模拟telnet协议C语言客户端程序

首先要了解telnet协议，一下两篇blog给了我初步的思路

https://www.cnblogs.com/liang-ling/p/5833489.html 这篇有比较基础的介绍 以及IAC命令含义解释

https://www.cnblogs.com/image-eye/archive/2012/03/28/2421726.html 这篇就很能抓住重点 另外这位博主使用的是C# 写的程序十分完整 我这里关于IAC指令处理有部分是直接借鉴他的程序 因此注释都没有修改仍然使用的是他的注释

以上两篇大致看过之后 就可以了解本次po出来的程序了 内容较为朴素

utils.h

#ifndef __UTILS__H
#define __UTILS__H

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <malloc.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <stdarg.h>
#include <fcntl.h>
#include <fcntl.h>
#include <sys/poll.h>
#include <signal.h>
#include <sys/wait.h>

typedef signed long int ssize_t;
typedef unsigned long int size_t;

ssize_t readn(int fd, void *vptr,size_t n);
ssize_t writen(int fd, const void *vptr, size_t n);
ssize_t readline(int fd,void *vptr,size_t maxlen);

#endif

utils.c

#include"utils.h"

ssize_t readn(int fd, void *vptr,size_t n)
{
	size_t nleft;
	ssize_t nread;
	char *ptr;

	ptr = vptr;
	nleft = n;
	while(nleft >0)
	{
		if((nread = read(fd,ptr,nleft))<0)
		{
			if(errno == EINTR)//error：为EAGAIN，表示在非阻塞下，此时无数据到达，立即返回。
                nread = 0;            //error：为EINTR，表示被信号中断了。
			else
				return (-1);
		}
		else if(nread == 0)
			break;
		else
			/* do nothing */
		nleft -= nread;
		ptr += nread;
	}
	return n-nleft;//实际读了多少字节
}

ssize_t writen(int fd, const void *vptr, size_t n)
{
	size_t nleft;
	ssize_t nwritten;
	const char *ptr;

	ptr = vptr;
	nleft = n;
	while(nleft > 0)
	{
		if((nwritten = write(fd,ptr,nleft)) < 0)
		{
			if(nwritten <0 &&  errno == EINTR)
			{
				nwritten = 0;
			}
			else
				return (-1);
		}
		else if(nwritten == 0)
			break;
		else //nwritten > 0
		{
			/*do nothing*/
		}
		nleft = nleft - nwritten;
		ptr = ptr + nwritten;
	}
	return (n- nleft);//实际写了多少字节
}

ssize_t readline(int fd,void *vptr,size_t maxlen)
{
	ssize_t n =0,rc;
	char c,*ptr;

	ptr = vptr;
	while(1)
	{
		if((rc = read(fd,&c,1)) == 1)
		{
			*ptr++ = c;
			n++;
			if(c == '\n')
				break;
		}
		else if (rc == 0)
		{
			*ptr = '\0';
			return (n -1);
		}
		else
		{
			if(errno == EINTR)
				continue;
			else
				return (-1);
		}
	}
	*ptr = '\0';
	return n;
}

telnet.h

#ifndef __TELNET__H
#define __TELNET__H

//<IAC CMD OP >

#define IAC   255

//command word
#define NUL        0
#define BEL        7
#define BS         8
#define HT         9
#define LF         10
#define VT         11
#define FF         12
#define CR         13
#define SE         240
#define NOP        241
#define DM         242
#define BRK        243
#define IP         244
#define AO         245
#define AYT        246
#define EC         247
#define EL         248
#define GA         249
#define SB         250
#define WILL       251
#define WONT       252
#define DO         253
#define DONT       254

typedef unsigned char uint8;
typedef unsigned int  uint32;

//operation options

typedef enum tagOPERATION_OPTIONS
{
    TOPT_BIN = 0,
    TOPT_ECHO = 1,
    TOPT_RECN = 2,
    TOPT_SUPP = 3

