1TELNETD(8) BSD System Manager's Manual TELNETD(8)
2
4 telnetd — DARPA telnet protocol server
5
7 /usr/sbin/in.telnetd [-hns] [-a authmode] [-D debugmode] [-L loginprg]
8 [-S tos] [-X authtype] [-edebug] [-debug port]
9
11 The telnetd program is a server which supports the DARPA telnet interac‐
12 tive communication protocol. Telnetd is normally invoked by the internet
13 server (see inetd(8)) for requests to connect to the telnet port as indi‐
14 cated by the /etc/services file (see services(5)). The -debug option may
15 be used to start up telnetd manually, instead of through inetd(8). If
16 started up this way, port may be specified to run telnetd on an alternate
17 TCP port number.
18 The telnetd program accepts the following options:
20 -a authmode This option may be used for specifying what mode should be
22 used for authentication. Note that this option is only use‐
23 ful if telnetd has been compiled with support for authenti‐
24 cation, which is not available in the current version. The
25 following values of authmode are understood:
26 debug Turns on authentication debugging code.
28 user Only allow connections when the remote user can pro‐
30 vide valid authentication information to identify the
31 remote user, and is allowed access to the specified
32 account without providing a password.
33 valid Only allow connections when the remote user can pro‐
35 vide valid authentication information to identify the
36 remote user. The login(1) command will provide any
37 additional user verification needed if the remote
38 user is not allowed automatic access to the specified
39 account.
40 other Only allow connections that supply some authentica‐
42 tion information. This option is currently not sup‐
43 ported by any of the existing authentication mecha‐
44 nisms, and is thus the same as specifying valid.
45 none This is the default state. Authentication informa‐
47 tion is not required. If no or insufficient authen‐
48 tication information is provided, then the login(1)
49 program will provide the necessary user verification.
50 off This disables the authentication code. All user ver‐
52 ification will happen through the login(1) program.
53 -c This option disables reverse dns checking. Of course that
55 security is lower with this option
56 -D debugmode
58 This option may be used for debugging purposes. This allows
59 telnetd to print out debugging information to the connec‐
60 tion, allowing the user to see what telnetd is doing. There
61 are several possible values for debugmode:
62 options Prints information about the negotiation of telnet
64 options.
65 report Prints the options information, plus some addi‐
67 tional information about what processing is going
68 on.
69 netdata Displays the data stream received by telnetd.
71 ptydata Displays data written to the pty.
73 exercise Has not been implemented yet.
75 -edebug If telnetd has been compiled with support for encryption,
77 then the -edebug option may be used to enable encryption
78 debugging code.
79 -h Disables the printing of host-specific information before
81 login has been completed.
82 -L loginprg This option may be used to specify a different login pro‐
84 gram. By default, /bin/login is used.
85 -n Disable TCP keep-alives. Normally telnetd enables the TCP
87 keep-alive mechanism to probe connections that have been
88 idle for some period of time to determine if the client is
89 still there, so that idle connections from machines that
90 have crashed or can no longer be reached may be cleaned up.
91 -s This option is only enabled if telnetd is compiled with sup‐
93 port for SecurID cards. It causes the -s option to be
94 passed on to login(1), and thus is only useful if login(1)
95 supports the -s flag to indicate that only SecurID validated
96 logins are allowed. This is usually useful for controlling
97 remote logins from outside of a firewall.
98 -S tos Sets the IP type-of-service (TOS) option for the telnet con‐
100 nection to the value tos.
101 -X authtype This option is only valid if telnetd has been built with
103 support for the authentication option. It disables the use
104 of authtype authentication, and can be used to temporarily
105 disable a specific authentication type without having to
106 recompile telnetd.
107 If the file /etc/issue.net is present, telnetd will display its contents
109 before the login prompt of a telnet session (see issue.net(5)).
110 Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for
112 a client, then creating a login process which has the slave side of the
113 pseudo-terminal as stdin, stdout, and stderr. Telnetd manipulates the
114 master side of the pseudo-terminal, implementing the telnet protocol and
115 passing characters between the remote client and the login process.
116 When a telnet session is started up, telnetd sends telnet options to the
118 client side indicating a willingness to do the following telnet options,
119 which are described in more detail below:
120 DO AUTHENTICATION
122 WILL ENCRYPT
123 DO TERMINAL TYPE
124 DO TSPEED
125 DO XDISPLOC
126 DO NEW-ENVIRON
127 DO ENVIRON
128 WILL SUPPRESS GO AHEAD
129 DO ECHO
130 DO LINEMODE
131 DO NAWS
132 WILL STATUS
133 DO LFLOW
134 DO TIMING-MARK
135 The pseudo-terminal allocated to the client is configured to operate in
137 cooked mode, and with XTABS CRMOD enabled (see tty(4)).
