cellos-supervisor 0.5.1

//! HTTP/1.x request-line + Host header extractor — pure logic, no I/O.
//!
//! The SEC-22 Phase 2 SNI proxy peeks the first bytes of a TCP connection
//! and, when the bytes look like a cleartext HTTP request (ASCII method
//! token like `GET ` / `POST` / `HEAD` / `OPTIONS` / `CONNECT` / `PUT`), this
//! module identifies the end of the header section and returns the `Host`
//! header value (RFC 9110 §7.2).
//!
//! # Honest scope
//!
//! - **HTTP/1.x cleartext only.** HTTP/2 (binary frames after `PRI * HTTP/2.0`
//!   preface) and HTTP/3 (QUIC) are out of scope. The HTTP/2 preface is
//!   detected and explicitly returned as `Ok(None)` so the proxy can flag the
//!   flow with `reasonCode: l7_unknown_protocol` rather than mis-parsing it.
//! - **No body reading.** We stop at the end-of-headers (`\r\n\r\n`) marker.
//! - **8 KiB header cap.** Defensive bound — header sections larger than this
//!   are rejected as malformed.
//! - **No CRLF injection laundering.** Raw bytes are walked; embedded NUL or
//!   bare LF inside the request line is rejected.

use std::fmt;

/// Header-section size cap. Apache, nginx, and most reverse proxies cap at
/// 8 KiB by default; we mirror that here so a workload can't blow our peek
/// buffer on a malicious giant header section.
pub const MAX_HEADER_SECTION_LEN: usize = 8192;

/// HTTP/2 connection preface (RFC 7540 §3.5).
const HTTP2_PREFACE: &[u8] = b"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n";

/// Errors produced by [`extract_http_host`].
#[derive(Debug, PartialEq, Eq, Clone)]
pub enum HttpParseError {
    /// Header section did not terminate with `\r\n\r\n` within
    /// [`MAX_HEADER_SECTION_LEN`].
    HeaderSectionOverflow,
    /// First line is not a recognisable HTTP/1.x request line.
    MalformedRequestLine,
    /// Embedded NUL / bare LF / non-ASCII byte in method or target.
    InvalidByte,
    /// HTTP/0.9 has no Host header semantics.
    Http09NotSupported,
}

impl fmt::Display for HttpParseError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            HttpParseError::HeaderSectionOverflow => write!(
                f,
                "HTTP header section exceeded {MAX_HEADER_SECTION_LEN} bytes without CRLFCRLF"
            ),
            HttpParseError::MalformedRequestLine => write!(f, "malformed HTTP/1.x request line"),
            HttpParseError::InvalidByte => write!(f, "invalid byte in request line"),
            HttpParseError::Http09NotSupported => write!(f, "HTTP/0.9 has no Host semantics"),
        }
    }
}

impl std::error::Error for HttpParseError {}

/// Extract the `Host` header value from buffered first bytes of a TCP
/// connection.
pub fn extract_http_host(first_bytes: &[u8]) -> Result<Option<String>, HttpParseError> {
    if first_bytes.len() >= HTTP2_PREFACE.len()
        && &first_bytes[..HTTP2_PREFACE.len()] == HTTP2_PREFACE
    {
        return Ok(None);
    }

    let scan_end = first_bytes.len().min(MAX_HEADER_SECTION_LEN);
    let header_end = match find_crlf_crlf(&first_bytes[..scan_end]) {
        Some(idx) => idx,
        None => {
            if first_bytes.len() >= MAX_HEADER_SECTION_LEN {
                return Err(HttpParseError::HeaderSectionOverflow);
            }
            return Err(HttpParseError::MalformedRequestLine);
        }
    };
    // Include the first CRLF of the terminating CRLFCRLF so each header line
    // — including the LAST header — has its trailing `\r\n` inside the slice
    // we walk. Without this, the loop's `find_crlf(&header_section[idx..])`
    // would miss the final header line (which has no CRLF before
    // `header_end`) and the parser would silently return `Ok(None)` for any
    // request whose only header is `Host:`. The +2 stays bounded: `header_end`
    // is the start offset of the four-byte `\r\n\r\n` we just located.
    let header_section_end = header_end + 2;
    let header_section = &first_bytes[..header_section_end];

    let line_end = find_crlf(header_section).ok_or(HttpParseError::MalformedRequestLine)?;
    let request_line = &header_section[..line_end];
    validate_request_line(request_line)?;
    let space_count = request_line.iter().filter(|&&b| b == b' ').count();
    if space_count < 2 {
        return Err(HttpParseError::Http09NotSupported);
    }

    let mut idx = line_end + 2;
    while idx < header_section.len() {
        let rel_end = match find_crlf(&header_section[idx..]) {
            Some(e) => e,
            None => break,
        };
        let line = &header_section[idx..idx + rel_end];
        if line.is_empty() {
            break;
        }
        if let Some(colon) = line.iter().position(|&b| b == b':') {
            let name = &line[..colon];
            if eq_ignore_ascii(name, b"Host") {
                let value = &line[colon + 1..];
                return Ok(Some(normalise_host_value(value)));
            }
        }
        idx += rel_end + 2;
    }
    Ok(None)
}

fn find_crlf(buf: &[u8]) -> Option<usize> {
    buf.windows(2).position(|w| w == b"\r\n")
}

fn find_crlf_crlf(buf: &[u8]) -> Option<usize> {
    buf.windows(4).position(|w| w == b"\r\n\r\n")
}

fn validate_request_line(line: &[u8]) -> Result<(), HttpParseError> {
    if line.is_empty() {
        return Err(HttpParseError::MalformedRequestLine);
    }
    for &b in line {
        if b == 0 || b == b'\n' || b == b'\r' {
            return Err(HttpParseError::InvalidByte);
        }
        if !(0x20..=0x7E).contains(&b) {
            return Err(HttpParseError::InvalidByte);
        }
    }
    Ok(())
}

