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b86f0198d

Denys Vlasenko

2026-02-24

SUCCESS Duration: 144s

Subject

telnet: fixes after testing against some MUD servers

Binary

No binary (build failed)

Architecture & Compiler Flags

Architecture: x86_64-linux-gnu Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

gcc -Wp,-MD,applets/.usage.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer      -Iinclude -Iinclude -o applets/usage applets/usage.c

Build Log

(log yok)

Commit Diff GitHub →

networking/telnet.c +193 -112

networking/telnet.c+193 -112

@@ -52,20 +52,14 @@

	//kbuild:lib-$(CONFIG_TELNET) += telnet.o

-	//usage:#if ENABLE_FEATURE_TELNET_AUTOLOGIN
	//usage:#define telnet_trivial_usage
-	//usage: "[-a] [-l USER] HOST [PORT]"
-	//usage:#define telnet_full_usage "\n\n"
-	//usage: "Connect to telnet server\n"
-	//usage: "\n -a Automatic login with $USER variable"
-	//usage: "\n -l USER Automatic login as USER"
-	//usage:
-	//usage:#else
-	//usage:#define telnet_trivial_usage
-	//usage: "HOST [PORT]"
+	//usage: IF_FEATURE_TELNET_AUTOLOGIN("[-a] [-l USER] ")"HOST [PORT]"
	//usage:#define telnet_full_usage "\n\n"
	//usage: "Connect to telnet server"
-	//usage:#endif
+	//usage: IF_FEATURE_TELNET_AUTOLOGIN("\n"
+	//usage: "\n -a Automatic login with $USER envvar"
+	//usage: "\n -l USER Automatic login as USER"
+	//usage: )

	#include <arpa/telnet.h>
	#include <netinet/in.h>
@@ -79,15 +73,24 @@
	# define DO 253 /* please, you use option */
	# define WONT 252 /* I won't use option */
	# define WILL 251 /* I will use option (if I see your "IAC DO <opt>" confirmation */
-	# define SB 250 /* interpret as subnegotiation */
-	# define SE 240 /* end sub negotiation */
+	# define SB 250 /* begin subnegotiation */
+	# define SE 240 /* end subnegotiation */
	# define TELOPT_ECHO 1 /* echo */
	# define TELOPT_SGA 3 /* suppress go ahead */
	# define TELOPT_TTYPE 24 /* terminal type */
	# define TELOPT_NAWS 31 /* window size */
	#endif

-	#define DEBUG 0
+	#define SUPPORT_FOR_LOCAL_OPTS (0 \
+	\|\| ENABLE_FEATURE_TELNET_WIDTH \
+	\|\| ENABLE_FEATURE_TELNET_TTYPE \
+	\|\| ENABLE_FEATURE_TELNET_AUTOLOGIN \
+	)
+
+	// -v option
+	#define VERBOSE 0
+	// lots of unconditional debug
+	#define DEBUG 0

	#if DEBUG
	# define dbg(...) bb_error_msg(__VA_ARGS__)
@@ -102,6 +105,14 @@ static char bin_to_hex(const void hash_value, unsigned hash_length)
	# define dbg(...) ((void)0)
	#endif

+	#if VERBOSE
+	# define log1(...) do { if (G.verbose) bb_error_msg(__VA_ARGS__); } while (0)
+	# define IF_VERBOSE(...) __VA_ARGS__
+	#else
+	# define log1(...) ((void)0)
+	# define IF_VERBOSE(...) /* nothing */
+	#endif
+
	enum {
	TS_NORMAL = 0,
	TS_IAC = 1,
@@ -110,7 +121,7 @@ enum {
	TS_SUB2 = 4,
	TS_CR = 5,

