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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | USAGE EXAMPLE | CONFIG FILES | FILTER EXAMPLE | PCAP FORMATS: | NOTE | BUGS | LEGAL | HISTORY | SEE ALSO | AUTHOR | COLOPHON | COLOPHON |
NETSNIFF-NG(8) netsniff-ng toolkit NETSNIFF-NG(8)
netsniff-ng - the packet sniffing beast
netsniff-ng { [options] [filter-expression] }
netsniff-ng is a fast, minimal tool to analyze network packets,
capture pcap files, replay pcap files, and redirect traffic between
interfaces with the help of zero-copy packet(7) sockets. netsniff-ng
uses both Linux specific RX_RING and TX_RING interfaces to perform
zero-copy. This is to avoid copy and system call overhead between
kernel and user address space. When we started working on netsniff-
ng, the pcap(3) library did not use this zero-copy facility.
netsniff-ng is Linux specific, meaning there is no support for other
operating systems. Therefore we can keep the code footprint quite
minimal and to the point. Linux packet(7) sockets and its RX_RING and
TX_RING interfaces bypass the normal packet processing path through
the networking stack. This is the fastest capturing or transmission
performance one can get from user space out of the box, without
having to load unsupported or non-mainline third-party kernel
modules. We explicitly refuse to build netsniff-ng on top of
ntop/PF_RING. Not because we do not like it (we do find it
interesting), but because of the fact that it is not part of the
mainline kernel. Therefore, the ntop project has to maintain and sync
out-of-tree drivers to adapt them to their DNA. Eventually, we went
for untainted Linux kernel, since its code has a higher rate of
review, maintenance, security and bug fixes.
netsniff-ng also supports early packet filtering in the kernel. It
has support for low-level and high-level packet filters that are
translated into Berkeley Packet Filter instructions.
netsniff-ng can capture pcap files in several different pcap formats
that are interoperable with other tools. The following pcap I/O
methods are supported for efficient to-disc capturing: scatter-
gather, mmap(2), read(2), and write(2). netsniff-ng is also able to
rotate pcap files based on data size or time intervals, thus, making
it a useful backend tool for subsequent traffic analysis.
netsniff-ng itself also supports analysis, replaying, and dumping of
raw 802.11 frames. For online or offline analysis, netsniff-ng has a
built-in packet dissector for the current 802.3 (Ethernet), 802.11*
(WLAN), ARP, MPLS, 802.1Q (VLAN), 802.1QinQ, LLDP, IPv4, IPv6,
ICMPv4, ICMPv6, IGMP, TCP and UDP, including GeoIP location analysis.
Since netsniff-ng does not establish any state or perform reassembly
during packet dissection, its memory footprint is quite low, thus,
making netsniff-ng quite efficient for offline analysis of large pcap
files as well.
Note that netsniff-ng is currently not multithreaded. However, this
does not prevent you from starting multiple netsniff-ng instances
that are pinned to different, non-overlapping CPUs and f.e. have
different BPF filters attached. Likely that at some point in time
your harddisc might become a bottleneck assuming you do not rotate
such pcaps in ram (and from there periodically scheduled move to
slower medias). You can then use mergecap(1) to transform all pcap
files into a single large pcap file. Thus, netsniff-ng then works
multithreaded eventually.
netsniff-ng can also be used to debug netlink traffic.
-i <dev|pcap|->, -d <dev|pcap|->, --in <dev|pcap|->, --dev
<dev|pcap|->
Defines an input device. This can either be a networking
device, a pcap file or stdin (“-”). In case of a pcap file,
the pcap type (-D option) is determined automatically by the
pcap file magic. In case of stdin, it is assumed that the
input stream is a pcap file. If the pcap link type is Netlink
and pcap type is default format (usec or nsec), then each
packet will be wrapped with pcap cooked header [2].