fn eq_ignore_ascii(a: &[u8], b: &[u8]) -> bool {
    if a.len() != b.len() {
        return false;
    }
    a.iter()
        .zip(b.iter())
        .all(|(x, y)| x.eq_ignore_ascii_case(y))
}

fn normalise_host_value(raw: &[u8]) -> String {
    let mut start = 0;
    while start < raw.len() && (raw[start] == b' ' || raw[start] == b'\t') {
        start += 1;
    }
    let mut end = raw.len();
    while end > start && (raw[end - 1] == b' ' || raw[end - 1] == b'\t') {
        end -= 1;
    }
    let v = &raw[start..end];
    let host_bytes = if v.first() == Some(&b'[') {
        if let Some(close) = v.iter().position(|&b| b == b']') {
            &v[..=close]
        } else {
            v
        }
    } else if let Some(colon) = v.iter().position(|&b| b == b':') {
        &v[..colon]
    } else {
        v
    };
    let mut s = String::from_utf8_lossy(host_bytes).to_string();
    s.make_ascii_lowercase();
    if s.ends_with('.') {
        s.pop();
    }
    s
}

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

    #[test]
    fn extracts_get_host() {
        let req = b"GET / HTTP/1.1\r\nHost: api.example.com\r\nUser-Agent: x\r\n\r\n";
        assert_eq!(
            extract_http_host(req).unwrap().as_deref(),
            Some("api.example.com")
        );
    }

    #[test]
    fn extracts_post_host() {
        let req = b"POST /v1/x HTTP/1.1\r\nHost: api.example.com\r\nContent-Length: 0\r\n\r\n";
        assert_eq!(
            extract_http_host(req).unwrap().as_deref(),
            Some("api.example.com")
        );
    }

    #[test]
    fn missing_host_returns_ok_none() {
        let req = b"GET / HTTP/1.1\r\nAccept: */*\r\n\r\n";
        assert_eq!(extract_http_host(req).unwrap(), None);
    }

    #[test]
    fn case_insensitive_host_header() {
        let req = b"GET / HTTP/1.1\r\nhost: api.example.com\r\n\r\n";
        assert_eq!(
            extract_http_host(req).unwrap().as_deref(),
            Some("api.example.com")
        );
        let req2 = b"GET / HTTP/1.1\r\nHOST: api.example.com\r\n\r\n";
        assert_eq!(
            extract_http_host(req2).unwrap().as_deref(),
            Some("api.example.com")
        );
    }

    #[test]
    fn host_with_port_strips_port() {
        let req = b"GET / HTTP/1.1\r\nHost: api.example.com:8443\r\n\r\n";
        assert_eq!(
            extract_http_host(req).unwrap().as_deref(),
            Some("api.example.com")
        );
    }

    #[test]
    fn ipv6_host_with_port_keeps_brackets() {
        let req = b"GET / HTTP/1.1\r\nHost: [::1]:443\r\n\r\n";
        assert_eq!(extract_http_host(req).unwrap().as_deref(), Some("[::1]"));
    }

    #[test]
    fn malformed_request_line_no_version() {
        let req = b"GET /\r\nHost: x.example.com\r\n\r\n";
        assert!(matches!(
            extract_http_host(req),
            Err(HttpParseError::Http09NotSupported)
        ));
    }

    #[test]
    fn oversized_header_section_rejected() {
        let mut buf: Vec<u8> = Vec::with_capacity(MAX_HEADER_SECTION_LEN + 16);
        buf.extend_from_slice(b"GET / HTTP/1.1\r\n");
        buf.extend_from_slice(b"X-Pad: ");
        while buf.len() < MAX_HEADER_SECTION_LEN + 8 {
            buf.push(b'a');
        }
        assert_eq!(
            extract_http_host(&buf),
            Err(HttpParseError::HeaderSectionOverflow)
        );
    }

    #[test]
    fn http2_preface_returns_ok_none() {
        let bytes = b"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n\x00\x00";
        assert_eq!(extract_http_host(bytes).unwrap(), None);
    }

    #[test]
    fn crlf_injection_in_request_line_rejected() {
        let req = b"GET /\rinjected HTTP/1.1\r\nHost: api.example.com\r\n\r\n";
        assert!(matches!(
            extract_http_host(req),
            Err(HttpParseError::InvalidByte) | Err(HttpParseError::MalformedRequestLine)
        ));
    }

    #[test]
    fn nul_byte_in_request_line_rejected() {
        let req = b"GET /\x00 HTTP/1.1\r\nHost: api.example.com\r\n\r\n";
        assert!(matches!(
            extract_http_host(req),
            Err(HttpParseError::InvalidByte)
        ));
    }

    #[test]
    fn trailing_dot_in_host_stripped() {
        let req = b"GET / HTTP/1.1\r\nHost: api.example.com.\r\n\r\n";
        assert_eq!(
            extract_http_host(req).unwrap().as_deref(),
            Some("api.example.com")
        );
    }

    #[test]
    fn host_lowercased() {
        let req = b"GET / HTTP/1.1\r\nHost: API.Example.COM\r\n\r\n";
        assert_eq!(
            extract_http_host(req).unwrap().as_deref(),
            Some("api.example.com")
        );
    }

    #[test]
    fn incomplete_request_no_crlfcrlf_is_malformed() {
        let req = b"GET / HTTP/1.1\r\nHost: api.example.com\r\n";
        assert_eq!(
            extract_http_host(req),
            Err(HttpParseError::MalformedRequestLine)
        );
    }
}