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libpcap官方入门教程 [复制链接]

sulilo

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发表于 2012-03-31 09:22 |只看该作者 |倒序浏览

今天学习了下抓包程序，使用libpcap库，官方这代码做为教程非常不错，程序从抓包到协议分析，短短五百行代码，相于当一个精小的tcpdump工具。做为入门教程可别错过了。

小提示:
编译的时候加上 -lpcap
运行的时候记得使用root,

/*
* sniffex.c
*
* Sniffer example of TCP/IP packet capture using libpcap.
*
* Version 0.1.1 (2005-07-05)
* Copyright (c) 2005 The Tcpdump Group
*
* This software is intended to be used as a practical example and
* demonstration of the libpcap library; available at:
* http://www.tcpdump.org/
*
****************************************************************************
*
* ..... 字数太长了，此处版权信息就省了
*
****************************************************************************
*
* Example compiler command-line for GCC:
* gcc -Wall -o sniffex sniffex.c -lpcap
*
****************************************************************************
*
* Code Comments
*
* This section contains additional information and explanations regarding
* comments in the source code. It serves as documentaion and rationale
* for why the code is written as it is without hindering readability, as it
* might if it were placed along with the actual code inline. References in
* the code appear as footnote notation (e.g. [1]).
*
* 1. Ethernet headers are always exactly 14 bytes, so we define this
* explicitly with "#define". Since some compilers might pad structures to a
* multiple of 4 bytes - some versions of GCC for ARM may do this -
* "sizeof (struct sniff_ethernet)" isn't used.
*
* 2. Check the link-layer type of the device that's being opened to make
* sure it's Ethernet, since that's all we handle in this example. Other
* link-layer types may have different length headers (see [1]).
*
* 3. This is the filter expression that tells libpcap which packets we're
* interested in (i.e. which packets to capture). Since this source example
* focuses on IP and TCP, we use the expression "ip", so we know we'll only
* encounter IP packets. The capture filter syntax, along with some
* examples, is documented in the tcpdump man page under "expression."
* Below are a few simple examples:
*
* Expression Description
* ---------- -----------
* ip Capture all IP packets.
* tcp Capture only TCP packets.
* tcp port 80 Capture only TCP packets with a port equal to 80.
* ip host 10.1.2.3 Capture all IP packets to or from host 10.1.2.3.
*
****************************************************************************
*
*/
#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>
/* 默认捕获长度 (每个包捕获的最大长度)
default snap length (maximum bytes per packet to capture) */
#define SNAP_LEN 1518
/* 以太网头部14个字节
ethernet headers are always exactly 14 bytes [1] */
#define SIZE_ETHERNET 14
/* 以太网地址6个字节
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;
}
/*
* 打印包的有效载荷数据（避免打印二进制数据）
* 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; /* 每行的字节数 | number of bytes per line */
int line_len;
int offset = 0; /* 从0开始的偏移计数器 | 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;
}
/* 数据跨越多行 data spans multiple lines */
for ( ;; ) {
/* 计算当前行的长度 | compute current line length */
line_len = line_width % len_rem;
/* 显示分割线 | print line */
print_hex_ascii_line(ch, line_len, offset);
/* 计算总剩余 | compute total remaining */
len_rem = len_rem - line_len;
/* 转移到打印的剩余字节的指针
shift pointer to remaining bytes to print */
ch = ch + line_len;
/* 添加偏移 | add offset */
offset = offset + line_width;
/* 检查是否有线宽字符或更少
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;
}
/*
* 解析/显示包
* dissect/print packet
*/
void
got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet)
{
static int count = 1; /* 包计数器 | packet counter */
/* declare pointers to packet headers */
const struct sniff_ethernet *ethernet; /* 以太网头部 | The ethernet header [1] */
const struct sniff_ip *ip; /* IP 头部 | The IP header */
const struct sniff_tcp *tcp; /* TCP 头部 | The TCP header */
const char *payload; /* Packet payload */
int size_ip;
int size_tcp;
int size_payload;
/* 显示包总数 */
printf("\nPacket number %d:\n", count);
count++;
/* 定义以太网头部
define ethernet header */
ethernet = (struct sniff_ethernet*)(packet);
/* 定义/计算 IP 头部偏移
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;
}
/* 显示源IP和目的IP
print source and destination IP addresses */
printf(" From: %s\n", inet_ntoa(ip->ip_src));
printf(" To: %s\n", inet_ntoa(ip->ip_dst));
/* 确定协议
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;
}
/*
* 好，这个包是TCP.
* OK, this packet is TCP.
*/
/* 定义/计算 TCP 头部偏移
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));
/* 定义/计算TCP有效载荷（段）偏移
define/compute tcp payload (segment) offset */
payload = (u_char *)(packet + SIZE_ETHERNET + size_ip + size_tcp);
/* 计算TCP有效载荷（段）的大小
compute tcp payload (segment) size */
size_payload = ntohs(ip->ip_len) - (size_ip + size_tcp);
/*
* 打印有效载荷数据，它可能是二进制的，所以不要只把它作为一个字符串。
* 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; /* 捕获设备的名称 | capture device name */
char errbuf[PCAP_ERRBUF_SIZE]; /* 错误的缓冲区 | error buffer */
pcap_t *handle; /* 数据包捕获句柄 | packet capture handle */
char filter_exp[] = "ip"; /* 过滤表达示 | filter expression [3] */
struct bpf_program fp; /* 编译过滤表达示 | compiled filter program (expression) */
bpf_u_int32 mask; /* 子网掩码 | subnet mask */
bpf_u_int32 net; /* IP 地址 | ip */
int num_packets = 10; /* 捕获的数据包数量 | number of packets to capture */
/* 显示程序版本信息 */
print_app_banner();
/* 检查来自命令行参数需要捕获设备的名称
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 {
/* 如果命令行参数没有指定, 则自动找到一个设备
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);
}
}
/* 获得捕获设备的网络号和掩码
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;
}
/* 显示捕获设备信息
print capture info */
printf("Device: %s\n", dev);
printf("Number of packets: %d\n", num_packets);
printf("Filter expression: %s\n", filter_exp);
/* 打开捕获设备
@1 捕获的设备
@2 每次捕获数据的最大长度
@3 1 启用混杂模式
@4 捕获时间, 单位ms
@5 错误缓冲区
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);
}
/* pcap_datalink();
返回数据链路层类型，例如DLT_EN10MB;
确保我们对以太网设备捕获
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);
}
/* 编译过滤表达式
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);
}
/* 应用过滤规则
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);
}
/* 设置回高函数并开始捕获包
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;
}