1TELNETD(8) BSD System Manager's Manual TELNETD(8)
2
4 telnetd — DARPA telnet protocol server
5
7 /usr/sbin/in.telnetd [-ihnNs] [-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 -D debugmode
55 This option may be used for debugging purposes. This allows
56 telnetd to print out debugging information to the connec‐
57 tion, allowing the user to see what telnetd is doing. There
58 are several possible values for debugmode:
59 options Prints information about the negotiation of telnet
61 options.
62 report Prints the options information, plus some addi‐
64 tional information about what processing is going
65 on.
66 netdata Displays the data stream received by telnetd.
68 ptydata Displays data written to the pty.
70 exercise Has not been implemented yet.
72 -edebug If telnetd has been compiled with support for encryption,
74 then the -edebug option may be used to enable encryption
75 debugging code.
76 -h Disables the printing of host-specific information before
78 login has been completed.
79 -i Disable reverse DNS lookups and use the numeric IP address
81 in logs and REMOTEHOST environment variable. (-i switch cor‐
82 responds to utilities like last)
83 -L loginprg This option may be used to specify a different login pro‐
85 gram. By default, /bin/login is used.
86 -n Disable TCP keep-alives. Normally telnetd enables the TCP
88 keep-alive mechanism to probe connections that have been
89 idle for some period of time to determine if the client is
90 still there, so that idle connections from machines that
91 have crashed or can no longer be reached may be cleaned up.
92 -N Do not use DNS to canonicalize the local hostname;
94 gethostname(2) returns a fully qualified name.
95 -s This option is only enabled if telnetd is compiled with sup‐
97 port for SecurID cards. It causes the -s option to be
98 passed on to login(1), and thus is only useful if login(1)
99 supports the -s flag to indicate that only SecurID validated
100 logins are allowed. This is usually useful for controlling
101 remote logins from outside of a firewall.
102 -S tos Sets the IP type-of-service (TOS) option for the telnet con‐
104 nection to the value tos.
105 -X authtype This option is only valid if telnetd has been built with
107 support for the authentication option. It disables the use
108 of authtype authentication, and can be used to temporarily
109 disable a specific authentication type without having to
110 recompile telnetd.
111 If the file /etc/issue.net is present, telnetd will display its contents
113 before the login prompt of a telnet session (see issue.net(5)).
114 Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for
116 a client, then creating a login process which has the slave side of the
117 pseudo-terminal as stdin, stdout, and stderr. Telnetd manipulates the
118 master side of the pseudo-terminal, implementing the telnet protocol and
119 passing characters between the remote client and the login process.
120 When a telnet session is started up, telnetd sends telnet options to the
122 client side indicating a willingness to do the following telnet options,
123 which are described in more detail below:
124 DO AUTHENTICATION
126 WILL ENCRYPT
127 DO TERMINAL TYPE
128 DO TSPEED
129 DO XDISPLOC
130 DO NEW-ENVIRON
131 DO ENVIRON
132 WILL SUPPRESS GO AHEAD
133 DO ECHO
134 DO LINEMODE
135 DO NAWS
136 WILL STATUS
137 DO LFLOW
138 DO TIMING-MARK
139 The pseudo-terminal allocated to the client is configured to operate in
141 cooked mode, and with XTABS CRMOD enabled (see tty(4)).
142 Telnetd has support for enabling locally the following telnet options:
144 WILL ECHO When the LINEMODE option is enabled, a WILL ECHO or
146 WONT ECHO will be sent to the client to indicate the
147 current state of terminal echoing. When terminal echo
148 is not desired, a WILL ECHO is sent to indicate that
149 telnetd will take care of echoing any data that needs
150 to be echoed to the terminal, and then nothing is
151 echoed. When terminal echo is desired, a WONT ECHO is
152 sent to indicate that telnetd will not be doing any
153 terminal echoing, so the client should do any terminal
154 echoing that is needed.
155 WILL BINARY Indicates that the client is willing to send a 8 bits
157 of data, rather than the normal 7 bits of the Network
158 Virtual Terminal.
159 WILL SGA Indicates that it will not be sending IAC GA, go
161 ahead, commands.
162 WILL STATUS Indicates a willingness to send the client, upon
164 request, of the current status of all TELNET options.
