astraceroute: use switch instead of lookup table for short proto id

[netsniff-ng.git] / netsniff-ng.8

.\" netsniff-ng - the packet sniffing beast
.\" Copyright 2013 Daniel Borkmann.
.\" Subject to the GPL, version 2.
.TH NETSNIFF-NG 8 "03 March 2013" "Linux" "netsniff-ng toolkit"
.SH NAME
netsniff-ng \- the packet sniffing beast
.PP
.SH SYNOPSIS
.PP
\fBnetsniff-ng\fR { [\fIoptions\fR] [\fIfilter-expression\fR] }
.PP
.SH DESCRIPTION
.PP
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.
.PP
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.
.PP
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.
.PP
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.
.PP
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.
.PP
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.
.PP
netsniff-ng can also be used to debug netlink traffic.
.PP
.SH OPTIONS
.PP
.SS -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 (\[lq]\-\[rq]). In case of a pcap file, the pcap type (\[lq]\-D\[rq]
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].
.PP
.SS -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 (\[lq]-\[rq]). In the case of a
pcap file that should not have the default pcap type (0xa1b2c3d4), the additional
option \[lq]\-T\[rq] 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 (\[lq]\-F\[rq] 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 \[lq].pcap\[rq] 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).
.PP
.SS -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.
.PP
.SS -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 (\[lq]hash\[rq]), in a round-robin manner (\[lq]lb\[rq]),
by CPU the packet arrived on (\[lq]cpu\[rq]), by random (\[lq]rnd\[rq]), by rolling
over sockets (\[lq]roll\[rq]) which means if one socket's queue is full, we move on
to the next one, or by NIC hardware queue mapping (\[lq]qm\[rq]).
.PP
.SS -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 \[lq]defrag\[rq], the kernel is
being told to defragment packets before delivering to user space, and \[lq]roll\[rq]
provides the same roll-over option as the \[lq]roll\[rq] fanout type, so that on any
different fanout type being used (e.g. \[lq]qm\[rq]) the socket may temporarily roll
over to the next fanout group member in case the original one's queue is full.
.PP
.SS -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 \[lq]filter example\[rq] or at pcap-filter(7). A filter
expression may also be passed to netsniff-ng without option \[lq]\-f\[rq] in case
there is no subsequent option following after the command-line filter expression.
.PP
.SS -t, --type <type>
This defines some sort of filtering mechanisms in terms of addressing. Possible
values for type are \[lq]host\[rq] (to us), \[lq]broadcast\[rq] (to all), \[lq]multicast\[rq] (to
group), \[lq]others\[rq] (promiscuous mode) or \[lq]outgoing\[rq] (from us).
.PP
.SS -F, --interval <size|time>
If the output device is a folder, with \[lq]\-F\[rq], 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 \[lq]<num>KiB/MiB/GiB\[rq]; As time parameter,
it can be \[lq]<num>s/sec/min/hrs\[rq].
.PP
.SS -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!
.PP
.SS -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.
.PP
.SS -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.
.PP
.SS -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 \[lq]dump\-\[rq]
followed by a Unix timestamp. Use \[lq]\-\-prefex ""\[rq] to set filename as
seconds since the Unix Epoch e.g. 1369179203.pcap
.PP
.SS -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 \[lq]\-D\[rq]. 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.
.PP
.SS -D, --dump-pcap-types
Dump all available pcap types with their capabilities and magic numbers that
can be used with option \[lq]\-T\[rq] to stdout and exit.
.PP
.SS -B, --dump-bpf
If a Berkeley Packet Filter is given, for example via option \[lq]\-f\[rq], then
dump the BPF disassembly to stdout during ring setup. This only serves for informative
or verification purposes.
.PP
.SS -r, --rand
If the input and output device are both networking devices, then this option will
randomize packet order in the output ring buffer.
.PP
.SS -M, --no-promisc
The networking interface will not be put into promiscuous mode. By default,
promiscuous mode is turned on.
.PP
.SS -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.
.PP
.SS -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.
.PP
.SS -m, --mmap
Use mmap(2) as pcap file I/O. This is the default when replaying pcap files.
.PP
.SS -G, --sg
Use scatter-gather as pcap file I/O. This is the default when capturing
pcap files.
.PP
.SS -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.
.PP
.SS -S <size>, --ring-size <size>
Manually define the RX_RING resp. TX_RING size in \[lq]<num>KiB/MiB/GiB\[rq]. By
default, the size is determined based on the network connectivity rate.
.PP
.SS -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.
.PP
.SS -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
\[lq]\-s\[rq] in case a middle to high packet rate is expected.
.PP
.SS -u <uid>, --user <uid> resp. -g <gid>, --group <gid>
After ring setup drop privileges to a non-root user/group combination.
.PP
.SS -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.
.PP
.SS -Q, --notouch-irq
Do not reassign the NIC's IRQ CPU affinity settings.
.PP
.SS -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.
.PP
.SS -q, --less
Print a less verbose one-line information for each packet to the terminal.
.PP
.SS -X, --hex
Only dump packets in hex format to the terminal.
.PP
.SS -l, --ascii
Only display ASCII printable characters.
