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