библиотека захвата пакетов (Packet Capture library)
Описание (Description)
The Packet Capture library provides a high level interface to
packet capture systems. All packets on the network, even those
destined for other hosts, are accessible through this mechanism.
It also supports saving captured packets to a ``savefile'', and
reading packets from a ``savefile''.
Initializing
pcap_init() initializes the library. It takes an argument giving
options; currently, the options are:
PCAP_CHAR_ENC_LOCAL
Treat all strings supplied as arguments, and return all
strings to the caller, as being in the local character
encoding.
PCAP_CHAR_ENC_UTF_8
Treat all strings supplied as arguments, and return all
strings to the caller, as being in UTF-8.
On UNIX-like systems, the local character encoding is assumed to
be UTF-8, so no character encoding transformations are done.
On Windows, the local character encoding is the local ANSI code
page.
If pcap_init() is called, the deprecated pcap_lookupdev() routine
always fails, so it should not be used, and, on Windows,
pcap_create() does not attempt to handle UTF-16LE strings.
If pcap_init() is not called, strings are treated as being in the
local ANSI code page on Windows, pcap_lookupdev() will succeed if
there is a device on which to capture, and pcap_create() makes an
attempt to check whether the string passed as an argument is a
UTF-16LE string - note that this attempt is unsafe, as it may run
past the end of the string - to handle pcap_lookupdev() returning
a UTF-16LE string. Programs that don't call pcap_init() should,
on Windows, call pcap_wsockinit() to initialize Winsock; this is
not necessary if pcap_init() is called, as pcap_init() will
initialize Winsock itself on Windows.
Routines
pcap_init(3PCAP)
initialize the library
Opening a capture handle for reading
To open a handle for a live capture, given the name of the
network or other interface on which the capture should be done,
call pcap_create(), set the appropriate options on the handle,
and then activate it with pcap_activate(). If pcap_activate()
fails, the handle should be closed with pcap_close().
To obtain a list of devices that can be opened for a live
capture, call pcap_findalldevs(); to free the list returned by
pcap_findalldevs(), call pcap_freealldevs(). pcap_lookupdev()
will return the first device on that list that is not a
``loopback`` network interface.
To open a handle for a ``savefile'' from which to read packets,
given the pathname of the ``savefile'', call pcap_open_offline();
to set up a handle for a ``savefile'', given a FILE * referring
to a file already opened for reading, call pcap_fopen_offline().
In order to get a ``fake'' pcap_t for use in routines that
require a pcap_t as an argument, such as routines to open a
``savefile'' for writing and to compile a filter expression, call
pcap_open_dead().
pcap_create(), pcap_open_offline(), pcap_fopen_offline(), and
pcap_open_dead() return a pointer to a pcap_t, which is the
handle used for reading packets from the capture stream or the
``savefile'', and for finding out information about the capture
stream or ``savefile''. To close a handle, use pcap_close().
The options that can be set on a capture handle include
snapshot length
If, when capturing, you capture the entire contents of the
packet, that requires more CPU time to copy the packet to
your application, more disk and possibly network bandwidth
to write the packet data to a file, and more disk space to
save the packet. If you don't need the entire contents of
the packet - for example, if you are only interested in
the TCP headers of packets - you can set the "snapshot
length" for the capture to an appropriate value. If the
snapshot length is set to snaplen, and snaplen is less
than the size of a packet that is captured, only the first
snaplen bytes of that packet will be captured and provided
as packet data.
A snapshot length of 65535 should be sufficient, on most
if not all networks, to capture all the data available
from the packet.
The snapshot length is set with pcap_set_snaplen().
promiscuous mode
On broadcast LANs such as Ethernet, if the network isn't
switched, or if the adapter is connected to a "mirror
port" on a switch to which all packets passing through the
switch are sent, a network adapter receives all packets on
the LAN, including unicast or multicast packets not sent
to a network address that the network adapter isn't
configured to recognize.
Normally, the adapter will discard those packets; however,
many network adapters support "promiscuous mode", which is
a mode in which all packets, even if they are not sent to
an address that the adapter recognizes, are provided to
the host. This is useful for passively capturing traffic
between two or more other hosts for analysis.
