ssh-copy-id(1)
NAME
ssh-copy-id --- use locally available keys to authorise logins on a remote
machine
SYNOPSIS
ssh-copy-id [-f] [-n] [-i [identity_file]] [-p port] [-o ssh_option]
[user@]hostname
ssh-copy-id -h | -?
DESCRIPTION
ssh-copy-id is a script that uses 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 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 ~/.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 'set ssh pka-dsa key ...'
command instead.
The options are as follows:
-i identity_file
Use only the key(s) contained in identity_file (rather than
looking for identities via ssh-add(1) or in the default_ID_file).
If the filename does not end in .pub this is added. If the
filename is omitted, the default_ID_file is used.
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.
-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.
-n do a dry-run. Instead of installing keys on the remote system
simply prints the key(s) that would have been installed.
-h, -? Print Usage summary
-p port, -o ssh_option
These two options are simply passed through untouched, along with
their argument, to allow one to set the port or other ssh(1)
options, respectively.
Rather than specifying these as command line options, it is often
better to use (per-host) settings in ssh(1)'s configuration file:
ssh_config(5).
Default behaviour without -i, is to check if '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 ssh-add(1) when
the key was loaded into your ssh-agent(1) rather than the comment
contained in that file, which is a bit of a shame. Otherwise, if
ssh-add(1) provides no keys contents of the default_ID_file will be used.
The default_ID_file is the most recent file that matches: ~/.ssh/id*.pub,
(excluding those that match ~/.ssh/*-cert.pub) so if you create a key
that is not the one you want ssh-copy-id to use, just use touch(1) on
your preferred key's .pub file to reinstate it as the most recent.
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 ssh-agent(1). Load the new key first, without
the -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 -A option
to allow agent forwarding:
user@newclient$ ssh-add
user@newclient$ ssh -A old.client
user@oldl$ ssh-add -c
... prompt for pass-phrase ...
user@old$ logoff
user@newclient$ ssh someserver
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
user@newclient$ ssh-copy-id -i someserver
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 .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 ssh-agent(1). Of course, you can specify
another id, or use the contents of the ssh-agent(1) as you prefer.
Having mentioned ssh-add(1)'s -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 ssh(1)'s ProxyCommand and -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
ssh-agent(1). A web search for 'ssh proxycommand nc' should prove
enlightening (N.B. the modern approach is to use the -W option, rather
than nc(1)).
SEE ALSO
ssh(1), ssh-agent(1), sshd(8)
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