PyBootd

Overview

PyBootd is a daemon supporting a subset of the BOOTP, DHCP, PXE, TFTP and HTTP protocols, with some handy extensions.

One of its main goals is to provide a simple solution to boot up any PXE-enabled personal computer, with no other tool required but a standard Python installation.

It is not designed to be feature-complete, but to be used as an easy modifiable code to develop custom boot solutions

Pybootd can be used for any network boot up, or to install an OS without any physical support such as a USB key or a CD/DVD.

Requirements

Python

• Python 3.5+ or above is required. Python 2.x is not longer supported.
• Netifaces Python module is required

Permissions

• DHCP protocol requires the daemon to listen on port 67.
• TFTP protocol requires the daemon to listen on port 69.
• HTTP optional daemon may be run on any port.

As these ports are within the server's range (<1024), the superuser privileges are required on Unix hosts (Linux, macOS, ...) to start up these daemons.

Status

This project is in beta development stage.

Supported features

• Access control:

1. None (any remote host can be served)
2. MAC address ACL
3. UUID based ACL - requires PXE protocol
4. HTTP forwarding - authorization is delegated to a remote server using simple HTTP GET requests

• Local or remote file serving:
• For example, it is possible to boot up a full Debian system directly from the Internet, without storing any file on the pybootd host machine
• Network notification of client requests through UDP messages
• File name translation
  • Files requested from TFTP clients can be filtered and transformed into local filenames using filters
• It is possible to use pybootd with only one of the services, either TFTP or DHCP
• A very basic HTTP server can be optionally enabled to serve files over HTTP for complex hosts that require additional files (such as a root file system) after the initial boot sequence.

Warning

There is no strong checking of permissions nor robust file path management, so it is recommended NOT to run this daemon on a host with sensitive content.

Although only read requests are implemented, there is no enforcement or strong validation of received data and strings from adversary remote clients.

FAQ

Common errors

pybootd.pxed.BootpError: Unable to detect network configuration

This error is often triggered when the pool_start address is not part of a valid network. Double check the network configuration and fix up the [bootpd] section so that it matches the actual network. If you don't want to allocate addresses dynamically from the pool (with pool_count = 0), you still need to specify pool_start to some address in the local network you want to serve (eg. the address of your local server).

error: Can't assign requested address

This errir is often triggered with an invalid listening address setting. Try listening on all IPv4 interfaces with address = 0.0.0.0 and use ACL to discard requests from network you do not want to serve.

DHCP client keeps requesting an address but seems to receive none

Some stupid clients - such as the ones implemented in BIOS/UEFI from Intel silently ignore proper network broadcast packets. They only consider global broadcast packets. The Ethernet MAC of such clients should be added to the [buggy_clients] section, so that global broadcast packets are generated for these clients.

Configuration

pybootd has a few option switches. The server offers two services: bootpd (which supports DHCP and PXE extensions) and tftpd. It is possible to disable either services.

Usage: pybootd.py [-h] [-c CONFIG] [-p] [-t] [-d]

Tiny BOOTP/DHCP/TFTP/PXE server

Options:

-h, --help	show this help message and exit
-c CONFIG, --config CONFIG
	configuration file
-p, --pxe	only enable BOOTP/DHCP/PXE server
-t, --tftp	only enable TFTP server
-H, --http	enable HTTP server (default: disabled)
-d, --debug	enable debug mode

pybootd daemon uses a configuration file, in .ini format, for all other options.

Some options accept a boolean value. The following values are recognized:

• true values: on, high, true, enable, enabled, yes,

  1

• false values: off, low, false, disable, disabled, no,

  0

The BOOTP daemon associates each MAC address to an assigned IP address. As long as the BOOTP daemon is running, the same IP address is always assigned to the same client. The address never gets back to the pool, i.e. it cannot be re-assigned to another machine even when the lease expires.

This is especially useful for a full network-based installation, where each client requests at least an IP address twice:

• when BIOS kicks off, its PXE ROM code requests an IP address, then requests an executable to run,
• when the executable runs, it usually boots up an OS (Linux, ...), which in turn requests an IP address to resume the installation.

[logger] section

type

The type of logger, if any. stderr, file, syslog or none.

level

The level of logger verbosity. critical, error, info or debug.

file

The path to the output log file, if type is set to file.

[bootpd] section

access

Type of access control list. If this option is not defined, all BOOTP requests are served, as long as the defined pool is not exhausted. It can be one among the following options:

• mac: incoming BOOTP requests are filtered out based on the MAC address of the requester.
• uuid: incoming PXE requests are filtered out based on the UUID of the request. UUIDs are not emitted from simple BOOTP or DHCP clients, so this option is only meaningful for PXE-enabled clients.
• http: incoming requests are forwarded to another host, through simple HTTP GET requests. The MAC address and the UUID if it exists, are sent to the HTTP server which replies to grant or deny access to the requester.

