PLUS 내부 세미나 1/22 The Libnet Library 이병영 2004/05/06

PLUS 내부 세미나 2/22 CONTENTS  Introduction of libnet  Building packets (with libnet-1.1)  Four steps to send a packet  Practical use example with libnet  ARP spoofing  TCP RESET attack

PLUS 내부 세미나 3/22 Introduction of Libnet  Libnet is a C library providing a high-level interface to packet injection.  Previous to libnet, programmers had to wrestle with confusing, obscure, and poorly documented interfaces to build and write network packets.

PLUS 내부 세미나 4/22 Introduction of Libnet

PLUS 내부 세미나 5/22 Introduction of Libnet  Libnet is a wonderful utility for writing security- related applications, tools and modules.  Many recent exploits and tools have been rapidly developed using libnet.

PLUS 내부 세미나 6/22 Building packets  Libnet contexts  typedef struct {... /* some declarations */ } libnet_t;

PLUS 내부 세미나 7/22 Building packets  Libnet pblock(packet block)  struct libnet_protocol_block { … /* some declarations */ }; typedef struct libnet_protocol_block libnet_pblock_t;

PLUS 내부 세미나 8/22 Building packets - our example’s plan  Suppose that we want to  build a simple UDP packet  work in link layer level  So we have to build a UDP, IPV4, ethernet header(pblock) in libnet contexts.

PLUS 내부 세미나 9/22

PLUS 내부 세미나 10/22 Building packets (1)Initializing the context  libnet_t* libnet_init ( int injection_type, char * device, char * err_buf)  injection type  Decides your working level  LIBNET_RAW4, LIBNET_LINK  device  Specify the device you will use  err_buf  Buffer to write the error message

PLUS 내부 세미나 11/22 Building packets (2)Building pblock (UDP)  Build a UDP header libnet_ptag_t libnet_build_udp( u_int16_t sp, // The src UDP port u_int16_t dp, // The dst UDP port u_int16_t len, // Length of UDP packet u_int16_t sum, // Checksum, 0 for libnet autofill u_int8_t *payload, // Optional payload u_int32_t payload_s, // Payload size libnet_t *l, // The libnet context pointer libnet_ptag_t ptag // Protocol tag );

PLUS 내부 세미나 12/22 Building packets (2)Building pblock (IPV4)  Build a IPV4 header libnet_ptag_t libnet_build_ipv4( u_int16_t len,// Length of IPV4 packet u_int8_t tos,// Type of service bits u_int16_t id, // IP identification u_int16_t frag, // Fragmentation bits u_int8_t ttl, // Time to live u_int8_t prot, // Upper layer protocol u_int16_t sum, // Checksum, 0 for libnet autofill u_int32_t src, // Src IP address u_int32_t dst, // Dst IP address u_int8_t *payload,// Optional payload u_int32_t payload_s,// Payload size libnet_t *l, // The libnet context pointer libnet_ptag_t ptag);// Protocol tag

PLUS 내부 세미나 13/22 Building packets (2)Building pblock (Ethernet)  Bulid a ethernet header libnet_ptag_t libnet_build_ethernet( u_int8_t *dst, // Dst ethernet address u_int8_t *src,// Src ethernet address u_int16_t type, // Upper layer type u_int8_t *payload, // Optional payload u_int32_t payload_s, // Payload size libnet_t *l,// The libnet context pointer libnet_ptag_t ptag);// Protocal tag

PLUS 내부 세미나 14/22 Building packets (3)Write the packet to wire  After building pblock, just call the function libnet_write with argument of libnet_t pointer.  int libnet_write ( libnet_t * )  Returns the amount of bytes written to the wire.

PLUS 내부 세미나 15/22 Building packets (4)Clean up a libnet context  void libnet_destroy ( libnet_t * )  This function frees memeory of libnet context.

PLUS 내부 세미나 16/22

PLUS 내부 세미나 17/22 Building packets - confirm with tcpdump

PLUS 내부 세미나 18/22 Practical use – (1) ARP spoofing

PLUS 내부 세미나 19/22 Practical use – (2) TCP RESET attack  In a traditional sequence number attack, the exact sequence number considered valid and accepted by the receiving TCP endpoint.  The utilization of the TCP window size to reduce the number of sequence numbers that must be guessed.  More details are in document “Slippling in the window : TCP Reset Attacks”.

PLUS 내부 세미나 20/22 Practical use – (2) TCP RESET attack

PLUS 내부 세미나 21/22 Practical use – (2) TCP RESET attack

PLUS 내부 세미나 22/22 References  1. Libnet reference manual   2. Building packets for dummies and others with libnet   3. TCP/IP Illustrated, Volume1 by Stevens  4. Slippling in the window : TCP Reset Attacks 