Switch from sanboot to kernel/initrd boot with PXE overlay for ISO download

sanboot makes the ISO visible to UEFI but invisible to the Linux kernel
after ExitBootServices(). Switch to direct kernel/initrd boot with a small
CPIO overlay containing /pxe-init — a shell script that loads NIC drivers,
configures DHCP, downloads the ISO via wget, and creates a loop device
before handing off to the Proxmox installer init.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/image/cpio.go

@@ -7,47 +7,60 @@ import (
 )
 
 const pxeInitScript = `#!/bin/sh
-mkdir -p /dev
-mount -t devtmpfs devtmpfs /dev 2>/dev/null
-losetup /dev/loop0 /proxmox.iso 2>/dev/null
+mount -t proc proc /proc 2>/dev/null
+mount -t sysfs sys /sys 2>/dev/null
+mount -t devtmpfs dev /dev 2>/dev/null
+mkdir -p /tmp
+
+ISO_URL=""
+for param in $(cat /proc/cmdline); do
+    case $param in
+        prov.iso=*) ISO_URL="${param#prov.iso=}" ;;
+    esac
+done
+
+if [ -n "$ISO_URL" ]; then
+    echo "=== PXE: Setting up network ==="
+    for mod in e1000 e1000e igb ixgbe i40e ice tg3 bnxt_en r8169 virtio_net; do
+        modprobe $mod 2>/dev/null
+    done
+    sleep 2
+
+    for iface in $(ls /sys/class/net/ 2>/dev/null | grep -v lo); do
+        ip link set "$iface" up 2>/dev/null
+        if udhcpc -i "$iface" -n -q -t 5 2>/dev/null; then
+            echo "PXE: Network up on $iface"
+            break
+        fi
+    done
+
+    echo "=== PXE: Downloading ISO ==="
+    wget -O /tmp/proxmox.iso "$ISO_URL" 2>&1
+
+    if [ -s /tmp/proxmox.iso ]; then
+        losetup /dev/loop0 /tmp/proxmox.iso
+        echo "PXE: ISO on /dev/loop0"
+    else
+        echo "PXE: ERROR - download failed"
+    fi
+fi
+
 umount /dev 2>/dev/null
+umount /sys 2>/dev/null
+umount /proc 2>/dev/null
 exec /init "$@"
 `
 
-func CreatePXEInitrd(origInitrdPath, isoPath, outputPath string) error {
+func CreatePXEOverlay(outputPath string) error {
 	out, err := os.Create(outputPath)
 	if err != nil {
-		return fmt.Errorf("create output: %w", err)
+		return fmt.Errorf("create overlay: %w", err)
 	}
 	defer out.Close()
 
-	orig, err := os.Open(origInitrdPath)
-	if err != nil {
-		return fmt.Errorf("open initrd: %w", err)
-	}
-	if _, err := io.Copy(out, orig); err != nil {
-		orig.Close()
-		return fmt.Errorf("copy initrd: %w", err)
-	}
-	orig.Close()
-
 	if err := writeCPIOEntry(out, "pxe-init", []byte(pxeInitScript), 0o100755, 1); err != nil {
 		return err
 	}
-
-	isoStat, err := os.Stat(isoPath)
-	if err != nil {
-		return fmt.Errorf("stat iso: %w", err)
-	}
-	isoFile, err := os.Open(isoPath)
-	if err != nil {
-		return fmt.Errorf("open iso: %w", err)
-	}
-	defer isoFile.Close()
-	if err := writeCPIOEntryFromReader(out, "proxmox.iso", isoFile, isoStat.Size(), 0o100644, 2); err != nil {
-		return err
-	}
-
 	return writeCPIOTrailer(out)
 }
 
@@ -66,21 +79,6 @@ func writeCPIOEntry(w io.Writer, name string, data []byte, mode uint32, ino uint
 	return nil
 }
 
-func writeCPIOEntryFromReader(w io.Writer, name string, r io.Reader, size int64, mode uint32, ino uint32) error {
-	if err := writeCPIOHeader(w, name, size, mode, ino); err != nil {
-		return err
-	}
-	if _, err := io.Copy(w, r); err != nil {
-		return err
-	}
-	if pad := (4 - int(size%4)) % 4; pad > 0 {
-		if _, err := w.Write(make([]byte, pad)); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
 func writeCPIOHeader(w io.Writer, name string, fileSize int64, mode uint32, ino uint32) error {
 	nameLen := len(name) + 1
 	hdr := fmt.Sprintf("070701%08X%08X%08X%08X%08X%08X%08X%08X%08X%08X%08X%08X%08X",

internal/image/service.go

@@ -55,12 +55,13 @@ func (s *Service) Upload(ctx context.Context, p UploadParams) (*model.Image, err
 		return nil, fmt.Errorf("extract ISO: %w", err)
 	}
 
-	if s.PublicURL != "" {
-		report := func(stage, detail string) {
-			if p.OnProgress != nil {
-				p.OnProgress(stage, detail)
-			}
+	report := func(stage, detail string) {
+		if p.OnProgress != nil {
+			p.OnProgress(stage, detail)
 		}
+	}
+
+	if s.PublicURL != "" {
 		report("packaging", "Configuring ISO for automated installation...")
 		isoFile := filepath.Join(destDir, result.ISOFilename)
 		answerURL := s.PublicURL + "/api/boot/auto-answer"
@@ -69,6 +70,12 @@ func (s *Service) Upload(ctx context.Context, p UploadParams) (*model.Image, err
 		}
 	}
 
+	report("packaging", "Creating PXE boot overlay...")
+	overlayPath := filepath.Join(destDir, "pxe-overlay.img")
+	if err := CreatePXEOverlay(overlayPath); err != nil {
+		log.Printf("image: PXE overlay creation failed (non-fatal): %v", err)
+	}
+
 	kernelPath := filepath.Join(p.Name, result.KernelFilename)
 	initrdPath := filepath.Join(p.Name, result.InitrdFilename)
 	isoPath := filepath.Join(p.Name, result.ISOFilename)

internal/pxe/ipxe.go

@@ -2,16 +2,26 @@ package pxe
 
 import (
 	"fmt"
+	"path/filepath"
+	"strings"
 
 	"provisioning/internal/model"
 )
 
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
-	isoURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.ISOPath)
+	base := fmt.Sprintf("%s/images/boot", publicURL)
+	kernelURL := fmt.Sprintf("%s/%s", base, img.KernelPath)
+	initrdURL := fmt.Sprintf("%s/%s", base, img.InitrdPath)
+	isoURL := fmt.Sprintf("%s/%s", base, img.ISOPath)
+	overlayURL := fmt.Sprintf("%s/%s/pxe-overlay.img", base,
+		strings.Split(filepath.ToSlash(img.KernelPath), "/")[0])
+	answerURL := fmt.Sprintf("%s/api/boot/auto-answer", publicURL)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
-echo Booting ISO...
-sanboot %s
-`, img.Name, isoURL)
+kernel %s ro ramdisk_size=16777216 rw quiet splash=silent proxmox-start-auto-installer proxmox-auto-installer-answer-url=%s prov.iso=%s rdinit=/pxe-init
+initrd %s
+initrd %s
+boot
+`, img.Name, kernelURL, answerURL, isoURL, initrdURL, overlayURL)
 }