Note that even if an application does not set promiscuous
mode, the adapter could well be in promiscuous mode for
some other reason.
For now, this doesn't work on the "any" device; if an
argument of "any" or NULL is supplied, the setting of
promiscuous mode is ignored.
Promiscuous mode is set with pcap_set_promisc().
monitor mode
On IEEE 802.11 wireless LANs, even if an adapter is in
promiscuous mode, it will supply to the host only frames
for the network with which it's associated. It might also
supply only data frames, not management or control frames,
and might not provide the 802.11 header or radio
information pseudo-header for those frames.
In "monitor mode", sometimes also called "rfmon mode" (for
"Radio Frequency MONitor"), the adapter will supply all
frames that it receives, with 802.11 headers, and might
supply a pseudo-header with radio information about the
frame as well.
Note that in monitor mode the adapter might disassociate
from the network with which it's associated, so that you
will not be able to use any wireless networks with that
adapter. This could prevent accessing files on a network
server, or resolving host names or network addresses, if
you are capturing in monitor mode and are not connected to
another network with another adapter.
Monitor mode is set with pcap_set_rfmon(), and
pcap_can_set_rfmon() can be used to determine whether an
adapter can be put into monitor mode.
packet buffer timeout
If, when capturing, packets are delivered as soon as they
arrive, the application capturing the packets will be
woken up for each packet as it arrives, and might have to
make one or more calls to the operating system to fetch
each packet.
If, instead, packets are not delivered as soon as they
arrive, but are delivered after a short delay (called a
"packet buffer timeout"), more than one packet can be
accumulated before the packets are delivered, so that a
single wakeup would be done for multiple packets, and each
set of calls made to the operating system would supply
multiple packets, rather than a single packet. This
reduces the per-packet CPU overhead if packets are
arriving at a high rate, increasing the number of packets
per second that can be captured.
The packet buffer timeout is required so that an
application won't wait for the operating system's capture
buffer to fill up before packets are delivered; if packets
are arriving slowly, that wait could take an arbitrarily
long period of time.
Not all platforms support a packet buffer timeout; on
platforms that don't, the packet buffer timeout is
ignored. A zero value for the timeout, on platforms that
support a packet buffer timeout, will cause a read to wait
forever to allow enough packets to arrive, with no
timeout. A negative value is invalid; the result of
setting the timeout to a negative value is unpredictable.
NOTE: the packet buffer timeout cannot be used to cause
calls that read packets to return within a limited period
of time, because, on some platforms, the packet buffer
timeout isn't supported, and, on other platforms, the
timer doesn't start until at least one packet arrives.
This means that the packet buffer timeout should NOT be
used, for example, in an interactive application to allow
the packet capture loop to ``poll'' for user input
periodically, as there's no guarantee that a call reading
packets will return after the timeout expires even if no
packets have arrived.
The packet buffer timeout is set with pcap_set_timeout().
immediate mode
In immediate mode, packets are always delivered as soon as
they arrive, with no buffering. Immediate mode is set
with pcap_set_immediate_mode().
buffer size
Packets that arrive for a capture are stored in a buffer,
so that they do not have to be read by the application as
soon as they arrive. On some platforms, the buffer's size
can be set; a size that's too small could mean that, if
too many packets are being captured and the snapshot
length doesn't limit the amount of data that's buffered,
packets could be dropped if the buffer fills up before the
application can read packets from it, while a size that's
too large could use more non-pageable operating system
memory than is necessary to prevent packets from being
dropped.
The buffer size is set with pcap_set_buffer_size().
timestamp type
On some platforms, the time stamp given to packets on live
captures can come from different sources that can have
different resolutions or that can have different
relationships to the time values for the current time
supplied by routines on the native operating system. See
pcap-tstamp(7) for a list of time stamp types.
The time stamp type is set with pcap_set_tstamp_type().
Reading packets from a network interface may require that you
have special privileges:
Under SunOS 3.x or 4.x with NIT or BPF:
You must have read access to /dev/nit or /dev/bpf*.
