SSH-COPY-ID(1)            BSD General Commands Manual           SSH-COPY-ID(1)

     ssh-copy-id -- use locally available keys to authorise logins on a remote

     ssh-copy-id [-f] [-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 in-
     stalled (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 re-
     mote system is a NetScreen, and using its 'set ssh pka-dsa key ...' com-
     mand instead.

     The options are as follows:

     -i identity_file
             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

     -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) op-
             tions, respectively.

             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.

     The default_ID_file is the most recent file that matches: ~/.ssh/id*.pub,
     (excluding those that match ~/.ssh/* 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.

     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 un-
     prompted, 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 en-
     sure 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 an-
     other 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 au-
     thentication. This way the middle hop(s) don't get access to your
     ssh-agent(1).  A web search for 'ssh proxycommand nc' should prove en-
     lightening (N.B. the modern approach is to use the -W option, rather than

     ssh(1), ssh-agent(1), sshd(8)

BSD                              June 17, 2010                             BSD
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