1 .\" netsniff-ng - the packet sniffing beast
2 .\" Copyright 2013 Daniel Borkmann.
3 .\" Subject to the GPL, version 2.
5 .TH NETSNIFF-NG 8 "03 March 2013" "Linux" "netsniff-ng toolkit"
7 netsniff-ng \- the packet sniffing beast
11 \fB netsniff-ng\fR { [\fIoptions\fR] [\fIfilter-expression\fR] }
15 netsniff-ng is a fast, minimal tool to analyze network packets, capture
16 pcap files, replay pcap files, and redirect traffic between interfaces
17 with the help of zero-copy packet(7) sockets. netsniff-ng uses both Linux
18 specific RX_RING and TX_RING interfaces to perform zero-copy. This is to avoid
19 copy and system call overhead between kernel and user address space. When we
20 started working on netsniff-ng, the pcap(3) library did not use this
23 netsniff-ng is Linux specific, meaning there is no support for other
24 operating systems. Therefore we can keep the code footprint quite minimal and to
25 the point. Linux packet(7) sockets and its RX_RING and TX_RING interfaces
26 bypass the normal packet processing path through the networking stack.
27 This is the fastest capturing or transmission performance one can get from user
28 space out of the box, without having to load unsupported or non-mainline
29 third-party kernel modules. We explicitly refuse to build netsniff-ng on top of
30 ntop/PF_RING. Not because we do not like it (we do find it interesting), but
31 because of the fact that it is not part of the mainline kernel. Therefore, the
32 ntop project has to maintain and sync out-of-tree drivers to adapt them to their
33 DNA. Eventually, we went for untainted Linux kernel, since its code has a higher
34 rate of review, maintenance, security and bug fixes.
36 netsniff-ng also supports early packet filtering in the kernel. It has support
37 for low-level and high-level packet filters that are translated into Berkeley
38 Packet Filter instructions.
40 netsniff-ng can capture pcap files in several different pcap formats that
41 are interoperable with other tools. It has different pcap I/O methods supported
42 (scatter-gather, mmap(2), read(2), and write(2)) for efficient to-disc capturing.
43 netsniff-ng is also able to rotate pcap files based on data size or time
44 intervals, thus, making it a useful backend tool for subsequent traffic
47 netsniff-ng itself also supports analysis, replaying, and dumping of raw 802.11
48 frames. For online or offline analysis, netsniff-ng has a built-in packet
49 dissector for the current 802.3 (Ethernet), 802.11* (WLAN), ARP, MPLS, 802.1Q
50 (VLAN), 802.1QinQ, LLDP, IPv4, IPv6, ICMPv4, ICMPv6, IGMP, TCP and UDP,
51 including GeoIP location analysis. Since netsniff-ng does not establish any
52 state or perform reassembly during packet dissection, its memory footprint is quite
53 low, thus, making netsniff-ng quite efficient for offline analysis of large
56 Note that netsniff-ng is currently not multithreaded. However, this does not
57 prevent you from starting multiple netsniff-ng instances that are pinned to
58 different, non-overlapping CPUs and f.e. have different BPF filters attached.
59 Likely that at some point in time your harddisc might become a bottleneck
60 assuming you do not rotate such pcaps in ram (and from there periodically
61 scheduled move to slower medias). You can then use mergecap(1) to transform
62 all pcaps into a single large pcap. Thus, netsniff-ng then works multithreaded
67 .SS -i <dev|pcap|->, -d <dev|pcap|->, --in <dev|pcap|->, --dev <dev|pcap|->
68 Defines an input device. This can either be a networking device, a pcap file
69 or stdin (\[lq]\-\[rq]). In case of a pcap file, the pcap type (\[lq]\-D\[rq]
70 option) is determined automatically by the pcap file magic. In case of stdin,
71 it is assumed that the input stream is a pcap file.
