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-rw-r--r--contrib/ssh-copy-id377
-rw-r--r--contrib/ssh-copy-id.1198
2 files changed, 575 insertions, 0 deletions
diff --git a/contrib/ssh-copy-id b/contrib/ssh-copy-id
new file mode 100644
index 0000000..cd122de
--- /dev/null
+++ b/contrib/ssh-copy-id
@@ -0,0 +1,377 @@
+#!/bin/sh
+
+# Copyright (c) 1999-2020 Philip Hands <phil@hands.com>
+# 2020 Matthias Blümel <blaimi@blaimi.de>
+# 2017 Sebastien Boyron <seb@boyron.eu>
+# 2013 Martin Kletzander <mkletzan@redhat.com>
+# 2010 Adeodato =?iso-8859-1?Q?Sim=F3?= <asp16@alu.ua.es>
+# 2010 Eric Moret <eric.moret@gmail.com>
+# 2009 Xr <xr@i-jeuxvideo.com>
+# 2007 Justin Pryzby <justinpryzby@users.sourceforge.net>
+# 2004 Reini Urban <rurban@x-ray.at>
+# 2003 Colin Watson <cjwatson@debian.org>
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+# OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+# IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+# Shell script to install your public key(s) on a remote machine
+# See the ssh-copy-id(1) man page for details
+
+# shellcheck shell=dash
+
+# check that we have something mildly sane as our shell, or try to find something better
+if false ^ printf "%s: WARNING: ancient shell, hunting for a more modern one... " "$0"
+then
+ SANE_SH=${SANE_SH:-/usr/bin/ksh}
+ if printf 'true ^ false\n' | "$SANE_SH"
+ then
+ printf "'%s' seems viable.\\n" "$SANE_SH"
+ exec "$SANE_SH" "$0" "$@"
+ else
+ cat <<-EOF
+ oh dear.
+
+ If you have a more recent shell available, that supports \$(...) etc.
+ please try setting the environment variable SANE_SH to the path of that
+ shell, and then retry running this script. If that works, please report
+ a bug describing your setup, and the shell you used to make it work.
+
+ EOF
+ printf '%s: ERROR: Less dimwitted shell required.\n' "$0"
+ exit 1
+ fi
+fi
+
+# shellcheck disable=SC2010
+DEFAULT_PUB_ID_FILE=$(ls -t "${HOME}"/.ssh/id*.pub 2>/dev/null | grep -v -- '-cert.pub$' | head -n 1)
+SSH="ssh -a -x"
+umask 0177
+
+usage () {
+ printf 'Usage: %s [-h|-?|-f|-n|-s] [-i [identity_file]] [-p port] [-F alternative ssh_config file] [[-o <ssh -o options>] ...] [user@]hostname\n' "$0" >&2
+ printf '\t-f: force mode -- copy keys without trying to check if they are already installed\n' >&2
+ printf '\t-n: dry run -- no keys are actually copied\n' >&2
+ printf '\t-s: use sftp -- use sftp instead of executing remote-commands. Can be useful if the remote only allows sftp\n' >&2
+ printf '\t-h|-?: print this help\n' >&2
+ exit 1
+}
+
+# escape any single quotes in an argument
+quote() {
+ printf '%s\n' "$1" | sed -e "s/'/'\\\\''/g"
+}
+
+use_id_file() {
+ L_ID_FILE="$1"
+
+ if [ -z "$L_ID_FILE" ] ; then
+ printf '%s: ERROR: no ID file found\n' "$0"
+ exit 1
+ fi
+
+ if expr "$L_ID_FILE" : '.*\.pub$' >/dev/null ; then
+ PUB_ID_FILE="$L_ID_FILE"
+ else
+ PUB_ID_FILE="$L_ID_FILE.pub"
+ fi
+
+ [ "$FORCED" ] || PRIV_ID_FILE=$(dirname "$PUB_ID_FILE")/$(basename "$PUB_ID_FILE" .pub)
+
+ # check that the files are readable
+ for f in "$PUB_ID_FILE" ${PRIV_ID_FILE:+"$PRIV_ID_FILE"} ; do
+ ErrMSG=$( { : < "$f" ; } 2>&1 ) || {