A section named after the selected option should exist to define the access list.

address

Specifies the network to listen to requesters for receiving incoming BOOTP requests. On most hosts, the only valid address is 0.0.0.0. Some hosts accept subnetworks (such as 192.168.1.0). It is recommended not to define this option, and use an ACL to reject clients. Hosts will multiple network interfaces, it might not be possible to listen to single network. Implementing such as feature would require to use RAW sockets, which falls out of scope for this simple server.

allow_simple_dhcp

The default behaviour is to expect PXE requests. In order to serve simple BOOTP or DHCP requests, this option should be enabled. This option accepts a boolean value.

domain

Domain part of the client FQDN, that is the network's domain name.

dns

IP addresses of DNS servers. Multiple addresses are separated with semicolon. Specify auto to re-use DNS addresses used by the server. Note that most DHCP clients will only consider the first DNS address if multiple are provided.

gateway

Specify gateway address in DHCP reply, default to DHCP server address

lease_time

Validity in seconds of a DHCP lease. Please note that the BOOTP daemon does not manage lease expiration; this value has therefore little meaning.

pool_start

First address to allocate for a BOOT client. This has to be an address in the local network you want to serve, even if pool_count is set to 0, in which case the address of the DHCP server is a good choice.

pool_count

The maximum number of IP addresses that can be dynamically allocated from the pool to BOOTP/DHCP clients. Set it to 0 to prevent server from dynamically allocating IP addresses from the pool and see static_dhcp below.

notify

When defined, the IP address and port (using a column separator: a.b.c.d:p) to which a UDP notification message should be sent whenever a client requests an IP address to the BOOTP daemon.

port

Alternative port for incoming BOOTP requests.

timeout

Timeout in seconds for a response from a remote authentication host to be received, when ACL is enabled and set to use the HTTP protocol. If no answer is received from the remote host, the BOOTP daemon ignores the incoming BOOTP/DHCP request.

servername

Name of the BOOTP server.

[mac] section

The [mac] section contains one entry for each MAC address to allow or block. The value for each entry is a boolean, i.e.:

AA-BB-CC-DD-EE-FF = enable

Note that due to a limitation of the configuration parser, ':' byte separator in MAC addresses is not allowed, please use '-' separator.

It is possible to use a mask syntax to specify a range of MACs addresses, for example:

AA-BB-CC/24 = enable

Accepts all Ethernet addresses whose OUI is AA:BB:CC.

[static_dhcp] section

The [static_dhcp] section contains one entry for each MAC address to associate with a specific IP address. The IP address can be any IPv4 address in dotted notation, i.e.:

AA-BB-CC-DD-EE-FF = 192.168.1.2

The MAC addresses specified here will automatically be allowed, unless [mac] section specifies otherwise.

[uuid] section

The [uuid] section contains one entry for each UUID to allow or block. The value for each entry is a boolean, i.e.:

xxxxxxxx-aaaa-bbbb-cccc-yyyyyyyyyyyy = enable

[http] section

location

The URL prefix to contact the remote server for boot permission.

pxe

The path to append to the URL prefix when the requester emits PXE information. A regular PC with PXE capability emits a PXE boot request when the BIOS kicks off. The remote HTTP server may therefore identify a BIOS boot sequence upon receiving this kind of request from the pybootd daemon.

dhcp

The path to append to the URL prefix when the requester emits simple DHCP information. A regular OS emits a simple DHCP request at start up. The remote HTTP server may therefore identify an OS boot sequence upon receiving this kind of request from the pybootd daemon.

The pxe/dhcp option pair enables the remote HTTP server to identify the boot phase: either a BIOS initialization or an OS boot sequence. When such differentiation is useless, both options may refer to the same path.

[bootfile] section

This section contains one entry for each supported architecture. It defines the name of the initial boot file the client should request, indexed on the architecture it reports, if any.

It should contain at least one entry, default, which map to the bootfile for clients that do no expose their architecture.

The bootfile is usually requested over TFTP to boot up after the client has been assigned a network address.

Each entry is the architecture string, with a filename value.

[buggy_clients] section

When a BOOTP client requests a network address, the BOOTP/DHCP server should broadcast on the client's LAN the DHCP offerring. Using the client's network is recommended, as it avoid broadcasting BOOTP/DHCP packets to other networks.

Some clients, notably the clients based on Intel firmwares, are stupid enough to ignore DHCP offering which is broadcasted to the network broadcast address. They do require the DHCP server to broadcast to the global 255.255.255.255 address.

This section lists the MAC of the clients that are so stupid they need this global broadcast address to work. If you use Intel BIOS/UEFI, this option is likely needed.