-	MAX_NAWS_SIZE = 13, /* pathological 65535x65535 case needs full escaping */
+	MAX_NAWS_SIZE = 13, // pathological 65535x65535 case needs full escaping

	netfd = 3,
	};
@@ -144,45 +155,51 @@ static void set_input_state(net_to_stdout_t *conn, int new_state, int c)
	#endif

	struct globals {
-	unsigned flags;
-	/* Set when server agreed to use NAWS: */
-	#define FLAGS_NAWS_ON (1 << 0)
-	/* SGA option seen and responded to, no longer look for it: */
-	#define FLAGS_SGA_SEEN (1 << 1)
-	/* Seen telnet protocol from server and sent our wishes: */
-	#define INITIAL_SENT (1 << 2)
-	#define DO_TERMIOS (1 << 3)
-
-	byte word_aligned_bytes[2];
-	#define changed word_aligned_bytes[0]
+	unsigned flags;
+	// Set when server agreed to use NAWS:
+	#define FLAGS_NAWS_ON (1 << 0)
+	// SGA option seen and responded to, no longer look for it:
+	#define FLAGS_SGA_SEEN (1 << 1)
+	// Seen telnet protocol from server and sent our WILLs:
+	#define INITIAL_SENT (1 << 2)
+	#define DO_TERMIOS (1 << 3)
+
+	byte word_aligned_bytes[2];
+	#define changed word_aligned_bytes[0]
	#define CHANGED_ECHO (1 << 0)
	#define CHANGED_NAWS (1 << 1)
-	/* These happen only once: */
+	// These happen only once:
	#define CHANGED_SGA (1 << 2)
	#define CHANGED_TTYPE (1 << 3)
	#define CHANGED_NEW_ENVIRON (1 << 4)
-	/* The second byte is changed async, by signal handler: */
-	#define got_SIGWINCH word_aligned_bytes[1]
-	#define G_changed_word ((uint16_t)G.word_aligned_bytes)
+	// The second byte is changed async, by signal handler:
+	#define got_SIGWINCH word_aligned_bytes[1]
+	#define G_changed_word ((uint16_t)G.word_aligned_bytes)

	byte optstate_ECHO;
+	#define OPT_ECHO_ON 1
+	// We operate tty in rawmode only when echo ON (IOW: we saw server say that)
+	#define OPT_ECHO_OFF 0
+	#define OPT_ECHO_UNKNOWN 0xff /* on program start */
+
	byte echo_sga_response_size;

+	IF_VERBOSE( unsigned verbose;)
	IF_FEATURE_TELNET_TTYPE( const char *ttype;)
	IF_FEATURE_TELNET_AUTOLOGIN(const char *autologin;)
-	ioloop_state_t io;
-	stdin_to_net_t conn_stdin2net;
+	ioloop_state_t io;
+	stdin_to_net_t conn_stdin2net;
	net_to_stdout_t conn_net2stdout;
-	struct termios termios_def;
-	struct termios termios_raw;
+	struct termios termios_def;
+	struct termios termios_raw;
	// buf[] arrays in conn structs are conceptually cleaner, but they
	// make G.member offsets larger -> larger code
-	#define BUF_TTY2NET ((byte*)bb_common_bufsiz1)
-	#define BUF_NET2TTY G.buf2
-	#define BUFSIZE 1024
-	/* Note: can't just increase BUFSIZE arbitrarily: common bufsiz1 is not guaranteed to be >1k! */
-	#define BUFMASK (BUFSIZE - 1)
-	byte buf2[BUFSIZE];
+	#define BUF_TTY2NET ((byte*)bb_common_bufsiz1)
+	#define BUF_NET2TTY G.buf2
+	#define BUFSIZE 1024
+	// Note: can't just increase BUFSIZE arbitrarily: common bufsiz1 is not guaranteed to be >1k!
+	#define BUFMASK (BUFSIZE - 1)
+	byte buf2[BUFSIZE];
	} FIX_ALIASING;
	#define G (*ptr_to_globals)
	#define INIT_G() do { \
@@ -197,7 +214,8 @@ static int remaining_free_bytes(int n)
	#if ENABLE_FEATURE_TELNET_WIDTH
	static void handle_SIGWINCH(int sig UNUSED_PARAM)
	{
-	if (G.flags & FLAGS_NAWS_ON) /* Only if server okayed NAWS */
+	// Only if server said DO NAWS and seen our WILL NAWS:
+	if ((G.flags & (FLAGS_NAWS_ON\|INITIAL_SENT)) == (FLAGS_NAWS_ON\|INITIAL_SENT))
	G.got_SIGWINCH = 1;
	}
	#endif
@@ -237,9 +255,7 @@ static void put_iac2_msb_lsb(unsigned x_y)
	}
	#define put_iac2_x_y(x,y) put_iac2_msb_lsb(((x)<<8) + (y))

