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24.Dd $Mdocdate: June 17 2010 $
25.Dt SSH-COPY-ID 1
26.Os
27.Sh NAME
28.Nm ssh-copy-id
29.Nd use locally available keys to authorise logins on a remote machine
30.Sh SYNOPSIS
31.Nm
32.Op Fl f
33.Op Fl n
34.Op Fl i Op Ar identity_file
35.Op Fl p Ar port
36.Op Fl o Ar ssh_option
37.Op Ar user Ns @ Ns
38.Ar hostname
39.Nm
40.Fl h | Fl ?
41.br
42.Sh DESCRIPTION
43.Nm
44is a script that uses
45.Xr ssh 1
46to log into a remote machine (presumably using a login password,
47so password authentication should be enabled, unless you've done some
48clever use of multiple identities).  It assembles a list of one or more
49fingerprints (as described below) and tries to log in with each key, to
50see if any of them are already installed (of course, if you are not using
51.Xr ssh-agent 1
52this may result in you being repeatedly prompted for pass-phrases).
53It then assembles a list of those that failed to log in, and using ssh,
54enables logins with those keys on the remote server.  By default it adds
55the keys by appending them to the remote user's
56.Pa ~/.ssh/authorized_keys
57(creating the file, and directory, if necessary).  It is also capable
58of detecting if the remote system is a NetScreen, and using its
59.Ql set ssh pka-dsa key ...
60command instead.
61.Pp
62The options are as follows:
63.Bl -tag -width Ds
64.It Fl i Ar identity_file
65Use only the key(s) contained in
66.Ar identity_file
67(rather than looking for identities via
68.Xr ssh-add 1
69or in the
70.Ic default_ID_file ) .
71If the filename does not end in
72.Pa .pub
73this is added.  If the filename is omitted, the
74.Ic default_ID_file
75is used.
76.Pp
77Note that this can be used to ensure that the keys copied have the
78comment one prefers and/or extra options applied, by ensuring that the
79key file has these set as preferred before the copy is attempted.
80.It Fl f
81Forced mode: doesn't check if the keys are present on the remote server.
82This means that it does not need the private key.  Of course, this can result
83in more than one copy of the key being installed on the remote system.
84.It Fl n
85do a dry-run.  Instead of installing keys on the remote system simply
86prints the key(s) that would have been installed.
87.It Fl h , Fl ?
88Print Usage summary
89.It Fl p Ar port , Fl o Ar ssh_option
90These two options are simply passed through untouched, along with their
91argument, to allow one to set the port or other
92.Xr ssh 1
93options, respectively.
94.Pp
95Rather than specifying these as command line options, it is often better to use (per-host) settings in
96.Xr ssh 1 Ns 's
97configuration file:
98.Xr ssh_config 5 .
99.El
100.Pp
101Default behaviour without
102.Fl i ,
103is to check if
104.Ql ssh-add -L
105provides any output, and if so those keys are used.  Note that this results in
106the comment on the key being the filename that was given to
107.Xr ssh-add 1
108when the key was loaded into your
109.Xr ssh-agent 1
110rather than the comment contained in that file, which is a bit of a shame.
111Otherwise, if
112.Xr ssh-add 1
113provides no keys contents of the
114.Ic default_ID_file
115will be used.
116.Pp
117The
118.Ic default_ID_file
119is the most recent file that matches:
120.Pa ~/.ssh/id*.pub ,
121(excluding those that match
122.Pa ~/.ssh/*-cert.pub )
123so if you create a key that is not the one you want
124.Nm
125to use, just use
126.Xr touch 1
127on your preferred key's
128.Pa .pub
129file to reinstate it as the most recent.
130.Pp
131.Sh EXAMPLES
132If you have already installed keys from one system on a lot of remote
133hosts, and you then create a new key, on a new client machine, say,
134it can be difficult to keep track of which systems on which you've
135installed the new key.  One way of dealing with this is to load both
136the new key and old key(s) into your
137.Xr ssh-agent 1 .
138Load the new key first, without the
139.Fl c
140option, then load one or more old keys into the agent, possibly by
141ssh-ing to the client machine that has that old key, using the
142.Fl A
143option to allow agent forwarding:
144.Pp
145.D1 user@newclient$ ssh-add
146.D1 user@newclient$ ssh -A old.client
147.D1 user@oldl$ ssh-add -c
148.D1 No   ... prompt for pass-phrase ...
149.D1 user@old$ logoff
150.D1 user@newclient$ ssh someserver
151.Pp
152now, if the new key is installed on the server, you'll be allowed in
153unprompted, whereas if you only have the old key(s) enabled, you'll be
154asked for confirmation, which is your cue to log back out and run
155.Pp
156.D1 user@newclient$ ssh-copy-id -i someserver
157.Pp
158The reason you might want to specify the -i option in this case is to
159ensure that the comment on the installed key is the one from the
160.Pa .pub
161file, rather than just the filename that was loaded into you agent.
162It also ensures that only the id you intended is installed, rather than
163all the keys that you have in your
164.Xr ssh-agent 1 .
165Of course, you can specify another id, or use the contents of the
166.Xr ssh-agent 1
167as you prefer.
168.Pp
169Having mentioned
170.Xr ssh-add 1 Ns 's
171.Fl c
172option, you might consider using this whenever using agent forwarding
173to avoid your key being hijacked, but it is much better to instead use
174.Xr ssh 1 Ns 's
175.Ar ProxyCommand
176and
177.Fl W
178option,
179to bounce through remote servers while always doing direct end-to-end
180authentication. This way the middle hop(s) don't get access to your
181.Xr ssh-agent 1 .
182A web search for
183.Ql ssh proxycommand nc
184should prove enlightening (N.B. the modern approach is to use the
185.Fl W
186option, rather than
187.Xr nc 1 ) .
188.Sh "SEE ALSO"
189.Xr ssh 1 ,
190.Xr ssh-agent 1 ,
191.Xr sshd 8
192