版权声明:本文为博主原创文章,未经博主允许不得转载。 https://blog.csdn.net/jason_holan/article/details/86540116
本实验来自中科大孟宁老师教的《Linux网络程序设计》课程实验部分。
课程通过搭建linux内核调试环境,将编写的http客户端集成到MenuOS中,从linux源码出发 学习网络中的协议具体是如何收发的。
实验代码及参考:
https://github.com/mengning/linuxnet/blob/master/np2018.md
实验内容:
编写一个简单的HTTP客户端访问github.com(此次访问的是科大的主页ustc.edu.cn)并集成到MenuOS里并跟踪分析路由和ARP解析的过程
编写一个简单的客户端通过DNS解析API解析github.com(或其他域名)获得其IP地址
客户端HTTP访问github.com对应的IP地址,能够获取HTTP响应
将前述客户端集成到MenuOS中,能正常输出github.com对应的HTTP响应
跟踪分析MenuOS中前述客户端的运行过程,总从体上分析DNS解析过程UDP在内核中的发送和接收过程, 以及HTTP通信TCP在内核中的发送和接收过程
实验环境:
qemu+linux-3.18.6+MenuOS,主机环境14.04
注意:
每次编译后需将生成的init, 放到rootfs.img中。
过程:
mkdir root
sudo make mount -o loop rootfs.img root
cp init root
sudo umount root
代码如下:
int main()
{
BringUpNetInterface();
SetDefaultGateway();
GetRoute();
PrintMenuOS();
SetPrompt("MenuOS>>");
MenuConfig("version","MenuOS V1.0(Based on Linux 3.18.6)",NULL);
MenuConfig("quit","Quit from MenuOS",Quit);
MenuConfig("replyhi", "Reply hi TCP Service", StartReplyhi);
MenuConfig("hello", "Hello TCP Client", Hello);
MenuConfig("httpReq", "DNS Query github.com", HttpRequest);
ExecuteMenu();
}
访问其他域名将unsigned char hostname[100] = "www.ustc.edu.cn";
更改即可。
//Header Files
#include<stdio.h> //printf
#include<string.h> //strlen
#include<stdlib.h> //malloc
#include<sys/socket.h> //you know what this is for
#include<arpa/inet.h> //inet_addr , inet_ntoa , ntohs etc
#include<netinet/in.h>
#include<unistd.h> //getpid
//List of DNS Servers registered on the system
char dns_servers[10][100];
int dns_server_count = 0;
//Types of DNS resource records :)
#define T_A 1 //Ipv4 address
#define T_NS 2 //Nameserver
#define T_CNAME 5 // canonical name
#define T_SOA 6 /* start of authority zone */
#define T_PTR 12 /* domain name pointer */
#define T_MX 15 //Mail server
//Function Prototypes
void ngethostbyname (unsigned char* , int);
void ChangetoDnsNameFormat (unsigned char*,unsigned char*);
unsigned char* ReadName (unsigned char*,unsigned char*,int*);
void get_dns_servers();
int Hellohttp();
//DNS header structure
char ip[20] = {0};
struct DNS_HEADER
{
unsigned short id; // identification number
unsigned char rd :1; // recursion desired
unsigned char tc :1; // truncated message
unsigned char aa :1; // authoritive answer
unsigned char opcode :4; // purpose of message
unsigned char qr :1; // query/response flag
unsigned char rcode :4; // response code
unsigned char cd :1; // checking disabled
unsigned char ad :1; // authenticated data
unsigned char z :1; // its z! reserved
unsigned char ra :1; // recursion available
unsigned short q_count; // number of question entries
unsigned short ans_count; // number of answer entries
unsigned short auth_count; // number of authority entries
unsigned short add_count; // number of resource entries
};
//Constant sized fields of query structure
struct QUESTION
{
unsigned short qtype;
unsigned short qclass;
};
//Constant sized fields of the resource record structure
#pragma pack(push, 1)
struct R_DATA
{
unsigned short type;
unsigned short _class;
unsigned int ttl;
unsigned short data_len;
