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Sniffing network traffic in Python

Sniffing network traffic in Python. Jose Nazario, Ph.D. <jose@monkey.org>. Why Python?. Interpreted language Bound to be slower than C Rapid development Easy data structure use Fewer LoC per tool Easy to manipulate strings http://www.python.org/. Marrying Python and Sniffing.

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Sniffing network traffic in Python

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  1. Sniffing network traffic in Python Jose Nazario, Ph.D. <jose@monkey.org>

  2. Why Python? • Interpreted language • Bound to be slower than C • Rapid development • Easy data structure use • Fewer LoC per tool • Easy to manipulate strings • http://www.python.org/

  3. Marrying Python and Sniffing • Librares in C • Often SWIGged, exported to Python • pcap, dnet, nids … • Modules • pypcap/pcappy – pcap for python • dpkt – packet deconstruction library • libdnet – packet construction library (has python bindings in the distribution) • pynids – connection reassembly tool

  4. libnids – reassemble IP streams NIDS “E” box (event generation box) Userland TCP/IP stack Based on Linux 2.0.36 IP stack Uses libpcap, libnet internally IP fragment reassembly

  5. Userland Kernel IP stack

  6. Userland Kernel IP stack Libnids IP stack

  7. libnids Basics • Initialize • nids_init() • Register callbacks • nids_register_tcp() • nids_regster_ip() • nids_regiser_udp() • Run! • nids_run() • React • nids_kill_tcp()

  8. nids_run() TCP callback UDP callback IP callback TCP stream object: - TCP state - client data - server data - source IP, port - dest IP, port - seq, ack, etc … UDP packet: - source IP, port - dest IP, port - UDP payload IP packet - struct IP packet - contains upper layers

  9. libnids TCP states • NIDS_JUST_ESTABLISHED • New TCP connected state (3WHS) • Must set stream->{client,server}.collect=1 to get stream payload collected • NIDS_DATA • Data within a known, established TCP connection • NIDS_RESET, NIDS_CLOSE, NIDS_TIMED_OUT • TCP connection is reset, closed gracefully, or was lost libnids doesn’t expose SYN_SENT, FIN_WAIT, etc …

  10. pynids Basics • Event driven interface (nids_run(), nids_next()) • TCP stream reassembly • TCP state exposure • Creates a TCP object • Holds addresses, data, etc • UDP and IP packet reassembly

  11. Basic pynids Steps • Initialize • nids_init() • Establish parameters • nids.param(“attribute”, value) • Register callbacks • nids.register_tcp(handleTcp) • def handleTcp(tcp): … • Go! • nids_run() • while 1: nids_next()

  12. pynids Order of Operations • Packets come in • TCP? • State exist? Create state or reuse state • Append data • Process based on state in callback • UDP or IP? • Use handler, pass packet in • You process in callback

  13. Code Example (Python) import nids <handleTcpStream> def main(): nids.param("scan_num_hosts", 0) if not nids.init(): print "error -", nids.errbuf() sys.exit(1) nids.register_tcp(handleTcpStream) try: nids.run() # loop forever except KeyboardInterrupt: sys.exit(1)

  14. Code Example (Python) cont def handleTcpStream(tcp): if tcp.nids_state == nids.NIDS_JUST_EST: if dport in (80, 8000, 8080): tcp.client.collect = 1 tcp.server.collect = 1 elif tcp.nids_state == nids.NIDS_DATA: tcp.discard(0) elif tcp.nids_state in end_states: print "addr:", tcp.addr # may be binary print "To server:“, tcp.server.data print "To client:“, tcp.client.data

  15. Code Example (C) int main(int argv, char *argv[]) { if (nids_init() == 0) err(1, “error, %s”, nids_errbuf); nids_register_tcp(handleTcp); nids_run(); exit(0); }

  16. Code Example (C), cont int handleTcp(struct tcp_stream *tcp) { switch (tcp->nids_state) { case ‘NIDS_JUST_EST’: if ((tcp->addr.dest == 80) || (tcp->addr.dest == 8000) || (tcp->addr.dest == 8080) { tcp.server.collect = 1; tcp.client.collect = 1; } break; case ‘NIDS_DATA’: nids_discard(tcp, 0); break; case ‘NIDS_CLOSE’: case ‘NIDS_RESET’: case ‘NIDS_TIMED_OUT’: printf(“((%s, %d), (%s, %d))\n”, inet_ntoa(tcp->saddr), tcp.srce, inet_ntoa(tcp->daddr), tcp.dest); printf(“%s\n”, tcp->server.data); printf(“%s\n”, tcp->client.data); break; } } About the same LoC, until we start string manipulation

