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1 .\" $NetBSD: rshd.8,v 1.18 2005/03/09 16:42:49 wiz Exp $
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30 .\" from: @(#)rshd.8 8.1 (Berkeley) 6/4/93
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32 .Dd March 9, 2005
33 .Dt RSHD 8
34 .Os
35 .Sh NAME
36 .Nm rshd
37 .Nd remote shell server
38 .Sh SYNOPSIS
39 .Nm
40 .Op Fl aLln
41 .Sh DESCRIPTION
42 The
43 .Nm
44 server is the server for the
45 .Xr rcmd 3
46 routine and, consequently, for the
47 .Xr rsh 1
48 program.
49 The server provides remote execution facilities
50 with authentication based on privileged port numbers from trusted hosts.
51 .Pp
52 The
53 .Nm
54 server listens for service requests at the port indicated in
55 the
56 .Dq cmd
57 service specification; see
58 .Xr services 5 .
59 When a service request is received the following protocol
60 is initiated:
61 .Bl -enum
62 .It
63 The server checks the client's source port.
64 If the port is not in the range 512-1023, the server
65 aborts the connection.
66 .It
67 The server reads characters from the socket up
68 to a null
69 .Pq Sq \e0
70 byte.
71 The resultant string is interpreted as an
72 .Tn ASCII
73 number, base 10.
74 .It
75 If the number received in step 2 is non-zero,
76 it is interpreted as the port number of a secondary
77 stream to be used for the
78 .Em stderr .
79 A second connection is then created to the specified
80 port on the client's machine.
81 The source port of this
82 second connection is also in the range 512-1023.
83 .It
84 The server checks the client's source address
85 and requests the corresponding host name (see
86 .Xr getnameinfo 3 ,
87 .Xr hosts 5 ,
88 and
89 .Xr named 8 ) .
90 If the hostname cannot be determined,
91 the dot-notation representation of the host address is used.
92 If the hostname is in the same domain as the server (according to
93 the last two components of the domain name), or if the
94 .Fl a
95 option is given,
96 the addresses for the hostname are requested,
97 verifying that the name and address correspond.
98 If address verification fails, the connection is aborted
99 with the message
100 .Dq Host address mismatch.
101 .It
102 A null terminated user name of at most 16 characters
103 is retrieved on the initial socket.
104 This user name is interpreted as the user identity on the
105 .Em client Ns 's
106 machine.
107 .It
108 A null terminated user name of at most 16 characters
109 is retrieved on the initial socket.
110 This user name is interpreted as a user identity to use on the
111 .Sy server Ns 's
112 machine.
113 .It
114 A null terminated command to be passed to a
115 shell is retrieved on the initial socket.
116 The length of the command is limited by the upper
117 bound on the size of the system's argument list.
118 .It
119 .Nm
120 then validates the user using
121 .Xr ruserok 3 ,
122 which uses the file
123 .Pa /etc/hosts.equiv
124 and the
125 .Pa .rhosts
126 file found in the user's home directory.
127 The
128 .Fl l
129 option prevents
130 .Xr ruserok 3
131 from doing any validation based on the user's
132 .Dq Pa .rhosts
133 file, unless the user is the superuser.
134 .It
135 If the file
136 .Pa /etc/nologin
137 exists and the user is not the superuser,
138 the connection is closed.
139 .It
140 A null byte is returned on the initial socket
141 and the command line is passed to the normal login
142 shell of the user.
143 The shell inherits the network connections established by
144 .Nm .
145 .El
146 .Pp
147 Transport-level keepalive messages are enabled unless the
148 .Fl n
149 option is present.
150 The use of keepalive messages allows sessions to be timed out
151 if the client crashes or becomes unreachable.
152 .Pp
153 The
154 .Fl L
155 option causes all successful accesses to be logged to
156 .Xr syslogd 8
157 as
158 .Li auth.info
159 messages.
160 .Sh DIAGNOSTICS
161 Except for the last one listed below,
162 all diagnostic messages
163 are returned on the initial socket,
164 after which any network connections are closed.
165 An error is indicated by a leading byte with a value of
166 1 (0 is returned in step 10 above upon successful completion
167 of all the steps prior to the execution of the login shell).
168 .Bl -tag -width indent
169 .It Sy Locuser too long.
170 The name of the user on the client's machine is
171 longer than 16 characters.
172 .It Sy Ruser too long.
173 The name of the user on the remote machine is
174 longer than 16 characters.
175 .It Sy Command too long .
176 The command line passed exceeds the size of the argument
177 list (as configured into the system).
178 .It Sy Login incorrect.
179 No password file entry for the user name existed.
180 .It Sy Remote directory.
181 The
182 .Xr chdir 2
183 to the home directory failed.
184 .It Sy Permission denied.
185 The authentication procedure described above failed.
186 .It Sy Can't make pipe.
187 The pipe needed for the
188 .Em stderr ,
189 wasn't created.
190 .It Sy Can't fork; try again.
191 A
192 .Xr fork 2
193 by the server failed.
194 .It Sy \*[Lt]shellname\*[Gt]: ...
195 The user's login shell could not be started.
196 This message is returned on the connection associated with the
197 .Em stderr ,
198 and is not preceded by a flag byte.
199 .El
200 .Sh SEE ALSO
201 .Xr rsh 1 ,
202 .Xr ssh 1 ,
203 .Xr rcmd 3 ,
204 .Xr ruserok 3 ,
205 .Xr hosts_access 5 ,
206 .Xr login.conf 5 ,
207 .Xr sshd 8
208 .Sh BUGS
209 The authentication procedure used here assumes the integrity
210 of every machine and every network that can reach the rshd/rlogind
211 ports on the server.
212 This is insecure, but is useful in an
213 .Dq open
214 environment.
215 .Xr sshd 8
216 or a Kerberized version of this server are much more secure.
217 .Pp
218 A facility to allow all data exchanges to be encrypted should be
219 present.
220 .Pp
221 A more extensible protocol (such as Telnet) should be used.
222 .Pp
223 .Nm
224 intentionally rejects accesses from IPv4 mapped address on top of
225 .Dv AF_INET6
226 socket, since IPv4 mapped address complicates
227 host-address based authentication.
228 If you would like to accept connections from IPv4 peers, you will
229 need to run
230 .Nm
231 on top of an
232 .Dv AF_INET
233 socket, not an
234 .Dv AF_INET6
235 socket.