165 WILL TIMING-MARK Whenever a DO TIMING-MARK command is received, it is
167 always responded to with a WILL TIMING-MARK
168 WILL LOGOUT When a DO LOGOUT is received, a WILL LOGOUT is sent in
170 response, and the TELNET session is shut down.
171 WILL ENCRYPT Only sent if telnetd is compiled with support for data
173 encryption, and indicates a willingness to decrypt the
174 data stream.
175 Telnetd has support for enabling remotely the following TELNET options:
177 DO BINARY Sent to indicate that telnetd is willing to receive an
179 8 bit data stream.
180 DO LFLOW Requests that the client handle flow control charac‐
182 ters remotely.
183 DO ECHO This is not really supported, but is sent to identify
185 a 4.2BSD telnet(1) client, which will improperly
186 respond with WILL ECHO. If a WILL ECHO is received, a
187 DONT ECHO will be sent in response.
188 DO TERMINAL-TYPE Indicates a desire to be able to request the name of
190 the type of terminal that is attached to the client
191 side of the connection.
192 DO SGA Indicates that it does not need to receive IAC GA, the
194 go ahead command.
195 DO NAWS Requests that the client inform the server when the
197 window (display) size changes.
198 DO TERMINAL-SPEED Indicates a desire to be able to request information
200 about the speed of the serial line to which the client
201 is attached.
202 DO XDISPLOC Indicates a desire to be able to request the name of
204 the X windows display that is associated with the tel‐
205 net client.
206 DO NEW-ENVIRON Indicates a desire to be able to request environment
208 variable information, as described in RFC 1572.
209 DO ENVIRON Indicates a desire to be able to request environment
211 variable information, as described in RFC 1408.
212 DO LINEMODE Only sent if telnetd is compiled with support for
214 linemode, and requests that the client do line by line
215 processing.
216 DO TIMING-MARK Only sent if telnetd is compiled with support for both
218 linemode and kludge linemode, and the client responded
219 with WONT LINEMODE. If the client responds with WILL
220 TM, the it is assumed that the client supports kludge
221 linemode. Note that the [-k] option can be used to
222 disable this.
223 DO AUTHENTICATION Only sent if telnetd is compiled with support for
225 authentication, and indicates a willingness to receive
226 authentication information for automatic login.
227 DO ENCRYPT Only sent if telnetd is compiled with support for data
229 encryption, and indicates a willingness to decrypt the
230 data stream. issue.net(5)).
231
233 /etc/services, /etc/issue.net
234
236 telnet(1), login(1), issue.net(5),
237
239 RFC-854 TELNET PROTOCOL SPECIFICATION
240 RFC-855 TELNET OPTION SPECIFICATIONS
241 RFC-856 TELNET BINARY TRANSMISSION
242 RFC-857 TELNET ECHO OPTION
243 RFC-858 TELNET SUPPRESS GO AHEAD OPTION
244 RFC-859 TELNET STATUS OPTION
245 RFC-860 TELNET TIMING MARK OPTION
246 RFC-861 TELNET EXTENDED OPTIONS - LIST OPTION
247 RFC-885 TELNET END OF RECORD OPTION
248 RFC-1073 Telnet Window Size Option
249 RFC-1079 Telnet Terminal Speed Option
250 RFC-1091 Telnet Terminal-Type Option
251 RFC-1096 Telnet X Display Location Option
252 RFC-1123 Requirements for Internet Hosts -- Application and Support
253 RFC-1184 Telnet Linemode Option
254 RFC-1372 Telnet Remote Flow Control Option
255 RFC-1416 Telnet Authentication Option
256 RFC-1411 Telnet Authentication: Kerberos Version 4
257 RFC-1412 Telnet Authentication: SPX
258 RFC-1571 Telnet Environment Option Interoperability Issues
259 RFC-1572 Telnet Environment Option
260
262 Some TELNET commands are only partially implemented.
263 Because of bugs in the original 4.2 BSD telnet(1), telnetd performs some
265 dubious protocol exchanges to try to discover if the remote client is, in
266 fact, a 4.2 BSD telnet(1).
267 Binary mode has no common interpretation except between similar operating
269 systems (Unix in this case).
270 The terminal type name received from the remote client is converted to
272 lower case.
273 Telnetd never sends TELNET IAC GA (go ahead) commands.
275 The source code is not comprehensible.
277Linux NetKit (0.17) December 29, 1996 Linux NetKit (0.17)