};
#pragma pack(pop)
//Pointers to resource record contents
struct RES_RECORD
{
unsigned char *name;
struct R_DATA *resource;
unsigned char *rdata;
};
//Structure of a Query
typedef struct
{
unsigned char *name;
struct QUESTION *ques;
} QUERY;
//int main( int argc , char *argv[])
int HttpRequest()
{
unsigned char hostname[100] = "www.ustc.edu.cn";
//Get the DNS servers from the resolv.conf file
get_dns_servers();
//Get the hostname from the terminal
//printf("Enter Hostname to Lookup : ");
//scanf("%s" , hostname);
//Now get the ip of this hostname , A record
ngethostbyname(hostname , T_A);
Hellohttp();
return 0;
}
/*
* Perform a DNS query by sending a packet
* */
void ngethostbyname(unsigned char *host , int query_type)
{
int jj=0;
unsigned char buf[65536],*qname,*reader;
int i , j , stop , s;
struct sockaddr_in a;
struct RES_RECORD answers[20],auth[20],addit[20]; //the replies from the DNS server
struct sockaddr_in dest;
struct DNS_HEADER *dns = NULL;
struct QUESTION *qinfo = NULL;
printf("Resolving %s" , host);
s = socket(AF_INET , SOCK_DGRAM , IPPROTO_UDP); //UDP packet for DNS queries
dest.sin_family = AF_INET;
dest.sin_port = htons(53);
dest.sin_addr.s_addr = inet_addr(dns_servers[0]); //dns servers
//Set the DNS structure to standard queries
dns = (struct DNS_HEADER *)&buf;
dns->id = (unsigned short) htons(getpid());
dns->qr = 0; //This is a query
dns->opcode = 0; //This is a standard query
dns->aa = 0; //Not Authoritative
dns->tc = 0; //This message is not truncated
dns->rd = 1; //Recursion Desired
dns->ra = 0; //Recursion not available! hey we dont have it (lol)
dns->z = 0;
dns->ad = 0;
dns->cd = 0;
dns->rcode = 0;
dns->q_count = htons(1); //we have only 1 question
dns->ans_count = 0;
dns->auth_count = 0;
dns->add_count = 0;
//point to the query portion
qname =(unsigned char*)&buf[sizeof(struct DNS_HEADER)];
ChangetoDnsNameFormat(qname , host);
qinfo =(struct QUESTION*)&buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname) + 1)]; //fill it
qinfo->qtype = htons( query_type ); //type of the query , A , MX , CNAME , NS etc
qinfo->qclass = htons(1); //its internet (lol)
printf("\nSending Packet...");
if( sendto(s,(char*)buf,sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION),0,(struct sockaddr*)&dest,sizeof(dest)) < 0)
{
perror("sendto failed");
}
printf("Done");
//Receive the answer
i = sizeof dest;
printf("\nReceiving answer...");
if(recvfrom (s,(char*)buf , 65536 , 0 , (struct sockaddr*)&dest , (socklen_t*)&i ) < 0)
{
perror("recvfrom failed");
}
printf("Done");
dns = (struct DNS_HEADER*) buf;
//move ahead of the dns header and the query field
reader = &buf[sizeof(struct DNS_HEADER) + (strlen((const char*)qname)+1) + sizeof(struct QUESTION)];
printf("\nThe response contains : ");
printf("\n %d Questions.",ntohs(dns->q_count));
printf("\n %d Answers.",ntohs(dns->ans_count));
printf("\n %d Authoritative Servers.",ntohs(dns->auth_count));
printf("\n %d Additional records.\n\n",ntohs(dns->add_count));
//Start reading answers
stop=0;
for(i=0;i<ntohs(dns->ans_count);i++)
{