}OPERATION_OPTIONS;

uint32 get_every_frame(uint8* recvbuf,uint32 len,uint8* sendbuf,uint32 sendlen);

#endif

telnet.c

#include"telnet.h"
#include<string.h>
#include<stdio.h>
#include <stdlib.h>

#define MAXLINE 1024
#define SEND    1
#define IS      0

static uint32 handle_telnetcmd_from_server(uint8* buf,uint32 len,uint8* resp,uint32 n);
static uint32 process_every_frame(uint8* startByte,uint8* endByte,uint8* sendbuf,uint32 startSendByte);

/* 2020-01-13 begin */
extern uint32 SOCKFD ;
extern void telnet_client_send_msg(int fd, const void *vptr, size_t n);
/* 2020-01-13 end*/

uint32 get_every_frame(uint8* recvbuf,uint32 len,uint8* sendbuf,uint32 sendlen)
{
	uint32 i =0,n=0,sum =0;
	//uint8* p = sendbuf;
	uint8* pRear = &recvbuf[len];
	uint8* startByte = recvbuf;
	uint8* endByte = recvbuf;
	uint8* strSendToMQTT = NULL;

	strSendToMQTT = (uint8*)malloc(MAXLINE);
	if(strSendToMQTT == NULL)
	{
		printf("==========strSendToMQTT malloc failed==============\n");
		return 0;
	}

#if 1
	printf("-sum-receivelen----%d-------\n",len);
	printf("receive :<*");

	for(i =0 ;i<len;i++)
	{
		printf("%x*",recvbuf[i]);
	}
	printf("*>\n");

#endif	

	while(startByte != pRear)
	{
		if(*startByte == IAC)
		{
			sum = sum + n;
			switch(*(++endByte))
			{
	    /*fa 250 */case SB:while(*(++endByte) != SE){};n = process_every_frame(startByte,endByte,sendbuf,sum);break;
		/*fb 251 */case WILL:endByte +=2;n = process_every_frame(startByte,endByte,sendbuf,sum);break;
		/*fc 252 */case WONT:endByte +=2;n = process_every_frame(startByte,endByte,sendbuf,sum);break;
        /*fd 253 */case DO:endByte +=2;n = process_every_frame(startByte,endByte,sendbuf,sum);break;
        /*fe 254 */case DONT:endByte +=2;n = process_every_frame(startByte,endByte,sendbuf,sum);break;
		/* 240 */case SE:break;
		/* sss */default : break;
			}
                        startByte = endByte;
		}
		else
		{
                        /* 2020-01-13 begin ：按照原来的写法 else分支会造成死循环，这里修改 */
			i = 0;
			memset(strSendToMQTT,0,MAXLINE);
			while((startByte != pRear)&&(*startByte != IAC ))
			{
				strSendToMQTT[i++] = *startByte ;
				++startByte;
			}
			strSendToMQTT[i] = '\0';
			/* 2020-01-13 strSendToMQTT的字符串应当也要发送出去 */
			telnet_client_send_msg(SOCKFD,strSendToMQTT,i+1);
                       /* 2020-01-13 end ：按照原来的写法 else分支会造成死循环，这里修改 */
		}
	}
	if(sum > sendlen)
	{
		printf("--error3---sum > MAXLINE-----\n");
	}
	printf("--------------sum is %d ----\n",sum);
	return sum;
}

static uint32 process_every_frame(uint8* startByte,uint8* endByte,uint8* sendbuf,uint32 startSendByte)
{
	uint8 n = 0 ;
	uint8* pstartByte = startByte;

	while(pstartByte != endByte)
	{
		n++;
		pstartByte++;
	}
	return handle_telnetcmd_from_server(startByte,n,&sendbuf[startSendByte],MAXLINE);
}

static uint32 handle_telnetcmd_from_server(uint8* buf,uint32 len,uint8* resp,uint32 n)
{
	uint32 i =0;
	uint8 *p = resp;
	OPERATION_OPTIONS optionCode;
	uint8 cmdCode,ch;
	uint32 resplen =0;
	memset(resp,0,len);
	//first display cmd from server in string

