1.. SPDX-License-Identifier: GPL-2.0+
2.. Copyright (c) 2018 Heinrich Schuchardt
3
4iSCSI booting with U-Boot and iPXE
5==================================
6
7Motivation
8----------
9
10U-Boot has only a reduced set of supported network protocols. The focus for
11network booting has been on UDP based protocols. A TCP stack and HTTP support
12are expected to be integrated in 2018 together with a wget command.
13
14For booting a diskless computer this leaves us with BOOTP or DHCP to get the
15address of a boot script. TFTP or NFS can be used to load the boot script, the
16operating system kernel and the initial file system (initrd).
17
18These protocols are insecure. The client cannot validate the authenticity
19of the contacted servers. And the server cannot verify the identity of the
20client.
21
22Furthermore the services providing the operating system loader or kernel are
23not the ones that the operating system typically will use. Especially in a SAN
24environment this makes updating the operating system a hassle. After installing
25a new kernel version the boot files have to be copied to the TFTP server
26directory.
27
28The HTTPS protocol provides certificate based validation of servers. Sensitive
29data like passwords can be securely transmitted.
30
31The iSCSI protocol is used for connecting storage attached networks. It
32provides mutual authentication using the CHAP protocol. It typically runs on
33a TCP transport.
34
35Thus a better solution than DHCP/TFTP/NFS boot would be to load a boot script
36via HTTPS and to download any other files needed for booting via iSCSI from the
37same target where the operating system is installed.
38
39An alternative to implementing these protocols in U-Boot is to use an existing
40software that can run on top of U-Boot. iPXE[1] is the "swiss army knife" of
41network booting. It supports both HTTPS and iSCSI. It has a scripting engine for
42fine grained control of the boot process and can provide a command shell.
43
44iPXE can be built as an EFI application (named snp.efi) which can be loaded and
45run by U-Boot.
46
47Boot sequence
48-------------
49
50U-Boot loads the EFI application iPXE snp.efi using the bootefi command. This
51application has network access via the simple network protocol offered by
52U-Boot.
53
54iPXE executes its internal script. This script may optionally chain load a
55secondary boot script via HTTPS or open a shell.
56
57For the further boot process iPXE connects to the iSCSI server. This includes
58the mutual authentication using the CHAP protocol. After the authentication iPXE
59has access to the iSCSI targets.
60
61For a selected iSCSI target iPXE sets up a handle with the block IO protocol. It
62uses the ConnectController boot service of U-Boot to request U-Boot to connect a
63file system driver. U-Boot reads from the iSCSI drive via the block IO protocol
64offered by iPXE. It creates the partition handles and installs the simple file
65protocol. Now iPXE can call the simple file protocol to load GRUB[2]. U-Boot
66uses the block IO protocol offered by iPXE to fulfill the request.
67
68Once GRUB is started it uses the same block IO protocol to load Linux. Via
69the EFI stub Linux is called as an EFI application::
70
71                  +--------+         +--------+
72                  |        | Runs    |        |
73                  | U-Boot |========>| iPXE   |
74                  | EFI    |         | snp.efi|
75    +--------+    |        | DHCP    |        |
76    |        |<===|********|<========|        |
77    | DHCP   |    |        | Get IP  |        |
78    | Server |    |        | Address |        |
79    |        |===>|********|========>|        |
80    +--------+    |        | Response|        |
81                  |        |         |        |
82                  |        |         |        |
83    +--------+    |        | HTTPS   |        |
84    |        |<===|********|<========|        |
85    | HTTPS  |    |        | Load    |        |
86    | Server |    |        | Script  |        |
87    |        |===>|********|========>|        |
88    +--------+    |        |         |        |
89                  |        |         |        |
90                  |        |         |        |
91    +--------+    |        | iSCSI   |        |
92    |        |<===|********|<========|        |
93    | iSCSI  |    |        | Auth    |        |
94    | Server |===>|********|========>|        |
95    |        |    |        |         |        |
96    |        |    |        | Loads   |        |
97    |        |<===|********|<========|        |       +--------+
98    |        |    |        | GRUB    |        | Runs  |        |
99    |        |===>|********|========>|        |======>| GRUB   |
100    |        |    |        |         |        |       |        |
101    |        |    |        |         |        |       |        |
102    |        |    |        |         |        | Loads |        |
103    |        |<===|********|<========|********|<======|        |      +--------+
104    |        |    |        |         |        | Linux |        | Runs |        |
105    |        |===>|********|========>|********|======>|        |=====>| Linux  |
106    |        |    |        |         |        |       |        |      |        |
107    +--------+    +--------+         +--------+       +--------+      |        |
108                                                                      |        |
109                                                                      |        |
110                                                                      | ~ ~ ~ ~|
111
112Security
113--------
114
115The iSCSI protocol is not encrypted. The traffic could be secured using IPsec
116but neither U-Boot nor iPXE does support this. So we should at least separate
117the iSCSI traffic from all other network traffic. This can be achieved using a
118virtual local area network (VLAN).
119
120Configuration
121-------------
122
123iPXE
124~~~~
125
126For running iPXE on arm64 the bin-arm64-efi/snp.efi build target is needed::
127
128    git clone http://git.ipxe.org/ipxe.git
129    cd ipxe/src
130    make bin-arm64-efi/snp.efi -j6 EMBED=myscript.ipxe
131
132The available commands for the boot script are documented at:
133
134http://ipxe.org/cmd
135
136Credentials are managed as environment variables. These are described here:
137
138http://ipxe.org/cfg
139
140iPXE by default will put the CPU to rest when waiting for input. U-Boot does
141not wake it up due to missing interrupt support. To avoid this behavior create
142file src/config/local/nap.h:
143
144.. code-block:: c
145
146    /* nap.h */
147    #undef NAP_EFIX86
148    #undef NAP_EFIARM
149    #define NAP_NULL
150
151The supported commands in iPXE are controlled by an include, too. Putting the
152following into src/config/local/general.h is sufficient for most use cases:
153
154.. code-block:: c
155
156    /* general.h */
157    #define NSLOOKUP_CMD            /* Name resolution command */
158    #define PING_CMD                /* Ping command */
159    #define NTP_CMD                 /* NTP commands */
160    #define VLAN_CMD                /* VLAN commands */
161    #define IMAGE_EFI               /* EFI image support */
162    #define DOWNLOAD_PROTO_HTTPS    /* Secure Hypertext Transfer Protocol */
163    #define DOWNLOAD_PROTO_FTP      /* File Transfer Protocol */
164    #define DOWNLOAD_PROTO_NFS      /* Network File System Protocol */
165    #define DOWNLOAD_PROTO_FILE     /* Local file system access */
166
167Open-iSCSI
168~~~~~~~~~~
169
170When the root file system is on an iSCSI drive you should disable pings and set
171the replacement timer to a high value in the configuration file [3]::
172
173    node.conn[0].timeo.noop_out_interval = 0
174    node.conn[0].timeo.noop_out_timeout = 0
175    node.session.timeo.replacement_timeout = 86400
176
177Links
178-----
179
180* [1] https://ipxe.org - iPXE open source boot firmware
181* [2] https://www.gnu.org/software/grub/ -
182  GNU GRUB (Grand Unified Bootloader)
183* [3] https://github.com/open-iscsi/open-iscsi/blob/master/README -
184  Open-iSCSI README
185