NETSNIFF-NG(8) netsniff-ng toolkit NETSNIFF-NG(8)NAME
netsniff-ng - the packet sniffing beast
SYNOPSIS
netsniff-ng { [options] [filter-expression] }
DESCRIPTION
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 mini‐
mal 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 per‐
formance one can get from user space out of the box, without having to
load unsupported or non-mainline third-party kernel modules. We explic‐
itly 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. It has different pcap I/O
methods supported (scatter-gather, mmap(2), read(2), and write(2)) for
efficient to-disc capturing. 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 net‐
sniff-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 pcaps into a single large pcap.
Thus, netsniff-ng then works multithreaded eventually.
netsniff-ng can also be used to debug netlink traffic.
OPTIONS-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.
-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 (“-”).
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 direc‐
tory 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). A trafgen configuration file can currently only
be specified if the input device is a pcap file. 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.
-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 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 exam‐
ple” 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.
-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 pre‐
fix 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 vari‐
ous 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.
-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 any‐
where, 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 combi‐
nation 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 termi‐
nal.
-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 mech‐
anism 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), differ‐
ent hosts or IP addresses can be placed into geoip.conf, separated by a
newline.
-V, --verbose
Be more verbose during startup i.e. show detailed ring setup informa‐
tion.
-v, --version
Show version information and exit.
-h, --help
Show user help and exit.
USAGE EXAMPLE
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 exten‐
sions (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 outgo‐
ing 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 wlan0 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 cap‐
ture, 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 automati‐
cally enabled and only print human readable packet data to the termi‐
nal, 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 bea‐
con.bpf. The BPF engine here is invoked in user space inside of net‐
sniff-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) configura‐
tion file to stdout.
modprobe nlmon
ip link add type nlmon
ip link set nlmon0 up
netsniff-ng -i nlmon0 -o dump.pcap -s
ip link set nlmon0 down
ip link del dev nlmon0
rmmod nlmon
In this example, netlink traffic is being captured. If not already
done, a netlink monitoring device needs to be set up before it can be
used to capture netlink socket buffers (iproute2's ip(1) commands are
given for nlmon device setup and teardown). netsniff-ng can then make
use of the nlmon device as an input device. In this example a pcap file
with netlink traffic is being recorded.
CONFIG FILES
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
FILTER EXAMPLE
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
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 from pcap-fil‐
ter(7):
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 Eth‐
ernet 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).
PCAP FORMATS:
netsniff-ng supports a couple of pcap formats, visible through ``net‐
sniff-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 origi‐
nal 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 origi‐
nal 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_proto‐
col), the packet type (sll_pkttype) and the hardware type (sll_hatype).
For further implementation details or format support in your applica‐
tion, have a look at pcap_io.h.
NOTE
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. Sim‐
ply letting netsniff-ng run without thinking about your underlying sys‐
tem 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 ker‐
nel, 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 technolo‐
gies 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" func‐
tion 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 net‐
sniff-ng as well.
BUGS
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 ker‐
nel is that some drivers already handle VLANs, others do not. Those who
handle it can have different implementations, such as hardware acceler‐
ation 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 noone 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
LEGAL
netsniff-ng is licensed under the GNU GPL version 2.0.
HISTORY
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>.
SEE ALSOtrafgen(8), mausezahn(8), ifpps(8), bpfc(8), flowtop(8), astracer‐
oute(8), curvetun(8)AUTHOR
Manpage was written by Daniel Borkmann.
COLOPHON
This page is part of the Linux netsniff-ng toolkit project. A descrip‐
tion of the project, and information about reporting bugs, can be found
at http://netsniff-ng.org/.
Linux 03 March 2013 NETSNIFF-NG(8)
