--- /dev/null
+# iSCSI booting with U-Boot and iPXE
+
+## Motivation
+
+U-Boot has only a reduced set of supported network protocols. The focus for
+network booting has been on UDP based protocols. A TCP stack and HTTP support
+are expected to be integrated in 2018 together with a wget command.
+
+For booting a diskless computer this leaves us with BOOTP or DHCP to get the
+address of a boot script. TFTP or NFS can be used to load the boot script, the
+operating system kernel and the initial file system (initrd).
+
+These protocols are insecure. The client cannot validate the authenticity
+of the contacted servers. And the server cannot verify the identity of the
+client.
+
+Furthermore the services providing the operating system loader or kernel are
+not the ones that the operating system typically will use. Especially in a SAN
+environment this makes updating the operating system a hassle. After installing
+a new kernel version the boot files have to be copied to the TFTP server
+directory.
+
+The HTTPS protocol provides certificate based validation of servers. Sensitive
+data like passwords can be securely transmitted.
+
+The iSCSI protocol is used for connecting storage attached networks. It
+provides mutual authentication using the CHAP protocol. It typically runs on
+a TCP transport.
+
+Thus a better solution than DHCP/TFTP/NFS boot would be to load a boot script
+via HTTPS and to download any other files needed for booting via iSCSI from the
+same target where the operating system is installed.
+
+An alternative to implementing these protocols in U-Boot is to use an existing
+software that can run on top of U-Boot. iPXE is the "swiss army knife" of
+network booting. It supports both HTTPS and iSCSI. It has a scripting engine for
+fine grained control of the boot process and can provide a command shell.
+
+iPXE can be built as an EFI application (named snp.efi) which can be loaded and
+run by U-Boot.
+
+## Boot sequence
+
+U-Boot loads the EFI application iPXE snp.efi using the bootefi command. This
+application has network access via the simple network protocol offered by
+U-Boot.
+
+iPXE executes its internal script. This script may optionally chain load a
+secondary boot script via HTTPS or open a shell.
+
+For the further boot process iPXE connects to the iSCSI server. This includes
+the mutual authentication using the CHAP protocol. After the authentication iPXE
+has access to the iSCSI targets.
+
+For a selected iSCSI target iPXE sets up a handle with the block IO protocol. It
+uses the ConnectController boot service of U-Boot to request U-Boot to connect a
+file system driver. U-Boot reads from the iSCSI drive via the block IO protocol
+offered by iPXE. It creates the partition handles and installs the simple file
+protocol. Now iPXE can call the simple file protocol to load Grub. U-Boot uses
+the block IO protocol offered by iPXE to fulfill the request.
+
+Once Grub is started it uses the same block IO protocol to load Linux. Via
+the EFI stub Linux is called as an EFI application.
+
+```
+ +--------+ +--------+
+ | | Runs | |
+ | U-Boot |=========>| iPXE |
+ | EFI | | snp.efi|
++--------+ | | DHCP | |
+| |<====|********|<=========| |
+| DHCP | | | Get IP | |
+| Server | | | Address | |
+| |====>|********|=========>| |
++--------+ | | Response | |
+ | | | |
+ | | | |
++--------+ | | HTTPS | |
+| |<====|********|<=========| |
+| HTTPS | | | Load | |
+| Server | | | Script | |
+| |====>|********|=========>| |
++--------+ | | | |
+ | | | |
+ | | | |
++--------+ | | iSCSI | |
+| |<====|********|<=========| |
+| iSCSI | | | Auth | |
+| Server |====>|********|=========>| |
+| | | | | |
+| | | | Loads | |
+| |<====|********|<=========| | +--------+
+| | | | Grub | | Runs | |
+| |====>|********|=========>| |=======>| Grub |
+| | | | | | | |
+| | | | | | | |
+| | | | | | Loads | |
+| |<====|********|<=========|********|<=======| | +--------+
+| | | | | | Linux | | Runs | |
+| |====>|********|=========>|********|=======>| |=====>| Linux |
+| | | | | | | | | |
++--------+ +--------+ +--------+ +--------+ | |
+ | |
+ | |
+ | ~ ~ ~ ~|
+```
+
+## Security
+
+The iSCSI protocol is not encrypted. The traffic could be secured using IPsec
+but neither U-Boot nor iPXE does support this. So we should at least separate
+the iSCSI traffic from all other network traffic. This can be achieved using a
+virtual local area network (VLAN).
+
+## Configuration
+
+### iPXE
+
+For running iPXE on arm64 the bin-arm64-efi/snp.efi build target is needed.
+
+ git clone http://git.ipxe.org/ipxe.git
+ cd ipxe/src
+ make bin-arm64-efi/snp.efi -j6 EMBED=myscript.ipxe
+
+The available commands for the boot script are documented at:
+
+http://ipxe.org/cmd
+
+Credentials are managed as environment variables. These are described here:
+
+http://ipxe.org/cfg
+
+iPXE by default will put the CPU to rest when waiting for input. U-Boot does
+not wake it up due to missing interrupt support. To avoid this behavior create
+file src/config/local/nap.h.
+
+ /* nap.h */
+ #undef NAP_EFIX86
+ #undef NAP_EFIARM
+ #define NAP_NULL
+
+The supported commands in iPXE are controlled by an include, too. Putting the
+following into src/config/local/general.h is sufficient for most use cases.
+
+ /* general.h */
+ #define NSLOOKUP_CMD /* Name resolution command */
+ #define PING_CMD /* Ping command */
+ #define NTP_CMD /* NTP commands */
+ #define VLAN_CMD /* VLAN commands */
+ #define IMAGE_EFI /* EFI image support */
+ #define DOWNLOAD_PROTO_HTTPS /* Secure Hypertext Transfer Protocol */
+ #define DOWNLOAD_PROTO_FTP /* File Transfer Protocol */
+ #define DOWNLOAD_PROTO_NFS /* Network File System Protocol */
+ #define DOWNLOAD_PROTO_FILE /* Local file system access */
+
+## Links
+
+* https://ipxe.org - iPXE open source boot firmware
+* https://www.gnu.org/software/grub/ - GNU Grub (Grand Unified Bootloader)
projects / u-boot.git / commitdiff