Under Solaris with DLPI:
You must have read/write access to the network pseudo
device, e.g. /dev/le. On at least some versions of
Solaris, however, this is not sufficient to allow tcpdump
to capture in promiscuous mode; on those versions of
Solaris, you must be root, or the application capturing
packets must be installed setuid to root, in order to
capture in promiscuous mode. Note that, on many (perhaps
all) interfaces, if you don't capture in promiscuous mode,
you will not see any outgoing packets, so a capture not
done in promiscuous mode may not be very useful.
In newer versions of Solaris, you must have been given the
net_rawaccess privilege; this is both necessary and
sufficient to give you access to the network pseudo-device
- there is no need to change the privileges on that
device. A user can be given that privilege by, for
example, adding that privilege to the user's defaultpriv
key with the usermod(8) command.
Under HP-UX with DLPI:
You must be root or the application capturing packets must
be installed setuid to root.
Under IRIX with snoop:
You must be root or the application capturing packets must
be installed setuid to root.
Under Linux:
You must be root or the application capturing packets must
be installed setuid to root, unless your distribution has
a kernel that supports capability bits such as CAP_NET_RAW
and code to allow those capability bits to be given to
particular accounts and to cause those bits to be set on a
user's initial processes when they log in, in which case
you must have CAP_NET_RAW in order to capture.
Under ULTRIX and Digital UNIX/Tru64 UNIX:
Any user may capture network traffic. However, no user
(not even the super-user) can capture in promiscuous mode
on an interface unless the super-user has enabled
promiscuous-mode operation on that interface using
pfconfig(8), and no user (not even the super-user) can
capture unicast traffic received by or sent by the machine
on an interface unless the super-user has enabled copy-
all-mode operation on that interface using pfconfig, so
useful packet capture on an interface probably requires
that either promiscuous-mode or copy-all-mode operation,
or both modes of operation, be enabled on that interface.
Under BSD (this includes macOS):
You must have read access to /dev/bpf* on systems that
don't have a cloning BPF device, or to /dev/bpf on systems
that do. On BSDs with a devfs (this includes macOS), this
might involve more than just having somebody with super-
user access setting the ownership or permissions on the
BPF devices - it might involve configuring devfs to set
the ownership or permissions every time the system is
booted, if the system even supports that; if it doesn't
support that, you might have to find some other way to
make that happen at boot time.
Reading a saved packet file doesn't require special privileges.
The packets read from the handle may include a ``pseudo-header''
containing various forms of packet meta-data, and probably
includes a link-layer header whose contents can differ for
different network interfaces. To determine the format of the
packets supplied by the handle, call pcap_datalink();
https://www.tcpdump.org/linktypes.html lists the values it
returns and describes the packet formats that correspond to those
values.
Do NOT assume that the packets for a given capture or
``savefile`` will have any given link-layer header type, such as
DLT_EN10MB for Ethernet. For example, the "any" device on Linux
will have a link-layer header type of DLT_LINUX_SLL or
DLT_LINUX_SLL2 even if all devices on the system at the time the
"any" device is opened have some other data link type, such as
DLT_EN10MB for Ethernet.
To obtain the FILE * corresponding to a pcap_t opened for a
``savefile'', call pcap_file().