  17. VersionDetect • Small python tool • Reports on headers • Fully passive • Support for: SSH (client, server), WWW (client, server), and SMTP clients • Motivation: coordinate data collection with TCP stack fingerprinting 63.236.16.161 SymbianOS 6048 (on Nokia 7650?) www 80/tcp 63.236.16.161: 80: Microsoft-IIS/6.0

  18. VersionDetect Output 192.168.1.7: 22: SSH-2.0-OpenSSH_3.5 192.168.1.101:http: Mozilla/5.0 (X11; U; OpenBSD i386; en- US; rv:1.5a) Gecko/20031030 Mozilla Firebird/0.6.1 168.75.65.85: 80: Microsoft-IIS/5.0 165.1.76.60: 80: Netscape-Enterprise/3.6 SP2 168.75.65.69: 80: Microsoft-IIS/5.0 168.75.65.87: 80: Microsoft-IIS/5.0 69.28.159.7: 80: ZEDO 3G 198.65.148.234: 80: Apache/1.3.29 (Unix) PHP/4.3.3 216.150.209.231: 80: Apache/1.3.31 (Unix) 212.187.153.30: 80: Apache/1.3.31 (Unix) 212.187.153.37: 80: Apache/1.3.31 (Unix) 212.187.153.32: 80: thttpd/2.25b 29dec2003 64.209.232.207: 80: Apache/1.3.27 (Unix) mod_perl/1.27 216.239.39.99: 80: CAFE/1.0

  19. http-graph • Small, passive python tool • Examines HTTP request header: GET /blog/styles-site.css HTTP/1.1 Host: www.jackcheng.com User-Agent: Mozilla/5.0 (X11; U; OpenBSD i386; en-US; rv:1.5a) Gecko/20031030 Mozilla Firebird/0.6.1Accept: text/css,*/*;q=0.1 Referer: http://www.jackcheng.com/blog/archives/2004/12/ipod_rumors.html

  20. http-graph • Directed graph history of browsing • Reconstructs graph from referrer and URL in the header: Referrer Request • Lets you view your history as you took it • Shows natural “hubs” of information • See also: http://www.uiweb.com.nyud.net:8090/issues/issue37.htm

  21. Displaying http-graph Output • Writes a small “dot” file • “dot” part of “graphviz” tool • Use “neato” to graph • Output formats: SVG, PS, PDF, image map • Can make fully interactive!

  22. Example http-graph Output

  23. Grabbing Data with pynids • tcp.{server, client}.data and just strings • Any string operations will work • Searching if “HTTP/1.0” in tcp.client.data: • Regular Expression searches if re.search(“HTTP/1.[10]”, tcp.client.data): • Rewriting string.replace(req, “GET HTTP/1.0”, “”, 1)

  24. More Fun! • Privacy invasion • Snarf mail • Log conversations • IRC, AIM, etc … • Steal files • FTP, P2P apps, HTTP downloads … • Disrupt sessions tcp.kill() New dsniff is written in Python …

  25. flowgrep • Marries sniffing with regular expressions • A lot like ngrep, tcpkill, and dsniff • Logs the whole connection, not just a packet • Look for data in streams using regular expressions • Log or kill selected streams • Dirt cheap IDS or IPS • Under 400 lines of code

  26. Resources • http://www.tcpdump.org/ • http://www.packetfactory.net/projects/libnids/ • http://monkey.org/~provos/libevent/ • http://monkey.org/~dugsong/{dpkt, pycap} • http://oss.coresecurity.com/projects/pcapy.html • http://monkey.org/~jose/software/flowgrep/ • http://pilcrow.madison.wi.us/pynids/

  27. Additional Resources • Stevens, TCP/IP Illustrated vols 1 and 2 • Schiffman, Building Open Source Network Security Tools • RFCs from the IETF

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