-o <dev|pcap|dir|cfg|->, --out <dev|pcap|dir|cfg|->
Defines the output device. This can either be a networking
device, a pcap file, a folder, a trafgen(8) configuration file
or stdout (“-”). If the output device is a pcap or trafgen(8)
configuration file, it may include a time format as defined by
strfime(3). If used in conjunction with the -F option, each
rotated file will have a unique time stamp. In the case of a
pcap file that should not have the default pcap type
(0xa1b2c3d4), the additional option -T must be provided. If a
directory is given, then, instead of a single pcap file,
multiple pcap files are generated with rotation based on
maximum file size or a given interval (-F option). Optionally,
sending the SIGHUP signal to the netsniff-ng process causes a
premature rotation of the file. A trafgen configuration file
can currently only be specified if the input device is a pcap
file. To specify a pcap file as the output device, the file
name must have “.pcap” as its extension. If stdout is given as
a device, then a trafgen configuration will be written to
stdout if the input device is a pcap file, or a pcap file if
the input device is a networking device. If the input device
is a Netlink monitor device and pcap type is default (usec or
nsec) then each packet will be wrapped with pcap cooked header
[2] to keep Netlink family number (Kuznetzov's and netsniff-ng
pcap types already contain family number in protocol number
field).
-C <id>, --fanout-group <id>
If multiple netsniff-ng instances are being started that all
have the same packet fanout group id, then the ingress network
traffic being captured is being distributed/load-balanced
among these group participants. This gives a much better
scaling than running multiple netsniff-ng processes without a
fanout group parameter in parallel, but only with a BPF filter
attached as a packet would otherwise need to be delivered to
all such capturing processes, instead of only once to such a
fanout member. Naturally, each fanout member can have its own
BPF filters attached.
-K <hash|lb|cpu|rnd|roll|qm>, --fanout-type <hash|lb|cpu|rnd|roll|qm>
This parameter specifies the fanout discipline, in other
words, how the captured network traffic is dispatched to the
fanout group members. Options are to distribute traffic by the
packet hash (“hash”), in a round-robin manner (“lb”), by CPU
the packet arrived on (“cpu”), by random (“rnd”), by rolling
over sockets (“roll”) which means if one socket's queue is
full, we move on to the next one, or by NIC hardware queue
mapping (“qm”).
-L <defrag|roll>, --fanout-opts <defrag|roll>
Defines some auxiliary fanout options to be used in addition
to a given fanout type. These options apply to any fanout
type. In case of “defrag”, the kernel is being told to
defragment packets before delivering to user space, and “roll”
provides the same roll-over option as the “roll” fanout type,
so that on any different fanout type being used (e.g. “qm”)
the socket may temporarily roll over to the next fanout group
member in case the original one's queue is full.
-f, --filter <bpf-file|-|expr>
Specifies to not dump all traffic, but to filter the network
packet haystack. As a filter, either a bpfc(8) compiled
file/stdin can be passed as a parameter or a tcpdump(1)-like
filter expression in quotes. For details regarding the bpf-
file have a look at bpfc(8), for details regarding a
tcpdump(1)-like filter have a look at section “filter example”
or at pcap-filter(7). A filter expression may also be passed
to netsniff-ng without option -f in case there is no
subsequent option following after the command-line filter
expression.
-t, --type <type>
This defines some sort of filtering mechanisms in terms of
addressing. Possible values for type are “host” (to us),
“broadcast” (to all), “multicast” (to group), “others”
(promiscuous mode) or “outgoing” (from us).
-F, --interval <size|time>
If the output device is a folder, with “-F”, it is possible to
define the pcap file rotation interval either in terms of size
or time. Thus, when the interval limit has been reached, a new
pcap file will be started. As size parameter, the following
values are accepted “<num>KiB/MiB/GiB”; As time parameter, it
can be “<num>s/sec/min/hrs”.
-J, --jumbo-support
By default, in pcap replay or redirect mode, netsniff-ng's
ring buffer frames are a fixed size of 2048 bytes. This means
that if you are expecting jumbo frames or even super jumbo
frames to pass through your network, then you need to enable
support for that by using this option. However, this has the
disadvantage of performance degradation and a bigger memory
footprint for the ring buffer. Note that this doesn't affect
(pcap) capturing mode, since tpacket in version 3 is used!