73 .SS -o <dev|pcap|dir|cfg|->, --out <dev|pcap|dir|cfg|->
74 Defines the output device. This can either be a networking device, a pcap file,
75 a folder, a trafgen(8) configuration file or stdout (\[lq]-\[rq]). In the case of a pcap
76 file that should not have the default pcap type (0xa1b2c3d4), the additional
77 option \[lq]\-T\[rq] must be provided. If a directory is given, then, instead of a
78 single pcap file, multiple pcap files are generated with rotation based on
79 maximum file size or a given interval (\[lq]\-F\[rq] option). A trafgen configuration
80 file can currently only be specified if the input device is a pcap file. If
81 stdout is given as a device, then a trafgen configuration will be written to
82 stdout if the input device is a pcap file, or a pcap file if the input device
83 is a networking device.
85 .SS -f, --filter <bpf-file|expr>
86 Specifies to not dump all traffic, but to filter the network packet haystack.
87 As a filter, either a bpfc(8) compiled file can be passed as a parameter or
88 a tcpdump(1)-like filter expression in quotes. For details regarding the
89 bpf-file have a look at bpfc(8), for details regarding a tcpdump(1)-like filter
90 have a look at section \[lq]filter example\[rq] or at pcap-filter(7). A filter
91 expression may also be passed to netsniff-ng without option \[lq]\-f\[rq] in case
92 there is no subsequent option following after the command-line filter expression.
95 This defines some sort of filtering mechanisms in terms of addressing. Possible
96 values for type are \[lq]host\[rq] (to us), \[lq]broadcast\[rq] (to all), \[lq]multicast\[rq] (to
97 group), \[lq]others\[rq] (promiscuous mode) or \[lq]outgoing\[rq] (from us).
99 .SS -F, --interval <size|time>
100 If the output device is a folder, with \[lq]\-F\[rq], it is possible to define the pcap
101 file rotation interval either in terms of size or time. Thus, when the interval
102 limit has been reached, a new pcap file will be started. As size parameter, the
103 following values are accepted \[lq]<num>KiB/MiB/GiB\[rq]; As time parameter,
104 it can be \[lq]<num>s/sec/min/hrs\[rq].
106 .SS -J, --jumbo-support
107 By default, in pcap replay or redirect mode, netsniff-ng's ring buffer frames
108 are a fixed size of 2048 bytes. This means that if you are expecting jumbo
109 frames or even super jumbo frames to pass through your network, then you need
110 to enable support for that by using this option. However, this has the
111 disadvantage of performance degradation and a bigger memory footprint for the
112 ring buffer. Note that this doesn't affect (pcap) capturing mode, since tpacket
113 in version 3 is used!
116 In case the input or output networking device is a wireless device, it is
117 possible with netsniff-ng to turn this into monitor mode and create a mon<X>
118 device that netsniff-ng will be listening on instead of wlan<X>, for instance.
119 This enables netsniff-ng to analyze, dump, or even replay raw 802.11 frames.
121 .SS -n <0|uint>, --num <0|uint>
122 Process a number of packets and then exit. If the number of packets is 0, then
123 this is equivalent to infinite packets resp. processing until interrupted.
124 Otherwise, a number given as an unsigned integer will limit processing.
126 .SS -P <name>, --prefix <name>
127 When dumping pcap files into a folder, a file name prefix can be defined with
128 this option. If not otherwise specified, the default prefix is \[lq]dump\-\[rq]
129 followed by a Unix timestamp. Use \[lq]\-\-prefex ""\[rq] to set filename as
130 seconds since the Unix Epoch e.g. 1369179203.pcap
132 .SS -T <pcap-magic>, --magic <pcap-magic>
133 Specify a pcap type for storage. Different pcap types with their various meta
134 data capabilities are shown with option \[lq]\-D\[rq]. If not otherwise
135 specified, the pcap-magic 0xa1b2c3d4, also known as a standard tcpdump-capable
136 pcap format, is used. Pcap files with swapped endianness are also supported.
138 .SS -D, --dump-pcap-types
139 Dump all available pcap types with their capabilities and magic numbers that
140 can be used with option \[lq]\-T\[rq] to stdout and exit.
143 If a Berkeley Packet Filter is given, for example via option \[lq]\-f\[rq], then
144 dump the BPF disassembly to stdout during ring setup. This only serves for informative
145 or verification purposes.
148 If the input and output device are both networking devices, then this option will
149 randomize packet order in the output ring buffer.