138 Telnetd has support for enabling locally the following telnet options:
140 WILL ECHO When the LINEMODE option is enabled, a WILL ECHO or
142 WONT ECHO will be sent to the client to indicate the
143 current state of terminal echoing. When terminal echo
144 is not desired, a WILL ECHO is sent to indicate that
145 telnetd will take care of echoing any data that needs
146 to be echoed to the terminal, and then nothing is
147 echoed. When terminal echo is desired, a WONT ECHO is
148 sent to indicate that telnetd will not be doing any
149 terminal echoing, so the client should do any terminal
150 echoing that is needed.
151 WILL BINARY Indicates that the client is willing to send a 8 bits
153 of data, rather than the normal 7 bits of the Network
154 Virtual Terminal.
155 WILL SGA Indicates that it will not be sending IAC GA, go
157 ahead, commands.
158 WILL STATUS Indicates a willingness to send the client, upon
160 request, of the current status of all TELNET options.
161 WILL TIMING-MARK Whenever a DO TIMING-MARK command is received, it is
163 always responded to with a WILL TIMING-MARK
164 WILL LOGOUT When a DO LOGOUT is received, a WILL LOGOUT is sent in
166 response, and the TELNET session is shut down.
167 WILL ENCRYPT Only sent if telnetd is compiled with support for data
169 encryption, and indicates a willingness to decrypt the
170 data stream.
171 Telnetd has support for enabling remotely the following TELNET options:
173 DO BINARY Sent to indicate that telnetd is willing to receive an
175 8 bit data stream.
176 DO LFLOW Requests that the client handle flow control charac‐
178 ters remotely.
179 DO ECHO This is not really supported, but is sent to identify
181 a 4.2BSD telnet(1) client, which will improperly
182 respond with WILL ECHO. If a WILL ECHO is received, a
183 DONT ECHO will be sent in response.
184 DO TERMINAL-TYPE Indicates a desire to be able to request the name of
186 the type of terminal that is attached to the client
187 side of the connection.
188 DO SGA Indicates that it does not need to receive IAC GA, the
190 go ahead command.
191 DO NAWS Requests that the client inform the server when the
193 window (display) size changes.
194 DO TERMINAL-SPEED Indicates a desire to be able to request information
196 about the speed of the serial line to which the client
197 is attached.
198 DO XDISPLOC Indicates a desire to be able to request the name of
200 the X windows display that is associated with the tel‐
201 net client.
202 DO NEW-ENVIRON Indicates a desire to be able to request environment
204 variable information, as described in RFC 1572.
205 DO ENVIRON Indicates a desire to be able to request environment
207 variable information, as described in RFC 1408.
208 DO LINEMODE Only sent if telnetd is compiled with support for
210 linemode, and requests that the client do line by line
211 processing.
212 DO TIMING-MARK Only sent if telnetd is compiled with support for both
214 linemode and kludge linemode, and the client responded
215 with WONT LINEMODE. If the client responds with WILL
216 TM, the it is assumed that the client supports kludge
217 linemode. Note that the [-k] option can be used to
218 disable this.
219 DO AUTHENTICATION Only sent if telnetd is compiled with support for
221 authentication, and indicates a willingness to receive
222 authentication information for automatic login.
223 DO ENCRYPT Only sent if telnetd is compiled with support for data
225 encryption, and indicates a willingness to decrypt the
226 data stream. issue.net(5)).
227
229 /etc/services, /etc/issue.net
230
232 telnet(1), login(1), issue.net(5),
233
235 RFC-854 TELNET PROTOCOL SPECIFICATION
236 RFC-855 TELNET OPTION SPECIFICATIONS
237 RFC-856 TELNET BINARY TRANSMISSION
238 RFC-857 TELNET ECHO OPTION
239 RFC-858 TELNET SUPPRESS GO AHEAD OPTION
240 RFC-859 TELNET STATUS OPTION
241 RFC-860 TELNET TIMING MARK OPTION
242 RFC-861 TELNET EXTENDED OPTIONS - LIST OPTION
243 RFC-885 TELNET END OF RECORD OPTION
244 RFC-1073 Telnet Window Size Option
245 RFC-1079 Telnet Terminal Speed Option
246 RFC-1091 Telnet Terminal-Type Option
247 RFC-1096 Telnet X Display Location Option
248 RFC-1123 Requirements for Internet Hosts -- Application and Support
249 RFC-1184 Telnet Linemode Option
250 RFC-1372 Telnet Remote Flow Control Option
251 RFC-1416 Telnet Authentication Option
252 RFC-1411 Telnet Authentication: Kerberos Version 4
253 RFC-1412 Telnet Authentication: SPX
254 RFC-1571 Telnet Environment Option Interoperability Issues
255 RFC-1572 Telnet Environment Option
256
258 Some TELNET commands are only partially implemented.
259 Because of bugs in the original 4.2 BSD telnet(1), telnetd performs some
261 dubious protocol exchanges to try to discover if the remote client is, in
262 fact, a 4.2 BSD telnet(1).
263 Binary mode has no common interpretation except between similar operating
265 systems (Unix in this case).
266 The terminal type name received from the remote client is converted to
268 lower case.
269 Telnetd never sends TELNET IAC GA (go ahead) commands.
271 The source code is not comprehensible.
273Linux NetKit (0.17) December 29, 1996 Linux NetKit (0.17)