-	#if ENABLE_FEATURE_TELNET_WIDTH \
-	\|\| ENABLE_FEATURE_TELNET_TTYPE \
-	\|\| ENABLE_FEATURE_TELNET_AUTOLOGIN
+	#if SUPPORT_FOR_LOCAL_OPTS
	static void put_iac4_msb_lsb(unsigned x_y_z_t)
	{
	put_iac2_msb_lsb(x_y_z_t >> 16);
@@ -303,6 +319,7 @@ static void put_iac_naws(void)

	/* Send width and height as 16-bit big-endian, escaping IAC bytes */
	get_terminal_width_height(0, &width, &height);
+	log1("C:SB NAWS %dx%d", width, height);
	put_iac_byte_escaped(width >> 8);
	put_iac_byte_escaped(width);
	put_iac_byte_escaped(height >> 8);
@@ -314,125 +331,175 @@ static void put_iac_naws(void)

	// Telnet option handling strategy:
	//
-	// We send nothing on startup by our own (think "telnet www.kernel.org 80").
-	// As soon as we see ECHO or SGA message, we respond to them and then
-	// send "IAC WILL NAWS", "IAC WILL TTYPE", "IAC WILL NEW_ENVIRON"
+	// We send nothing on startup on our own (think "telnet www.kernel.org 80").
+	// As soon as we see any DO X or WILL X message known to us, we respond
+	// to them: send DO/DONT ECHO, DO SGA if server talked about them, then send
+	// WILL NAWS, WILL TTYPE, WILL NEW_ENVIRON
	// without waiting for server to advertise those options.
-	// (Why? servers may decide to not advertise all 123 options they know).
+	// (Why? Some servers may decide to not advertise all 123 options they know).
	//
	// TELOPT_ECHO (1) - remote: server echoes our keystrokes back to us
-	// - Server says: "IAC WILL ECHO" or "IAC WONT ECHO"
+	// - Server says: WILL ECHO or WONT ECHO
	// - If we don't see it, or see WONT: line mode (local terminal echoes), aka cooked mode
	// - When active: We're in character mode (server echoes), aka raw mode
	// - Response logic:
-	// - Server says WILL ECHO: we send "IAC DO ECHO", enter raw mode
-	// (this is the most usual case for non-ancient servers)
-	// - Server says WONT ECHO: we send "IAC DONT ECHO", enter cooked mode
-	// - We mirror server's changes, we do allow it to change during session
-	// unlike one-shot options which are negotiated once during startup.
-	// - If user changes mode from ^] menu, we send corresponding "IAC DO/DONT ECHO".
+	// - Server says WILL ECHO: we send DO ECHO, enter raw mode
+	// (this is the most usual case for non-ancient servers)
+	// - Server says WONT ECHO: we send DONT ECHO, enter cooked mode
+	// - We mirror server's changes, we do allow it to change during session
+	// unlike one-shot options which are negotiated once during startup.
+	// - If user changes mode from ^] menu, we send corresponding DO/DONT ECHO.
	//
	// TELOPT_SGA (3) - remote: "Suppress Go Ahead", full-duplex
-	// - Server says: "IAC WILL SGA" or "IAC WONT SGA"
+	// - Server says: WILL SGA or WONT SGA
	// - Response logic: one-shot.
-	// - We expect all sane servers to say "IAC WILL SGA" at startup.
-	// - If that happens, we send "IAC DO SGA". After that, we never react to any
-	// further SGA messages (we expect them to not happen in practice)
+	// - We expect all sane servers to choose WILL SGA at startup.
+	// - If that happens, we send DO SGA. After that, we never react to any
+	// further SGA messages (we expect them to not repeat in practice)
	//
	// TELOPT_NAWS (31) - Window Size - local: we send our window size
-	// - If server says: "IAC DO NAWS", it understands NAWS. We check this.
+	// - If server says: DO NAWS, it understands NAWS. We require this before
+	// - sending SB NAWS ...
	// - We send window size during initial handshake and on window resize
	//
	// TELOPT_TTYPE (24) - Terminal Type - local: we send our terminal type
	// - Response logic: one-shot.
-	// - If server says DO TTYPE: send "IAC SB TTYPE 0 <$TERM> IAC SE"
+	// - If server says DO TTYPE: send "IAC SB TTYPE 0 '$TERM' IAC SE"
	//
	// TELOPT_NEW_ENVIRON (39) - Environment (Autologin) - local: we send our username
	// - Response logic: one-shot.
-	// - If server says DO NEW_ENVIRON: send "IAC SB NEW_ENVIRON IS VAR USER VALUE <name> IAC SE"
+	// - If server says DO NEW_ENVIRON: send "IAC SB NEW_ENVIRON IS VAR 'USER' VALUE 'name' IAC SE"
	//
	// Unknown options:
	// Response: NONE - just ignore them
	// By protocol definition, an option is not in effect until BOTH sides agree.
	// We don't respond to unknown options - they simply won't be in effect.
+	//
+	// Testing at discworld.atuin.net:23 reveal servers often do NOT negotiate ECHO/SGA
+	// in initial handshake while supporting NAWS and TTYPE:
+	//telnet: S:DO 24 <--TTYPE
+	//telnet: TTYPE:'xterm-256color' setting CHANGED_TTYPE
+	//LPmud version : DW OS v1.02 on port 4242.
+	//telnet: changed:8 flags:8
+	//telnet: C:SB TTYPE 'xterm-256color'
+	//telnet: S:DO 31 <--NAWS
+	//telnet: S:WILL 86
+	//telnet: S:DO 91
+	//telnet: S:WILL 70
+	//telnet: S:WILL 93
+	//telnet: S:DO 39
+	//telnet: S:WILL 201
+	//......text.....
+	//telnet: changed:2 flags:8
+	//telnet: C:SB NAWS 226x53
+	//...text....
+	//telnet: S:SB 24 <--TTYPE
+	//...text....
+	//Setting your network terminal type to "xterm-256color".
+	//> cols
+	//Columns currently set to 79.
+	//> rows
+	//Rows currently set to 24.
+	//^^^ server understood TTYPE but not NAWS. ?!
	static void handle_changes_in_options(stdin_to_net_t *conn)
	{
	int count;