-R, --rfraw
In case the input or output networking device is a wireless
device, it is possible with netsniff-ng to turn this into
monitor mode and create a mon<X> device that netsniff-ng will
be listening on instead of wlan<X>, for instance. This
enables netsniff-ng to analyze, dump, or even replay raw
802.11 frames.
-n <0|uint>, --num <0|uint>
Process a number of packets and then exit. If the number of
packets is 0, then this is equivalent to infinite packets
resp. processing until interrupted. Otherwise, a number given
as an unsigned integer will limit processing.
-O <N>, --overwrite <N>
A number from 0 to N-1 will be used in the file name instead
of a Unix timestamp. The previous file will be overwritten
when number wraps around. The maximum value is 2^32 - 1.
Intended for rotating capture files when used with options -F
and -P.
-P <name>, --prefix <name>
When dumping pcap files into a folder, a file name prefix can
be defined with this option. If not otherwise specified, the
default prefix is “dump-” followed by a Unix timestamp. Use
“--prefex ""” to set filename as seconds since the Unix Epoch
e.g. 1369179203.pcap
-T <pcap-magic>, --magic <pcap-magic>
Specify a pcap type for storage. Different pcap types with
their various meta data capabilities are shown with option -D.
If not otherwise specified, the pcap-magic 0xa1b2c3d4, also
known as a standard tcpdump-capable pcap format, is used. Pcap
files with swapped endianness are also supported.
-D, --dump-pcap-types
Dump all available pcap types with their capabilities and
magic numbers that can be used with option “-T” to stdout and
exit.
-B, --dump-bpf
If a Berkeley Packet Filter is given, for example via option
“-f”, then dump the BPF disassembly to stdout during ring
setup. This only serves for informative or verification
purposes.
-r, --rand
If the input and output device are both networking devices,
then this option will randomize packet order in the output
ring buffer.
-M, --no-promisc
The networking interface will not be put into promiscuous
mode. By default, promiscuous mode is turned on.
-N, --no-hwtimestamp
Disable taking hardware time stamps for RX packets. By
default, if the network device supports hardware time
stamping, the hardware time stamps will be used when writing
packets to pcap files. This option disables this behavior and
forces (kernel based) software time stamps to be used, even if
hardware time stamps are available.
-A, --no-sock-mem
On startup and shutdown, netsniff-ng tries to increase socket
read and write buffers if appropriate. This option will
prevent netsniff-ng from doing so.
-m, --mmap
Use mmap(2) as pcap file I/O. This is the default when
replaying pcap files.
-G, --sg
Use scatter-gather as pcap file I/O. This is the default when
capturing pcap files.
-c, --clrw
Use slower read(2) and write(2) I/O. This is not the default
case anywhere, but in some situations it could be preferred as
it has a lower latency on write-back to disc.
-S <size>, --ring-size <size>
Manually define the RX_RING resp. TX_RING size in
“<num>KiB/MiB/GiB”. By default, the size is determined based
on the network connectivity rate.
-k <uint>, --kernel-pull <uint>
Manually define the interval in micro-seconds where the kernel
should be triggered to batch process the ring buffer frames.
By default, it is every 10us, but it can manually be
prolonged, for instance.
-b <cpu>, --bind-cpu <cpu>
Pin netsniff-ng to a specific CPU and also pin resp. migrate
the NIC's IRQ CPU affinity to this CPU. This option should be
preferred in combination with -s in case a middle to high
packet rate is expected.
-u <uid>, --user <uid> resp. -g <gid>, --group <gid>
After ring setup drop privileges to a non-root user/group
combination.
-H, --prio-high
Set this process as a high priority process in order to
achieve a higher scheduling rate resp. CPU time. This is
however not the default setting, since it could lead to
starvation of other processes, for example low priority kernel
threads.
-Q, --notouch-irq
Do not reassign the NIC's IRQ CPU affinity settings.
-s, --silent
Do not enter the packet dissector at all and do not print any
packet information to the terminal. Just shut up and be
silent. This option should be preferred in combination with
pcap recording or replay, since it will not flood your
terminal which causes a significant performance degradation.
-q, --less
Print a less verbose one-line information for each packet to
the terminal.
-X, --hex
Only dump packets in hex format to the terminal.
