To audit a terminal-connected device on Linux, inspect the exact /dev node your application opens, check its owner, group, mode bits and ACL, then trace the udev rules that assign its settings. If you also need a record of later access, assess Linux Audit separately: audit rules monitor events, not the node’s permission mode. There is no single correct permission or group for every device and distribution.
1. Identify the device node the application uses
Start with the exact path opened by the terminal application. For a USB serial adapter, that might be /dev/ttyUSB0, but use your application’s configured path rather than assuming a familiar name is correct.
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If the application uses a symlink, inspect both the path it opens and the target it resolves to. A link and its target are different filesystem objects; permissions on the device node are what matter for access to the device. Keep the path you actually need to audit consistent through the commands below.
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2. Check the live node’s owner, group and mode
Run these read-only checks, replacing the example path with the actual node:
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ls -l /dev/ttyUSB0
stat /dev/ttyUSB0The listing shows the object type, owner, group and mode bits. A device node should appear as a character or block device, as appropriate for the hardware. stat provides file status information; consult ls(1) and stat(2) for details.
Read the mode in terms of its owner, group and other-user classes. A group permission may be intentional if a specific user group is meant to access the device, but the right group and access scope depend on local policy and device use. Do not assume a mode or group is a universal Linux default.
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3. Inspect ACLs and effective access
Mode bits do not tell the whole story when a node has an extended access control list (ACL). Check it with:
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getfacl /dev/ttyUSB0Look for named user or group entries as well as the ACL mask. The mask limits the effective permissions of certain ACL entries, so an entry that appears to grant access may provide less in practice. Use any effective-rights annotations in the output when interpreting the result. See getfacl(1) for how ACL entries and masks are reported.
4. Find the udev properties and rules behind the node
The owner, group, mode and related device-node properties may be assigned or reset by udev when the kernel reports device events. Query the current device information and inspect its attributes:
udevadm info --query=all --name=/dev/ttyUSB0
udevadm info --attribute-walk --name=/dev/ttyUSB0The first command reports information associated with the named node; the attribute walk shows attributes on the device and its parents that may help identify matching rules. Exact options can vary with the installed udevadm version, so check the host’s udevadm(8) manual.
Review candidate rule files in these directories:
/etc/udev/rules.d/run/udev/rules.d/usr/local/lib/udev/rules.d/usr/lib/udev/rules.d
Search for matches involving the device’s subsystem, kernel name and attributes—for example, SUBSYSTEM, KERNEL, ATTR and ATTRS—and for assignments or links such as OWNER, GROUP, MODE, tags and symlinks. udev collects rules from system and local directories and processes them in lexicographic order; a local file with the same name as a vendor file can replace it. The rules and ordering model are described in udev(7).
Use the observed attributes to trace the rule that applies to this specific device. A rule should match the intended hardware closely enough to avoid granting access to unrelated devices. A one-time chmod does not explain or necessarily persist the effective configuration if udev assigns permissions again when the device is added or an event occurs.
5. Decide whether the current access is appropriate
Compare the live result—including ACL effective rights—with the least-privilege policy for this device and its users. A permissive group setting may be deliberate, but the generic Linux documentation does not establish one correct group or mode for all terminal-connected devices. Device class, distribution configuration and organizational policy matter.
The checks above inspect state; they do not change it. Treat changes to a mode, ACL or udev rule as a separate, authorized configuration decision. If an administrator changes an ACL, setfacl may also change mode bits when the filesystem cannot represent the requested ACL as given. Re-run the inspection after an authorized change; see setfacl(1).
6. Monitor later access with Linux Audit, if needed
A one-time permissions audit does not require an audit rule. If you need to record later activity, consider a Linux Audit filesystem watch for the device path and the event types relevant to your policy, such as reads, writes, execution or attribute changes. An audit rule’s perm filter refers to access types and syscall behavior; it is not the Unix mode value shown by ls -l. The distinction is covered in audit.rules(7) and auditctl(8).
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Which tool answers which question?
| Tool | What it reveals | Best use |
|---|---|---|
ls -l or stat |
Node type, owner, group and mode metadata | Quick current-state check |
getfacl |
Extended ACL entries and the mask that can limit effective access | Checking grants beyond ordinary mode bits |
udevadm info |
Device properties and attributes useful for tracing udev matches | Finding the policy behind assigned node settings |
| Linux Audit rules | Configured events on a path or relevant system calls | Ongoing event monitoring, subject to local audit policy |
These tools cover different layers: current filesystem metadata, ACL grants, device-management policy and event observation. For a focused audit, inspect them in that order and use monitoring only when you need an event record.
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