diff options
Diffstat (limited to 'networking/telnetd.c')
| -rw-r--r-- | networking/telnetd.c | 610 |
1 files changed, 610 insertions, 0 deletions
diff --git a/networking/telnetd.c b/networking/telnetd.c new file mode 100644 index 0000000..46dfb31 --- /dev/null +++ b/networking/telnetd.c @@ -0,0 +1,610 @@ +/* vi: set sw=4 ts=4: */ +/* + * Simple telnet server + * Bjorn Wesen, Axis Communications AB (bjornw@axis.com) + * + * Licensed under GPLv2 or later, see file LICENSE in this tarball for details. + * + * --------------------------------------------------------------------------- + * (C) Copyright 2000, Axis Communications AB, LUND, SWEDEN + **************************************************************************** + * + * The telnetd manpage says it all: + * + * Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for + * a client, then creating a login process which has the slave side of the + * pseudo-terminal as stdin, stdout, and stderr. Telnetd manipulates the + * master side of the pseudo-terminal, implementing the telnet protocol and + * passing characters between the remote client and the login process. + * + * Vladimir Oleynik <dzo@simtreas.ru> 2001 + * Set process group corrections, initial busybox port + */ + +#define DEBUG 0 + +#include "libbb.h" +#include <syslog.h> + +#if DEBUG +#define TELCMDS +#define TELOPTS +#endif +#include <arpa/telnet.h> + +/* Structure that describes a session */ +struct tsession { + struct tsession *next; + int sockfd_read, sockfd_write, ptyfd; + int shell_pid; + + /* two circular buffers */ + /*char *buf1, *buf2;*/ +/*#define TS_BUF1 ts->buf1*/ +/*#define TS_BUF2 TS_BUF2*/ +#define TS_BUF1 ((unsigned char*)(ts + 1)) +#define TS_BUF2 (((unsigned char*)(ts + 1)) + BUFSIZE) + int rdidx1, wridx1, size1; + int rdidx2, wridx2, size2; +}; + +/* Two buffers are directly after tsession in malloced memory. + * Make whole thing fit in 4k */ +enum { BUFSIZE = (4 * 1024 - sizeof(struct tsession)) / 2 }; + + +/* Globals */ +static int maxfd; +static struct tsession *sessions; +static const char *loginpath = "/bin/login"; +static const char *issuefile = "/etc/issue.net"; + + +/* + Remove all IAC's from buf1 (received IACs are ignored and must be removed + so as to not be interpreted by the terminal). Make an uninterrupted + string of characters fit for the terminal. Do this by packing + all characters meant for the terminal sequentially towards the end of buf. + + Return a pointer to the beginning of the characters meant for the terminal. + and make *num_totty the number of characters that should be sent to + the terminal. + + Note - If an IAC (3 byte quantity) starts before (bf + len) but extends + past (bf + len) then that IAC will be left unprocessed and *processed + will be less than len. + + FIXME - if we mean to send 0xFF to the terminal then it will be escaped, + what is the escape character? We aren't handling that situation here. + + CR-LF ->'s CR mapping is also done here, for convenience. + + NB: may fail to remove iacs which wrap around buffer! + */ +static unsigned char * +remove_iacs(struct tsession *ts, int *pnum_totty) +{ + unsigned char *ptr0 = TS_BUF1 + ts->wridx1; + unsigned char *ptr = ptr0; + unsigned char *totty = ptr; + unsigned char *end = ptr + MIN(BUFSIZE - ts->wridx1, ts->size1); + int num_totty; + + while (ptr < end) { + if (*ptr != IAC) { + char c = *ptr; + + *totty++ = c; + ptr++; + /* We map \r\n ==> \r for pragmatic reasons. + * Many client implementations send \r\n when + * the user hits the CarriageReturn key. + */ + if (c == '\r' && ptr < end && (*ptr == '\n' || *ptr == '\0')) + ptr++; + continue; + } + + if ((ptr+1) >= end) + break; + if (ptr[1] == NOP) { /* Ignore? (putty keepalive, etc.) */ + ptr += 2; + continue; + } + if (ptr[1] == IAC) { /* Literal IAC? (emacs M-DEL) */ + *totty++ = ptr[1]; + ptr += 2; + continue; + } + + /* + * TELOPT_NAWS support! + */ + if ((ptr+2) >= end) { + /* only the beginning of the IAC is in the + buffer we were asked to process, we can't + process this char. */ + break; + } + /* + * IAC -> SB -> TELOPT_NAWS -> 4-byte -> IAC -> SE + */ + if (ptr[1] == SB && ptr[2] == TELOPT_NAWS) { + struct winsize ws; + if ((ptr+8) >= end) + break; /* incomplete, can't process */ + ws.ws_col = (ptr[3] << 8) | ptr[4]; + ws.ws_row = (ptr[5] << 8) | ptr[6]; + ioctl(ts->ptyfd, TIOCSWINSZ, (char *)&ws); + ptr += 9; + continue; + } + /* skip 3-byte IAC non-SB cmd */ +#if DEBUG + fprintf(stderr, "Ignoring IAC %s,%s\n", + TELCMD(ptr[1]), TELOPT(ptr[2])); +#endif + ptr += 3; + } + + num_totty = totty - ptr0; + *pnum_totty = num_totty; + /* the difference between ptr and totty is number of iacs + we removed from the stream. Adjust buf1 accordingly. */ + if ((ptr - totty) == 0) /* 99.999% of cases */ + return ptr0; + ts->wridx1 += ptr - totty; + ts->size1 -= ptr - totty; + /* move chars meant for the terminal towards the end of the buffer */ + return memmove(ptr - num_totty, ptr0, num_totty); +} + + +static struct tsession * +make_new_session( + USE_FEATURE_TELNETD_STANDALONE(int sock) + SKIP_FEATURE_TELNETD_STANDALONE(void) +) { + const char *login_argv[2]; + struct termios termbuf; + int fd, pid; + char tty_name[GETPTY_BUFSIZE]; + struct tsession *ts = xzalloc(sizeof(struct tsession) + BUFSIZE * 2); + + /*ts->buf1 = (char *)(ts + 1);*/ + /*ts->buf2 = ts->buf1 + BUFSIZE;*/ + + /* Got a new connection, set up a tty. */ + fd = xgetpty(tty_name); + if (fd > maxfd) + maxfd = fd; + ts->ptyfd = fd; + ndelay_on(fd); +#if ENABLE_FEATURE_TELNETD_STANDALONE + ts->sockfd_read = sock; + /* SO_KEEPALIVE by popular demand */ + setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE, &const_int_1, sizeof(const_int_1)); + ndelay_on(sock); + if (!sock) { /* We are called with fd 0 - we are in inetd mode */ + sock++; /* so use fd 1 for output */ + ndelay_on(sock); + } + ts->sockfd_write = sock; + if (sock > maxfd) + maxfd = sock; +#else + /* SO_KEEPALIVE by popular demand */ + setsockopt(0, SOL_SOCKET, SO_KEEPALIVE, &const_int_1, sizeof(const_int_1)); + /* ts->sockfd_read = 0; - done by xzalloc */ + ts->sockfd_write = 1; + ndelay_on(0); + ndelay_on(1); +#endif + /* Make the telnet client understand we will echo characters so it + * should not do it locally. We don't tell the client to run linemode, + * because we want to handle line editing and tab completion and other + * stuff that requires char-by-char support. */ + { + static const char iacs_to_send[] ALIGN1 = { + IAC, DO, TELOPT_ECHO, + IAC, DO, TELOPT_NAWS, + IAC, DO, TELOPT_LFLOW, + IAC, WILL, TELOPT_ECHO, + IAC, WILL, TELOPT_SGA + }; + memcpy(TS_BUF2, iacs_to_send, sizeof(iacs_to_send)); + ts->rdidx2 = sizeof(iacs_to_send); + ts->size2 = sizeof(iacs_to_send); + } + + fflush(NULL); /* flush all streams */ + pid = vfork(); /* NOMMU-friendly */ + if (pid < 0) { + free(ts); + close(fd); + /* sock will be closed by caller */ + bb_perror_msg("vfork"); + return NULL; + } + if (pid > 0) { + /* Parent */ + ts->shell_pid = pid; + return ts; + } + + /* Child */ + /* Careful - we are after vfork! */ + + /* make new session and process group */ + setsid(); + + /* Restore default signal handling */ + bb_signals((1 << SIGCHLD) + (1 << SIGPIPE), SIG_DFL); + + /* open the child's side of the tty. */ + /* NB: setsid() disconnects from any previous ctty's. Therefore + * we must open child's side of the tty AFTER setsid! */ + close(0); + xopen(tty_name, O_RDWR); /* becomes our ctty */ + xdup2(0, 1); + xdup2(0, 2); + tcsetpgrp(0, getpid()); /* switch this tty's process group to us */ + + /* The pseudo-terminal allocated to the client is configured to operate in + * cooked mode, and with XTABS CRMOD enabled (see tty(4)). */ + tcgetattr(0, &termbuf); + termbuf.c_lflag |= ECHO; /* if we use readline we dont want this */ + termbuf.c_oflag |= ONLCR | XTABS; + termbuf.c_iflag |= ICRNL; + termbuf.c_iflag &= ~IXOFF; + /*termbuf.c_lflag &= ~ICANON;*/ + tcsetattr_stdin_TCSANOW(&termbuf); + + /* Uses FILE-based I/O to stdout, but does fflush(stdout), + * so should be safe with vfork. + * I fear, though, that some users will have ridiculously big + * issue files, and they may block writing to fd 1, + * (parent is supposed to read it, but parent waits + * for vforked child to exec!) */ + print_login_issue(issuefile, tty_name); + + /* Exec shell / login / whatever */ + login_argv[0] = loginpath; + login_argv[1] = NULL; + /* exec busybox applet (if PREFER_APPLETS=y), if that fails, + * exec external program */ + BB_EXECVP(loginpath, (char **)login_argv); + /* _exit is safer with vfork, and we shouldn't send message + * to remote clients anyway */ + _exit(EXIT_FAILURE); /*bb_perror_msg_and_die("execv %s", loginpath);*/ +} + +/* Must match getopt32 string */ +enum { + OPT_WATCHCHILD = (1 << 2), /* -K */ + OPT_INETD = (1 << 3) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -i */ + OPT_PORT = (1 << 4) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -p */ + OPT_FOREGROUND = (1 << 6) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -F */ +}; + +#if ENABLE_FEATURE_TELNETD_STANDALONE + +static void +free_session(struct tsession *ts) +{ + struct tsession *t = sessions; + + if (option_mask32 & OPT_INETD) + exit(EXIT_SUCCESS); + + /* Unlink this telnet session from the session list */ + if (t == ts) + sessions = ts->next; + else { + while (t->next != ts) + t = t->next; + t->next = ts->next; + } + +#if 0 + /* It was said that "normal" telnetd just closes ptyfd, + * doesn't send SIGKILL. When we close ptyfd, + * kernel sends SIGHUP to processes having slave side opened. */ + kill(ts->shell_pid, SIGKILL); + wait4(ts->shell_pid, NULL, 0, NULL); +#endif + close(ts->ptyfd); + close(ts->sockfd_read); + /* We do not need to close(ts->sockfd_write), it's the same + * as sockfd_read unless we are in inetd mode. But in inetd mode + * we do not reach this */ + free(ts); + + /* Scan all sessions and find new maxfd */ + maxfd = 0; + ts = sessions; + while (ts) { + if (maxfd < ts->ptyfd) + maxfd = ts->ptyfd; + if (maxfd < ts->sockfd_read) + maxfd = ts->sockfd_read; +#if 0 + /* Again, sockfd_write == sockfd_read here */ + if (maxfd < ts->sockfd_write) + maxfd = ts->sockfd_write; +#endif + ts = ts->next; + } +} + +#else /* !FEATURE_TELNETD_STANDALONE */ + +/* Used in main() only, thus "return 0" actually is exit(EXIT_SUCCESS). */ +#define free_session(ts) return 0 + +#endif + +static void handle_sigchld(int sig UNUSED_PARAM) +{ + pid_t pid; + struct tsession *ts; + + /* Looping: more than one child may have exited */ + while (1) { + pid = wait_any_nohang(NULL); + if (pid <= 0) + break; + ts = sessions; + while (ts) { + if (ts->shell_pid == pid) { + ts->shell_pid = -1; + break; + } + ts = ts->next; + } + } +} + +int telnetd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE; +int