152 The networking interface will not be put into promiscuous mode. By default,
153 promiscuous mode is turned on.
155 .SS -A, --no-sock-mem
156 On startup and shutdown, netsniff-ng tries to increase socket read and
157 write buffers if appropriate. This option will prevent netsniff-ng from doing
161 Use mmap(2) as pcap file I/O. This is the default when replaying pcap files.
164 Use scatter-gather as pcap file I/O. This is the default when capturing
168 Use slower read(2) and write(2) I/O. This is not the default case anywhere, but in
169 some situations it could be preferred as it has a lower latency on write-back
172 .SS -S <size>, --ring-size <size>
173 Manually define the RX_RING resp. TX_RING size in \[lq]<num>KiB/MiB/GiB\[rq]. By
174 default, the size is determined based on the network connectivity rate.
176 .SS -k <uint>, --kernel-pull <uint>
177 Manually define the interval in micro-seconds where the kernel should be triggered
178 to batch process the ring buffer frames. By default, it is every 10us, but it can
179 manually be prolonged, for instance.
181 .SS -b <cpu>, --bind-cpu <cpu>
182 Pin netsniff-ng to a specific CPU and also pin resp. migrate the NIC's IRQ
183 CPU affinity to this CPU. This option should be preferred in combination with
184 \[lq]\-s\[rq] in case a middle to high packet rate is expected.
186 .SS -u <uid>, --user <uid> resp. -g <gid>, --group <gid>
187 After ring setup drop privileges to a non-root user/group combination.
190 Set this process as a high priority process in order to achieve a higher
191 scheduling rate resp. CPU time. This is however not the default setting, since
192 it could lead to starvation of other processes, for example low priority kernel
195 .SS -Q, --notouch-irq
196 Do not reassign the NIC's IRQ CPU affinity settings.
199 Do not enter the packet dissector at all and do not print any packet information
200 to the terminal. Just shut up and be silent. This option should be preferred in
201 combination with pcap recording or replay, since it will not flood your terminal
202 which causes a significant performance degradation.
205 Print a less verbose one-line information for each packet to the terminal.
208 Only dump packets in hex format to the terminal.
211 Only display ASCII printable characters.
214 If geographical IP location is used, the built-in database update
215 mechanism will be invoked to get Maxmind's latest database. To configure
216 search locations for databases, the file /etc/netsniff-ng/geoip.conf contains
217 possible addresses. Thus, to save bandwidth or for mirroring of Maxmind's
218 databases (to bypass their traffic limit policy), different hosts or IP
219 addresses can be placed into geoip.conf, separated by a newline.
222 Be more verbose during startup i.e. show detailed ring setup information.
225 Show version information and exit.
228 Show user help and exit.
233 The most simple command is to just run \[lq]netsniff-ng\[rq]. This will start
234 listening on all available networking devices in promiscuous mode and dump
235 the packet dissector output to the terminal. No files will be recorded.
237 .SS netsniff-ng --in eth0 --out dump.pcap -s -T 0xa1e2cb12 -b 0 tcp or udp
238 Capture TCP or UDP traffic from the networking device eth0 into the pcap file
239 named dump.pcap, which has netsniff-ng specific pcap extensions (see
240 \[lq]netsniff-ng \-D\[rq] for capabilities). Also, do not print the content to
241 the terminal and pin the process and NIC IRQ affinity to CPU 0. The pcap write
242 method is scatter-gather I/O.
244 .SS netsniff-ng --in wlan0 --rfraw --out dump.pcap --silent --bind-cpu 0
245 Put the wlan0 device into monitoring mode and capture all raw 802.11 frames
246 into the file dump.pcap. Do not dissect and print the content to the terminal
247 and pin the process and NIC IRQ affinity to CPU 0. The pcap write method is
250 .SS netsniff-ng --in dump.pcap --mmap --out eth0 -k1000 --silent --bind-cpu 0
251 Replay the pcap file dump.pcap which is read through mmap(2) I/O and send
252 the packets out via the eth0 networking device. Do not dissect and print the
253 content to the terminal and pin the process and NIC IRQ affinity to CPU 0.