-l, --ascii
Only display ASCII printable characters.
-U, --update
If geographical IP location is used, the built-in database
update mechanism will be invoked to get Maxmind's latest
database. To configure search locations for databases, the
file /etc/netsniff-ng/geoip.conf contains possible addresses.
Thus, to save bandwidth or for mirroring of Maxmind's
databases (to bypass their traffic limit policy), different
hosts or IP addresses can be placed into geoip.conf, separated
by a newline.
-w, --cooked
Replace each frame link header with Linux "cooked" header [3]
which keeps info about link type and protocol. It allows to
dump and dissect frames captured from different link types
when -i "any" was specified, for example.
-V, --verbose
Be more verbose during startup i.e. show detailed ring setup
information.
-v, --version
Show version information and exit.
-h, --help
Show user help and exit.
netsniff-ng
The most simple command is to just run “netsniff-ng”. This
will start listening on all available networking devices in
promiscuous mode and dump the packet dissector output to the
terminal. No files will be recorded.
netsniff-ng --in eth0 --out dump.pcap -s -T 0xa1e2cb12 -b 0 tcp or
udp
Capture TCP or UDP traffic from the networking device eth0
into the pcap file named dump.pcap, which has netsniff-ng
specific pcap extensions (see “netsniff-ng -D” for
capabilities). Also, do not print the content to the terminal
and pin the process and NIC IRQ affinity to CPU 0. The pcap
write method is scatter-gather I/O.
netsniff-ng --in wlan0 --rfraw --out dump.pcap --silent --bind-cpu 0
Put the wlan0 device into monitoring mode and capture all raw
802.11 frames into the file dump.pcap. Do not dissect and
print the content to the terminal and pin the process and NIC
IRQ affinity to CPU 0. The pcap write method is scatter-gather
I/O.
netsniff-ng --in dump.pcap --mmap --out eth0 -k1000 --silent --bind-
cpu 0
Replay the pcap file dump.pcap which is read through mmap(2)
I/O and send the packets out via the eth0 networking device.
Do not dissect and print the content to the terminal and pin
the process and NIC IRQ affinity to CPU 0. Also, trigger the
kernel every 1000us to traverse the TX_RING instead of every
10us. Note that the pcap magic type is detected automatically
from the pcap file header.
netsniff-ng --in eth0 --out eth1 --silent --bind-cpu 0 --type host -r
Redirect network traffic from the networking device eth0 to
eth1 for traffic that is destined for our host, thus ignore
broadcast, multicast and promiscuous traffic. Randomize the
order of packets for the outgoing device and do not print any
packet contents to the terminal. Also, pin the process and NIC
IRQ affinity to CPU 0.
netsniff-ng --in team0 --out /opt/probe/ -s -m --interval 100MiB -b 0
Capture on an aggregated team0 networking device and dump
packets into multiple pcap files that are split into 100MiB
each. Use mmap(2) I/O as a pcap write method, support for
super jumbo frames is built-in (does not need to be configured
here), and do not print the captured data to the terminal.
Pin netsniff-ng and NIC IRQ affinity to CPU 0. The default
pcap magic type is 0xa1b2c3d4 (tcpdump-capable pcap).
netsniff-ng --in vlan0 --out dump.pcap -c -u `id -u bob` -g `id -g
bob`
Capture network traffic on device vlan0 into a pcap file
called dump.pcap by using normal read(2), write(2) I/O for the
pcap file (slower but less latency). Also, after setting up
the RX_RING for capture, drop privileges from root to the user
and group “bob”. Invoke the packet dissector and print packet
contents to the terminal for further analysis.
netsniff-ng --in any --filter http.bpf -B --ascii -V
Capture from all available networking interfaces and install a
low-level filter that was previously compiled by bpfc(8) into
http.bpf in order to filter HTTP traffic. Super jumbo frame
support is automatically enabled and only print human readable
packet data to the terminal, and also be more verbose during
setup phase. Moreover, dump a BPF disassembly of http.bpf.
netsniff-ng --in dump.pcap --out dump.cfg --silent
Convert the pcap file dump.pcap into a trafgen(8)
configuration file dump.cfg. Do not print pcap contents to the
terminal.