#if 1
	printf("--receivelen----%d-------\n",len);
	printf("receive :<*");
	for(i =0 ;i<len;i++)
	{
		printf("%x*",buf[i]);
	}
	printf("*>\n");

#endif

	if(len < 3)
	{
		printf("IAC command length is %d less then 3\n",len);
		return -1;
	}

	//获得命令码
	cmdCode = buf[1];
	//获得选项码
	optionCode = buf[2];

	//response requests from server

	*p = IAC;
	resplen++;
	if( optionCode == TOPT_SUPP)//if(optionCode == TOPT_ECHO || optionCode == TOPT_SUPP)
	{
		if (cmdCode == DO)
		{
			//我设置我应答的命令码为 251(WILL) 即为支持 回显或抑制继续进行
			ch = WILL;
			*(++p) = ch;
			*(++p)= optionCode;
			resplen += 2;

		}
		//如果命令码为 254(DONT)
		else if (cmdCode == DONT)
		{
			//我设置我应答的命令码为 252(WONT) 即为我也会"拒绝启动" 回显或抑制继续进行
			ch = WONT;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;

		}
		//如果命令码为251(WILL)
		else if (cmdCode == WILL)
		{
			//我设置我应答的命令码为 253(DO) 即为我认可你使用回显或抑制继续进行
			ch = DO;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;
			//break;
		}
		//如果接受到的命令码为251(WONT)
		else if (cmdCode == WONT)
		{
			//应答  我也拒绝选项请求回显或抑制继续进行
			ch = DONT;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;
			//    break;
		}
		//如果接受到250(sb,标志子选项开始)
		else if (cmdCode == SB)
		{
			/*
			 * 因为启动了子标志位,命令长度扩展到了4字节,
			 * 取最后一个标志字节为选项码
			 * 如果这个选项码字节为1(send)
			 * 则回发为 250(SB子选项开始) + 获取的第二个字节 + 0(is) + 255(标志位IAC) + 240(SE子选项结束)
			*/
			ch = buf[3];
			if (ch == SEND)
			{
				ch = SB;
				*(++p)= ch;
				*(++p)= optionCode;
				*(++p)= IS;
				*(++p)= IAC;
				*(++p)= SE;
				resplen += 5;
			}
			else
			{
				printf("ch != SEND\n");
			}
		}
		else
		{
			/* do nothing */
		}
	}
	else/* 如果选项码不是1 或者3  */
	{
		// 底下一系列代表,无论你发那种请求,我都不干
		if (cmdCode == DO)
		{
			ch = WONT;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;
		}
		else if (cmdCode == DONT)
		{
			ch = WONT;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;
		}
		else if (cmdCode == WILL)
		{
			ch = DONT;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;
		}
		else if (cmdCode == WONT)
		{
			ch = DONT;
			*(++p)= ch;
			*(++p)= optionCode;
			resplen += 2;
		}
		else
		{
			/* do nothing */
		}

	}

#if 1
	printf("--resplen---%d-------\n",resplen);
	printf("response :<*");
	for(i =0 ;i<resplen;i++)
	{
		printf("%x*",resp[i]);
	}
	printf("*>\n");
#endif	

	if(n < resplen )
	{
		printf("error n < resplen !!! \n");
	}
	if(resplen < 3 )
	{
		printf("resplen < 3 \n");
	}
	return resplen;
}

client.c

//gcc client.c -o client

#include"utils.h"
#include"telnet.h"

