule | Responsibility |
//! |---|---|
//! | [`reader`] | Bounds-checked cursor over `bytes::Bytes` for parsing |
//! | [`writer`] | Append-only output buffer over `bytes::BytesMut` for synthesis |
//! | [`framing`] | UDP/TCP message framing (length-prefix encode/decode) |
//! | [`header`] | 12-byte DNS header, [`header::Opcode`], [`header::Rcode`] |
//! | [`name`] | [`Name`] type, QNAME reader/writer, RR name-skip helper |
//! | [`synth`] | EDNS/OPT scan, block/local/error response synthesis |
//! | [`ttl`] | Bounded TTL scan — min-TTL and per-RR TTL field offsets for in-place patching |
//!
//! All submodules share this single [`Error`] type; the crate-level
//! [`crate::error::Error`] wraps it via `#[from]`.

pub mod framing;
pub mod header;
pub mod message;
pub mod name;
pub mod reader;
pub mod synth;
pub mod ttl;
pub mod writer;

/// Errors that can occur while parsing or serializing DNS wire format.
#[derive(Debug, thiserror::Error)]
pub enum Error {
    /// The message was too short to contain a valid DNS header.
    #[error("message too short: need at least 12 bytes, got {0}")]
    MessageTooShort(usize),

    /// The question count was not exactly 1.
    #[error("expected QDCOUNT=1, got {0}")]
    InvalidQuestionCount(u16),

    /// A compression pointer was found in the question section.
    #[error("compression pointer in question section is not allowed")]
    CompressionPointerInQuestion,

    /// A read attempted to consume more bytes than are available in the buffer.
    ///
    /// Returned whenever a `read_u8`, `read_u16`, `read_u32`, or `read_slice`
    /// call would go past the end of the underlying buffer — never panics.
    #[error(
        "unexpected end of buffer at offset {offset}: need {needed} bytes, {available} available"
    )]
    UnexpectedEof {
        /// Byte offset at which the read was attempted.
        offset: usize,
        /// Number of bytes the attempted read needed.
        needed: usize,
        /// Number of bytes actually remaining from that offset.
        available: usize,
    },

    /// A TCP length-prefix frame was truncated: the 2-byte prefix was not fully
    /// received.
    #[error("TCP length prefix is truncated: need 2 bytes, got {0}")]
    TruncatedLengthPrefix(usize),

    /// A DNS name label exceeded the 63-byte maximum (RFC 1035 §2.3.4).
    ///
    /// Returned when constructing a [`name::Name`] from a string or from the
    /// wire if a label length byte exceeds 63.
    #[error("label too long: {0} bytes (maximum is 63)")]
    LabelTooLong(usize),

    /// A DNS name exceeded the 255-byte total wire-format length limit
    /// (RFC 1035 §2.3.4).
    ///
    /// Returned when constructing a [`name::Name`] from a string or from the
    /// wire if the cumulative encoded length would exceed 255 bytes.
    #[error("name too long: {0} bytes (maximum is 255)")]
    NameTooLong(usize),

    /// The name-skip helper in the RR section exceeded the hop or byte cap
    /// while following compression pointers.
    ///
    /// This indicates either a pointer loop or a pathologically deep chain
    /// crafted to exhaust resources.  The skip is aborted with this error
    /// rather than looping indefinitely.
    #[error("name skip limit exceeded while following compression pointers")]
    NameSkipLimitExceeded,

    /// A compression pointer in the RR section pointed to an offset that is
    /// not strictly before the current position (i.e. forward or out-of-range
    /// pointer).  Such pointers are rejected defensively.
    #[error("compression pointer target {target} is invalid (message length {msg_len})")]
    InvalidPointerTarget {
        /// The pointer offset that was rejected.
        target: u16,
        /// Total length of the message buffer used to validate the range.
        msg_len: usize,
    },

    /// A domain name string could not be parsed because it contained an
    /// empty label in a non-root position (e.g. `"foo..bar"`).
    #[error("invalid domain name: empty label")]
    EmptyLabel,

    /// A DNS message exceeded the 65535-byte maximum wire-format length.
    ///
    /// DNS over TCP uses a 2-byte length prefix (u16), so no message can
    /// exceed 65535 bytes.  Inputs larger than this are rejected defensively
    /// before any other parsing work.
    #[error("message too long: {0} bytes (maximum is 65535)")]
    MessageTooLong(usize),
}

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

    #[test]
    fn error_variants_display() {
        assert!(Error::MessageTooShort(4).to_string().contains('4'));
        assert!(Error::InvalidQuestionCount(2).to_string().contains('2'));
        assert!(!Error::CompressionPointerInQuestion.to_string().is_empty());

        let eof = Error::UnexpectedEof {
            offset: 10,
            needed: 4,
            available: 2,
        };
        let s = eof.to_string();
        assert!(s.contains("10"), "offset should appear in message: {s}");
        assert!(s.contains('4'), "needed should appear in message: {s}");
        assert!(s.contains('2'), "available should appear in message: {s}");

        assert!(Error::TruncatedLengthPrefix(1).to_string().contains('1'));
        assert!(Error::MessageTooLong(70000).to_string().contains("70000"));
    }
}