实验一：网络嗅探器
一：实验背景
为了深入了解和掌握TCP/IP协议栈以及数据包的格式，练习使用libpcap对网络数据包进行解析。
libpcap提供的接口函数主要实现和封装了与数据包截获有关的过程。利用libpcap的C函数库的接口，网络安全工具开发人员可以很方便地编写出具有结构化强、健壮性好、可移植性高等特点的程序。因此，这些函数库在网络安全工具的开发中具有很大的价值，在scanner、sniffer、firewall、IDS等领域都获得了极其广泛的应用，著名的tcpdump软件、ethereal软件等就是在libpcap的基础上开发的。
二：实验步骤
实验步骤：
结合上传的代码，学习和理解Libpcap中主要API。
1. char *pcap_lookupdev(char *errbuf)
该函数用于返回可被pcap_open_live()或pcap_lookupnet()函数调用的网络设备名（一个字符串指针）。如果函数出错，则返回NULL，同时errbuf中存放相关的错误消息。
2. int pcap_lookupnet(char *device, bpf_u_int32 *netp,bpf_u_int32 *maskp, char *errbuf)
获得指定网络设备的网络号和掩码。netp参数和maskp参数都是bpf_u_int32指针。如果函数出错，则返回-1，同时errbuf中存放相关的错误消息。
3. 打开设备
pcap_t *pcap_open_live(char *device, int snaplen,int promisc, int to_ms,char *ebuf)
获得用于捕获网络数据包的数据包捕获描述字。device参数为指定打开的网络设备名。snaplen参数定义捕获数据的最大字节数。promisc指定是否将网络接口置于混杂模式。to_ms参数指定超时时间（毫秒）。ebuf参数则仅在pcap_open_live()函数出错返回NULL时用于传递错误消息。
4. 编译和设置过滤器
int pcap_compile(pcap_t *p, struct bpf_program *fp,
char *str, int optimize, bpf_u_int32 netmask)
将str参数指定的字符串编译到过滤程序中。fp是一个bpf_program结构的指针，在pcap_compile()函数中被赋值。optimize参数控制结果代码的优化。netmask参数指定本地网络的网络掩码。
int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
指定一个过滤程序。fp参数是bpf_program结构指针，通常取自pcap_compile()函数调用。出错时返回-1；成功时返回0。抓取下一个数据包
5. 抓取数据包
int pcap_dispatch(pcap_t *p, int cnt,pcap_handler callback, u_char *user)
捕获并处理数据包。cnt参数指定函数返回前所处理数据包的最大值。cnt=-1表示在一个缓冲区中处理所有的数据包。cnt=0表示处理所有数据包，直到产生以下错误之一：读取到EOF；超时读取。callback参数指定一个带有三个参数的回调函数，这三个参数为：一个从pcap_dispatch()函数传递过来的u_char指针，一个pcap_pkthdr结构的指针，和一个数据包大小的u_char指针。如果成功则返回读取到的字节数。读取到EOF时则返回零值。出错时则返回-1，此时可调用pcap_perror()或pcap_geterr()函数获取错误消息。
回调函数的第一个参数对应pcap_loop中的最后一个参数，不管给pcap_loop的最后一个参数传递什么值，当回调函数被调用时，它都会作为第一个参数传递给回调函数，第二个参数是pcap头，它包含一些信息：抓包时间，包多大，等等，这个结构在pcap.h中定义，如下
struct pcap_pkthdr {
struct timeval ts; /* time stamp */
bpf_u_int32 caplen; /* length of portion present */
bpf_u_int32 len; /* length this packet (off wire) */
};
第三个参数则是数据内容，主要是对其进行分析。
int pcap_loop(pcap_t *p, int cnt, pcap_handler callback, u_char *user)
功能基本与pcap_dispatch()函数相同，只不过此函数在cnt个数据包被处理或出现错误时才返回，但读取超时不会返回。而如果为pcap_open_live()函数指定了一个非零值的超时设置，然后调用pcap_dispatch()函数，则当超时发生时pcap_dispatch()函数会返回。cnt参数为负值时pcap_loop()函数将始终循环运行，除非出现错误。
u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
返回指向下一个数据包的u_char指针。
6. void pcap_close(pcap_t *p)
关闭p参数相应的文件，并释放资源。
正确使用以上接口，能较为容易地获取经过网卡的数据包。值得注意的是，运行程序时需要使用超级权限；以及代码编译时需要使用-lpcap选项，指明链接额外的函数库。
struct ip * ip = (struct ip *)(packet + ETHER_SIZE);
7.packet为回调函数中的第三个参数，是指向从以太层开始的数据包头的u_char类型的指针；通过将指针后移ETHER_SIZE字节，可以找到IP头部。通过对之后的数据进行强制类型转换(struct ip*)，可以将接下来的20个字节转换为具有格式的结构体。ip头部的具体内容可以参见/usr/include/netinet中的头文件ip.h中的内容。
8.类似地，struct tcphdr *tcp = (struct tcphdr *)(packet + 14 + ip_hl);可以获得数据包中tcp的包头部。
代码如下
#define APP_NAME "sniffex" #define APP_DESC "Sniffer example using libpcap" #define APP_COPYRIGHT "Copyright (c) 2005 The Tcpdump Group" #define APP_DISCLAIMER "THERE IS ABSOLUTELY NO WARRANTY FOR THIS PROGRAM." #include <pcap.h> #include <stdio.h> #include <string.h> #include <stdlib.h> #include <ctype.h> #include <errno.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> /* default snap length (maximum bytes per packet to capture) */ #define SNAP_LEN 1518 /* ethernet headers are always exactly 14 bytes [1] */ #define SIZE_ETHERNET 14 /* Ethernet addresses are 6 bytes */ #define ETHER_ADDR_LEN 6 /* Ethernet header */ struct sniff_ethernet { u_char ether_dhost[ETHER_ADDR_LEN]; /* destination host address */ u_char ether_shost[ETHER_ADDR_LEN]; /* source host address */ u_short ether_type; /* IP? ARP? RARP? etc */ }; /* IP header */ struct sniff_ip { u_char ip_vhl; /* version << 4 | header length >> 2 */ u_char ip_tos; /* type of service */ u_short ip_len; /* total length */ u_short ip_id; /* identification */ u_short ip_off; /* fragment offset field */ #define IP_RF 0x8000 /* reserved