#define IP_ADDRESS   "127.0.0.1"
#define IP_PORT      23
#define SERV_PORT    3333
#define MAXLINE      1024

typedef struct sockaddr SA;

void str_cli(FILE *fp,uint32 sockfd);
uint32 max(uint32 a,uint32 b);
void ERR_EXIT(char* s);

uint32 main(uint32 argc,uint32 **argv)
{
	uint32 sockfd,isReady=0;
	struct sockaddr_in servaddr;
	uint32 hname[128];

	sockfd = socket(AF_INET,SOCK_STREAM,0);
	bzero(&servaddr,sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_port = htons(IP_PORT);
	servaddr.sin_addr.s_addr = inet_addr(IP_ADDRESS);
	printf("servaddr: IP is %s, Port is %d\n",inet_ntoa(servaddr.sin_addr), ntohs(servaddr.sin_port));

	while(connect(sockfd,(SA*)&servaddr,sizeof(servaddr))){};
	printf("connect has been ready\n");

	str_cli(stdin,sockfd);
	exit(0);
	return 0;
}
void ERR_EXIT(char* s)
{
	perror(s);
	exit(EXIT_FAILURE);
}
void INFO_PRINT(char* s)
{
	printf("%s",s);
}
uint32 max(uint32 a,uint32 b)
{
	return (a>b?a:b);
}

void str_cli(FILE *fp,uint32 sockfd)
{
	uint32 maxfdp1,nready;//stdineof;
	fd_set rset;
	uint8 buf[MAXLINE];
	uint8 respbuff[MAXLINE] = {0};;
	uint32 resplen;
	uint32 n;
	uint8 echo_cmd[] = {0xff,0xfb,0x01};
	//stdineof = 0;
	FD_ZERO(&rset);
	writen(sockfd,echo_cmd,3);

	for(;;)
	{
		//if(stdineof == 0)
		FD_SET(fileno(fp),&rset);
		FD_SET(sockfd,&rset);
		maxfdp1 = max(fileno(fp),sockfd)+1;
		nready = select(maxfdp1,&rset,NULL,NULL,NULL);

		if(nready < 0)
		{
			ERR_EXIT("ERROR!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
		}

		if(FD_ISSET(sockfd,&rset))
		{
			memset(buf,0,MAXLINE);
			if((n = read(sockfd,buf,MAXLINE))==0)
			{
				ERR_EXIT("str_cli:server termination prematurely");
			}
			buf[n] = '\0';
			//printf("FD_ISSET(sockfd,&rset)-------------%s\n",buf);

			if(buf[0] == IAC)
			{
				memset(respbuff,0,MAXLINE);
				resplen = get_every_frame(buf,n,respbuff,MAXLINE);
				writen(sockfd,respbuff,resplen);
			}
			else
			{
				telnet_client_send_msg(fileno(stdout),(char *)buf,n);
			}

			//writen(fileno(stdout),buf,n);
		}
		if(FD_ISSET(fileno(fp),&rset))
		{
			memset(buf,0,MAXLINE);
			if((n = readline(fileno(fp),(char *)buf,MAXLINE)) == 0)
			{
				//stdineof = 1;//此时碰到EOF 并且马上要发生FIN序列 所以标准输入不可读了
				shutdown(sockfd,SHUT_WR);
				FD_CLR(fileno(fp),&rset);
				INFO_PRINT("nothing input!");
				continue;
			}
			else if(n >0)
			{
				/* do nothing */
			}
			else
			{
				ERR_EXIT("some error occurred ");
			}
			//printf("FD_ISSET(fileno(fp),&rset)----%d--\n",n);
			//memset(buf,0,MAXLINE);
			telnet_client_send_msg(sockfd,(char *)buf,n);
		}
	}
}

makefile

all:Client_telnet
	@echo ""
	@echo "This is telnet Client compile......."
	@echo ""

Client_telnet:client.o utils.o telnet.o
	gcc -g -o Client_telnet client.o utils.o telnet.o

client.o:client.c utils.h telnet.h
	gcc -g -c client.c

utils.o:utils.c utils.h
	gcc -g -c utils.c 

telnet.o:telnet.c telnet.h
	gcc -g -c telnet.c

clean :
	-rm client.o utils.o telnet.o Client_telnet

以上为本次程序使用的源码 程序在Linux系统上运行方式为 ./Client_telnet

然后是运行截图如此下：

以上