telnetd_main(int argc UNUSED_PARAM, char **argv) +{ + fd_set rdfdset, wrfdset; + unsigned opt; + int count; + struct tsession *ts; +#if ENABLE_FEATURE_TELNETD_STANDALONE +#define IS_INETD (opt & OPT_INETD) + int master_fd = master_fd; /* be happy, gcc */ + unsigned portnbr = 23; + char *opt_bindaddr = NULL; + char *opt_portnbr; +#else + enum { + IS_INETD = 1, + master_fd = -1, + portnbr = 23, + }; +#endif + /* Even if !STANDALONE, we accept (and ignore) -i, thus people + * don't need to guess whether it's ok to pass -i to us */ + opt = getopt32(argv, "f:l:Ki" USE_FEATURE_TELNETD_STANDALONE("p:b:F"), + &issuefile, &loginpath + USE_FEATURE_TELNETD_STANDALONE(, &opt_portnbr, &opt_bindaddr)); + if (!IS_INETD /*&& !re_execed*/) { + /* inform that we start in standalone mode? + * May be useful when people forget to give -i */ + /*bb_error_msg("listening for connections");*/ + if (!(opt & OPT_FOREGROUND)) { + /* DAEMON_CHDIR_ROOT was giving inconsistent + * behavior with/without -F, -i */ + bb_daemonize_or_rexec(0 /*was DAEMON_CHDIR_ROOT*/, argv); + } + } + /* Redirect log to syslog early, if needed */ + if (IS_INETD || !(opt & OPT_FOREGROUND)) { + openlog(applet_name, 0, LOG_USER); + logmode = LOGMODE_SYSLOG; + } + USE_FEATURE_TELNETD_STANDALONE( + if (opt & OPT_PORT) + portnbr = xatou16(opt_portnbr); + ); + + /* Used to check access(loginpath, X_OK) here. Pointless. + * exec will do this for us for free later. */ + +#if ENABLE_FEATURE_TELNETD_STANDALONE + if (IS_INETD) { + sessions = make_new_session(0); + if (!sessions) /* pty opening or vfork problem, exit */ + return 1; /* make_new_session prints error message */ + } else { + master_fd = create_and_bind_stream_or_die(opt_bindaddr, portnbr); + xlisten(master_fd, 1); + } +#else + sessions = make_new_session(); + if (!sessions) /* pty opening or vfork problem, exit */ + return 1; /* make_new_session prints error message */ +#endif + + /* We don't want to die if just one session is broken */ + signal(SIGPIPE, SIG_IGN); + + if (opt & OPT_WATCHCHILD) + signal(SIGCHLD, handle_sigchld); + else /* prevent dead children from becoming zombies */ + signal(SIGCHLD, SIG_IGN); + +/* + This is how the buffers are used. The arrows indicate the movement + of data. + +-------+ wridx1++ +------+ rdidx1++ +----------+ + | | <-------------- | buf1 | <-------------- | | + | | size1-- +------+ size1++ | | + | pty | | socket | + | | rdidx2++ +------+ wridx2++ | | + | | --------------> | buf2 | --------------> | | + +-------+ size2++ +------+ size2-- +----------+ + + size1: "how many bytes are buffered for pty between rdidx1 and wridx1?" + size2: "how many bytes are buffered for socket between rdidx2 and wridx2?" + + Each session has got two buffers. Buffers are circular. If sizeN == 0, + buffer is empty. If sizeN == BUFSIZE, buffer is full. In both these cases + rdidxN == wridxN. +*/ + again: + FD_ZERO(&rdfdset); + FD_ZERO(&wrfdset); + + /* Select on the master socket, all telnet sockets and their + * ptys if there is room in their session buffers. + * NB: scalability problem: we recalculate entire bitmap + * before each select. Can be a problem with 500+ connections. */ + ts = sessions; + while (ts) { + struct tsession *next = ts->next; /* in case we free ts. */ + if (ts->shell_pid == -1) { + /* Child died and we detected that */ + free_session(ts); + } else { + if (ts->size1 > 0) /* can write to pty */ + FD_SET(ts->ptyfd, &wrfdset); + if (ts->size1 < BUFSIZE) /* can read from socket */ + FD_SET(ts->sockfd_read, &rdfdset); + if (ts->size2 > 0) /* can write