首页 > 代码库 > 构建tcpdump/wireshark pcap文件
构建tcpdump/wireshark pcap文件
pcap文件格式是bpf保存原始数据包的格式,很多软件都在使用,比如tcpdump、wireshark等等,了解pcap格式可以加深对原始数据包的了解,自己也可以手工构造任意的数据包进行测试。
pcap文件的格式为:
文件头 24字节
数据包头 + 数据包 数据包头为16字节,后面紧跟数据包
数据包头 + 数据包 ……
pcap.h里定义了文件头的格式
- struct pcap_file_header {
- bpf_u_int32 magic;
- u_short version_major;
- u_short version_minor;
- bpf_int32 thiszone; /* gmt to local correction */
- bpf_u_int32 sigfigs; /* accuracy of timestamps */
- bpf_u_int32 snaplen; /* max length saved portion of each pkt */
- bpf_u_int32 linktype; /* data link type (LINKTYPE_*) */
- };
看一下各字段的含义:
magic: 4字节 pcap文件标识 目前为“d4 c3 b2 a1”
major: 2字节 主版本号 #define PCAP_VERSION_MAJOR 2
minor: 2字节 次版本号 #define PCAP_VERSION_MINOR 4
thiszone:4字节 时区修正 并未使用,目前全为0
sigfigs: 4字节 精确时间戳 并未使用,目前全为0
snaplen: 4字节 抓包最大长度 如果要抓全,设为0x0000ffff(65535),
tcpdump -s 0就是设置这个参数,缺省为68字节
linktype:4字节 链路类型 一般都是1:ethernet
常用链路类型:
0 BSD loopback devices, except for later OpenBSD
1 Ethernet, and Linux loopback devices
6 802.5 Token Ring
7 ARCnet
8 SLIP
9 PPP
10 FDDI
100 LLC/SNAP-encapsulated ATM
101 "raw IP", with no link
102 BSD/OS SLIP
103 BSD/OS PPP
104 Cisco HDLC
105 802.11
108 later OpenBSD loopback devices (with the AF_value in network byte order)
113 special Linux "cooked" capture
114 LocalTalk
=======================================================================================
| magic |major | minor | thiszone | sigfigs | snaplen | linktype
| d4 c3 b2 a1 | 02 00 | 04 00 | 00 00 00 00 | 00 00 00 00 | ff ff 00 00 | 01 00 00 00
=======================================================================================
数据包头的格式
- 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) */
- };
- struct timeval {
- long tv_sec; /* seconds (XXX should be time_t) */
- suseconds_t tv_usec; /* and microseconds */
- };
ts: 8字节 抓包时间 4字节表示秒数,4字节表示微秒数
caplen:4字节 保存下来的包长度(最多是snaplen,比如68字节)
len: 4字节 数据包的真实长度,如果文件中保存的不是完整数据包,可能比caplen大
了解了pcap文件格式,就可以自己手工构造任意数据包了,可以以录好的包为基础,
构建pcap文件示例:
含有两套构建方法, 因起先不知道可以通过pcap_open_dead & pcap_dump_open 构建文件头, 所以开始是自己创建一个文件并写入文件头.
