contrib/tcp_wrappers/safe_finger.c

   1  /*
   2   * safe_finger - finger client wrapper that protects against nasty stuff
   3   * from finger servers. Use this program for automatic reverse finger
   4   * probes, not the raw finger command.
   5   *
   6   * Build with: cc -o safe_finger safe_finger.c
   7   *
   8   * The problem: some programs may react to stuff in the first column. Other
   9   * programs may get upset by thrash anywhere on a line. File systems may
  10   * fill up as the finger server keeps sending data. Text editors may bomb
  11   * out on extremely long lines. The finger server may take forever because
  12   * it is somehow wedged. The code below takes care of all this badness.
  13   *
  14   * Author: Wietse Venema, Eindhoven University of Technology, The Netherlands.
  15   */
  16
  17 #ifndef lint
  18 static char sccsid[] = "@(#) safe_finger.c 1.4 94/12/28 17:42:41";
  19 #endif
  20
  21 /* System libraries */
  22
  23 #include <sys/types.h>
  24 #include <sys/stat.h>
  25 #include <signal.h>
  26 #include <stdio.h>
  27 #include <ctype.h>
  28 #include <pwd.h>
  29
  30 extern void exit();
  31
  32 /* Local stuff */
  33
  34 char    path[] = "PATH=/bin:/usr/bin:/usr/ucb:/usr/bsd:/etc:/usr/etc:/usr/sbin";
  35
  36 #define TIME_LIMIT      60              /* Do not keep listinging forever */
  37 #define INPUT_LENGTH    100000          /* Do not keep listinging forever */
  38 #define LINE_LENGTH     128             /* Editors can choke on long lines */
  39 #define FINGER_PROGRAM  "finger"        /* Most, if not all, UNIX systems */
  40 #define UNPRIV_NAME     "nobody"        /* Preferred privilege level */
  41 #define UNPRIV_UGID     32767           /* Default uid and gid */
  42
  43 int     finger_pid;
  44
  45 void    cleanup(sig)
  46 int     sig;
  47 {
  48     kill(finger_pid, SIGKILL);
  49     exit(0);
  50 }
  51
  52 main(argc, argv)
  53 int     argc;
  54 char  **argv;
  55 {
  56     int     c;
  57     int     line_length = 0;
  58     int     finger_status;
  59     int     wait_pid;
  60     int     input_count = 0;
  61     struct passwd *pwd;
  62
  63     /*
  64      * First of all, let's don't run with superuser privileges.
  65      */
  66     if (getuid() == 0 || geteuid() == 0) {
  67         if ((pwd = getpwnam(UNPRIV_NAME)) && pwd->pw_uid > 0) {
  68             setgid(pwd->pw_gid);
  69             setuid(pwd->pw_uid);
  70         } else {
  71             setgid(UNPRIV_UGID);
  72             setuid(UNPRIV_UGID);
  73         }
  74     }
  75
  76     /*
  77      * Redirect our standard input through the raw finger command.
  78      */
  79     if (putenv(path)) {
  80         fprintf(stderr, "%s: putenv: out of memory", argv[0]);
  81         exit(1);
  82     }
  83     argv[0] = FINGER_PROGRAM;
  84     finger_pid = pipe_stdin(argv);
  85
  86     /*
  87      * Don't wait forever (Peter Wemm <peter@gecko.DIALix.oz.au>).
  88      */
  89     signal(SIGALRM, cleanup);
  90     (void) alarm(TIME_LIMIT);
  91
  92     /*
  93      * Main filter loop.
  94      */
  95     while ((c = getchar()) != EOF) {
  96         if (input_count++ >= INPUT_LENGTH) {    /* don't listen forever */
  97             fclose(stdin);
  98             printf("\n\n Input truncated to %d bytes...\n", input_count - 1);
  99             break;
 100         }
 101         if (c == '\n') {                        /* good: end of line */
 102             putchar(c);
 103             line_length = 0;
 104         } else {
 105             if (line_length >= LINE_LENGTH) {   /* force end of line */
 106                 printf("\\\n");
 107                 line_length = 0;
 108             }
 109             if (line_length == 0) {             /* protect left margin */
 110                 putchar(' ');
 111                 line_length++;
 112             }
 113             if (isascii(c) && (isprint(c) || isspace(c))) {     /* text */
 114                 if (c == '\\') {
 115                     putchar(c);
 116                     line_length++;
 117                 }
 118                 putchar(c);
 119                 line_length++;
 120             } else {                            /* quote all other thash */
 121                 printf("\\%03o", c & 0377);
 122                 line_length += 4;
 123             }
 124         }
 125     }
 126
 127     /*
 128      * Wait until the finger child process has terminated and account for its
 129      * exit status. Which will always be zero on most systems.
 130      */
 131     while ((wait_pid = wait(&finger_status)) != -1 && wait_pid != finger_pid)
 132          /* void */ ;
 133     return (wait_pid != finger_pid || finger_status != 0);
 134 }
 135
 136 /* perror_exit - report system error text and terminate */
 137
 138 void    perror_exit(text)
 139 char   *text;
 140 {
 141     perror(text);
 142     exit(1);
 143 }
 144
 145 /* pipe_stdin - pipe stdin through program (from my ANSI to OLD C converter) */
 146
 147 int     pipe_stdin(argv)
 148 char  **argv;
 149 {
 150     int     pipefds[2];
 151     int     pid;
 152     int     i;
 153     struct stat st;
 154
 155     /*
 156      * The code that sets up the pipe requires that file descriptors 0,1,2
 157      * are already open. All kinds of mysterious things will happen if that
 158      * is not the case. The following loops makes sure that descriptors 0,1,2
 159      * are set up properly.
 160      */
 161
 162     for (i = 0; i < 3; i++) {
 163         if (fstat(i, &st) == -1 && open("/dev/null", 2) != i)
 164             perror_exit("open /dev/null");
 165     }
 166
 167     /*
 168      * Set up the pipe that interposes the command into our standard input
 169      * stream.
 170      */
 171
 172     if (pipe(pipefds))
 173         perror_exit("pipe");
 174
 175     switch (pid = fork()) {
 176     case -1:                                    /* error */
 177         perror_exit("fork");
 178         /* NOTREACHED */
 179     case 0:                                     /* child */
 180         (void) close(pipefds[0]);               /* close reading end */
 181         (void) close(1);                        /* connect stdout to pipe */
 182         if (dup(pipefds[1]) != 1)
 183             perror_exit("dup");
 184         (void) close(pipefds[1]);               /* close redundant fd */
 185         (void) execvp(argv[0], argv);
 186         perror_exit(argv[0]);
 187         /* NOTREACHED */
 188     default:                                    /* parent */
 189         (void) close(pipefds[1]);               /* close writing end */
 190         (void) close(0);                        /* connect stdin to pipe */
 191         if (dup(pipefds[0]) != 0)
 192             perror_exit("dup");
 193         (void) close(pipefds[0]);               /* close redundant fd */
 194         return (pid);
 195     }
 196 }