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>


#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);


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;
    
    printf("-sum-receivelen----%d-------\n",len);
    printf("receive :<*");
    
    for(i =0 ;i<len;i++)
    {
        printf("%x*",recvbuf[i]);
    }
    printf("*>\n");
    
    
    
    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;
    }
    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
    
    printf("--receivelen----%d-------\n",len);
    printf("receive :<*");
    for(i =0 ;i<len;i++)
    {
        printf("%x*",buf[i]);
    }
    printf("*>\n");
    
    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_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 */
        }
            
    }
    
    printf("--resplen---%d-------\n",resplen);
    printf("response :<*");
    for(i =0 ;i<resplen;i++)
    {
        printf("%x*",resp[i]);
    }
    printf("*>\n");
    
    
    
    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
            {
                writen(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);
            writen(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