254 Also, trigger the kernel every 1000us to traverse the TX_RING instead of every
255 10us. Note that the pcap magic type is detected automatically from the pcap
258 .SS netsniff-ng --in eth0 --out eth1 --silent --bind-cpu 0 --type host -r
259 Redirect network traffic from the networking device eth0 to eth1 for traffic
260 that is destined for our host, thus ignore broadcast, multicast and promiscuous
261 traffic. Randomize the order of packets for the outgoing device and do not
262 print any packet contents to the terminal. Also, pin the process and NIC IRQ
265 .SS netsniff-ng --in team0 --out /opt/probe/ -s -m --interval 100MiB -b 0
266 Capture on an aggregated team0 networking device and dump packets into multiple
267 pcap files that are split into 100MiB each. Use mmap(2) I/O as a pcap write
268 method, support for super jumbo frames is built-in (does not need to be
269 configured here), and do not print the captured data to the terminal. Pin
270 netsniff-ng and NIC IRQ affinity to CPU 0. The default pcap magic type is
271 0xa1b2c3d4 (tcpdump-capable pcap).
273 .SS netsniff-ng --in vlan0 --out dump.pcap -c -u `id -u bob` -g `id -g bob`
274 Capture network traffic on device wlan0 into a pcap file called dump.pcap
275 by using normal read(2), write(2) I/O for the pcap file (slower but less
276 latency). Also, after setting up the RX_RING for capture, drop privileges
277 from root to the user and group \[lq]bob\[rq]. Invoke the packet dissector and print
278 packet contents to the terminal for further analysis.
280 .SS netsniff-ng --in any --filter http.bpf -B --ascii -V
281 Capture from all available networking interfaces and install a low-level
282 filter that was previously compiled by bpfc(8) into http.bpf in order to
283 filter HTTP traffic. Super jumbo frame support is automatically enabled and
284 only print human readable packet data to the terminal, and also be more
285 verbose during setup phase. Moreover, dump a BPF disassembly of http.bpf.
287 .SS netsniff-ng --in dump.pcap --out dump.cfg --silent
288 Convert the pcap file dump.pcap into a trafgen(8) configuration file dump.cfg.
289 Do not print pcap contents to the terminal.
291 .SS netsniff-ng -i dump.pcap -f beacon.bpf -o -
292 Convert the pcap file dump.pcap into a trafgen(8) configuration file and write
293 it to stdout. However, do not dump all of its content, but only the one that
294 passes the low-level filter for raw 802.11 from beacon.bpf. The BPF engine
295 here is invoked in user space inside of netsniff-ng, so Linux extensions
298 .SS cat foo.pcap | netsniff-ng -i - -o -
299 Read a pcap file from stdin and convert it into a trafgen(8) configuration
304 Files under /etc/netsniff-ng/ can be modified to extend netsniff-ng's
307 * oui.conf - OUI/MAC vendor database
308 * ether.conf - Ethernet type descriptions
309 * tcp.conf - TCP port/services map
310 * udp.conf - UDP port/services map
311 * geoip.conf - GeoIP database mirrors
315 netsniff-ng supports both, low-level and high-level filters that are
316 attached to its packet(7) socket. Low-level filters are described in
317 the bpfc(8) man page.
319 Low-level filters can be used with netsniff-ng in the following way:
322 2. netsniff-ng \-f bar
324 Here, foo is the bpfc program that will be translated into a netsniff-ng
325 readable \[lq]opcodes\[rq] file and passed to netsniff-ng through the \-f
328 Similarly, high-level filter can be either passed through the \-f option,
329 e.g. \-f "tcp or udp" or at the end of all options without the \[lq]\-f\[rq].
331 The filter syntax is the same as in tcpdump(8), which is described in
332 the man page pcap-filter(7). Just to quote some examples from pcap-filter(7):
335 To select all packets arriving at or departing from sundown.
337 .SS host helios and \( hot or ace \)
338 To select traffic between helios and either hot or ace.
340 .SS ip host ace and not helios
341 To select all IP packets between ace and any host except helios.
344 To select all traffic between local hosts and hosts at Berkeley.
346 .SS gateway snup and (port ftp or ftp-data)
347 To select all FTP traffic through Internet gateway snup.
349 .SS ip and not net localnet
350 To select traffic neither sourced from, nor destined for, local hosts. If you
351 have a gateway to another network, this traffic should never make it onto
354 .SS tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet
355 To select the start and end packets (the SYN and FIN packets) of each TCP
356 conversation that involve a non-local host.