fragment flag */ #define IP_DF 0x4000 /* dont fragment flag */ #define IP_MF 0x2000 /* more fragments flag */ #define IP_OFFMASK 0x1fff /* mask for fragmenting bits */ u_char ip_ttl; /* time to live */ u_char ip_p; /* protocol */ u_short ip_sum; /* checksum */ struct in_addr ip_src,ip_dst; /* source and dest address */ }; #define IP_HL(ip) (((ip)->ip_vhl) & 0x0f) #define IP_V(ip) (((ip)->ip_vhl) >> 4) /* TCP header */ typedef u_int tcp_seq; struct sniff_tcp { u_short th_sport; /* source port */ u_short th_dport; /* destination port */ tcp_seq th_seq; /* sequence number */ tcp_seq th_ack; /* acknowledgement number */ u_char th_offx2; /* data offset, rsvd */ #define TH_OFF(th) (((th)->th_offx2 & 0xf0) >> 4) u_char th_flags; #define TH_FIN 0x01 #define TH_SYN 0x02 #define TH_RST 0x04 #define TH_PUSH 0x08 #define TH_ACK 0x10 #define TH_URG 0x20 #define TH_ECE 0x40 #define TH_CWR 0x80 #define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR) u_short th_win; /* window */ u_short th_sum; /* checksum */ u_short th_urp; /* urgent pointer */ }; void got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet); void print_payload(const u_char *payload, int len); void print_hex_ascii_line(const u_char *payload, int len, int offset); void print_app_banner(void); void print_app_usage(void); /* * app name/banner */ void print_app_banner(void) { printf("%s - %s\n", APP_NAME, APP_DESC); printf("%s\n", APP_COPYRIGHT); printf("%s\n", APP_DISCLAIMER); printf("\n"); return; } /* * print help text */ void print_app_usage(void) { printf("Usage: %s [interface]\n", APP_NAME); printf("\n"); printf("Options:\n"); printf(" interface Listen on <interface> for packets.\n"); printf("\n"); return; } /* * print data in rows of 16 bytes: offset hex ascii * * 00000 47 45 54 20 2f 20 48 54 54 50 2f 31 2e 31 0d 0a GET / HTTP/1.1.. */ void print_hex_ascii_line(const u_char *payload, int len, int offset) { int i; int gap; const u_char *ch; /* offset */ printf("%05d ", offset); /* hex */ ch = payload; for(i = 0; i < len; i++) { printf("%02x ", *ch); ch++; /* print extra space after 8th byte for visual aid */ if (i == 7) printf(" "); } /* print space to handle line less than 8 bytes */ if (len < 8) printf(" "); /* fill hex gap with spaces if not full line */ if (len < 16) { gap = 16 - len; for (i = 0; i < gap; i++) { printf(" "); } } printf(" "); /* ascii (if printable) */ ch = payload; for(i = 0; i < len; i++) { if (isprint(*ch)) printf("%c", *ch); else printf("."); ch++; } printf("\n"); return; } /* * print packet payload data (avoid printing binary data) */ void print_payload(const u_char *payload, int len) { int len_rem = len; int line_width = 16; /* number of bytes per line */ int line_len; int offset = 0; /* zero-based offset counter */ const u_char *ch = payload; if (len <= 0) return; /* data fits on one line */ if (len <= line_width) { print_hex_ascii_line(ch, len, offset); return; } /* data spans multiple lines */ for ( ;; ) { /* compute current line length */ line_len = line_width % len_rem; /* print line */ print_hex_ascii_line(ch, line_len, offset); /* compute total remaining */ len_rem = len_rem - line_len; /* shift pointer to remaining bytes to print */ ch = ch + line_len; /* add offset */ offset = offset + line_width; /* check if we have line width chars or less */ if (len_rem <= line_width) { /* print last line and get out */ print_hex_ascii_line(ch, len_rem, offset); break; } } return; } /* * dissect/print packet */ void got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet) { static int count = 1; /* packet counter */ /* declare pointers to packet headers */ const struct sniff_ethernet *ethernet; /* The ethernet header [1] */ const struct sniff_ip *ip; /* The IP header */ const struct sniff_tcp *tcp; /* The TCP header */ const