openssh/contrib/ssh-copy-id.1

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..
.Dd $Mdocdate: June 17 2010 $
.Dt SSH-COPY-ID 1
.Os
.Sh NAME
.Nm ssh-copy-id
.Nd use locally available keys to authorise logins on a remote machine
.Sh SYNOPSIS
.Nm
.Op Fl f
.Op Fl n
.Op Fl i Op Ar identity_file
.Op Fl p Ar port
.Op Fl o Ar ssh_option
.Op Ar user Ns @ Ns
.Ar hostname
.Nm
.Fl h | Fl ?
.br
.Sh DESCRIPTION
.Nm
is a script that uses
.Xr ssh 1
to log into a remote machine (presumably using a login password,
so password authentication should be enabled, unless you've done some
clever use of multiple identities).  It assembles a list of one or more
fingerprints (as described below) and tries to log in with each key, to
see if any of them are already installed (of course, if you are not using
.Xr ssh-agent 1
this may result in you being repeatedly prompted for pass-phrases).
It then assembles a list of those that failed to log in, and using ssh,
enables logins with those keys on the remote server.  By default it adds
the keys by appending them to the remote user's
.Pa ~/.ssh/authorized_keys
(creating the file, and directory, if necessary).  It is also capable
of detecting if the remote system is a NetScreen, and using its
.Ql set ssh pka-dsa key ...
command instead.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl i Ar identity_file
Use only the key(s) contained in
.Ar identity_file
(rather than looking for identities via
.Xr ssh-add 1
or in the
.Ic default_ID_file ) .
If the filename does not end in
.Pa .pub
this is added.  If the filename is omitted, the 
.Ic default_ID_file
is used.
.Pp
Note that this can be used to ensure that the keys copied have the
comment one prefers and/or extra options applied, by ensuring that the
key file has these set as preferred before the copy is attempted.
.It Fl f
Forced mode: doesn't check if the keys are present on the remote server.
This means that it does not need the private key.  Of course, this can result
in more than one copy of the key being installed on the remote system.
.It Fl n
do a dry-run.  Instead of installing keys on the remote system simply
prints the key(s) that would have been installed.
.It Fl h , Fl ?
Print Usage summary
.It Fl p Ar port , Fl o Ar ssh_option
These two options are simply passed through untouched, along with their
argument, to allow one to set the port or other
.Xr ssh 1
options, respectively.
.Pp
Rather than specifying these as command line options, it is often better to use (per-host) settings in
.Xr ssh 1 Ns 's
configuration file:
.Xr ssh_config 5 .
.El
.Pp
Default behaviour without
.Fl i ,
is to check if
.Ql ssh-add -L
provides any output, and if so those keys are used.  Note that this results in
the comment on the key being the filename that was given to
.Xr ssh-add 1
when the key was loaded into your
.Xr ssh-agent 1
rather than the comment contained in that file, which is a bit of a shame.
Otherwise, if
.Xr ssh-add 1
provides no keys contents of the 
.Ic default_ID_file
will be used.
.Pp
The
.Ic default_ID_file
is the most recent file that matches:
.Pa ~/.ssh/id*.pub ,
(excluding those that match
.Pa ~/.ssh/*-cert.pub )
so if you create a key that is not the one you want
.Nm
to use, just use
.Xr touch 1
on your preferred key's 
.Pa .pub
file to reinstate it as the most recent.
.Pp
.Sh EXAMPLES
If you have already installed keys from one system on a lot of remote
hosts, and you then create a new key, on a new client machine, say,
it can be difficult to keep track of which systems on which you've
installed the new key.  One way of dealing with this is to load both
the new key and old key(s) into your
.Xr ssh-agent 1 .
Load the new key first, without the
.Fl c
option, then load one or more old keys into the agent, possibly by
ssh-ing to the client machine that has that old key, using the
.Fl A
option to allow agent forwarding:
.Pp
.D1 user@newclient$ ssh-add
.D1 user@newclient$ ssh -A old.client
.D1 user@oldl$ ssh-add -c
.D1 No   ... prompt for pass-phrase ...
.D1 user@old$ logoff
.D1 user@newclient$ ssh someserver
.Pp
now, if the new key is installed on the server, you'll be allowed in
unprompted, whereas if you only have the old key(s) enabled, you'll be
asked for confirmation, which is your cue to log back out and run
.Pp
.D1 user@newclient$ ssh-copy-id -i someserver
.Pp
The reason you might want to specify the -i option in this case is to
ensure that the comment on the installed key is the one from the
.Pa .pub
file, rather than just the filename that was loaded into you agent.
It also ensures that only the id you intended is installed, rather than
all the keys that you have in your
.Xr ssh-agent 1 .
Of course, you can specify another id, or use the contents of the
.Xr ssh-agent 1
as you prefer.
.Pp
Having mentioned
.Xr ssh-add 1 Ns 's
.Fl c
option, you might consider using this whenever using agent forwarding
to avoid your key being hijacked, but it is much better to instead use
.Xr ssh 1 Ns 's
.Ar ProxyCommand
and 
.Fl W
option,
to bounce through remote servers while always doing direct end-to-end
authentication. This way the middle hop(s) don't get access to your
.Xr ssh-agent 1 .
A web search for
.Ql ssh proxycommand nc
should prove enlightening (N.B. the modern approach is to use the
.Fl W
option, rather than
.Xr nc 1 ) .
.Sh "SEE ALSO"
.Xr ssh 1 ,
.Xr ssh-agent 1 ,
.Xr sshd 8