358 .SS tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)
359 To select all IPv4 HTTP packets to and from port 80, that is to say, print only packets
360 that contain data, not, for example, SYN and FIN packets and ACK-only packets.
361 (IPv6 is left as an exercise for the reader.)
363 .SS gateway snup and ip[2:2] > 576
364 To select IP packets longer than 576 bytes sent through gateway snup.
366 .SS ether[0] & 1 = 0 and ip[16] >= 224
367 To select IP broadcast or multicast packets that were not sent via Ethernet
368 broadcast or multicast.
370 .SS icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply
371 To select all ICMP packets that are not echo requests or replies
372 (that is to say, not "ping" packets).
376 netsniff-ng supports a couple of pcap formats, visible through ``netsniff-ng \-D'':
378 .SS tcpdump-capable pcap (default)
379 Pcap magic number is encoded as 0xa1b2c3d4 resp. 0xd4c3b2a1. As packet meta data
380 this format contains the timeval in microseconds, the original packet length and
381 the captured packet length.
383 .SS tcpdump-capable pcap with ns resolution
384 Pcap magic number is encoded as 0xa1b23c4d resp. 0x4d3cb2a1. As packet meta data
385 this format contains the timeval in nanoseconds, the original packet length and
386 the captured packet length.
388 .SS Alexey Kuznetzov's pcap
389 Pcap magic number is encoded as 0xa1b2cd34 resp. 0x34cdb2a1. As packet meta data
390 this format contains the timeval in microseconds, the original packet length,
391 the captured packet length, the interface index (sll_ifindex), the packet's
392 protocol (sll_protocol), and the packet type (sll_pkttype).
395 Pcap magic number is encoded as 0xa1e2cb12 resp. 0x12cbe2a1. As packet meta data
396 this format contains the timeval in nanoseconds, the original packet length,
397 the captured packet length, the timestamp hw/sw source, the interface index
398 (sll_ifindex), the packet's protocol (sll_protocol), the packet type (sll_pkttype)
399 and the hardware type (sll_hatype).
401 For further implementation details or format support in your application,
402 have a look at pcap_io.h.
405 For introducing bit errors, delays with random variation and more
406 while replaying pcaps, make use of tc(8) with its disciplines such
409 netsniff-ng does only some basic, architecture generic tuning on
410 startup. If you are considering to do high performance capturing,
411 you need to carefully tune your machine, both hardware and software.
412 Simply letting netsniff-ng run without thinking about your underlying
413 system might not necessarily give you the desired performance. Note
414 that tuning your system is always a tradeoff and fine-grained
415 balancing act (throughput versus latency). You should know what
418 One recommendation for software-based tuning is tuned(8). Besides
419 that, there are many other things to consider. Just to throw you
420 a few things that you might want to look at: NAPI networking drivers,
421 tickless kernel, I/OAT DMA engine, Direct Cache Access, RAM-based
422 file systems, multi-queues, and many more things. Also, you might
423 want to read the kernel's Documentation/networking/scaling.txt file
424 regarding technologies such as RSS, RPS, RFS, aRFS and XPS. Also
425 check your ethtool(8) settings, for example regarding offloading or
426 Ethernet pause frames.
428 Moreover, to get a deeper understanding of netsniff-ng internals
429 and how it interacts with the Linux kernel, the kernel documentation
430 under Documentation/networking/{packet_mmap.txt, filter.txt,
431 multiqueue.txt} might be of interest.
433 How do you sniff in a switched environment? I rudely refer to dSniff's
434 documentation that says:
436 The easiest route is simply to impersonate the local gateway, stealing
437 client traffic en route to some remote destination. Of course, the traffic
438 must be forwarded by your attacking machine, either by enabling kernel IP
439 forwarding or with a userland program that accomplishes the same
442 Several people have reportedly destroyed connectivity on their LAN to the
443 outside world by ARP spoofing the gateway, and forgetting to enable IP
444 forwarding on the attacking machine. Do not do this. You have been warned.