Routines
pcap_create(3PCAP)
get a pcap_t for live capture
pcap_activate(3PCAP)
activate a pcap_t for live capture
pcap_findalldevs(3PCAP)
get a list of devices that can be opened for a live
capture
pcap_freealldevs(3PCAP)
free list of devices
pcap_lookupdev(3PCAP)
get first non-loopback device on that list
pcap_open_offline(3PCAP)
open a pcap_t for a ``savefile'', given a pathname
pcap_open_offline_with_tstamp_precision(3PCAP)
open a pcap_t for a ``savefile'', given a pathname,
and specify the precision to provide for packet
time stamps
pcap_fopen_offline(3PCAP)
open a pcap_t for a ``savefile'', given a FILE *
pcap_fopen_offline_with_tstamp_precision(3PCAP)
open a pcap_t for a ``savefile'', given a FILE *,
and specify the precision to provide for packet
time stamps
pcap_open_dead(3PCAP)
create a ``fake'' pcap_t
pcap_close(3PCAP)
close a pcap_t
pcap_set_snaplen(3PCAP)
set the snapshot length for a not-yet-activated
pcap_t for live capture
pcap_snapshot(3PCAP)
get the snapshot length for a pcap_t
pcap_set_promisc(3PCAP)
set promiscuous mode for a not-yet-activated pcap_t
for live capture
pcap_set_protocol_linux(3PCAP)
set capture protocol for a not-yet-activated pcap_t
for live capture (Linux only)
pcap_set_rfmon(3PCAP)
set monitor mode for a not-yet-activated pcap_t for
live capture
pcap_can_set_rfmon(3PCAP)
determine whether monitor mode can be set for a
pcap_t for live capture
pcap_set_timeout(3PCAP)
set packet buffer timeout for a not-yet-activated
pcap_t for live capture
pcap_set_immediate_mode(3PCAP)
set immediate mode for a not-yet-activated pcap_t
for live capture
pcap_set_buffer_size(3PCAP)
set buffer size for a not-yet-activated pcap_t for
live capture
pcap_set_tstamp_type(3PCAP)
set time stamp type for a not-yet-activated pcap_t
for live capture
pcap_list_tstamp_types(3PCAP)
get list of available time stamp types for a not-
yet-activated pcap_t for live capture
pcap_free_tstamp_types(3PCAP)
free list of available time stamp types
pcap_tstamp_type_val_to_name(3PCAP)
get name for a time stamp type
pcap_tstamp_type_val_to_description(3PCAP)
get description for a time stamp type
pcap_tstamp_type_name_to_val(3PCAP)
get time stamp type corresponding to a name
pcap_set_tstamp_precision(3PCAP)
set time stamp precision for a not-yet-activated
pcap_t for live capture
pcap_get_tstamp_precision(3PCAP)
get the time stamp precision of a pcap_t for live
capture
pcap_datalink(3PCAP)
get link-layer header type for a pcap_t
pcap_file(3PCAP)
get the FILE * for a pcap_t opened for a
``savefile''
pcap_is_swapped(3PCAP)
determine whether a ``savefile'' being read came
from a machine with the opposite byte order
pcap_major_version(3PCAP)
pcap_minor_version(3PCAP)
get the major and minor version of the file format
version for a ``savefile''
Selecting a link-layer header type for a live capture
Some devices may provide more than one link-layer header type.
To obtain a list of all link-layer header types provided by a
device, call pcap_list_datalinks() on an activated pcap_t for the
device. To free a list of link-layer header types, call
pcap_free_datalinks(). To set the link-layer header type for a
device, call pcap_set_datalink(). This should be done after the
device has been activated but before any packets are read and
before any filters are compiled or installed.
Routines
pcap_list_datalinks(3PCAP)
get a list of link-layer header types for a device
pcap_free_datalinks(3PCAP)
free list of link-layer header types
pcap_set_datalink(3PCAP)
set link-layer header type for a device
pcap_datalink_val_to_name(3PCAP)
get name for a link-layer header type
pcap_datalink_val_to_description(3PCAP)
pcap_datalink_val_to_description_or_dlt(3PCAP)
get description for a link-layer header type
pcap_datalink_name_to_val(3PCAP)
get link-layer header type corresponding to a name
Reading packets
Packets are read with pcap_dispatch() or pcap_loop(), which
process one or more packets, calling a callback routine for each
packet, or with pcap_next() or pcap_next_ex(), which return the
next packet. The callback for pcap_dispatch() and pcap_loop() is
supplied a pointer to a struct pcap_pkthdr, which includes the
following members:
ts a struct timeval containing the time when the
packet was captured
caplen a bpf_u_int32 giving the number of bytes of the
packet that are available from the capture
len a bpf_u_int32 giving the length of the packet, in
bytes (which might be more than the number of bytes
available from the capture, if the length of the
packet is larger than the maximum number of bytes
to capture).