char *payload; /* Packet payload */ int size_ip; int size_tcp; int size_payload; printf("\nPacket number %d:\n", count); count++; /* define ethernet header */ ethernet = (struct sniff_ethernet*)(packet); /* define/compute ip header offset */ ip = (struct sniff_ip*)(packet + SIZE_ETHERNET); size_ip = IP_HL(ip)*4; if (size_ip < 20) { printf(" * Invalid IP header length: %u bytes\n", size_ip); return; } /* print source and destination IP addresses */ printf(" From: %s\n", inet_ntoa(ip->ip_src)); printf(" To: %s\n", inet_ntoa(ip->ip_dst)); /* determine protocol */ switch(ip->ip_p) { case IPPROTO_TCP: printf(" Protocol: TCP\n"); break; case IPPROTO_UDP: printf(" Protocol: UDP\n"); return; case IPPROTO_ICMP: printf(" Protocol: ICMP\n"); return; case IPPROTO_IP: printf(" Protocol: IP\n"); return; default: printf(" Protocol: unknown\n"); return; } /* * OK, this packet is TCP. */ /* define/compute tcp header offset */ tcp = (struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip); size_tcp = TH_OFF(tcp)*4; if (size_tcp < 20) { printf(" * Invalid TCP header length: %u bytes\n", size_tcp); return; } printf(" Src port: %d\n", ntohs(tcp->th_sport)); printf(" Dst port: %d\n", ntohs(tcp->th_dport)); /* define/compute tcp payload (segment) offset */ payload = (u_char *)(packet + SIZE_ETHERNET + size_ip + size_tcp); /* compute tcp payload (segment) size */ size_payload = ntohs(ip->ip_len) - (size_ip + size_tcp); /* * Print payload data; it might be binary, so don't just * treat it as a string. */ if (size_payload > 0) { printf(" Payload (%d bytes):\n", size_payload); print_payload(payload, size_payload); } return; } int main(int argc, char **argv) { char *dev = NULL; /* capture device name */ char errbuf[PCAP_ERRBUF_SIZE]; /* error buffer */ pcap_t *handle; /* packet capture handle */ char filter_exp[] = "ip"; /* filter expression [3] */ struct bpf_program fp; /* compiled filter program (expression) */ bpf_u_int32 mask; /* subnet mask */ bpf_u_int32 net; /* ip */ int num_packets = 100; /* number of packets to capture */ print_app_banner(); /* check for capture device name on command-line */ if (argc == 2) { dev = argv[1]; } else if (argc > 2) { fprintf(stderr, "error: unrecognized command-line options\n\n"); print_app_usage(); exit(EXIT_FAILURE); } else { /* find a capture device if not specified on command-line */ dev = pcap_lookupdev(errbuf); if (dev == NULL) { fprintf(stderr, "Couldn't find default device: %s\n", errbuf); exit(EXIT_FAILURE); } } /* get network number and mask associated with capture device */ if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) { fprintf(stderr, "Couldn't get netmask for device %s: %s\n", dev, errbuf); net = 0; mask = 0; } /* print capture info */ printf("Device: %s\n", dev); printf("Number of packets: %d\n", num_packets); printf("Filter expression: %s\n", filter_exp); /* open capture device */ handle = pcap_open_live(dev, SNAP_LEN, 1, 1000, errbuf); if (handle == NULL) { fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf); exit(EXIT_FAILURE); } /* make sure we're capturing on an Ethernet device [2] */ if (pcap_datalink(handle) != DLT_EN10MB) { fprintf(stderr, "%s is not an Ethernet\n", dev); exit(EXIT_FAILURE); } /* compile the filter expression */ if (pcap_compile(handle, &fp, filter_exp, 0, net) == -1) { fprintf(stderr, "Couldn't parse filter %s: %s\n", filter_exp, pcap_geterr(handle)); exit(EXIT_FAILURE); } /* apply the compiled filter */ if (pcap_setfilter(handle, &fp) == -1) { fprintf(stderr, "Couldn't install filter %s: %s\n", filter_exp, pcap_geterr(handle)); exit(EXIT_FAILURE); } /* now we can set our callback function */ pcap_loop(handle, num_packets, got_packet, NULL); /* cleanup */ pcap_freecode(&fp); pcap_close(handle); printf("\nCapture complete.\n"); return 0; }