netsniff-ng -i dump.pcap -f beacon.bpf -o -
Convert the pcap file dump.pcap into a trafgen(8)
configuration file and write it to stdout. However, do not
dump all of its content, but only the one that passes the low-
level filter for raw 802.11 from beacon.bpf. The BPF engine
here is invoked in user space inside of netsniff-ng, so Linux
extensions are not available.
cat foo.pcap | netsniff-ng -i - -o -
Read a pcap file from stdin and convert it into a trafgen(8)
configuration file to stdout.
netsniff-ng -i nlmon0 -o dump.pcap -s
Capture netlink traffic to a pcap file. This command needs a
netlink monitoring device to be set up beforehand using the
follwing commands using ip(1) from the iproute2 utility
collection:
modprobe nlmon
ip link add type nlmon
ip link set nlmon0 up
To tear down the nlmon0 device, use the following commands:
ip link set nlmon0 down
ip link del dev nlmon0
rmmod nlmon
netsniff-ng --fanout-group 1 --fanout-type cpu --fanout-opts defrag
--bind-cpu 0 --notouch-irq --silent --in em1 --out /var/cap/cpu0/
--interval 120sec
Start two netsniff-ng fanout instances. Both are assigned into
the same fanout group membership and traffic is splitted among
them by incoming cpu. Furthermore, the kernel is supposed to
defragment possible incoming fragments. First instance is
assigned to CPU 0 and the second one to CPU 1, IRQ bindings
are not altered as they might have been adapted to this
scenario by the user a-priori, and traffic is captured on
interface em1, and written out in 120 second intervals as pcap
files into /var/cap/cpu0/. Tools like mergecap(1) will be able
to merge the cpu0/1 split back together if needed.
Files under /etc/netsniff-ng/ can be modified to extend netsniff-ng's
functionality:
* oui.conf - OUI/MAC vendor database
* ether.conf - Ethernet type descriptions
* tcp.conf - TCP port/services map
* udp.conf - UDP port/services map
* geoip.conf - GeoIP database mirrors
netsniff-ng supports both, low-level and high-level filters that are
attached to its packet(7) socket. Low-level filters are described in
the bpfc(8) man page.
Low-level filters can be used with netsniff-ng in the following way:
1. bpfc foo > bar
2. netsniff-ng -f bar
3. bpfc foo | netsniff-ng -i nlmon0 -f -
Here, foo is the bpfc program that will be translated into a
netsniff-ng readable “opcodes” file and passed to netsniff-ng through
the -f option.
Similarly, high-level filter can be either passed through the -f
option, e.g. -f "tcp or udp" or at the end of all options without the
“-f”.
The filter syntax is the same as in tcpdump(8), which is described in
the man page pcap-filter(7). Just to quote some examples:
host sundown
To select all packets arriving at or departing from sundown.
host helios and (hot or ace)
To select traffic between helios and either hot or ace.
ip host ace and not helios
To select all IP packets between ace and any host except
helios.
net ucb-ether
To select all traffic between local hosts and hosts at
Berkeley.
gateway snup and (port ftp or ftp-data)
To select all FTP traffic through Internet gateway snup.
ip and not net localnet
To select traffic neither sourced from, nor destined for,
local hosts. If you have a gateway to another network, this
traffic should never make it onto your local network.
tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net
localnet
To select the start and end packets (the SYN and FIN packets)
of each TCP conversation that involve a non-local host.
tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2))
!= 0)
To select all IPv4 HTTP packets to and from port 80, that is
to say, print only packets that contain data, not, for
example, SYN and FIN packets and ACK-only packets. (IPv6 is
left as an exercise for the reader.)
gateway snup and ip[2:2] > 576
To select IP packets longer than 576 bytes sent through
gateway snup.
ether[0] & 1 = 0 and ip[16] >= 224
To select IP broadcast or multicast packets that were not sent
via Ethernet broadcast or multicast.
icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply
To select all ICMP packets that are not echo requests or
replies (that is to say, not "ping" packets).
netsniff-ng supports a couple of pcap formats, visible through
``netsniff-ng -D'':
tcpdump-capable pcap (default)
Pcap magic number is encoded as 0xa1b2c3d4 resp. 0xd4c3b2a1.