+ L_PRIVMSG=""
+ [ "$f" = "$PRIV_ID_FILE" ] && L_PRIVMSG=" (to install the contents of '$PUB_ID_FILE' anyway, look at the -f option)"
+ printf "\\n%s: ERROR: failed to open ID file '%s': %s\\n" "$0" "$f" "$(printf '%s\n%s\n' "$ErrMSG" "$L_PRIVMSG" | sed -e 's/.*: *//')"
+ exit 1
+ }
+ done
+ printf '%s: INFO: Source of key(s) to be installed: "%s"\n' "$0" "$PUB_ID_FILE" >&2
+ GET_ID="cat \"$PUB_ID_FILE\""
+}
+
+if [ -n "$SSH_AUTH_SOCK" ] && ssh-add -L >/dev/null 2>&1 ; then
+ GET_ID="ssh-add -L"
+fi
+
+while getopts "i:o:p:F:fnsh?" OPT
+do
+ case "$OPT" in
+ i)
+ [ "${SEEN_OPT_I}" ] && {
+ printf '\n%s: ERROR: -i option must not be specified more than once\n\n' "$0"
+ usage
+ }
+ SEEN_OPT_I="yes"
+ use_id_file "${OPTARG:-$DEFAULT_PUB_ID_FILE}"
+ ;;
+ o|p|F)
+ SSH_OPTS="${SSH_OPTS:+$SSH_OPTS }-$OPT '$(quote "${OPTARG}")'"
+ ;;
+ f)
+ FORCED=1
+ ;;
+ n)
+ DRY_RUN=1
+ ;;
+ s)
+ SFTP=sftp
+ ;;
+ h|\?)
+ usage
+ ;;
+ esac
+done
+#shift all args to keep only USER_HOST
+shift $((OPTIND-1))
+
+if [ $# = 0 ] ; then
+ usage
+fi
+if [ $# != 1 ] ; then
+ printf '%s: ERROR: Too many arguments. Expecting a target hostname, got: %s\n\n' "$0" "$SAVEARGS" >&2
+ usage
+fi
+
+# drop trailing colon
+USER_HOST="$*"
+# tack the hostname onto SSH_OPTS
+SSH_OPTS="${SSH_OPTS:+$SSH_OPTS }'$(quote "$USER_HOST")'"
+# and populate "$@" for later use (only way to get proper quoting of options)
+eval set -- "$SSH_OPTS"
+
+# shellcheck disable=SC2086
+if [ -z "$(eval $GET_ID)" ] && [ -r "${PUB_ID_FILE:=$DEFAULT_PUB_ID_FILE}" ] ; then
+ use_id_file "$PUB_ID_FILE"
+fi
+
+# shellcheck disable=SC2086
+if [ -z "$(eval $GET_ID)" ] ; then
+ printf '%s: ERROR: No identities found\n' "$0" >&2
+ exit 1
+fi
+
+# filter_ids()
+# tries to log in using the keys piped to it, and filters out any that work
+filter_ids() {
+ L_SUCCESS="$1"
+ L_TMP_ID_FILE="$SCRATCH_DIR"/popids_tmp_id
+ L_OUTPUT_FILE="$SCRATCH_DIR"/popids_output
+
+ # repopulate "$@" inside this function
+ eval set -- "$SSH_OPTS"
+
+ while read -r ID || [ "$ID" ] ; do
+ printf '%s\n' "$ID" > "$L_TMP_ID_FILE"
+
+ # the next line assumes $PRIV_ID_FILE only set if using a single id file - this
+ # assumption will break if we implement the possibility of multiple -i options.
+ # The point being that if file based, ssh needs the private key, which it cannot
+ # find if only given the contents of the .pub file in an unrelated tmpfile
+ $SSH -i "${PRIV_ID_FILE:-$L_TMP_ID_FILE}" \
+ -o ControlPath=none \
+ -o LogLevel=INFO \
+ -o PreferredAuthentications=publickey \
+ -o IdentitiesOnly=yes "$@" exit >"$L_OUTPUT_FILE" 2>&1 </dev/null
+ if [ "$?" = "$L_SUCCESS" ] || {
+ [ "$SFTP" ] && grep 'allows sftp connections only' "$L_OUTPUT_FILE" >/dev/null
+ # this error counts as a success if we're setting up an sftp connection
+ }
+ then
+ : > "$L_TMP_ID_FILE"
+ else
+ grep 'Permission denied' "$L_OUTPUT_FILE" >/dev/null || {
+ sed -e 's/^/ERROR: /' <"$L_OUTPUT_FILE" >"$L_TMP_ID_FILE"
+ cat >/dev/null #consume the other keys, causing loop to end
+ }
+ fi
+
+ cat "$L_TMP_ID_FILE"
+ done
+}
+
+# populate_new_ids() uses several global variables ($USER_HOST, $SSH_OPTS ...)
