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* Introduction
The name "usbmon" in lowercase refers to a facility in kernel which is
used to collect traces of I/O on the USB bus. This function is analogous
to a packet socket used by network monitoring tools such as tcpdump(1)
or Ethereal. Similarly, it is expected that a tool such as usbdump or
USBMon (with uppercase letters) is used to examine raw traces produced
by usbmon.
The usbmon reports requests made by peripheral-specific drivers to Host
Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
usbmon may not correspond to bus transactions precisely. This is the same
situation as with tcpdump.
* How to use usbmon to collect raw text traces
Unlike the packet socket, usbmon has an interface which provides traces
in a text format. This is used for two purposes. First, it serves as a
common trace exchange format for tools while most sophisticated formats
are finalized. Second, humans can read it in case tools are not available.
To collect a raw text trace, execute following steps.
1. Prepare
Mount debugfs (it has to be enabled in your kernel configuration), and
load the usbmon module (if built as module). The second step is skipped
if usbmon is built into the kernel.
# mount -t debugfs none_debugs /sys/kernel/debug
# modprobe usbmon
Verify that bus sockets are present.
# ls /sys/kernel/debug/usbmon
1s 1t 2s 2t 3s 3t 4s 4t
2. Find which bus connects to the desired device
Run "cat /proc/bus/usb/devices", and find the T-line which corresponds to
the device. Usually you do it by looking for the vendor string. If you have
many similar devices, unplug one and compare two /proc/bus/usb/devices outputs.
The T-line will have a bus number. Example:
T: Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 0
D: Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
P: Vendor=0557 ProdID=2004 Rev= 1.00
S: Manufacturer=ATEN
S: Product=UC100KM V2.00
Bus=03 means it's bus 3.
3. Start 'cat'
# cat /sys/kernel/debug/usbmon/3t > /tmp/1.mon.out
This process will be reading until killed. Naturally, the output can be
redirected to a desirable location. This is preferred, because it is going
to be quite long.
4. Perform the desired operation on the USB bus
This is where you do something that creates the traffic: plug in a flash key,
copy files, control a webcam, etc.
5. Kill cat
Usually it's done with a keyboard interrupt (Control-C).
At this point the output file (/tmp/1.mon.out in this example) can be saved,
sent by e-mail, or inspected with a text editor. In the last case make sure
that the file size is not excessive for your favourite editor.
* Raw text data format
The '1t' type data consists of a stream of events, such as URB submission,
URB callback, submission error. Every event is a text line, which consists
of whitespace separated words. The number of position of words may depend
on the event type, but there is a set of words, common for all types.
Here is the list of words, from left to right:
- URB Tag. This is used to identify URBs is normally a kernel mode address
of the URB structure in hexadecimal.
- Timestamp in microseconds, a decimal number. The timestamp's resolution
depends on available clock, and so it can be much worse than a microsecond
(if the implementation uses jiffies, for example).
- Event Type. This type refers to the format of the event, not URB type.
Available types are: S - submission, C - callback, E - submission error.
- "Pipe". The pipe concept is deprecated. This is a composite word, used to
be derived from information in pipes. It consists of three fields, separated
by colons: URB type and direction, Device address, Endpoint number.
Type and direction are encoded with two bytes in the following manner:
Ci Co Control input and output
Zi Zo Isochronous input and output
Ii Io Interrupt input and output
Bi Bo Bulk input and output
Device address and Endpoint number are 3-digit and 2-digit (respectively)
decimal numbers, with leading zeroes.
- URB Status. In most cases, this field contains a number, sometimes negative,
which represents a "status" field of the URB. This field makes no sense for
submissions, but is present anyway to help scripts with parsing. When an
error occurs, the field contains the error code. In case of a submission of
a Control packet, this field contains a Setup Tag instead of an error code.
It is easy to tell whether the Setup Tag is present because it is never a
number. Thus if scripts find a number in this field, they proceed to read
Data Length. If they find something else, like a letter, they read the setup
packet before reading the Data Length.
- Setup packet, if present, consists of 5 words: one of each for bmRequestType,
bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
These words are safe to decode if Setup Tag was 's'. Otherwise, the setup
packet was present, but not captured, and the fields contain filler.
- Data Length. For submissions, this is the requested length. For callbacks,
this is the actual length.
- Data tag. The usbmon may not always capture data, even if length is nonzero.
The data words are present only if this tag is '='.
- Data words follow, in big endian hexadecimal format. Notice that they are
not machine words, but really just a byte stream split into words to make
it easier to read. Thus, the last word may contain from one to four bytes.
The length of collected data is limited and can be less than the data length
report in Data Length word.
Here is an example of code to read the data stream in a well known programming
class ParsedLine {
int data_len; /* Available length of data */
byte data[];
void parseData(StringTokenizer st) {
int availwords = st.countTokens();
data = new byte[availwords * 4];
data_len = 0;
while (st.hasMoreTokens()) {
String data_str = st.nextToken();
int len = data_str.length() / 2;
int i;
int b; // byte is signed, apparently?! XXX
for (i = 0; i < len; i++) {
// data[data_len] = Byte.parseByte(
// data_str.substring(i*2, i*2 + 2),
// 16);
b = Integer.parseInt(
data_str.substring(i*2, i*2 + 2),
if (b >= 128)
b *= -1;
data[data_len] = (byte) b;
This format may be changed in the future.
An input control transfer to get a port status.
d5ea89a0 3575914555 S Ci:001:00 s a3 00 0000 0003 0004 4 <
d5ea89a0 3575914560 C Ci:001:00 0 4 = 01050000
An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper
to a storage device at address 5:
dd65f0e8 4128379752 S Bo:005:02 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000
dd65f0e8 4128379808 C Bo:005:02 0 31 >
* Raw binary format and API