/*
 * An ASCII filter for tcpdump.
 * 
 * "tcpdump -x" prints the content of network packets in hex. Read
 * this data and format it in ASCII. Non-printable chars are output
 * as NON_PRINTABLE_CHAR, defined below.
 *
 * The terminal window size is used for displayed lines length.
 *
 * Typical usage:
 *    tcpdump -lx -s 512 | tcpdump2ascii
 * 
 * NOTE
 * Newish versions of tcpdump implement a "-X" flag that does just this.
 *
 * Renaud Waldura <renaud@waldura.org> * Tue Jul 25 15:24:75 PDT 2000
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <signal.h>
#include <sys/ttycom.h>          /* for struct winsize */

#define DEFAULT_LINE_LENGTH	80
#define MAX_LINE_LENGTH		256
#define NON_PRINTABLE_CHAR	' '

#define hex(c)	((isdigit(c)) ? (c) - '0' : tolower(c) - 'a' + 10)


int line_length = -1;


int get_window_columns (int fd)
{
   struct winsize ws;
   int e = ioctl(fd, TIOCGWINSZ, &ws);
   return (e == 0) ? ws.ws_col : 0;
}

void get_options (int argc, const char* argv[])
{
   int columns;
  
   if (argc > 1)
      line_length = atoi(argv[1]);

   if (line_length < 1 && (columns = get_window_columns(1)))
      line_length = columns;

   if (line_length < 1 || line_length > MAX_LINE_LENGTH)
      line_length = DEFAULT_LINE_LENGTH;
}

void winch_handler (int sig)
{
   int columns = get_window_columns(1);
   
   if (columns)
      line_length = columns;
}

main (int argc, const char* argv[])
{
   char inbuf[MAX_LINE_LENGTH], outbuf[MAX_LINE_LENGTH];
   char *s;
   int n = 0;

   /* get options */
   get_options(argc, argv);   

   /* install signal handler for window size change */
   signal(SIGWINCH, winch_handler);
   
   while (s = fgets(inbuf, MAX_LINE_LENGTH, stdin))
   {
      /* look for lines starting with whitespace */
      if (!isspace(*s++)) continue;

      while (*s)
      {
	 char c;

	 if (!isxdigit(*s))		/* skip junk */
	 {  
	    s++;
	    continue;	
	 }

         /* convert pair of hex digits to char */
         c = 16 * hex(s[0]) + hex(s[1]);
	 s += 2;

	 /* store in buffer */
	 outbuf[n++] = (isprint(c)) ? c : NON_PRINTABLE_CHAR;

	 if (n > line_length - 1)	/* print buffer if full */
	 {
	    outbuf[n] = '\0';
	    puts(outbuf);
	    n = 0;
	 }
      }
   }

   /* flush output buffer */
   outbuf[n] = '\0';
   puts(outbuf);

   return 0;
}