Ubuntu Manpage: usbguard-rules.conf - USBGuard rule set file.

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 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. If 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 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. The device will remain in a blocked state until it is
authorized.

• reject - Deauthorize and Remove the device from the system. The device will have to be
re-inserted to become visible to the system again.

Device Specification
Except the target, all the other fields of a rule are optional. A rule where only the
target is specified will match any device. That allows the policy administator 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. The port might change after the system reboots or when
certain kernel modules are reloaded. 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 ... }
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 type. 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, ...) in case of a USB device.

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> for details. 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(true)=0.5.

random(p_true)
Evaluates to true with the specified probability p(true)=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 time 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:

               $ sudo 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. A desktop applet listening to USBGuard events can
ask the user for a decision if an implicit target was applied.

2. Allow a specific Yubikey device to be connected via a specific port (and 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 provide an useful hint for later inspection of the policy. On the other
hand, the hash is the most specific value we can use to identify a device. 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 that 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. 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 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

NAME

DESCRIPTION

RULE LANGUAGE

INITIAL POLICY

EXAMPLE POLICIES

SEE ALSO