answers[i].name=ReadName(reader,buf,&stop);
reader = reader + stop;
answers[i].resource = (struct R_DATA*)(reader);
reader = reader + sizeof(struct R_DATA);
if(ntohs(answers[i].resource->type) == 1) //if its an ipv4 address
{
answers[i].rdata = (unsigned char*)malloc(ntohs(answers[i].resource->data_len));
for(j=0 ; j<ntohs(answers[i].resource->data_len) ; j++)
{
answers[i].rdata[j]=reader[j];
}
answers[i].rdata[ntohs(answers[i].resource->data_len)] = '\0';
reader = reader + ntohs(answers[i].resource->data_len);
}
else
{
answers[i].rdata = ReadName(reader,buf,&stop);
reader = reader + stop;
}
}
//read authorities
for(i=0;i<ntohs(dns->auth_count);i++)
{
auth[i].name=ReadName(reader,buf,&stop);
reader+=stop;
auth[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);
auth[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}
//read additional
for(i=0;i<ntohs(dns->add_count);i++)
{
addit[i].name=ReadName(reader,buf,&stop);
reader+=stop;
addit[i].resource=(struct R_DATA*)(reader);
reader+=sizeof(struct R_DATA);
if(ntohs(addit[i].resource->type)==1)
{
addit[i].rdata = (unsigned char*)malloc(ntohs(addit[i].resource->data_len));
for(j=0;j<ntohs(addit[i].resource->data_len);j++)
addit[i].rdata[j]=reader[j];
addit[i].rdata[ntohs(addit[i].resource->data_len)]='\0';
reader+=ntohs(addit[i].resource->data_len);
}
else
{
addit[i].rdata=ReadName(reader,buf,&stop);
reader+=stop;
}
}
//print answers
printf("\nAnswer Records : %d \n" , ntohs(dns->ans_count) );
for(i=0 ; i < ntohs(dns->ans_count) ; i++)
{
printf("Name : %s ",answers[i].name);
if( ntohs(answers[i].resource->type) == T_A) //IPv4 address
{
long *p;
p=(long*)answers[i].rdata;
a.sin_addr.s_addr=(*p); //working without ntohl
printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
strcpy(ip, (char*)inet_ntoa(a.sin_addr));
printf("my ustc ip");
}
if(ntohs(answers[i].resource->type)==5)
{
//Canonical name for an alias
printf("has alias name : %s",answers[i].rdata);
}
printf("\n");
}
//print authorities
printf("\nAuthoritive Records : %d \n" , ntohs(dns->auth_count) );
for( i=0 ; i < ntohs(dns->auth_count) ; i++)
{
printf("Name : %s ",auth[i].name);
if(ntohs(auth[i].resource->type)==2)
{
printf("has nameserver : %s",auth[i].rdata);
}
printf("\n");
}
//print additional resource records
printf("\nAdditional Records : %d \n" , ntohs(dns->add_count) );
for(i=0; i < ntohs(dns->add_count) ; i++)
{
printf("Name : %s ",addit[i].name);
if(ntohs(addit[i].resource->type)==1)
{
long *p;
p=(long*)addit[i].rdata;
a.sin_addr.s_addr=(*p);
printf("has IPv4 address : %s",inet_ntoa(a.sin_addr));
}
printf("\n");
}
return;
}
/*
*
* */
u_char* ReadName(unsigned char* reader,unsigned char* buffer,int* count)
{
unsigned char *name;
unsigned int p=0,jumped=0,offset;
int i , j;
*count = 1;
name = (unsigned char*)malloc(256);
name[0]='\0';
//read the names in 3www6google3com format
while(*reader!=0)
{
if(*reader>=192)
{
offset = (*reader)*256 + *(reader+1) - 49152; //49152 = 11000000 00000000 ;)
reader = buffer + offset - 1;
jumped = 1; //we have jumped to another location so counting wont go up!
