telnetd: treat all 2-byte IACs in 240..249 range as NOPs.

A bit of future-proofing. Some of them can stand just being ignored.

function                                             old     new   delta
telnetd_main                                        1791    1798      +7

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 24 insertions, 18 deletions

diff --git a/networking/telnetd.c b/networking/telnetd.c
index 0da29410e..eaaf29bce 100644
--- a/networking/telnetd.c
+++ b/networking/telnetd.c
@@ -169,12 +169,10 @@ safe_write_to_pty_decode_iac(struct tsession *ts)
  handle_iac:
         /* 2-byte commands (240..250 and 255):
          * IAC IAC (255) Literal 255. Supported.
+         * IAC SE  (240) End of subnegotiation. Treated as NOP.
          * IAC NOP (241) NOP. Supported.
          * IAC BRK (243) Break. Like serial line break. TODO via tcsendbreak()?
          * IAC AYT (246) Are you there. Send back evidence that AYT was seen. TODO (send NOP back)?
-         *  Implemented only as part of NAWS:
-         * IAC SB  (250) Subnegotiation of an option follows.
-         * IAC SE  (240) End of subnegotiation.
          *  These don't look useful:
          * IAC DM  (242) Data mark. What is this?
          * IAC IP  (244) Suspend, interrupt or abort the process. (Ancient cousin of ^C).
@@ -182,8 +180,11 @@ safe_write_to_pty_decode_iac(struct tsession *ts)
          * IAC EC  (247) Erase character. The receiver should delete the last received char.
          * IAC EL  (248) Erase line. The receiver should delete everything up tp last newline.
          * IAC GA  (249) Go ahead. For half-duplex lines: "now you talk".
+         *  Implemented only as part of NAWS:
+         * IAC SB  (250) Subnegotiation of an option follows.
          */
-        if (buf[1] == IAC) { /* Literal 255 (emacs M-DEL) */
+        if (buf[1] == IAC) {
+                /* Literal 255 (emacs M-DEL) */
                 //bb_error_msg("255!");
                 rc = safe_write(ts->ptyfd, &buf[1], 1);
                 if (rc <= 0)
@@ -191,7 +192,9 @@ safe_write_to_pty_decode_iac(struct tsession *ts)
                 rc = 2;
                 goto update_and_return;
         }
-        if (buf[1] == NOP) { /* NOP (241). Ignore (putty keepalive, etc) */
+        if (buf[1] >= 240 && buf[1] <= 249) {
+                /* NOP (241). Ignore (putty keepalive, etc) */
+                /* All other 2-byte commands also treated as NOPs here */
                 rc = 2;
                 goto update_and_return;
         }
@@ -205,24 +208,27 @@ safe_write_to_pty_decode_iac(struct tsession *ts)
                 goto update_and_return;
         }
 
-        /* TELOPT_NAWS support */
-        /* IAC SB, TELOPT_NAWS, 4-byte, IAC SE */
-        if (buf[1] == SB && buf[2] == TELOPT_NAWS) {
-                struct winsize ws;
-                if (wr <= 8) {
+        if (buf[1] == SB) {
+                if (buf[2] == TELOPT_NAWS) {
+                        /* IAC SB, TELOPT_NAWS, 4-byte, IAC SE */
+                        struct winsize ws;
+                        if (wr <= 6) {
 /* BUG: incomplete, can't process */
-                        rc = wr;
+                                rc = wr;
+                                goto update_and_return;
+                        }
+                        memset(&ws, 0, sizeof(ws)); /* pixel sizes are set to 0 */
+                        ws.ws_col = (buf[3] << 8) | buf[4];
+                        ws.ws_row = (buf[5] << 8) | buf[6];
+                        ioctl(ts->ptyfd, TIOCSWINSZ, (char *)&ws);
+                        rc = 7;
+                        /* trailing IAC SE will be eaten separately, as 2-byte NOP */
                         goto update_and_return;
                 }
-                memset(&ws, 0, sizeof(ws));
-                ws.ws_col = (buf[3] << 8) | buf[4];
-                ws.ws_row = (buf[5] << 8) | buf[6];
-                ioctl(ts->ptyfd, TIOCSWINSZ, (char *)&ws);
-                rc = 9;
-                goto update_and_return;
+                /* else: other subnegs not supported yet */
         }
 
-        /* Skip 3-byte cmds (assume they are WILL/WONT/DO/DONT 251..254 codes) */
+        /* Assume it is a 3-byte WILL/WONT/DO/DONT 251..254 command and skip it */
 #if DEBUG
         fprintf(stderr, "Ignoring IAC %s,%s\n",
                         TELCMD(buf[1]), TELOPT(buf[2]));