to socket */ + FD_SET(ts->sockfd_write, &wrfdset); + if (ts->size2 < BUFSIZE) /* can read from pty */ + FD_SET(ts->ptyfd, &rdfdset); + } + ts = next; + } + if (!IS_INETD) { + FD_SET(master_fd, &rdfdset); + /* This is needed because free_session() does not + * take master_fd into account when it finds new + * maxfd among remaining fd's */ + if (master_fd > maxfd) + maxfd = master_fd; + } + + count = select(maxfd + 1, &rdfdset, &wrfdset, NULL, NULL); + if (count < 0) + goto again; /* EINTR or ENOMEM */ + +#if ENABLE_FEATURE_TELNETD_STANDALONE + /* First check for and accept new sessions. */ + if (!IS_INETD && FD_ISSET(master_fd, &rdfdset)) { + int fd; + struct tsession *new_ts; + + fd = accept(master_fd, NULL, NULL); + if (fd < 0) + goto again; + /* Create a new session and link it into our active list */ + new_ts = make_new_session(fd); + if (new_ts) { + new_ts->next = sessions; + sessions = new_ts; + } else { + close(fd); + } + } +#endif + + /* Then check for data tunneling. */ + ts = sessions; + while (ts) { /* For all sessions... */ + struct tsession *next = ts->next; /* in case we free ts. */ + + if (/*ts->size1 &&*/ FD_ISSET(ts->ptyfd, &wrfdset)) { + int num_totty; + unsigned char *ptr; + /* Write to pty from buffer 1. */ + ptr = remove_iacs(ts, &num_totty); + count = safe_write(ts->ptyfd, ptr, num_totty); + if (count < 0) { + if (errno == EAGAIN) + goto skip1; + goto kill_session; + } + ts->size1 -= count; + ts->wridx1 += count; + if (ts->wridx1 >= BUFSIZE) /* actually == BUFSIZE */ + ts->wridx1 = 0; + } + skip1: + if (/*ts->size2 &&*/ FD_ISSET(ts->sockfd_write, &wrfdset)) { + /* Write to socket from buffer 2. */ + count = MIN(BUFSIZE - ts->wridx2, ts->size2); + count = safe_write(ts->sockfd_write, TS_BUF2 + ts->wridx2, count); + if (count < 0) { + if (errno == EAGAIN) + goto skip2; + goto kill_session; + } + ts->size2 -= count; + ts->wridx2 += count; + if (ts->wridx2 >= BUFSIZE) /* actually == BUFSIZE */ + ts->wridx2 = 0; + } + skip2: + /* Should not be needed, but... remove_iacs is actually buggy + * (it cannot process iacs which wrap around buffer's end)! + * Since properly fixing it requires writing bigger code, + * we rely instead on this code making it virtually impossible + * to have wrapped iac (people don't type at 2k/second). + * It also allows for bigger reads in common case. */ + if (ts->size1 == 0) { + ts->rdidx1 = 0; + ts->wridx1 = 0; + } + if (ts->size2 == 0) { + ts->rdidx2 = 0; + ts->wridx2 = 0; + } + + if (/*ts->size1 < BUFSIZE &&*/ FD_ISSET(ts->sockfd_read, &rdfdset)) { + /* Read from socket to buffer 1. */ + count = MIN(BUFSIZE - ts->rdidx1, BUFSIZE - ts->size1); + count = safe_read(ts->sockfd_read, TS_BUF1 + ts->rdidx1, count); + if (count <= 0) { + if (count < 0 && errno == EAGAIN) + goto skip3; + goto kill_session; + } + /* Ignore trailing NUL if it is there */ + if (!TS_BUF1[ts->rdidx1 + count - 1]) { + --count; + } + ts->size1 += count; + ts->rdidx1 += count; + if (ts->rdidx1 >= BUFSIZE) /* actually == BUFSIZE */ + ts->rdidx1 = 0; + } + skip3: + if (/*ts->size2 < BUFSIZE &&*/ FD_ISSET(ts->ptyfd, &rdfdset)) { + /* Read from pty to buffer 2. */ + count = MIN(BUFSIZE - ts->rdidx2, BUFSIZE - ts->size2); + count = safe_read(ts->ptyfd, TS_BUF2 + ts->rdidx2, count); + if (count <= 0) { + if (count < 0 && errno == EAGAIN) + goto skip4; + goto kill_session; + } + ts->size2 += count; + ts->rdidx2 += count; + if (ts->rdidx2 >= BUFSIZE) /* actually == BUFSIZE */ + ts->rdidx2 = 0; + } + skip4: + ts = next; + continue; + kill_session: + free_session(ts); + ts = next; + } + + goto again; +} |