- #include <stdio.h>
- #include <stdlib.h>
- #include <string.h>
- #include <unistd.h>
- #include <stdint.h>
- #include <errno.h>
- #include <pcap.h>
- #include "common.h"
- #define TCPDUMP_MAGIC 0xa1b2c3d4
- #ifndef PCAP_VERSION_MAJOR
- #define PCAP_VERSION_MAJOR 2
- #define
- #define PCAP_VERSION_MINOR
- #define PCAP_VERSION_MINOR 4
- #endif
- #define LINKTYPE_NULL DLT_NULL
- #define LINKTYPE_ETHERNET DLT_EN10MB /* also for 100Mb and up */
- #define LINKTYPE_EXP_ETHERNET DLT_EN3MB /* 3Mb experimental Ethernet */
- #define LINKTYPE_AX25 DLT_AX25
- #define LINKTYPE_PRONET DLT_PRONET
- #define LINKTYPE_CHAOS DLT_CHAOS
- #define LINKTYPE_TOKEN_RING DLT_IEEE802 /* DLT_IEEE802 is used for Token Ring */
- #define LINKTYPE_ARCNET DLT_ARCNET /* BSD-style headers */
- #define LINKTYPE_SLIP DLT_SLIP
- #define LINKTYPE_PPP DLT_PPP
- #define LINKTYPE_FDDI DLT_FDDI
- static int
- pcap_write_header(FILE *fp, int linktype, int thiszone, int snaplen)
- {
- struct pcap_file_header hdr;
- hdr.magic = TCPDUMP_MAGIC;
- hdr.version_major = PCAP_VERSION_MAJOR;
- hdr.version_minor = PCAP_VERSION_MINOR;
- hdr.thiszone = thiszone;
- hdr.snaplen = snaplen;
- hdr.sigfigs = 0;
- hdr.linktype = linktype;
- if (fwrite((char *)&hdr, sizeof(hdr), 1, fp) != 1)
- return (-1);
- return (0);
- }
- #define FILE_SAVE "pcap_write.pcap"
- uint8_t l2_data[] = {
- 0x00, 0x0c, 0x29, 0x99, 0xfc, 0xa6, 0x00, 0x0c, 0x29, 0xd7, 0xc1, 0xf2, 0x08, 0x00, 0x45, 0x00,
- 0x00, 0x46, 0x87, 0x8a, 0x00, 0x00, 0x40, 0x11, 0x6e, 0xa5, 0xc0, 0xa8, 0x01, 0x31, 0xc0, 0xa8,
- 0x01, 0xf6, 0x7e, 0x75, 0x00, 0x35, 0x00, 0x32, 0x89, 0x42, 0x0a, 0x5d, 0x00, 0x00, 0x00, 0x01,
- 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x03, 0x6e, 0x73, 0x31, 0x05, 0x67, 0x75, 0x61, 0x72, 0x64,
- 0x03, 0x63, 0x6f, 0x6d, 0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x29, 0x10, 0x00, 0x00, 0x00,
- 0x80, 0x00, 0x00, 0x00
- };
- int main(int argc, char **argv)
- {
- #if 0
- FILE *fp = NULL;
- struct pcap_pkthdr h;
- fp = fopen(FILE_SAVE, "wb");
- if (!fp){
- fprintf(stderr, "fopen %s for write failed. errno=%d desc=%s\n",
- FILE_SAVE, errno, strerror(errno));
- return 1;
- }
- pcap_write_header(fp, LINKTYPE_ETHERNET, 0x0, 0x0000ffff);
- gettimeofday(&h.ts, NULL);
- h.caplen = sizeof(l2_data);
- h.len = sizeof(l2_data);
- pcap_dump((uint8_t *)fp, &h, l2_data);
- fflush(fp);
- fclose(fp);
- #else
- pcap_t *p = NULL;
- pcap_dumper_t *fp = NULL;
- struct pcap_pkthdr h;
- p = pcap_open_dead(LINKTYPE_ETHERNET, 0x0000ffff);
- if (NULL == p){
- fprintf(stderr, "pcap_open_dead failed.\n");
- return 1;
- }
- fp = pcap_dump_open(p, FILE_SAVE);
- if (NULL == fp){
- fprintf(stderr, "pcap_dump_open failed.\n");
- return 1;
- }
- gettimeofday(&h.ts, NULL);
- h.caplen = sizeof(l2_data);
- h.len = sizeof(l2_data);
- pcap_dump((uint8_t *)fp, &h, l2_data);
- pcap_dump_close(fp);
- #endif
- return 0;
- }
编译 & 链接
# gcc pcap_write.c -o pcap_write -lpcap
构建tcpdump/wireshark pcap文件