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snapshot length
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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.
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|
|
A snapshot length of 65535 should be sufficient, on most if not all
networks, to capture all the data available from the packet.
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|
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The snapshot length is set with
pcap_set_snaplen(). |
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promiscuous mode
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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.
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|
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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.
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|
|
Note that even if an application does not set promiscuous mode, the
adapter could well be in promiscuous mode for some other reason.
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|
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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.
|
|
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Promiscuous mode is set with
pcap_set_promisc(). |
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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.
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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.
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|
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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.
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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.
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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.
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|
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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.
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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.
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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.
|
|
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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 and
CAP_NET_ADMIN to enumerate network devices with, for example, the
-D flag).
|
|
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 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 |
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