+# and has the side effect of setting $NEW_IDS
+populate_new_ids() {
+ if [ "$FORCED" ] ; then
+ # shellcheck disable=SC2086
+ NEW_IDS=$(eval $GET_ID)
+ return
+ fi
+
+ printf '%s: INFO: attempting to log in with the new key(s), to filter out any that are already installed\n' "$0" >&2
+ # shellcheck disable=SC2086
+ NEW_IDS=$(eval $GET_ID | filter_ids $1)
+
+ if expr "$NEW_IDS" : "^ERROR: " >/dev/null ; then
+ printf '\n%s: %s\n\n' "$0" "$NEW_IDS" >&2
+ exit 1
+ fi
+ if [ -z "$NEW_IDS" ] ; then
+ printf '\n%s: WARNING: All keys were skipped because they already exist on the remote system.\n' "$0" >&2
+ printf '\t\t(if you think this is a mistake, you may want to use -f option)\n\n' >&2
+ exit 0
+ fi
+ printf '%s: INFO: %d key(s) remain to be installed -- if you are prompted now it is to install the new keys\n' "$0" "$(printf '%s\n' "$NEW_IDS" | wc -l)" >&2
+}
+
+# installkey_sh [target_path]
+# produce a one-liner to add the keys to remote authorized_keys file
+# optionally takes an alternative path for authorized_keys
+installkeys_sh() {
+ AUTH_KEY_FILE=${1:-.ssh/authorized_keys}
+ AUTH_KEY_DIR=$(dirname "${AUTH_KEY_FILE}")
+
+ # In setting INSTALLKEYS_SH:
+ # the tr puts it all on one line (to placate tcsh)
+ # (hence the excessive use of semi-colons (;) )
+ # then in the command:
+ # cd to be at $HOME, just in case;
+ # the -z `tail ...` checks for a trailing newline. The echo adds one if was missing
+ # the cat adds the keys we're getting via STDIN
+ # and if available restorecon is used to restore the SELinux context
+ INSTALLKEYS_SH=$(tr '\t\n' ' ' <<-EOF
+ cd;
+ umask 077;
+ mkdir -p "${AUTH_KEY_DIR}" &&
+ { [ -z \`tail -1c ${AUTH_KEY_FILE} 2>/dev/null\` ] ||
+ echo >> "${AUTH_KEY_FILE}" || exit 1; } &&
+ cat >> "${AUTH_KEY_FILE}" || exit 1;
+ if type restorecon >/dev/null 2>&1; then
+ restorecon -F "${AUTH_KEY_DIR}" "${AUTH_KEY_FILE}";
+ fi
+ EOF
+ )
+
+ # to defend against quirky remote shells: use 'exec sh -c' to get POSIX;
+ printf "exec sh -c '%s'" "${INSTALLKEYS_SH}"
+}
+
+#shellcheck disable=SC2120 # the 'eval set' confuses this
+installkeys_via_sftp() {
+
+ # repopulate "$@" inside this function
+ eval set -- "$SSH_OPTS"
+
+ L_KEYS=$SCRATCH_DIR/authorized_keys
+ L_SHARED_CON=$SCRATCH_DIR/master-conn
+ $SSH -f -N -M -S "$L_SHARED_CON" "$@"
+ L_CLEANUP="$SSH -S $L_SHARED_CON -O exit 'ignored' >/dev/null 2>&1 ; $SCRATCH_CLEANUP"
+ #shellcheck disable=SC2064
+ trap "$L_CLEANUP" EXIT TERM INT QUIT
+ sftp -b - -o "ControlPath=$L_SHARED_CON" "ignored" <<-EOF || return 1
+ -get .ssh/authorized_keys $L_KEYS
+ EOF
+ # add a newline or create file if it's missing, same like above
+ [ -z "$(tail -1c "$L_KEYS" 2>/dev/null)" ] || echo >> "$L_KEYS"
+ # append the keys being piped in here
+ cat >> "$L_KEYS"
+ sftp -b - -o "ControlPath=$L_SHARED_CON" "ignored" <<-EOF || return 1
+ -mkdir .ssh