446 A safer option than ARP spoofing would be to use a "port mirror" function
447 if your switch hardware supports it and if you have access to the switch.
449 If you do not need to dump all possible traffic, you have to consider
450 running netsniff-ng with a BPF filter for the ingress path. For that
451 purpose, read the bpfc(8) man page.
453 Also, to aggregate multiple NICs that you want to capture on, you
454 should consider using team devices, further explained in libteam resp.
457 The following netsniff-ng pcap magic numbers are compatible with other
458 tools, at least tcpdump or Wireshark:
460 0xa1b2c3d4 (tcpdump-capable pcap)
461 0xa1b23c4d (tcpdump-capable pcap with ns resolution)
462 0xa1b2cd34 (Alexey Kuznetzov's pcap)
464 Pcap files with different meta data endianness are supported by netsniff-ng
469 When replaying pcap files, the timing information from the pcap packet
470 header is currently ignored.
472 Also, when replaying pcap files, demultiplexing traffic among multiple
473 networking interfaces does not work. Currently, it is only sent via the
474 interface that is given by the \-\-out parameter.
476 When performing traffic capture on the Ethernet interface, the pcap file
477 is created and packets are received but without a 802.1Q header. When one
478 uses tshark, all headers are visible, but netsniff-ng removes 802.1Q
479 headers. Is that normal behavior?
481 Yes and no. The way VLAN headers are handled in PF_PACKET sockets by the
482 kernel is somewhat \[lq]problematic\[rq] [1]. The problem in the Linux kernel
483 is that some drivers already handle VLANs, others do not. Those who handle it
484 can have different implementations, such as hardware acceleration and so on.
485 So in some cases the VLAN tag is even stripped before entering the protocol
486 stack, in some cases probably not. The bottom line is that a "hack" was
487 introduced in PF_PACKET so that a VLAN ID is visible in some helper data
488 structure that is accessible from the RX_RING.
490 Then it gets really messy in the user space to artificially put the VLAN
491 header back into the right place. Not to mention the resulting performance
492 implications on all of libpcap(3) tools since parts of the packet need to
493 be copied for reassembly via memmove(3).
495 A user reported the following, just to demonstrate this mess: some tests were
496 made with two machines, and it seems that results depend on the driver ...
499 ethtool \-k eth0 gives "rx-vlan-offload: on"
500 - wireshark gets the vlan header
501 - netsniff-ng doesn't get the vlan header
502 ethtool \-K eth0 rxvlan off
503 - wireshark gets a QinQ header even though noone sent QinQ
504 - netsniff-ng gets the vlan header
507 ethtool \-k eth0 gives "rx-vlan-offload: on"
508 - wireshark gets the vlan header
509 - netsniff-ng doesn't get the vlan header
510 ethtool \-K eth0 rxvlan off
511 - wireshark gets the vlan header
512 - netsniff-ng doesn't get the vlan header
514 Even if we agreed on doing the same workaround as libpcap, we still will
515 not be able to see QinQ, for instance, due to the fact that only one VLAN tag
516 is stored in the kernel helper data structure. We think that there should be
517 a good consensus on the kernel space side about what gets transferred to
520 Update (28.11.2012): the Linux kernel and also bpfc(8) has built-in support
521 for hardware accelerated VLAN filtering, even though tags might not be visible
522 in the payload itself as reported here. However, the filtering for VLANs works
523 reliable if your NIC supports it. See bpfc(8) for an example.
525 [1] http://lkml.indiana.edu/hypermail/linux/kernel/0710.3/3816.html
528 netsniff-ng is licensed under the GNU GPL version 2.0.
532 was originally written for the netsniff-ng toolkit by Daniel Borkmann. Bigger
533 contributions were made by Emmanuel Roullit, Markus Amend, Tobias Klauser and
534 Christoph Jaeger. It is currently maintained by Tobias Klauser
535 <tklauser@distanz.ch> and Daniel Borkmann <dborkma@tik.ee.ethz.ch>.
543 .BR astraceroute (8),
547 Manpage was written by Daniel Borkmann.