1SSH-COPY-ID(1) BSD General Commands Manual SSH-COPY-ID(1)
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4 ssh-copy-id — use locally available keys to authorise logins on a remote
5 machine
6
8 ssh-copy-id [-f] [-n] [-i [identity_file]] [-p port] [-o ssh_option]
9 [user@]hostname
10 ssh-copy-id -h | -?
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13 ssh-copy-id is a script that uses ssh(1) to log into a remote machine
14 (presumably using a login password, so password authentication should be
15 enabled, unless you've done some clever use of multiple identities). It
16 assembles a list of one or more fingerprints (as described below) and
17 tries to log in with each key, to see if any of them are already
18 installed (of course, if you are not using ssh-agent(1) this may result
19 in you being repeatedly prompted for pass-phrases). It then assembles a
20 list of those that failed to log in, and using ssh, enables logins with
21 those keys on the remote server. By default it adds the keys by append‐
22 ing them to the remote user's ~/.ssh/authorized_keys (creating the file,
23 and directory, if necessary). It is also capable of detecting if the
24 remote system is a NetScreen, and using its ‘set ssh pka-dsa key ...’
25 command instead.
26 The options are as follows:
28 -i identity_file
30 Use only the key(s) contained in identity_file (rather than look‐
31 ing for identities via ssh-add(1) or in the default_ID_file). If
32 the filename does not end in .pub this is added. If the filename
33 is omitted, the default_ID_file is used.
34 Note that this can be used to ensure that the keys copied have
36 the comment one prefers and/or extra options applied, by ensuring
37 that the key file has these set as preferred before the copy is
38 attempted.
39 -f Forced mode: doesn't check if the keys are present on the remote
41 server. This means that it does not need the private key. Of
42 course, this can result in more than one copy of the key being
43 installed on the remote system.
44 -n do a dry-run. Instead of installing keys on the remote system
46 simply prints the key(s) that would have been installed.
47 -h, -? Print Usage summary
49 -p port, -o ssh_option
51 These two options are simply passed through untouched, along with
52 their argument, to allow one to set the port or other ssh(1)
53 options, respectively.
54 Rather than specifying these as command line options, it is often
56 better to use (per-host) settings in ssh(1)'s configuration file:
57 ssh_config(5).
58 Default behaviour without -i, is to check if ‘ssh-add -L’ provides any
60 output, and if so those keys are used. Note that this results in the
61 comment on the key being the filename that was given to ssh-add(1) when
62 the key was loaded into your ssh-agent(1) rather than the comment con‐
63 tained in that file, which is a bit of a shame. Otherwise, if ssh-add(1)
64 provides no keys contents of the default_ID_file will be used.
65 The default_ID_file is the most recent file that matches: ~/.ssh/id*.pub,
67 (excluding those that match ~/.ssh/*-cert.pub) so if you create a key
68 that is not the one you want ssh-copy-id to use, just use touch(1) on
69 your preferred key's .pub file to reinstate it as the most recent.
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72 If you have already installed keys from one system on a lot of remote
73 hosts, and you then create a new key, on a new client machine, say, it
74 can be difficult to keep track of which systems on which you've installed
75 the new key. One way of dealing with this is to load both the new key
76 and old key(s) into your ssh-agent(1). Load the new key first, without
77 the -c option, then load one or more old keys into the agent, possibly by
78 ssh-ing to the client machine that has that old key, using the -A option
79 to allow agent forwarding:
80 user@newclient$ ssh-add
82 user@newclient$ ssh -A old.client
83 user@oldl$ ssh-add -c
84 ... prompt for pass-phrase ...
85 user@old$ logoff
86 user@newclient$ ssh someserver
87 now, if the new key is installed on the server, you'll be allowed in
89 unprompted, whereas if you only have the old key(s) enabled, you'll be
90 asked for confirmation, which is your cue to log back out and run
91 user@newclient$ ssh-copy-id -i someserver
93 The reason you might want to specify the -i option in this case is to
95 ensure that the comment on the installed key is the one from the .pub
96 file, rather than just the filename that was loaded into your agent. It
97 also ensures that only the id you intended is installed, rather than all
98 the keys that you have in your ssh-agent(1). Of course, you can specify
99 another id, or use the contents of the ssh-agent(1) as you prefer.
100 Having mentioned ssh-add(1)'s -c option, you might consider using this
102 whenever using agent forwarding to avoid your key being hijacked, but it
103 is much better to instead use ssh(1)'s ProxyCommand and -W option, to
104 bounce through remote servers while always doing direct end-to-end
105 authentication. This way the middle hop(s) don't get access to your
106 ssh-agent(1). A web search for ‘ssh proxycommand nc’ should prove
107 enlightening (N.B. the modern approach is to use the -W option, rather
108 than nc(1)).
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111 ssh(1), ssh-agent(1), sshd(8)
112BSD May 10, 2020 BSD