Package: scapy-nflog-capture3
scapy-nflog-capture3Driver forscapy network manipulation
toolto allow capturing packets viaLinux NFLOG interface.InstallationIt's a regular package for Python 3Usingpipis the best way:% pip install scapy-nflog-capture3If you don't have it, use:% easy_install pip
% pip install scapy-nflog-capture3Alternatively (see
also):% curl https://raw.github.com/pypa/pip/master/contrib/get-pip.py | python
% pip install scapy-nflog-capture3Or, if you absolutely must:% easy_install scapy-nflog-capture3But, you really shouldn't do that.Current-git version can be installed like this:% pip install 'git+https://github.com/thekix/scapy-nflog-capture.git#egg=scapy-nflog-capture'RequirementsPython 3scapyCFFI(for libnetfilter_log bindings)libnetfilter_logCFFI 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.UsageTwo 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()
>>> sniff(prn=handler)install_nflog_listener above is a one-line function, doingconf.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 NFLOGNFLOG is a Linuxnetfiltersubsystem 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 (thinkiptablesrules) packets
to userspace.To export all sent/received packets via nflog:iptables -t raw -I PREROUTING -j NFLOG
iptables -t raw -I OUTPUT -j NFLOGOf course, any arbitrary filters can be added there, to dump only packets
matching specific protocol, port or whatever arbitrary netfilter matcher - seeiptables manpage(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, unlesshandle_overflows=Falseis 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_cffiscapy_nflog is based on nflog_cffi module, which can be used from any python
code (scapy shell included):fromnflog_cffiimportNFLOG# without extra_attrs just packet payload (possibly truncated) is returnednflog=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/libpkt,pkt_len,pkt_ts=next(nflog)print('Got packet, len:{}, ts:{}'.format(pkt_len,pkt_ts))forpkt,pkt_len,pkt_tsinnflog:# do stuff with each captured packetModule uses libnetfilter_log via CFFI.NFLOG generator has the keywords to control parameters of netlink socket that
are passed to libnetfilter_log, seelibnetfilter_log
documentationfor more verbose description of these.Not alllibnetfilter_log-exposed
attributesare exposed through bindings.AcknowledgmentsThis source code is based in the original source code of Mike Kazantsevmk.fraggod@gmail.com.
The original source, for Python2, is athttps://github.com/mk-fg/scapy-nflog-captureand it was deprecated by the author.