The callback is also supplied a const u_char pointer to the first
caplen (as given in the struct pcap_pkthdr mentioned above) bytes
of data from the packet. This won't necessarily be the entire
packet; to capture the entire packet, you will have to provide a
value for snaplen in your call to pcap_set_snaplen() that is
sufficiently large to get all of the packet's data - a value of
65535 should be sufficient on most if not all networks). When
reading from a ``savefile'', the snapshot length specified when
the capture was performed will limit the amount of packet data
available.
pcap_next() is passed an argument that points to a struct
pcap_pkthdr structure, and fills it in with the time stamp and
length values for the packet. It returns a const u_char to the
first caplen bytes of the packet on success, and NULL on error.
pcap_next_ex() is passed two pointer arguments, one of which
points to a structpcap_pkthdr* and one of which points to a const
u_char*. It sets the first pointer to point to a struct
pcap_pkthdr structure with the time stamp and length values for
the packet, and sets the second pointer to point to the first
caplen bytes of the packet.
To force the loop in pcap_dispatch() or pcap_loop() to terminate,
call pcap_breakloop().
By default, when reading packets from an interface opened for a
live capture, pcap_dispatch(), pcap_next(), and pcap_next_ex()
will, if no packets are currently available to be read, block
waiting for packets to become available. On some, but not all,
platforms, if a packet buffer timeout was specified, the wait
will terminate after the packet buffer timeout expires;
applications should be prepared for this, as it happens on some
platforms, but should not rely on it, as it does not happen on
other platforms. Note that the wait might, or might not,
terminate even if no packets are available; applications should
be prepared for this to happen, but must not rely on it
happening.
A handle can be put into ``non-blocking mode'', so that those
routines will, rather than blocking, return an indication that no
packets are available to read. Call pcap_setnonblock() to put a
handle into non-blocking mode or to take it out of non-blocking
mode; call pcap_getnonblock() to determine whether a handle is in
non-blocking mode. Note that non-blocking mode does not work
correctly in Mac OS X 10.6.
Non-blocking mode is often combined with routines such as
select(2) or poll(2) or other routines a platform offers to wait
for any of a set of descriptors to be ready to read. To obtain,
for a handle, a descriptor that can be used in those routines,
call pcap_get_selectable_fd(). If the routine indicates that
data is available to read on the descriptor, an attempt should be
made to read from the device.
Not all handles have such a descriptor available;
pcap_get_selectable_fd() will return -1 if no such descriptor is
available. If no such descriptor is available, this may be
because the device must be polled periodically for packets; in
that case, pcap_get_required_select_timeout() will return a
pointer to a struct timeval whose value can be used as a timeout
in those routines. When the routine returns, an attmept should
be made to read packets from the device. If
pcap_get_required_select_timeout() returns NULL, no such timeout
is available, and those routines cannot be used with the device.
In addition, for various reasons, one or more of those routines
will not work properly with the descriptor; the documentation for
pcap_get_selectable_fd() gives details. Note that, just as an
attempt to read packets from a pcap_t may not return any packets
if the packet buffer timeout expires, a select(), poll(), or
other such call may, if the packet buffer timeout expires,
indicate that a descriptor is ready to read even if there are no
packets available to read.
Routines
pcap_dispatch(3PCAP)
read a bufferful of packets from a pcap_t open for
a live capture or the full set of packets from a
pcap_t open for a ``savefile''
pcap_loop(3PCAP)
read packets from a pcap_t until an interrupt or
error occurs
pcap_next(3PCAP)
read the next packet from a pcap_t without an
indication whether an error occurred
pcap_next_ex(3PCAP)
read the next packet from a pcap_t with an error
indication on an error
pcap_breakloop(3PCAP)
prematurely terminate the loop in pcap_dispatch()
or pcap_loop()
pcap_setnonblock(3PCAP)
set or clear non-blocking mode on a pcap_t
pcap_getnonblock(3PCAP)
get the state of non-blocking mode for a pcap_t
pcap_get_selectable_fd(3PCAP)
attempt to get a descriptor for a pcap_t that can
be used in calls such as select(2) and poll(2)
pcap_get_required_select_timeout(3PCAP)
attempt to get a timeout required for using a
pcap_t in calls such as select(2) and poll(2)
Filters
In order to cause only certain packets to be returned when
reading packets, a filter can be set on a handle. For a live
capture, the filtering will be performed in kernel mode, if
possible, to avoid copying ``uninteresting'' packets from the
kernel to user mode.