Each entry is a MAC address, using the - byte separator, with a boolean value.

This section also accepts mask syntax, see [mac] section for details.

[tftpd] section

address

Address to listen to incoming TFTP requests. When the BOOTP daemon is enabled this option is better omitted, as the address is automatically received from the BOOTP daemon.

blocksize

Size of each exchanged data block. It is recommended to leave the default value, as some clients may not accept other values.

port

Alternative port for incoming TFTP request.

timeout

Timeout in seconds for an acknowledgment from the TFTP client to be received. If the timeout expires the TFTP server retransmits the last packet. It can be expressed as a real value.

root

Base directory for the TFTP service. This path is automatically prepended to the pathname issued from the TFTP client. It can either be:

• a relative path to the daemon directory, when the root option starts with ./,
• an absolute path, when the root option starts with /,
• a URL prefix, to access remote files.

[httpd] section

address

Address to listen to incoming HTTP requests. When the BOOTP daemon is enabled this option is better omitted, as the address is automatically received from the BOOTP daemon.

port

Alternative port for incoming HTTP request, default to 80

root

Base directory for the HTTP service. This path is automatically prepended to the pathname issued from the TFTP client. It can either point to a local directory for now.

check_ip

Whether to enforce HTTP client IP or not. When enabled, requests from clients that have not obtained an IP address from the BOOTP daemon are rejected.

[filters] section

The filters section allows on-the-fly pathnames transformation. When a TFTP client requests some specific filenames, the tftpd server can translate them to other ones.

This option is useful to serve the very same configuration file (''e.g.'' pxelinux.cfg) whatever the remote client, thus speeding up the boot process. This option also enables to access files that are not stored within the currently configured path (see the root option).

Each option of the filters section represents a file pattern to match. It accepts standard wildcard characters: * and ?. The option's value defines the translated path.

The value part can contain variables. Variables are written with enclosing braces, such as {varname}.

For now, the only supported variable is filename, which is replaced with the actual requested filename.

The value part can also contain a special marker, that tells the tftpd server to read the replacement pattern from a file. This special marker should be written with enclosing brackets, such as [file].

Examples

The following filter:

pxelinux.cfg/* = pybootd/etc/pxe.cfg

tells the tftpd server that all client requests matching the pxelinux.cfg/* pattern should be served the pybootd/etc/pxe.cfg file instead. This prevents the client to perform the usual time-costing fallback requests using UUID, MAC, and suffix addresses before eventually falling back to the simple pxelinux.cfg file.

The following filter:

startup = [dir/{filename}.cfg]

tells the tftpd server that when the startup file is requested, it should read out the actual filename from the dir/startup.cfg file.

HTTP-based authentication

This option enabled the delegation of the BOOTP authorization to a remote web server. As pybootd emits standard HTTP GET requests and expects standard HTTP reply codes, any web server may be used to manage authorizations.

This web server receives HTTP GET requests with URLs formatted as follows:

http://server/path?mac=AA-BB-CC-DD-EE-FF&uuid=xxxxxxxx-aaaa-bbbb-cccc-yyyyyyyyyyyy

where:

• http://server matches the location option,
• /path matches the pxe or dhcp options of the [http] section.

The web server should reply either with:

• 200 Ok result if the requester is to be assigned an IP address, or
• 401 Unauthorized result if it is to be ignored.

The pybootd package contains a minimalist HTTP server that demonstrates this feature. It can be found within the tests/ subdirectory. See the config.ini file for this test daemon. The test daemon expects the pxe path to be set to /boot and the dhcp path to /linux.

Sample configurations

The pybootd.ini would contain:

[logger]
; show requests on the standard error output of the daemon
type = stderr
; show informative and error messages only (disable verbose mode)
level = info

[bootpd]
; do not force a full PXE boot-up cycle to accept the client
allow_simple_dhcp = enable
; First BOOTP/DHCP address to generate
pool_start = 192.168.1.100
; Google DNS
dns = 8.8.8.8

[bootfile]
; boot-up executable the client should request through TFTP (BIOS)
default = pxelinux.0
; boot-up executable the client should request through TFTP (UEFI x86-64)
00007 = shimx64.efi

[tftpd]
root = http://example.com/installer-amd64/images/netboot

[filters]
; serve a simple configuration file to the linux PXE helper
pxelinux.cfg/* = pybootd/etc/pxe.cfg

The pool_start parameter should be a valid address on the host's networks, and the root URL may be changed to use alternative mirror and path.

Please note that to complete the network installation, the client should be able to access the remote resources on its own - as with a network ISO image installation. There are two ways to achieve this:

• either enable IP forwarding on the pybootd host (see forward.sh script within the pybootd package), or
• be sure to connect the network cable of the client to a LAN that has direct access to the Internet, once the first installation stage is complete.