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