usbguard-rules.conf(5)
NAME
usbguard-rules.conf -- USBGuard rule set file
DESCRIPTION
The usbguard-rules.conf file is loaded by the USBGuard daemon after it parses the main configuration file, usbguard-daemon.conf(5). The daemon expects the file to contain rules written in a rule language which is described in the Rule Language section below.
Rule Language
The USBGuard daemon decides which USB device to authorize based on a
policy defined by a set of rules. When a USB device is inserted into
the system, the daemon scans the existing rules sequentially and when a
matching rule is found, it either authorizes (allows), deauthorizes
(blocks) or removes (rejects) the device, based on the rule target. If
no matching rule is found, the decision is based on an implicit default
target. This implicit default is to block the device until a decision
is made by the user. The rule language grammar, expressed in a
BNF-like syntax, is the following:
rule ::= target attributes.
target ::= "allow" | "block" | "reject".
attributes ::= attributes | attribute.
attributes ::= .
Rule attributes specify which devices to match or what condition have
to be met for the rule to be applicable. See the Device Attributes
section for the list of available attributes and Conditions for the
list of supported rule conditions.
Targets
The target of a rule specifies whether the device will be authorized
for use or not. Three types of target are recognized:
* allow -- Authorize the device. The device and its interfaces will be
allowed to communicate with the system.
* block -- Deauthorize the device, i.e. don't talk to the device for
now.
* reject -- Remove the device from the system, i.e. ignore the device
as if didn't exist.
Device Specification
Except the target, all the other fields of a rule need not be
specified. Such a minimal rule will match any device and allows the
policy creator to write an explicit default target. If no rule from
the policy is applicable to the device, an implicit target configured
in usbguard-daemon.conf(5) will be used. However, if one wants to
narrow the applicability of a rule to a set of devices or one device
only, it's possible to do so with device attributes and rule
conditions.
Device Attributes
Device attributes are specific values read from the USB device after
it's inserted to the system. Which attributes are available is defined
below. Some of the attributes are derived and some are based on
attributes read directly from the device. All attributes support two
forms:
* single-valued with a syntax name value
* multi-valued with a syntax name [operator] { value1 value2 ... }
where the optional operator is one of:
* all-of -- The device attribute set must contain all of the specified
values for the rule to match.
* one-of -- The device attribute set must contain at least one of the
specified values for the rule to match.
* none-of -- The device attribute set must not contain any of the
specified values for the rule to match.
* equals -- The device attribute set must contain exactly the same set
of values for the rule to match.
* equals-ordered -- The device attribute set must contain exactly the
same set of values in the same order for the rule to match.
If the operator is not specified it is set to equals.
List of attributes:
id usb-device-id
Match a USB device ID.
id [operator] { usb-device-id ... }
Match a set of USB device IDs.
hash "value"
Match a hash computed from the device attribute values and the
USB descriptor data. The hash is computed for every device by
USBGuard.
hash [operator] { "value" ... }
Match a set of device hashes.
parent-hash "value"
Match a hash of the parent device.
parent-hash [operator] { "value" ... }
Match a set of parent device hashes.
name "device-name"
Match the USB device name attribute.
name [operator] { "device-name" ... }
Match a set of USB device names.
serial "serial-number"
Match the USB iSerial device attribute.
serial [operator] { "serial-number" ... }
Match a set of USB iSerial device attributes.
via-port "port-id"
Match the USB port through which the device is connected. Note
that some systems have unstable port numbering which change
after the system reboots or certain kernel modules are reloaded
(and maybe in other cases). Use the parent-hash attribute if
you want to ensure that a device is connected via a specific
parent device.
via-port [operator] { "port-id" ... }
Match a set of USB ports.
with-interface interface-type
Match an interface type that the USB device provides.
with-interface [operator] { interface-type interface-type ... }
Match a set of interface types against the set of interfaces
that the USB device provides.
The usb-device-id is a colon delimited pair in the form
vendor_id:product_id. All USB devices have this ID assigned by the
manufacturer and it should uniquely identify a USB product. Both
vendor_id and product_id are 16-bit numbers represented in hexadecimal
base. It's possible to use an asterisk character to match either any
device ID *:* or any product ID from a specific vendor, e.g. 1234:*.
The port-id value is a platform specific USB port identification. On
Linux it's in the form of "usbN" in case of a USB controller (more
accurately a "root hub") or "bus-port[.port[.port ...]]" (e.g. "1-2",
"1-2.1", ...).
The interface-type represents a USB interface and should be formatted
as three 8-bit numbers in hexadecimal base delimited by a colon
character, i.e. cc:ss:pp. The numbers represent the interface class
(cc), subclass (ss) and protocol (pp) as assigned by the USB-IF
(http://www.usb.org/about). See the list of assigned classes,
subclasses and protocols (http://www.usb.org/developers/defined_class).
Instead of the subclass and protocol number, you may write an asterisk
character (*) to match all subclasses or protocols. Matching a
specific class and a specific protocol is not allowed, i.e. if you use
an asterisk as the subclass number, you have to use an asterisk for the
protocol too.
Conditions
Whether a rule that matches a device will be applied or not can be
further restricted using rule conditions. If the condition expression
is met at the rule evaluation time, then the rule target is applied for
the device. A condition expression is met if it evaluates to true.