-	//bb_error_msg("changed:%x flags:%x", G.changed, G.flags);
+	log1("changed:%x flags:%x", G.changed, G.flags);

	count = remaining_free_bytes(conn->size);
-	/* As soon as we see either ECHO or SGA from server,
-	* we assume it is a telnet server
-	* (not in "telnet www.kernel.org 80" scenario),
-	* and we respond to them, also expressing our own
-	* wishes: "WILL NAWS" etc.
-	*/
-	if ((G.changed & (CHANGED_ECHO\|CHANGED_SGA))
+	// As soon as we see any DO/DONT/WILL/WONT known to us,
+	// we assume it is a telnet server
+	// (we are not in "telnet www.kernel.org 80" scenario),
+	// and we respond to them, also expressing our own
+	// wishes: WILL NAWS etc. We send our WILLs once, all of them.
+	// We send actual SB with option contents after WILLs
+	// only if that option was requested.
+	// We repeatedly send only NAWS (when our window changes).
+	// Repeated DO requests are ignored.
+	if (G.changed
	&& (count >= G.echo_sga_response_size)
	) {
	if (G.changed & CHANGED_ECHO) {
-	/* Server said WILL/WONT - confirm */
+	// Server said WILL/WONT ECHO - confirm every time
+	log1("C:%s ECHO", G.optstate_ECHO ? "DO" : "DONT");
	put_iac3_IAC_x_y(G.optstate_ECHO ? DO : DONT, TELOPT_ECHO);
	}
	if (G.changed & CHANGED_SGA) {
-	/* Server said WILL - send DO */
+	// Server said WILL SGA - confirm once
+	log1("C:DO SGA");
	put_iac3_IAC_x_y(DO, TELOPT_SGA);
-	G.flags \|= FLAGS_SGA_SEEN; /* remember we did it */
+	G.flags \|= FLAGS_SGA_SEEN; // remember we did it
	G.changed -= CHANGED_SGA;
	}
	G.changed &= ~(CHANGED_ECHO\|CHANGED_SGA);

