自己动手学TCP/IP–http协议(http报文头)

在前面的一篇文章中，简单了介绍了HTTP报文格式，详情参考http://www.firefoxbug.net/?cat=47。

这里大概介绍下基本的，常见的HTTP包头格式。

POST /report/getComment.jsp HTTP/1.1
Host: yeeg.com
Connection: keep-alive
Content-Length: 161
Origin: http://www.1g1g.com
User-Agent: Mozilla/5.0 (X11; Linux i686) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.77 Safari/535.7
content-type: application/x-www-form-urlencoded
Accept: */*
Referer: http://www.1g1g.com/player/loader.swf?uid=0.8106261373031884
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
Cookie: JSESSIONID=C3F105F72E3602D6292D3E4561E8E400

上面是一个POST包的包头，其中Content-Length字段里面的值就是POST包数据段的长度。可以用

wireshark抓取POST包，会发现，post包是把报文头和数据内容分开来发的，会被TCP分片，然后重组。

具体这里不详细讨论。

GET /enclosure/2010-09-10T02_51_05-07_00.mp3 HTTP/1.1
Host: 805665086.podomatic.com
Connection: keep-alive
User-Agent: Mozilla/5.0 (X11; Linux i686) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.77 Safari/535.7
Accept: */*
Referer: http://www.1g1g.com/player/loader.swf?uid=0.8106261373031884
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

上面是一个GET包，GET包请求的资源都是在URL里面的，所以数据段也就没有了，可以通过抓包分析。

HTTP/1.1 200 OK
Date: Tue, 10 Jul 2012 09:12:52 GMT
Server: Apache/2.2.14 (Ubuntu)
Last-Modified: Thu, 23 Dec 2010 19:29:26 GMT
ETag: "960fcf-4a6459-49818e3486374"
Accept-Ranges: bytes
Content-Length: 487
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: audio/mpeg

上面是一个http响应包，Content-Length指明了数据段的大小。

下面是我今天用C写了解析HTTP报文头的程序。注意：下面代码只主要用libcap实现实现了部分功能，具体

是解析GET包头，POST包头，还有HTTP相应包头，可能存在一些不足，希望大家多多交流。

/*
    capture http packet by firefoxbug
*/
#include <pcap.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <net/ethernet.h>
#include <netinet/ip_icmp.h>  //Provides declarations for icmp header
#include <netinet/udp.h>      //Provides declarations for udp header
#include <netinet/tcp.h>      //Provides declarations for tcp header
#include <netinet/ip.h>       //Provides declarations for ip header

#define BUFFSIZE 1500

typedef struct port{
    unsigned int src_port;
    unsigned int des_port;
}Port;

Port port;
int tcp=0;
FILE *logfile;
int size;

void process_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *buffer);
char *print_tcp_packet(const char *Buffer);
void get_line(char *data,char *buff,int length);
void print_http_req_packet(char *data);
void print_http_ans_packet(char *data);

int main(int argc,char *argv[])
{
    pcap_if_t *alldevsp , *device;
    pcap_t *handle; //Handle of the device that shall be sniffed

    char errbuf[100] , *devname , devs[100][100];
    int count = 1 , n;

    //First get the list of available devices
    printf("Finding available devices ... ");
    if( pcap_findalldevs( &alldevsp , errbuf) )
    {
        printf("Error finding devices : %s" , errbuf);
        exit(1);
    }
    printf("Done");

    //Print the available devices
    printf("
Available Devices are :
");
    for(device = alldevsp ; device != NULL ; device = device->next)
    {
        printf("%d. %s - %s
" , count , device->name , device->description);
        if(device->name != NULL)
        {
            strcpy(devs[count] , device->name);
        }
        count++;
    }

    //Ask user which device to sniff
    printf("Enter the number of the device you want to sniff : ");
    scanf("%d" , &n);
    devname = devs[n];

    //Open the device for sniffing
    printf("Opening device %s for sniffing ... " , devname);
    handle = pcap_open_live(devname , 65536 , 1 , 0 , errbuf);

    if (handle == NULL)
    {
        fprintf(stderr, "Couldn't open device %s : %s
" , devname , errbuf);
        exit(1);
    }
    printf("Done
");

    logfile=fopen("log.txt","w");
    if(logfile==NULL)
    {
        printf("Unable to create file.");
    }

    //Put the device in sniff loop
    pcap_loop(handle , -1 , process_packet , NULL);

    return 0;
}

void process_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *buffer)
{
    size = header->len;
//  fprintf(logfile,"length of packet : %d 
",size);
    //Get the IP Header part of this packet , excluding the ethernet header
    struct iphdr *iph = (struct iphdr*)(buffer + sizeof(struct ethhdr));
    switch (iph->protocol) //Check the Protocol and do accordingly...
    {

        case 6:  //TCP Protocol
            ++tcp;
        //  printf("TCP : %d 
", tcp);
            unsigned char *data = print_tcp_packet(buffer);
            if (size <= 0)
                break;
            if (port.des_port == 80)
            {
                print_http_req_packet(data);
            }
            else if ( port.src_port == 80 )
            {
                print_http_ans_packet(data);
            }
            break;
    }

}

char *print_tcp_packet(const char *Buffer)
{
    //IP header
    struct iphdr *iph = (struct iphdr *)( Buffer  + sizeof(struct ethhdr) );
    unsigned int iphdrlen = iph->ihl*4;
    //TCP header
    struct tcphdr *tcph=(struct tcphdr*)(Buffer + iphdrlen + sizeof(struct ethhdr));

    port.src_port = ntohs(tcph->source);
    port.des_port = ntohs(tcph->dest);

    // mac_header + ip_header + tcp_header
    int header_size =  sizeof(struct ethhdr) + iphdrlen + tcph->doff*4;
    size = size - header_size;
//  fprintf(logfile,"length of header : %d 
",header_size );
    return (char *)(Buffer + header_size);
}

void print_http_req_packet(char *data)
{
    if( strncmp(data,"GET",3) == 0 || strncmp(data,"POST",4) == 0 )
    {
        fprintf(logfile,"
/***********************length of data : %d**********************/ 
",size);
        fprintf(logfile,"From %d To %d 
",port.src_port,port.des_port);
        int i = 0;
        for( ; i < size ; ++i)
        {
            fprintf(logfile,"%c",*(data + i));
        }
        fprintf(logfile,"/***************************** end *******************************/ 

");
    }
    return ;
}

void print_http_ans_packet(char *data)
{
    if( strncmp(data,"HTTP",4) == 0 )
    {
        fprintf(logfile,"
/***********************length of data : %d**********************/ 
",size);
        fprintf(logfile,"From %d To %d 
",port.src_port,port.des_port);
        char buff[BUFFSIZE] = {''};
        get_line(data,buff,size);
        fprintf(logfile,"%s",buff);
        unsigned int off = strlen(buff);
        size = size - off;
        while(strcmp(buff,"
") != 0)
        {
            memset(buff,'',sizeof(buff));
            get_line(data + off,buff,size);
            fprintf(logfile,"%s",buff);
            off = off + strlen(buff);
            size = size - off;
        }
        fprintf(logfile,"/***************************** end *******************************/ 

");
    }
}

void get_line(char *data,char *buff,int length)
{
    int i = 0;
    char ch;
    for ( ; i < length;++i)
    {
        ch = *(data + i);
        *(buff + i) = ch;
        if(ch == '
')
            break;
    }
}