NETSNIFF-NG(8)               netsniff-ng toolkit              NETSNIFF-NG(8)

NAME         top

       netsniff-ng - the packet sniffing beast

SYNOPSIS         top

       netsniff-ng { [options] [filter-expression] }

DESCRIPTION         top

       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. 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 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 pcaps
       into a single large pcap. Thus, netsniff-ng then works multithreaded

       netsniff-ng can also be used to debug netlink traffic.

OPTIONS         top

   -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 (“-”).
       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. In case 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

   -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

   -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

   -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
       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

   -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

   -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

   -u <uid>, --user <uid> resp. -g <gid>, --group <gid>
       After ring setup drop privileges to a non-root user/group

   -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

   -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

   -v, --version
       Show version information and exit.

   -h, --help
       Show user help and exit.

USAGE EXAMPLE         top

       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

   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

   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.

   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.

   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
   netsniff-ng --fanout-group 1 --fanout-type cpu --fanout-opts defrag
       --bind-cpu 1 --notouch-irq --silent --in em1 --out /var/cap/cpu1/
       --interval 120sec
       Starts 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.

CONFIG FILES         top

       Files under /etc/netsniff-ng/ can be modified to extend netsniff-ng's

           * 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         top

       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

       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-

   host sundown
       To select all packets arriving at or departing from sundown.

   host helios and ˛t 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)) !=
       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

   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).

PCAP FORMATS:         top

       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.

NOTE         top

       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

       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.

BUGS         top

       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 ...

             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

             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.


LEGAL         top

       netsniff-ng is licensed under the GNU GPL version 2.0.

HISTORY         top

       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 <> and Daniel
       Borkmann <>.

SEE ALSO         top

       trafgen(8), mausezahn(8), ifpps(8), bpfc(8), flowtop(8),
       astraceroute(8), curvetun(8)

AUTHOR         top

       Manpage was written by Daniel Borkmann.

COLOPHON         top

       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

COLOPHON         top

       This page is part of the netsniff-ng (a free Linux networking
       toolkit) project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for this manual
       page, send it to  This page was
       obtained from the project's upstream Git repository
       ⟨git://⟩ on 2018-04-30.  (At
       that time, the date of the most recent commit that was found in the
       repository was 2018-04-19.)  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

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)