	if (!(G.flags & INITIAL_SENT)) {
+	// From now on, we'll only do DO/DONT ECHO and maybe DO SGA
+	// in the "if (G.changed)" block.
	G.flags \|= INITIAL_SENT;
-	G.echo_sga_response_size = 6; /* from now on, we'll need only 6 */
+	G.echo_sga_response_size = (G.flags & FLAGS_SGA_SEEN) ? 3 : 6;

-	/* Send initial IAC sequences for local options we want to advertise */
+	// Send initial IAC sequences for local options we want to advertise
+	// (there may be servers which send DO X only after we say WILL X)
	#if ENABLE_FEATURE_TELNET_WIDTH
+	log1("C:WILL %s", "NAWS");
	put_iac3_IAC_x_y(WILL, TELOPT_NAWS);
	#endif
	#if ENABLE_FEATURE_TELNET_TTYPE
-	if (G.ttype)
+	if (G.ttype) {
+	log1("C:WILL %s", "TTYPE");
	put_iac3_IAC_x_y(WILL, TELOPT_TTYPE);
+	}
	#endif
	#if ENABLE_FEATURE_TELNET_AUTOLOGIN
-	if (G.autologin)
+	if (G.autologin) {
+	log1("C:WILL %s", "NEW_ENVIRON");
	put_iac3_IAC_x_y(WILL, TELOPT_NEW_ENVIRON);
+	}
	#endif
	}
	}

+	// If any of the checks below are true, then we know we
+	// went through INITIAL_SENT code path above.
+	// Therefore we always send WILL X before SB X...
	#if ENABLE_FEATURE_TELNET_WIDTH
	if (remaining_free_bytes(conn->size) > MAX_NAWS_SIZE) {
	if (G.changed & CHANGED_NAWS) {
-	G.flags \|= FLAGS_NAWS_ON; /* remember we did it */
+	G.flags \|= FLAGS_NAWS_ON; // remember we did it
	G.changed -= CHANGED_NAWS;
	goto generate_naws;
	}
-	/* Handle window resize: send updated NAWS if we have room */
+	// Handle window resize: send updated NAWS
	if (G.got_SIGWINCH) {
+	// we know that INITIAL_SENT is set, signal handler checks that
	generate_naws:
+	// Clear the flag before put_iac_naws() to avoid race
	G.got_SIGWINCH = 0;
-	/* Clear the flag before put_iac_naws() to avoid race! */
+	//log1("C:WILL %s", "NAWS, the second time, I'm telling you it's fine!");
+	//put_iac3_IAC_x_y(WILL, TELOPT_NAWS);
	put_iac_naws();
	}
	}
	#endif
	#if ENABLE_FEATURE_TELNET_TTYPE
	if ((G.changed & CHANGED_TTYPE)
-	&& remaining_free_bytes(conn->size) > 2 * strlen(G.ttype)
+	&& remaining_free_bytes(conn->size) > 6 + 2 * strlen(G.ttype)
	) {
+	log1("C:SB %s '%s'", "TTYPE", G.ttype);
	put_iac_subopt(TELOPT_TTYPE, G.ttype);
-	G.ttype = NULL; /* remember we did it */
+	G.ttype = NULL; // remember we did it
	G.changed -= CHANGED_TTYPE;
	}
	#endif
	#if ENABLE_FEATURE_TELNET_AUTOLOGIN
	if ((G.changed & CHANGED_NEW_ENVIRON)
-	&& remaining_free_bytes(conn->size) > 2 * strlen(G.autologin)
+	&& remaining_free_bytes(conn->size) > 12 + 2 * strlen(G.autologin)
	) {
+	log1("C:SB %s '%s'", "NEW_ENVIRON", G.autologin);
	put_iac_subopt_autologin(G.autologin);
-	G.autologin = NULL; /* remember we did it */
+	G.autologin = NULL; // remember we did it
	G.changed -= CHANGED_NEW_ENVIRON;
	}
	#endif
@@ -477,20 +544,19 @@ static void show_menu(void)