+ chmod 700 .ssh
+ put $L_KEYS .ssh/authorized_keys
+ chmod 600 .ssh/authorized_keys
+ EOF
+ #shellcheck disable=SC2064
+ eval "$L_CLEANUP" && trap "$SCRATCH_CLEANUP" EXIT TERM INT QUIT
+}
+
+
+# create a scratch dir for any temporary files needed
+if SCRATCH_DIR=$(mktemp -d ~/.ssh/ssh-copy-id.XXXXXXXXXX) &&
+ [ "$SCRATCH_DIR" ] && [ -d "$SCRATCH_DIR" ]
+then
+ chmod 0700 "$SCRATCH_DIR"
+ SCRATCH_CLEANUP="rm -rf \"$SCRATCH_DIR\""
+ #shellcheck disable=SC2064
+ trap "$SCRATCH_CLEANUP" EXIT TERM INT QUIT
+else
+ printf '%s: ERROR: failed to create required temporary directory under ~/.ssh\n' "$0" >&2
+ exit 1
+fi
+
+REMOTE_VERSION=$($SSH -v -o PreferredAuthentications=',' -o ControlPath=none "$@" 2>&1 |
+ sed -ne 's/.*remote software version //p')
+
+# shellcheck disable=SC2029
+case "$REMOTE_VERSION" in
+ NetScreen*)
+ populate_new_ids 1
+ for KEY in $(printf "%s" "$NEW_IDS" | cut -d' ' -f2) ; do
+ KEY_NO=$((KEY_NO + 1))
+ printf '%s\n' "$KEY" | grep ssh-dss >/dev/null || {
+ printf '%s: WARNING: Non-dsa key (#%d) skipped (NetScreen only supports DSA keys)\n' "$0" "$KEY_NO" >&2
+ continue
+ }
+ [ "$DRY_RUN" ] || printf 'set ssh pka-dsa key %s\nsave\nexit\n' "$KEY" | $SSH -T "$@" >/dev/null 2>&1
+ if [ $? = 255 ] ; then
+ printf '%s: ERROR: installation of key #%d failed (please report a bug describing what caused this, so that we can make this message useful)\n' "$0" "$KEY_NO" >&2
+ else
+ ADDED=$((ADDED + 1))
+ fi
+ done
+ if [ -z "$ADDED" ] ; then
+ exit 1
+ fi
+ ;;
+ dropbear*)
+ populate_new_ids 0
+ [ "$DRY_RUN" ] || printf '%s\n' "$NEW_IDS" | \
+ $SSH "$@" "$(installkeys_sh /etc/dropbear/authorized_keys)" \
+ || exit 1
+ ADDED=$(printf '%s\n' "$NEW_IDS" | wc -l)
+ ;;
+ *)
+ # Assuming that the remote host treats ~/.ssh/authorized_keys as one might expect
+ populate_new_ids 0
+ if ! [ "$DRY_RUN" ] ; then
+ printf '%s\n' "$NEW_IDS" | \
+ if [ "$SFTP" ] ; then
+ #shellcheck disable=SC2119
+ installkeys_via_sftp
+ else
+ $SSH "$@" "$(installkeys_sh)"
+ fi || exit 1
+ fi
+ ADDED=$(printf '%s\n' "$NEW_IDS" | wc -l)
+ ;;
+esac
+
+if [ "$DRY_RUN" ] ; then
+ cat <<-EOF
+ =-=-=-=-=-=-=-=
+ Would have added the following key(s):
+
+ $NEW_IDS
+ =-=-=-=-=-=-=-=
+ EOF
+else
+ cat <<-EOF
+
+ Number of key(s) added: $ADDED
+
+ Now try logging into the machine, with: "${SFTP:-ssh} $SSH_OPTS"
+ and check to make sure that only the key(s) you wanted were added.
+
+ EOF
+fi
+
+# =-=-=-=
diff --git a/contrib/ssh-copy-id.1 b/contrib/ssh-copy-id.1
new file mode 100644
index 0000000..c141a29
--- /dev/null
+++ b/contrib/ssh-copy-id.1
@@ -0,0 +1,198 @@
+.ig \" -*- nroff -*-
+Copyright (c) 1999-2020 hands.com Ltd. <http://hands.com/>
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions
+are met:
+1. Redistributions of source code must retain the above copyright
+ notice, this list of conditions and the following disclaimer.