}
else
{
name[p++]=*reader;
}
reader = reader+1;
if(jumped==0)
{
*count = *count + 1; //if we havent jumped to another location then we can count up
}
}
name[p]='\0'; //string complete
if(jumped==1)
{
*count = *count + 1; //number of steps we actually moved forward in the packet
}
//now convert 3www6google3com0 to www.google.com
for(i=0;i<(int)strlen((const char*)name);i++)
{
p=name[i];
for(j=0;j<(int)p;j++)
{
name[i]=name[i+1];
i=i+1;
}
name[i]='.';
}
name[i-1]='\0'; //remove the last dot
return name;
}
/*
* Get the DNS servers from /etc/resolv.conf file on Linux
* */
void get_dns_servers()
{
FILE *fp;
char line[200] , *p;
if((fp = fopen("/etc/resolv.conf" , "r")) == NULL)
{
printf("Failed opening /etc/resolv.conf file \n");
}
while(fgets(line , 200 , fp))
{
printf("%s",line);
if(line[0] == '#')
{
continue;
}
if(strncmp(line , "nameserver" , 10) == 0)
{
p = strtok(line , " ");
p = strtok(NULL , " ");
//p now is the dns ip :)
printf("dns ip:%s",p);
}
}
strcpy(dns_servers[0] , "10.0.2.3");
strcpy(dns_servers[1] , "208.67.220.220");
}
/*
* This will convert www.google.com to 3www6google3com
* got it :)
* */
void ChangetoDnsNameFormat(unsigned char* dns,unsigned char* host)
{
int lock = 0 , i;
strcat((char*)host,".");
for(i = 0 ; i < strlen((char*)host) ; i++)
{
if(host[i]=='.')
{
*dns++ = i-lock;
for(;lock<i;lock++)
{
*dns++=host[lock];
}
lock++; //or lock=i+1;
}
}
*dns++='\0';
}
#include<stdio.h>
#include<stdlib.h>
#include<string.h> //strlen
#include<sys/socket.h>
#include<arpa/inet.h> //inet_addr
#include<netdb.h>
#include<errno.h>
int Hellohttp()
{
int socket_desc;
struct sockaddr_in server;
char *message;
//char ip[20] = {0};
int i = 0;
//Create socket
socket_desc = socket(AF_INET, SOCK_STREAM , 0);
if (socket_desc == -1) {
printf("Could not create socket");
}
printf("Ip rev!");
//print ip address
for(i=0;i<=20;i++){
printf("%c",ip[i]);
}
printf("\n");
server.sin_addr.s_addr = inet_addr(ip);
server.sin_family = AF_INET;
server.sin_port = htons(80);
printf("%s", ip);
//Connect to remote server
if (connect(socket_desc, (struct sockaddr *)&server, sizeof(server)) < 0) {
printf("connect error: %d", errno);
return 1;
}
puts("Connected\n");
//Send some data
//http 协议
message = "GET /index.html HTTP/1.1\r\nHost: www.ustc.edu.cn\r\n\r\n";
//向服务器发送数据
if (send(socket_desc, message, strlen(message) , 0) < 0) {
puts("Send failed");
return 1;
}
puts("Data Send\n");
struct timeval timeout = {3, 0};
setsockopt(socket_desc, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout, sizeof(struct timeval));
//Receive a reply from the server
//loop
int size_recv, total_size = 0;
char chunk[512];
while(1) {
memset(chunk , 0 , 512); //clear the variable
//获取数据
if ((size_recv = recv(socket_desc, chunk, 512, 0) ) == -1) {
if (errno == EWOULDBLOCK || errno == EAGAIN) {
printf("recv timeout ...\n");
break;
} else if (errno == EINTR) {
printf("interrupt by signal...\n");
continue;
} else if (errno == ENOENT) {
printf("recv RST segement...\n");
break;
} else {
printf("unknown error: %d\n", errno);
exit(1);
}
} else if (size_recv == 0) {
printf("peer closed ...\n");
break;
} else {
total_size += size_recv;
printf("%s" , chunk);
}
}
printf("Reply received, total_size = %d bytes\n", total_size);
return 0;
}
通过科大域名发送dns请求,获取ustc.edu.cn的ip地址
并通过ip访问科大主页。代码跟踪左图。
UDP详细的收发过程见:https://blog.csdn.net/jason_holan/article/details/85119483