	switch (b) {
	case 'l':
-	if (G.optstate_ECHO) {
-	G.optstate_ECHO = 0;
-	if (G.flags & INITIAL_SENT)
-	G.changed \|= CHANGED_ECHO; /* inform the server at next send */
+	if (G.optstate_ECHO == OPT_ECHO_ON) { /* are we in rawmode? */
+	G.optstate_ECHO = OPT_ECHO_OFF;
	announce_and_switch_to_cookmode();
-	return;
+	goto echo_changed;
	}
	break;
	case 'c':
-	if (!G.optstate_ECHO) {
-	G.optstate_ECHO = 1;
+	if (G.optstate_ECHO != OPT_ECHO_ON) { /* OFF or UNKNOWN? (both operate as linemode) */
+	G.optstate_ECHO = OPT_ECHO_ON;
+	announce_rawmode(); /* no "_and_switch_": we are already in rawmode */
+	echo_changed:
	if (G.flags & INITIAL_SENT)
	G.changed \|= CHANGED_ECHO; /* inform the server at next send */
-	announce_rawmode(); /* no "_and_switch_": we are already in rawmode */
	return;
	}
	break;
@@ -503,7 +569,7 @@ static void show_menu(void)

	full_write1_str("continuing...\r\n");

-	if (!G.optstate_ECHO)
+	if (G.optstate_ECHO != OPT_ECHO_ON)
	cookmode();
	}

@@ -722,7 +788,8 @@ static int read_from_net(void *this)
	} else if (c == SB) {
	conn->negotiation_verb = 0xff; /* reuse as counter */
	SET_INPUT_STATE(conn, TS_SUB1, c);
-	} else if (c == DO \|\| c == DONT \|\| c == WILL \|\| c == WONT) {
+	log1("S:SB %d", count ? *src : 0);
+	} else if (c == WILL \|\| c == WONT \|\| c == DO \|\| c == DONT) { /* 251-254 */
	conn->negotiation_verb = c;
	SET_INPUT_STATE(conn, TS_OPT, c);
	} else {
@@ -732,6 +799,12 @@ static int read_from_net(void *this)
	break;

	case TS_OPT:
+	#if VERBOSE
+	{
+	static const char verbs[] ALIGN1 = "WILL\0""WONT\0""DO\0""DONT";
+	log1("S:%s %d", nth_string(verbs, conn->negotiation_verb - WILL), c);
+	}
+	#endif
	/* Process option negotiation */
	will = (conn->negotiation_verb == WILL);
	if (will \|\| conn->negotiation_verb == WONT) {
@@ -748,6 +821,7 @@ static int read_from_net(void *this)
	switch (c) {
	#if ENABLE_FEATURE_TELNET_TTYPE
	case TELOPT_TTYPE: /* Local option: we send terminal type */
+	log1("TTYPE:'%s' %ssetting CHANGED_TTYPE", G.ttype, G.ttype ? "" : "not ");
	if (G.ttype)
	G.changed \|= CHANGED_TTYPE;
	break;
@@ -775,7 +849,7 @@ static int read_from_net(void *this)
	* if IAC SE is never seen (buggy server response?).
	*/
	if (--conn->negotiation_verb == 0) {
-	dbg("unterminated SB seen");
+	log1("unterminated SB (server bug?)");
	SET_INPUT_STATE(conn, TS_NORMAL, c);
	} else
	/* Skip over subnegotiation bytes until we see IAC */
@@ -801,7 +875,7 @@ static int read_from_net(void *this)
	/* Tell net writer to generate a confirmation */
	G.changed \|= CHANGED_ECHO;
	/* Print the banner and set termios */
-	if (G.optstate_ECHO)
+	if (G.optstate_ECHO == OPT_ECHO_ON)
	announce_and_switch_to_rawmode();
	else
	announce_and_switch_to_cookmode();
@@ -859,30 +933,35 @@ int telnet_main(int argc UNUSED_PARAM, char **argv)
	{
	char *host;
	int port;
+	int opts;

