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https://github.com/mk-fg/scapy-nflog-capture

Driver for scapy to allow capturing packets via Linux NFLOG interface
https://github.com/mk-fg/scapy-nflog-capture

cffi linux module netfilter networking nflog python scapy security sniffer

Last synced: 3 months ago
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Driver for scapy to allow capturing packets via Linux NFLOG interface

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README 

        
scapy-nflog-capture

--------------------



**Deprecation Warning:**

I haven't used or looked at this module for a while,

and it's obviously long-obsolete python2 code, so unless it's for similar legacy/obsolete uses,

I'd recommend to avoid using this module, or look at one of its more up-to-date forks, if any.



Driver for [scapy network manipulation tool](http://www.secdev.org/projects/scapy/)

to allow capturing packets via

[Linux NFLOG interface](http://wiki.wireshark.org/CaptureSetup/NFLOG).



Installation

--------------------



It's a regular package for Python 2.7 (not 3.X).



Using [pip](http://pip-installer.org/) is the best way:



	% pip install scapy-nflog-capture



If you don't have it, use:



	% easy_install pip

	% pip install scapy-nflog-capture



Alternatively ([see

also](http://www.pip-installer.org/en/latest/installing.html)):



	% curl https://raw.github.com/pypa/pip/master/contrib/get-pip.py | python

	% pip install scapy-nflog-capture



Or, if you absolutely must:



	% easy_install scapy-nflog-capture



But, you really shouldn't do that.



Current-git version can be installed like this:



	% pip install 'git+https://github.com/mk-fg/scapy-nflog-capture.git#egg=scapy-nflog-capture'



### Requirements



* Python 2.7 with ctypes support

* [scapy](http://www.secdev.org/projects/scapy/)

* [CFFI](http://cffi.readthedocs.org) (for libnetfilter_log bindings)

* [libnetfilter_log](http://netfilter.org/projects/libnetfilter_log)



CFFI uses C compiler to generate bindings, so gcc (or other compiler) should be

available if module is being built from source or used from checkout tree.



Usage

--------------------



Two python modules are installed: scapy_nflog and nflog_ctypes.



scapy_nflog has NFLOGListenSocket class (implementing SuperSocket), which can be

installed as default L2 listener like this:



	>>> import scapy_nflog

	>>> scapy_nflog.install_nflog_listener()



install_nflog_listener above is a one-line function, doing `conf.L2listen =

NFLOGListenSocket`, so if you're building custom module on scapy,

NFLOGListenSocket class can just be passed directly to scapy internals a

listening socket without setting it to be default one.



IDs of NFLOG queues to grab packets from can be controlled via passing optional

"queues" keyword on instance init (int or a list of ints) or by overriding

default "queues" class attribute in a subclass and setting that one as listener

class instead.



Note that NFLOG actually returns L3 packets, so despite the listener being

installed as L2 above, it will always return instances of IP class, not Ether or

such.



### Linux NFLOG



NFLOG is a Linux [netfilter](http://www.netfilter.org/) subsystem target,

somewhat like old and simple LOG target, which dumped info for each packet to

kmsg, but using special netlink queues to export netfilter-matched (think

[iptables](http://www.netfilter.org/projects/iptables/index.html) rules) packets

to userspace.



To export all sent/received packets via nflog:



	iptables -t raw -I PREROUTING -j NFLOG

	iptables -t raw -I OUTPUT -j NFLOG



Of course, any arbitrary filters can be added there, to dump only packets

matching specific protocol, port or whatever arbitrary netfilter matcher - see

[iptables manpage](http://ipset.netfilter.org/iptables.man.html) (or

iptables-extensions(8)) for the list/info on the ones shipped with mainline

linux.



Note that it's safe to add the above catch-all rules, as with no listeners

(nothing queries nflog for these packets), they'll just be discarded regardless

of these rules and won't be wasting much ram, cpu or anything like that.



Userspace readers (like this module) can subscribe to receive these packets,

setting how many bytes of these will be buffered in-kernel for later recv()

calls (optional "nlbufsiz" keyword to nflog_generator), the rest will be just

dropped (producing python logging warning by default, unless

`handle_overflows=False` is passed) until userspace catches up.



NFLOG itself is configurable with parameters like --nflog-group and

--nflog-range (see iptables-extensions(8)), allowing to have multiple nflog

queues for different apps and not passing lots of useless L7 data around.



Performance - especially coupled with in-kernel noise filtering and packet

truncation - seem to be more efficient than simpler approaches like using

AF_PACKET/SOCK_RAW sockets, but it's highly unlikely to be any kind of a

bottleneck with scapy sitting on top of it anyway.



One interesting advantage over libpcap is the ability to capture tunneled

packets after decryption (traffic coming from ipsec, pptp, openvpn, ssh, etc) or

transformation (stripping of ipip wrapping, netlink re-injection and such) here.



### nflog_cffi



scapy_nflog is based on nflog_cffi module, which can be used from any python

code (scapy shell included):



```python

from nflog_cffi import NFLOG



# without extra_attrs just packet payload (possibly truncated) is returned

nflog = NFLOG().generator(0, extra_attrs=['len', 'ts'], nlbufsiz=2*2**20)

fd = next(nflog) # netlink fd to do select/poll on, if necessary



# pkt_len is the *real* length, before nflog-truncation (if any)

# pkt_ts is the packet timestamp, as reported by kernel/lib

pkt, pkt_len, pkt_ts = next(nflog)

print('Got packet, len: {}, ts: {}'.format(pkt_len, pkt_ts))



for pkt, pkt_len, pkt_ts in nflog: # do stuff with each captured packet

```



Module uses libnetfilter_log via CFFI.



NFLOG generator has the keywords to control parameters of netlink socket that

are passed to libnetfilter_log, see [libnetfilter_log

documentation](http://www.netfilter.org/projects/libnetfilter_log/doxygen/group__Log.html)

for more verbose description of these.



Not all [libnetfilter_log-exposed

attributes](http://www.netfilter.org/projects/libnetfilter_log/doxygen/group__Parsing.html)

are exposed through bindings.