Manual: ssh-copy-id
SSH-COPY-ID(1) General Commands Manual 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] [-s] [-x] [-i [identity_file]] [-t target_path] [-F ssh_config] [[-o ssh_option] ...] [-p port]
[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(1), 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.
-s SFTP mode: usually the public keys are installed by executing commands on the remote side. With this option
the user's ~/.ssh/authorized_keys file will be downloaded, modified locally and uploaded with sftp. This op‐
tion is useful if the server has restrictions on commands which can be used on the remote side.
-t target_path
the path on the target system where the keys should be added (defaults to ".ssh/authorized_keys")
-p port
Specifies the port to connect to on the remote host.
-F ssh_config, -o ssh_option
These options are simply passed through untouched (with their argument) to ssh/sftp, allowing one to set an
alternative config file, or other 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).
-x This option is for debugging the ssh-copy-id script itself. It sets the shell's -x flag, so that you can see
the commands being run.
-h, -? Print Usage summary
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 your 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 re‐
mote 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 (NB the modern approach is to
use the -W option, rather than nc(1)).
SEE ALSO
ssh(1), ssh-agent(1), sshd(8)
Debian June 17, 2010 SSH-COPY-ID(1)