	INIT_G();

	#if ENABLE_FEATURE_TELNET_TTYPE
	G.ttype = getenv("TERM");
	#endif
+	opts = getopt32(argv, ""IF_VERBOSE("^")
+	IF_FEATURE_TELNET_AUTOLOGIN("al:")IF_VERBOSE("v")
+	IF_VERBOSE("\0" "vv")
+	IF_FEATURE_TELNET_AUTOLOGIN(, &G.autologin)
+	IF_VERBOSE(, &G.verbose)
+	);
	#if ENABLE_FEATURE_TELNET_AUTOLOGIN
-	if (1 == getopt32(argv, "al:", &G.autologin)) {
-	/* Only -a without -l USER picks $USER from envvar */
+	if ((opts & 3) == 1) {
+	// -a without -l USER picks USER from envvar:
	G.autologin = getenv("USER");
	}
-	argv += optind;
-	#else
-	argv++;
	#endif
+	argv += optind;
	if (!*argv)
	bb_show_usage();
	host = *argv++;
	port = argv ? bb_lookup_port(argv++, "tcp", 23)
	: bb_lookup_std_port("telnet", "tcp", 23);
-	if (argv) / extra params?? */
+	if (*argv) // extra params??
	bb_show_usage();

-	/* Save our termios */
+	// Save our termios
	if (tcgetattr(0, &G.termios_def) >= 0) {
	G.flags \|= DO_TERMIOS;
	G.termios_raw = G.termios_def;
@@ -899,10 +978,10 @@ int telnet_main(int argc UNUSED_PARAM, char **argv)
	signal(SIGWINCH, handle_SIGWINCH);
	#endif
	signal(SIGINT, record_signo);
-	/* Without this, SIGPIPE was seen on loopback connections: */
+	// Without this, SIGPIPE was seen on loopback connections:
	signal(SIGPIPE, SIG_IGN);

-	/* Initialize connections */
+	// Initialize connections
	G.conn_stdin2net.have_buffer_to_read_into = have_buffer_to_read_from_stdin;
	G.conn_stdin2net.have_data_to_write = have_data_to_write_to_net;
	G.conn_stdin2net.read = read_from_stdin;
@@ -924,18 +1003,20 @@ int telnet_main(int argc UNUSED_PARAM, char **argv)
	ioloop_insert_conn(&G.io, (connection_t*)&G.conn_net2stdout);

	G.echo_sga_response_size = 3 * (2
-	+ ENABLE_FEATURE_TELNET_WIDTH
+	IF_FEATURE_TELNET_WIDTH( + 1)
	IF_FEATURE_TELNET_TTYPE(+ !!G.ttype)
	IF_FEATURE_TELNET_AUTOLOGIN(+ !!G.autologin)
	);
-	#if DEBUG
-	/* Terminal can change to raw mode, fix line printing */
+	// Make any ECHO negotiation from server, positive or negative,
+	// trigger "we have a change" logic:
+	G.optstate_ECHO = OPT_ECHO_UNKNOWN;
+	#if DEBUG \|\| VERBOSE
+	// Terminal can change to raw mode, fix line printing
	msg_eol = "\r\n";
	#endif
-	/* EINTR flag and looping is only needed to handle ^C
-	* in line mode, otherwise just a call to ioloop_run() would do.
-	* TODO: replace primitive line mode with read_line_input()!!!
-	*/
+	// EINTR flag and looping is only needed to handle ^C
+	// in line mode, otherwise just a call to ioloop_run() would do.
+	// TODO: replace primitive line mode with read_line_input()!!!
	G.io.flags \|= IOLOOP_FLAG_EXIT_IF_EINTR;
	for (;;) {
	int rc = ioloop_run(&G.io);