A filter can be specified as a text string; the syntax and
semantics of the string are as described by pcap-filter(7). A
filter string is compiled into a program in a pseudo-machine-
language by pcap_compile() and the resulting program can be made
a filter for a handle with pcap_setfilter(). The result of
pcap_compile() can be freed with a call to pcap_freecode().
pcap_compile() may require a network mask for certain expressions
in the filter string; pcap_lookupnet() can be used to find the
network address and network mask for a given capture device.
A compiled filter can also be applied directly to a packet that
has been read using pcap_offline_filter().
Routines
pcap_compile(3PCAP)
compile filter expression to a pseudo-machine-
language code program
pcap_freecode(3PCAP)
free a filter program
pcap_setfilter(3PCAP)
set filter for a pcap_t
pcap_lookupnet(3PCAP)
get network address and network mask for a capture
device
pcap_offline_filter(3PCAP)
apply a filter program to a packet
Incoming and outgoing packets
By default, libpcap will attempt to capture both packets sent by
the machine and packets received by the machine. To limit it to
capturing only packets received by the machine or, if possible,
only packets sent by the machine, call pcap_setdirection().
Routines
pcap_setdirection(3PCAP)
specify whether to capture incoming packets,
outgoing packets, or both
Capture statistics
To get statistics about packets received and dropped in a live
capture, call pcap_stats().
Routines
pcap_stats(3PCAP)
get capture statistics
Opening a handle for writing captured packets
To open a ``savefile`` to which to write packets, given the
pathname the ``savefile'' should have, call pcap_dump_open(). To
open a ``savefile`` to which to write packets, given the pathname
the ``savefile'' should have, call pcap_dump_open(); to set up a
handle for a ``savefile'', given a FILE * referring to a file
already opened for writing, call pcap_dump_fopen(). They each
return pointers to a pcap_dumper_t, which is the handle used for
writing packets to the ``savefile''. If it succeeds, it will
have created the file if it doesn't exist and truncated the file
if it does exist. To close a pcap_dumper_t, call
pcap_dump_close().
Routines
pcap_dump_open(3PCAP)
open a pcap_dumper_t for a ``savefile``, given a
pathname
pcap_dump_fopen(3PCAP)
open a pcap_dumper_t for a ``savefile``, given a
FILE *
pcap_dump_close(3PCAP)
close a pcap_dumper_t
pcap_dump_file(3PCAP)
get the FILE * for a pcap_dumper_t opened for a
``savefile''
Writing packets
To write a packet to a pcap_dumper_t, call pcap_dump(). Packets
written with pcap_dump() may be buffered, rather than being
immediately written to the ``savefile''. Closing the
pcap_dumper_t will cause all buffered-but-not-yet-written packets
to be written to the ``savefile''. To force all packets written
to the pcap_dumper_t, and not yet written to the ``savefile''
because they're buffered by the pcap_dumper_t, to be written to
the ``savefile'', without closing the pcap_dumper_t, call
pcap_dump_flush().
Routines
pcap_dump(3PCAP)
write packet to a pcap_dumper_t
pcap_dump_flush(3PCAP)
flush buffered packets written to a pcap_dumper_t
to the ``savefile''
pcap_dump_ftell(3PCAP)
get current file position for a pcap_dumper_t
Injecting packets
If you have the required privileges, you can inject packets onto
a network with a pcap_t for a live capture, using pcap_inject()
or pcap_sendpacket(). (The two routines exist for compatibility
with both OpenBSD and WinPcap/Npcap; they perform the same
function, but have different return values.)
Routines
pcap_inject(3PCAP)
pcap_sendpacket(3PCAP)
transmit a packet
Reporting errors
Some routines return error or warning status codes; to convert
them to a string, use pcap_statustostr().
Routines
pcap_statustostr(3PCAP)
get a string for an error or warning status code
Getting library version information
To get a string giving version information about libpcap, call
pcap_lib_version().
Routines
pcap_lib_version(3PCAP)
get library version string