Otherwise, the rule evaluation continues with the next rule. A rule
conditions has the following syntax:
if [!]condition
if [operator] { [!]conditionA [!]conditionB ... }
Optionally, an exclamation mark (!) can be used to negate the result of
a condition.
Interpretation of the set operator:
* all-of -- Evaluate to true if all of the specified conditions
evaluated to true.
* one-of -- Evaluate to true if one of the specified conditions
evaluated to true.
* none-of -- Evaluate to true if none of the specified conditions
evaluated to true.
* equals -- Same as all-of.
* equals-ordered -- Same as all-of.
List of conditions:
localtime(time_range)
Evaluates to true if the local time is in the specified time
range. time_range can be written either as HH:MM[:SS] or
HH:MM[:SS]-HH:MM[:SS].
allowed-matches(query)
Evaluates to true if an allowed device matches the specified
query. The query uses the rule syntax. Conditions in the query
are not evaluated.
rule-applied
Evaluates to true if the rule currently being evaluated ever
matched a device.
rule-applied(past_duration)
Evaluates to true if the rule currently being evaluated matched
a device in the past duration of time specified by the
parameter. past_duration can be written as HH:MM:SS, HH:MM, or
SS.
rule-evaluated
Evaluates to true if the rule currently being evaluated was ever
evaluated before.
rule-evaluated(past_duration)
Evaluates to true if the rule currently being evaluated was
evaluated in the pas duration of time specified by the
parameter. past_duration can be written as HH:MM:SS, HH:MM, or
SS.
random Evaluates to true/false with a probability of p=0.5.
random(p_true)
Evaluates to true with the specified probability p_true.
true Evaluates always to true.
false Evaluates always to false.
Initial policy
Using the usbguard CLI tool and its generate-policy subcommand, you can
generate an initial policy for your system instead of writing one from
scratch. The tool generates an allow policy for all devices connected
to the system at the moment of execution. It has several options to
tweak the resulting policy, see usbguard(1) for further details.
The policy will be printed out on the standard output. It's a good
idea to review the generated rules before using them on a system. The
typical workflow for generating an initial policy could look like this:
# usbguard generate-policy > rules.conf
# vi rules.conf
(review/modify the rule set)
# sudo install -m 0600 -o root -g root rules.conf /etc/usbguard/rules.conf
# sudo systemctl restart usbguard
Example Policies
The following examples show what to put into the rules.conf file in
order to implement the given policy.
1) Allow USB mass storage devices (USB flash disks) and block
everything else
This policy will block any device that isn't just a mass storage
device. Devices with a hidden keyboard interface in a USB flash disk
will be blocked. Only devices with a single mass storage interface
will be allowed to interact with the operating system. The policy
consists of a single rule:
allow with-interface equals { 08:*:* }
The blocking is implicit in this case because we didn't write a block
rule. Implicit blocking is useful to desktop users because a desktop
applet listening to USBGuard events can ask the user for a decision if
an implicit target was selected for a device.
2) Allow a specific Yubikey device to be connected via a specific port.
Reject everything else on that port
allow 1050:0011 name "Yubico Yubikey II" serial "0001234567" via-port "1-2" hash "044b5e168d40ee0245478416caf3d998"
reject via-port "1-2"
We could use just the hash to match the device. However, using the
name and serial attributes allows the policy creator to quickly assign
rules to specific devices without computing the hash. On the other
hand, the hash is the most specific value we can use to identify a
device in USBGuard so it's the best attribute to use if you want a rule
to match just one device.
3) Reject devices with suspicious combination of interfaces
A USB flash disk which implements a keyboard or a network interface is
very suspicious. The following set of rules forms a policy which
allows USB flash disks and explicitly rejects devices with an
additional and suspicious (as defined before) interface.
allow with-interface equals { 08:*:* }
reject with-interface all-of { 08:*:* 03:00:* }
reject with-interface all-of { 08:*:* 03:01:* }
reject with-interface all-of { 08:*:* e0:*:* }
reject with-interface all-of { 08:*:* 02:*:* }
The policy rejects all USB flash disk devices with an interface from
the HID/Keyboard, Communications and Wireless classes. Please note
that blacklisting is the wrong approach and you shouldn't just
blacklist a set of devices and allow the rest. The policy above
assumes that blocking is the implicit default. Rejecting a set of
devices considered as "bad" is a good approach how to limit the
exposure of the OS to such devices as much as possible.
4) Allow a keyboard-only USB device only if there isn't already a USB
device with a keyboard interface allowed
allow with-interface one-of { 03:00:01 03:01:01 } if !allowed-matches(with-interface one-of { 03:00:01 03:01:01 })
5) Play "Russian roulette" with USB devices
allow if random(0.1666)
reject
BUGS
If you find a bug in this software or if you'd like to request a feature to be implemented, please file a ticket at <https://github.com/dkopecek/usbguard/issues/new>.
COPYRIGHT
Copyright 2016 Red Hat, Inc. License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
SEE ALSO
usbguard-daemon(8), usbguard-daemon.conf(5)
AUTHORS
Daniel Kopeek <[email protected]>. June 2016 USBGUARD-RULES.CONF(5)
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