OpenPXE/crates/dhcp-proxy/src/reply.rs

//! Build DHCP proxy replies.
//!
//! Proxy replies look like a normal DHCPOFFER/ACK except:
//!   - `yiaddr` (your IP) is 0 — we don't lease.
//!   - `siaddr` (server IP) is us — the client will TFTP from here.
//!   - option 60 (vendor class) MUST be echoed as `PXEClient` or clients drop.
//!   - option 66 (TFTP server name) points at us.
//!   - option 67 (bootfile name) is per-architecture iPXE binary on first
//!     pass, or the HTTP URL of the boot script once iPXE has chained.

use dhcproto::v4::{DhcpOption, Message, MessageType, Opcode, OptionCode};
use openpxe_core::{ClientArch, FirmwareClass};
use std::net::Ipv4Addr;

/// Where the reply directs the client next.
#[derive(Debug, Clone)]
pub enum BootDirective {
    /// Serve an iPXE binary over TFTP (first-stage chainload).
    TftpIpxe { filename: String },
    /// Serve an iPXE boot script directly over HTTP. Used when the client is
    /// iPXE itself (option 77 = "iPXE") or UEFI HTTP boot (option 60 starts
    /// with "HTTPClient").
    HttpScript { url: String },
    /// Refuse to respond (architecture we don't have a binary for, or
    /// not-a-PXE-client). Caller should skip sending anything.
    Ignore,
}

pub struct ReplyContext<'a> {
    pub request: &'a Message,
    pub our_ip: Ipv4Addr,
    pub arch: ClientArch,
    pub class: FirmwareClass,
    /// Public base URL (scheme://host[:port]) used in HTTP directives.
    pub public_base_url: &'a str,
}

/// Decide what to do for an incoming request. Pure function — easy to unit
/// test. Does NOT send anything.
#[must_use]
pub fn decide(ctx: &ReplyContext<'_>) -> BootDirective {
    match ctx.class {
        FirmwareClass::IpxeUserClass => BootDirective::HttpScript {
            // Pass the client's MAC in the query string so the HTTP
            // layer can short-circuit to a per-MAC binding when one
            // exists. iPXE substitutes `${mac}` literally before issuing
            // the GET, so this stays static across firmwares.
            url: format!(
                "{}/boot.ipxe?mac=${{mac}}",
                ctx.public_base_url.trim_end_matches('/')
            ),
        },
        FirmwareClass::HttpClient => {
            // UEFI HTTP boot: client wants an http:// URL in option 67
            // pointing at an EFI executable. We serve ipxe.efi over HTTP;
            // it'll then do the same script-fetch the iPXE path does.
            let name = ctx.arch.ipxe_bootfile().unwrap_or("snponly.efi");
            BootDirective::HttpScript {
                url: format!(
                    "{}/ipxe/{}",
                    ctx.public_base_url.trim_end_matches('/'),
                    name
                ),
            }
        }
        FirmwareClass::PxeClient => match ctx.arch.ipxe_bootfile() {
            Some(name) => BootDirective::TftpIpxe {
                filename: name.to_string(),
            },
            None => BootDirective::Ignore,
        },
        FirmwareClass::Other => BootDirective::Ignore,
    }
}

/// Build the outgoing DHCPOFFER (or ACK, matching request type) for a
/// directive. Caller is responsible for sending the bytes on the wire.
pub fn build_reply(ctx: &ReplyContext<'_>, directive: &BootDirective) -> Option<Message> {
    let reply_type = match request_message_type(ctx.request)? {
        MessageType::Discover => MessageType::Offer,
        MessageType::Request | MessageType::Inform => MessageType::Ack,
        _ => return None,
    };

    let mut msg = Message::default();
    msg.set_opcode(Opcode::BootReply)
        .set_htype(ctx.request.htype())
        .set_hops(0)
        .set_xid(ctx.request.xid())
        .set_secs(0)
        .set_flags(ctx.request.flags())
        .set_ciaddr(Ipv4Addr::UNSPECIFIED)
        .set_yiaddr(Ipv4Addr::UNSPECIFIED) // proxy does not lease
        .set_siaddr(ctx.our_ip)
        .set_giaddr(ctx.request.giaddr())
        .set_chaddr(ctx.request.chaddr());

    // Set the BOOTP `file` field for legacy PXE stacks before we take the
    // options borrow (the two borrows can't overlap).
    if let BootDirective::TftpIpxe { filename } = directive {
        msg.set_fname_str(filename);
    }

    let class_echo: &[u8] = match ctx.class {
        FirmwareClass::HttpClient => b"HTTPClient",
        _ => b"PXEClient",
    };
    let opts = msg.opts_mut();
    opts.insert(DhcpOption::MessageType(reply_type));
    opts.insert(DhcpOption::ServerIdentifier(ctx.our_ip));
    // Echo the vendor class — REQUIRED by spec for the client to accept.
    opts.insert(DhcpOption::ClassIdentifier(class_echo.to_vec()));

    match directive {
        BootDirective::TftpIpxe { filename } => {
            opts.insert(DhcpOption::TFTPServerName(
                ctx.our_ip.to_string().into_bytes(),
            ));
            opts.insert(DhcpOption::BootfileName(filename.as_bytes().to_vec()));
        }
        BootDirective::HttpScript { url } => {
            opts.insert(DhcpOption::BootfileName(url.as_bytes().to_vec()));
            opts.insert(DhcpOption::TFTPServerName(
                ctx.our_ip.to_string().into_bytes(),
            ));
        }
        BootDirective::Ignore => return None,
    }

    opts.insert(DhcpOption::End);
    Some(msg)
}

fn request_message_type(m: &Message) -> Option<MessageType> {
    m.opts().get(OptionCode::MessageType).and_then(|o| match o {
        DhcpOption::MessageType(t) => Some(*t),
        _ => None,
    })
}