.PP
.SS -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.
.PP
.SS -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.
.PP
.SS -V, --verbose
Be more verbose during startup i.e. show detailed ring setup information.
.PP
.SS -v, --version
Show version information and exit.
.PP
.SS -h, --help
Show user help and exit.
.PP
.SH USAGE EXAMPLE
.PP
.SS netsniff-ng
The most simple command is to just run \[lq]netsniff-ng\[rq]. 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.
.PP
.SS  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
\[lq]netsniff-ng \-D\[rq] 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.
.PP
.SS  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.
.PP
.SS  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.
.PP
.SS  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.
.PP
.SS  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).
.PP
.SS  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 \[lq]bob\[rq]. Invoke the packet dissector and print
packet contents to the terminal for further analysis.
.PP
.SS  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.
.PP
.SS 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.
.PP
.SS 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.
.PP
.SS cat foo.pcap | netsniff-ng -i - -o -
Read a pcap file from stdin and convert it into a trafgen(8) configuration
file to stdout.
.PP
.SS modprobe nlmon
.SS ip link add type nlmon
.SS ip link set nlmon0 up
.SS netsniff-ng -i nlmon0 -o dump.pcap -s
.SS ip link set nlmon0 down
.SS ip link del dev nlmon0
.SS 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.
.PP
.SS 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
.SS 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.
.PP
.SH CONFIG FILES
.PP
Files under /etc/netsniff-ng/ can be modified to extend netsniff-ng's
functionality:
.PP
    * 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
.PP
.SH FILTER EXAMPLE
.PP
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.
.PP
Low-level filters can be used with netsniff-ng in the following way:
.PP
    1. bpfc foo > bar
    2. netsniff-ng \-f bar
    3. bpfc foo | netsniff-ng -i nlmon0 -f -
.PP
Here, foo is the bpfc program that will be translated into a netsniff-ng
readable \[lq]opcodes\[rq] file and passed to netsniff-ng through the \-f
option.
.PP
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 \[lq]\-f\[rq].
.PP
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-filter(7):
.PP
.SS host sundown
To select all packets arriving at or departing from sundown.
.PP
.SS host helios and \(hot or ace\)
To select traffic between helios and either hot or ace.
.PP
.SS ip host ace and not helios
To select all IP packets between ace and any host except helios.
.PP
.SS net ucb-ether
To select all traffic between local hosts and hosts at Berkeley.
.PP
.SS gateway snup and (port ftp or ftp-data)
To select all FTP traffic through Internet gateway snup.
.PP
.SS 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.
.PP
.SS 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.
.PP
.SS 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.)
.PP
.SS gateway snup and ip[2:2] > 576
To select IP packets longer than 576 bytes sent through gateway snup.
.PP
.SS ether[0] & 1 = 0 and ip[16] >= 224
To select IP broadcast or multicast packets that were not sent via Ethernet
broadcast or multicast.
.PP
.SS 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).
.PP
.SH PCAP FORMATS:
.PP
netsniff-ng supports a couple of pcap formats, visible through ``netsniff-ng \-D'':
.PP
.SS 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.
.PP
.SS 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.
.PP
.SS 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).
.PP
.SS 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).
.PP
For further implementation details or format support in your application,
have a look at pcap_io.h.
.PP
.SH NOTE
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.
.PP
For introducing bit errors, delays with random variation and more
while replaying pcaps, make use of tc(8) with its disciplines such
as netem.
.PP
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!
.PP
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.
.PP
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.
.PP
How do you sniff in a switched environment? I rudely refer to dSniff's
documentation that says:
.PP
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).
.PP
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.
.PP
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.
.PP
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.
.PP
Also, to aggregate multiple NICs that you want to capture on, you
should consider using team devices, further explained in libteam resp.
teamd(8).
.PP
The following netsniff-ng pcap magic numbers are compatible with other
tools, at least tcpdump or Wireshark:
.PP
    0xa1b2c3d4 (tcpdump-capable pcap)
    0xa1b23c4d (tcpdump-capable pcap with ns resolution)
    0xa1b2cd34 (Alexey Kuznetzov's pcap)
.PP
Pcap files with different meta data endianness are supported by netsniff-ng
as well.
.PP
.SH BUGS
.PP
When replaying pcap files, the timing information from the pcap packet
header is currently ignored.
.PP
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.
.PP
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?
.PP
Yes and no. The way VLAN headers are handled in PF_PACKET sockets by the
kernel is somewhat \[lq]problematic\[rq] [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.
.PP
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).
.PP
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 ...
.PP
    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
.PP
    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
.PP
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.
.PP
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.
.PP
   [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
.PP
.SH LEGAL
netsniff-ng is licensed under the GNU GPL version 2.0.
.PP
.SH HISTORY
.B 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>.
.PP
.SH SEE ALSO
.BR trafgen (8),
.BR mausezahn (8),
.BR ifpps (8),
.BR bpfc (8),
.BR flowtop (8),
.BR astraceroute (8),
.BR curvetun (8)
.PP
.SH AUTHOR
Manpage was written by Daniel Borkmann.
.PP
.SH COLOPHON
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/.