As packet meta data this format contains the timeval in
microseconds, the original packet length and the captured
packet length.
tcpdump-capable pcap with ns resolution
Pcap magic number is encoded as 0xa1b23c4d resp. 0x4d3cb2a1.
As packet meta data this format contains the timeval in
nanoseconds, the original packet length and the captured
packet length.
Alexey Kuznetzov's pcap
Pcap magic number is encoded as 0xa1b2cd34 resp. 0x34cdb2a1.
As packet meta data this format contains the timeval in
microseconds, the original packet length, the captured packet
length, the interface index (sll_ifindex), the packet's
protocol (sll_protocol), and the packet type (sll_pkttype).
netsniff-ng pcap
Pcap magic number is encoded as 0xa1e2cb12 resp. 0x12cbe2a1.
As packet meta data this format contains the timeval in
nanoseconds, the original packet length, the captured packet
length, the timestamp hw/sw source, the interface index
(sll_ifindex), the packet's protocol (sll_protocol), the
packet type (sll_pkttype) and the hardware type (sll_hatype).
For further implementation details or format support in your
application, have a look at pcap_io.h in the netsniff-ng sources.
To avoid confusion, it should be noted that there is another network
analyzer with a similar name, called NetSniff, that is unrelated to
the netsniff-ng project.
For introducing bit errors, delays with random variation and more
while replaying pcaps, make use of tc(8) with its disciplines such as
netem.
netsniff-ng does only some basic, architecture generic tuning on
startup. If you are considering to do high performance capturing, you
need to carefully tune your machine, both hardware and software.
Simply letting netsniff-ng run without thinking about your underlying
system might not necessarily give you the desired performance. Note
that tuning your system is always a tradeoff and fine-grained
balancing act (throughput versus latency). You should know what you
are doing!
One recommendation for software-based tuning is tuned(8). Besides
that, there are many other things to consider. Just to throw you a
few things that you might want to look at: NAPI networking drivers,
tickless kernel, I/OAT DMA engine, Direct Cache Access, RAM-based
file systems, multi-queues, and many more things. Also, you might
want to read the kernel's Documentation/networking/scaling.txt file
regarding technologies such as RSS, RPS, RFS, aRFS and XPS. Also
check your ethtool(8) settings, for example regarding offloading or
Ethernet pause frames.
Moreover, to get a deeper understanding of netsniff-ng internals and
how it interacts with the Linux kernel, the kernel documentation
under Documentation/networking/{packet_mmap.txt, filter.txt,
multiqueue.txt} might be of interest.
How do you sniff in a switched environment? I rudely refer to
dSniff's documentation that says:
The easiest route is simply to impersonate the local gateway,
stealing client traffic en route to some remote destination. Of
course, the traffic must be forwarded by your attacking machine,
either by enabling kernel IP forwarding or with a userland program
that accomplishes the same (fragrouter -B1).
Several people have reportedly destroyed connectivity on their LAN to
the outside world by ARP spoofing the gateway, and forgetting to
enable IP forwarding on the attacking machine. Do not do this. You
have been warned.
A safer option than ARP spoofing would be to use a "port mirror"
function if your switch hardware supports it and if you have access
to the switch.
If you do not need to dump all possible traffic, you have to consider
running netsniff-ng with a BPF filter for the ingress path. For that
purpose, read the bpfc(8) man page.
Also, to aggregate multiple NICs that you want to capture on, you
should consider using team devices, further explained in libteam
resp. teamd(8).
The following netsniff-ng pcap magic numbers are compatible with
other tools, at least tcpdump or Wireshark:
0xa1b2c3d4 (tcpdump-capable pcap)
0xa1b23c4d (tcpdump-capable pcap with ns resolution)
0xa1b2cd34 (Alexey Kuznetzov's pcap)
Pcap files with different meta data endianness are supported by
netsniff-ng as well.
When replaying pcap files, the timing information from the pcap
packet header is currently ignored.
