ssh-copy-id [-n] [-i [identity_file]] [-p port] [-o ssh_option]
ssh-copy-id -h | -?
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 append-
ing 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 ...'
The options are as follows:
Use only the key(s) contained in identity_file (rather than look-
ing 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
-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)
Rather than specifying these as command line options, it is often
better to use (per-host) settings in ssh(1)'s configuration file:
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 con-
tained 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.
ssh-ing to the client machine that has that old key, using the -A option
to allow agent forwarding:
user@newclient$ ssh -A old.client
user@oldl$ ssh-add -c
... prompt for pass-phrase ...
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
ssh(1), ssh-agent(1), sshd(8)
BSD February 20, 2019 BSD
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