doc/README.iscsi

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