+2. Redistributions in binary form must reproduce the above copyright
+ notice, this list of conditions and the following disclaimer in the
+ documentation and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+..
+.Dd $Mdocdate: June 17 2010 $
+.Dt SSH-COPY-ID 1
+.Os
+.Sh NAME
+.Nm ssh-copy-id
+.Nd use locally available keys to authorise logins on a remote machine
+.Sh SYNOPSIS
+.Nm
+.Op Fl f
+.Op Fl n
+.Op Fl s
+.Op Fl i Op Ar identity_file
+.Op Fl p Ar port
+.Op Fl o Ar ssh_option
+.Op Ar user Ns @ Ns
+.Ar hostname
+.Nm
+.Fl h | Fl ?
+.br
+.Sh DESCRIPTION
+.Nm
+is a script that uses
+.Xr 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
+.Xr 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
+.Pa ~/.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
+.Ql set ssh pka-dsa key ...
+command instead.
+.Pp
+The options are as follows:
+.Bl -tag -width Ds
+.It Fl i Ar identity_file
+Use only the key(s) contained in
+.Ar identity_file
+(rather than looking for identities via
+.Xr ssh-add 1
+or in the
+.Ic default_ID_file ) .
+If the filename does not end in
+.Pa .pub
+this is added. If the filename is omitted, the
+.Ic default_ID_file
+is used.
+.Pp
+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.
+.It Fl 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.
+.It Fl n
+do a dry-run. Instead of installing keys on the remote system simply
+prints the key(s) that would have been installed.
+.It Fl s
+SFTP mode: usually the public keys are installed by executing commands on the remote side.
+With this option the user's
+.Pa ~/.ssh/authorized_keys
+file will be downloaded, modified locally and uploaded with sftp.
+This option is useful if the server has restrictions on commands which can be used on the remote side.
+.It Fl h , Fl ?
+Print Usage summary
+.It Fl p Ar port , Fl o Ar ssh_option
+These two options are simply passed through untouched, along with their
+argument, to allow one to set the port or other
+.Xr ssh 1
+options, respectively.
+.Pp
+Rather than specifying these as command line options, it is often better to use (per-host) settings in
+.Xr ssh 1 Ns 's
+configuration file:
+.Xr ssh_config 5 .
+.El
+.Pp
+Default behaviour without
+.Fl i ,
+is to check if
+.Ql 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
+.Xr ssh-add 1
+when the key was loaded into your
+.Xr ssh-agent 1
+rather than the comment contained in that file, which is a bit of a shame.
+Otherwise, if
+.Xr ssh-add 1
+provides no keys contents of the
+.Ic default_ID_file
+will be used.
+.Pp
+The
+.Ic default_ID_file
+is the most recent file that matches:
+.Pa ~/.ssh/id*.pub ,
+(excluding those that match
+.Pa ~/.ssh/*-cert.pub )
+so if you create a key that is not the one you want
+.Nm
+to use, just use
+.Xr touch 1
+on your preferred key's
+.Pa .pub
+file to reinstate it as the most recent.
+.Pp
+.Sh 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
+.Xr ssh-agent 1 .
+Load the new key first, without the
+.Fl 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
+.Fl A
+option to allow agent forwarding:
+.Pp
+.D1 user@newclient$ ssh-add
+.D1 user@newclient$ ssh -A old.client
+.D1 user@oldl$ ssh-add -c
+.D1 No ... prompt for pass-phrase ...
+.D1 user@old$ logoff
+.D1 user@newclient$ ssh someserver
+.Pp
+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
+.Pp
+.D1 user@newclient$ ssh-copy-id -i someserver
+.Pp
+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
+.Pa .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
+.Xr ssh-agent 1 .
+Of course, you can specify another id, or use the contents of the
+.Xr ssh-agent 1
+as you prefer.
+.Pp
+Having mentioned
+.Xr ssh-add 1 Ns 's
+.Fl 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
+.Xr ssh 1 Ns 's
+.Ar ProxyCommand
+and
+.Fl 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
+.Xr ssh-agent 1 .
+A web search for
+.Ql ssh proxycommand nc
+should prove enlightening (N.B. the modern approach is to use the
+.Fl W
+option, rather than
+.Xr nc 1 ) .
+.Sh "SEE ALSO"
+.Xr ssh 1 ,
+.Xr ssh-agent 1 ,
+.Xr sshd 8