| # 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[1] 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[2]. 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 */ |
| |
| ### Open-iSCSI |
| |
| When the root file system is on an iSCSI drive you should disable pings and set |
| the replacement timer to a high value [3]: |
| |
| node.conn[0].timeo.noop_out_interval = 0 |
| node.conn[0].timeo.noop_out_timeout = 0 |
| node.session.timeo.replacement_timeout = 86400 |
| |
| ## Links |
| |
| * [1](https://ipxe.org) https://ipxe.org - iPXE open source boot firmware |
| * [2](https://www.gnu.org/software/grub/) https://www.gnu.org/software/grub/ - |
| GNU GRUB (Grand Unified Bootloader) |
| * [3](https://github.com/open-iscsi/open-iscsi/blob/master/README) |
| https://github.com/open-iscsi/open-iscsi/blob/master/README - |
| Open-iSCSI README |
