|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
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 “--prefix ""” 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
⟨https://github.com/netsniff-ng/netsniff-ng⟩ on 2025-08-11. (At
that time, the date of the most recent commit that was found in
the repository was 2025-06-11.) 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)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|