Also, when replaying pcap files, demultiplexing traffic among
multiple networking interfaces does not work. Currently, it is only
sent via the interface that is given by the --out parameter.
When performing traffic capture on the Ethernet interface, the pcap
file is created and packets are received but without a 802.1Q header.
When one uses tshark, all headers are visible, but netsniff-ng
removes 802.1Q headers. Is that normal behavior?
Yes and no. The way VLAN headers are handled in PF_PACKET sockets by
the kernel is somewhat “problematic” [1]. The problem in the Linux
kernel is that some drivers already handle VLANs, others do not.
Those who handle it can have different implementations, such as
hardware acceleration and so on. So in some cases the VLAN tag is
even stripped before entering the protocol stack, in some cases
probably not. The bottom line is that a "hack" was introduced in
PF_PACKET so that a VLAN ID is visible in some helper data structure
that is accessible from the RX_RING.
Then it gets really messy in the user space to artificially put the
VLAN header back into the right place. Not to mention the resulting
performance implications on all of libpcap(3) tools since parts of
the packet need to be copied for reassembly via memmove(3).
A user reported the following, just to demonstrate this mess: some
tests were made with two machines, and it seems that results depend
on the driver ...
AR8131:
ethtool -k eth0 gives "rx-vlan-offload: on"
- wireshark gets the vlan header
- netsniff-ng doesn't get the vlan header
ethtool -K eth0 rxvlan off
- wireshark gets a QinQ header even though no one sent QinQ
- netsniff-ng gets the vlan header
RTL8111/8168B:
ethtool -k eth0 gives "rx-vlan-offload: on"
- wireshark gets the vlan header
- netsniff-ng doesn't get the vlan header
ethtool -K eth0 rxvlan off
- wireshark gets the vlan header
- netsniff-ng doesn't get the vlan header
Even if we agreed on doing the same workaround as libpcap, we still
will not be able to see QinQ, for instance, due to the fact that only
one VLAN tag is stored in the kernel helper data structure. We think
that there should be a good consensus on the kernel space side about
what gets transferred to userland first.
Update (28.11.2012): the Linux kernel and also bpfc(8) has built-in
support for hardware accelerated VLAN filtering, even though tags
might not be visible in the payload itself as reported here. However,
the filtering for VLANs works reliable if your NIC supports it. See
bpfc(8) for an example.
[1]
http://lkml.indiana.edu/hypermail/linux/kernel/0710.3/3816.html
[2] http://www.tcpdump.org/linktypes/LINKTYPE_NETLINK.html
[3] http://www.tcpdump.org/linktypes/LINKTYPE_LINUX_SLL.html
netsniff-ng is licensed under the GNU GPL version 2.0.
netsniff-ng was originally written for the netsniff-ng toolkit by
Daniel Borkmann. Bigger contributions were made by Emmanuel Roullit,
Markus Amend, Tobias Klauser and Christoph Jaeger. It is currently
maintained by Tobias Klauser <tklauser@distanz.ch> and Daniel
Borkmann <dborkma@tik.ee.ethz.ch>.
trafgen(8), mausezahn(8), ifpps(8), bpfc(8), flowtop(8),
astraceroute(8), curvetun(8)
Manpage was written by Daniel Borkmann.
This page is part of the Linux netsniff-ng toolkit project. A
description of the project, and information about reporting bugs, can
be found at http://netsniff-ng.org/.
This page is part of the netsniff-ng (a free Linux networking
toolkit) project. Information about the project can be found at
⟨http://netsniff-ng.org/⟩. If you have a bug report for this manual
page, send it to netsniff-ng@googlegroups.com. This page was
obtained from the project's upstream Git repository
⟨git://github.com/netsniff-ng/netsniff-ng.git⟩ on 2020-08-13. (At
that time, the date of the most recent commit that was found in the
repository was 2020-06-19.) If you discover any rendering problems
in this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to man-pages@man7.org
Linux 03 March 2013 NETSNIFF-NG(8)
Pages that refer to this page: astraceroute(8) , bpfc(8) , curvetun(8) , flowtop(8) , ifpps(8) , mausezahn(8) , trafgen(8)