input2@ubuntu:~$ cat input.c

 #include <stdio.h>

 #include <stdlib.h>

 #include <string.h>

 #include <sys/socket.h>

 #include <arpa/inet.h>

 int main(int argc, char* argv[], char* envp[]){

     printf("Welcome to pwnable.kr\n");

     printf("Let's see if you know how to give input to program\n");

     printf("Just give me correct inputs then you will get the flag :)\n");

     // argv

     if(argc != ) return ;

     if(strcmp(argv['A'],"\x00")) return ;

     if(strcmp(argv['B'],"\x20\x0a\x0d")) return ;

     printf("Stage 1 clear!\n");    

     // stdio

     char buf[];

     read(, buf, );

     if(memcmp(buf, "\x00\x0a\x00\xff", )) return ;

     read(, buf, );

         if(memcmp(buf, "\x00\x0a\x02\xff", )) return ;

     printf("Stage 2 clear!\n");

     // env

     if(strcmp("\xca\xfe\xba\xbe", getenv("\xde\xad\xbe\xef"))) return ;

     printf("Stage 3 clear!\n");

     // file

     FILE* fp = fopen("\x0a", "r");

     if(!fp) return ;

     if( fread(buf, , , fp)!= ) return ;

     if( memcmp(buf, "\x00\x00\x00\x00", ) ) return ;

     fclose(fp);

     printf("Stage 4 clear!\n");    

     // network

     int sd, cd;

     struct sockaddr_in saddr, caddr;

     sd = socket(AF_INET, SOCK_STREAM, );

     if(sd == -){

         printf("socket error, tell admin\n");

         return ;

     }

     saddr.sin_family = AF_INET;

     saddr.sin_addr.s_addr = INADDR_ANY;

     saddr.sin_port = htons( atoi(argv['C']) );

     if(bind(sd, (struct sockaddr*)&saddr, sizeof(saddr)) < ){

         printf("bind error, use another port\n");

             return ;

     }

     listen(sd, );

     int c = sizeof(struct sockaddr_in);

     cd = accept(sd, (struct sockaddr *)&caddr, (socklen_t*)&c);

     if(cd < ){

         printf("accept error, tell admin\n");

         return ;

     }

     if( recv(cd, buf, , ) !=  ) return ;

     if(memcmp(buf, "\xde\xad\xbe\xef", )) return ;

     printf("Stage 5 clear!\n");

     // here's your flag

     system("/bin/cat flag");

     return ;

 }