1iSCSI booting with U-Boot and iPXE 2================================== 3 4Motivation 5---------- 6 7U-Boot has only a reduced set of supported network protocols. The focus for 8network booting has been on UDP based protocols. A TCP stack and HTTP support 9are expected to be integrated in 2018 together with a wget command. 10 11For booting a diskless computer this leaves us with BOOTP or DHCP to get the 12address of a boot script. TFTP or NFS can be used to load the boot script, the 13operating system kernel and the initial file system (initrd). 14 15These protocols are insecure. The client cannot validate the authenticity 16of the contacted servers. And the server cannot verify the identity of the 17client. 18 19Furthermore the services providing the operating system loader or kernel are 20not the ones that the operating system typically will use. Especially in a SAN 21environment this makes updating the operating system a hassle. After installing 22a new kernel version the boot files have to be copied to the TFTP server 23directory. 24 25The HTTPS protocol provides certificate based validation of servers. Sensitive 26data like passwords can be securely transmitted. 27 28The iSCSI protocol is used for connecting storage attached networks. It 29provides mutual authentication using the CHAP protocol. It typically runs on 30a TCP transport. 31 32Thus a better solution than DHCP/TFTP/NFS boot would be to load a boot script 33via HTTPS and to download any other files needed for booting via iSCSI from the 34same target where the operating system is installed. 35 36An alternative to implementing these protocols in U-Boot is to use an existing 37software that can run on top of U-Boot. iPXE[1] is the "swiss army knife" of 38network booting. It supports both HTTPS and iSCSI. It has a scripting engine for 39fine grained control of the boot process and can provide a command shell. 40 41iPXE can be built as an EFI application (named snp.efi) which can be loaded and 42run by U-Boot. 43 44Boot sequence 45------------- 46 47U-Boot loads the EFI application iPXE snp.efi using the bootefi command. This 48application has network access via the simple network protocol offered by 49U-Boot. 50 51iPXE executes its internal script. This script may optionally chain load a 52secondary boot script via HTTPS or open a shell. 53 54For the further boot process iPXE connects to the iSCSI server. This includes 55the mutual authentication using the CHAP protocol. After the authentication iPXE 56has access to the iSCSI targets. 57 58For a selected iSCSI target iPXE sets up a handle with the block IO protocol. It 59uses the ConnectController boot service of U-Boot to request U-Boot to connect a 60file system driver. U-Boot reads from the iSCSI drive via the block IO protocol 61offered by iPXE. It creates the partition handles and installs the simple file 62protocol. Now iPXE can call the simple file protocol to load GRUB[2]. U-Boot 63uses the block IO protocol offered by iPXE to fulfill the request. 64 65Once GRUB is started it uses the same block IO protocol to load Linux. Via 66the EFI stub Linux is called as an EFI application:: 67 68 +--------+ +--------+ 69 | | Runs | | 70 | U-Boot |========>| iPXE | 71 | EFI | | snp.efi| 72 +--------+ | | DHCP | | 73 | |<===|********|<========| | 74 | DHCP | | | Get IP | | 75 | Server | | | Address | | 76 | |===>|********|========>| | 77 +--------+ | | Response| | 78 | | | | 79 | | | | 80 +--------+ | | HTTPS | | 81 | |<===|********|<========| | 82 | HTTPS | | | Load | | 83 | Server | | | Script | | 84 | |===>|********|========>| | 85 +--------+ | | | | 86 | | | | 87 | | | | 88 +--------+ | | iSCSI | | 89 | |<===|********|<========| | 90 | iSCSI | | | Auth | | 91 | Server |===>|********|========>| | 92 | | | | | | 93 | | | | Loads | | 94 | |<===|********|<========| | +--------+ 95 | | | | GRUB | | Runs | | 96 | |===>|********|========>| |======>| GRUB | 97 | | | | | | | | 98 | | | | | | | | 99 | | | | | | Loads | | 100 | |<===|********|<========|********|<======| | +--------+ 101 | | | | | | Linux | | Runs | | 102 | |===>|********|========>|********|======>| |=====>| Linux | 103 | | | | | | | | | | 104 +--------+ +--------+ +--------+ +--------+ | | 105 | | 106 | | 107 | ~ ~ ~ ~| 108 109Security 110-------- 111 112The iSCSI protocol is not encrypted. The traffic could be secured using IPsec 113but neither U-Boot nor iPXE does support this. So we should at least separate 114the iSCSI traffic from all other network traffic. This can be achieved using a 115virtual local area network (VLAN). 116 117Configuration 118------------- 119 120iPXE 121^^^^ 122 123For running iPXE on arm64 the bin-arm64-efi/snp.efi build target is needed:: 124 125 git clone http://git.ipxe.org/ipxe.git 126 cd ipxe/src 127 make bin-arm64-efi/snp.efi -j6 EMBED=myscript.ipxe 128 129The available commands for the boot script are documented at: 130 131http://ipxe.org/cmd 132 133Credentials are managed as environment variables. These are described here: 134 135http://ipxe.org/cfg 136 137iPXE by default will put the CPU to rest when waiting for input. U-Boot does 138not wake it up due to missing interrupt support. To avoid this behavior create 139file src/config/local/nap.h:: 140 141 /* nap.h */ 142 #undef NAP_EFIX86 143 #undef NAP_EFIARM 144 #define NAP_NULL 145 146The supported commands in iPXE are controlled by an include, too. Putting the 147following into src/config/local/general.h is sufficient for most use cases:: 148 149 /* general.h */ 150 #define NSLOOKUP_CMD /* Name resolution command */ 151 #define PING_CMD /* Ping command */ 152 #define NTP_CMD /* NTP commands */ 153 #define VLAN_CMD /* VLAN commands */ 154 #define IMAGE_EFI /* EFI image support */ 155 #define DOWNLOAD_PROTO_HTTPS /* Secure Hypertext Transfer Protocol */ 156 #define DOWNLOAD_PROTO_FTP /* File Transfer Protocol */ 157 #define DOWNLOAD_PROTO_NFS /* Network File System Protocol */ 158 #define DOWNLOAD_PROTO_FILE /* Local file system access */ 159 160Links 161----- 162 163* [1](https://ipxe.org) https://ipxe.org - iPXE open source boot firmware 164* [2](https://www.gnu.org/software/grub/) https://www.gnu.org/software/grub/ - 165 GNU GRUB (Grand Unified Bootloader) 166