- (djm) [contrib/ssh-copy-id contrib/ssh-copy-id.1] Updated to Phil

    Hands' greatly revised version.

3 files changed, 459 insertions, 105 deletions

diff --git a/ChangeLog b/ChangeLog
index 80429f84..d69fd350 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+20120322
+ - (djm) [contrib/ssh-copy-id contrib/ssh-copy-id.1] Updated to Phil
+   Hands' greatly revised version.
+
 20120318
  - (djm) [configure.ac log.c scp.c sshconnect2.c openbsd-compat/vis.c]
    [openbsd-compat/vis.h] FreeBSD's strnvis isn't compatible with OpenBSD's
diff --git a/contrib/ssh-copy-id b/contrib/ssh-copy-id
index 9451acee..af18a192 100644
--- a/contrib/ssh-copy-id
+++ b/contrib/ssh-copy-id
@@ -1,54 +1,293 @@
 #!/bin/sh
 
-# Shell script to install your public key on a remote machine
-# Takes the remote machine name as an argument.
-# Obviously, the remote machine must accept password authentication,
-# or one of the other keys in your ssh-agent, for this to work.
-
-ID_FILE="${HOME}/.ssh/id_rsa.pub"
-
-if [ "-i" = "$1" ]; then
-  shift
-  # check if we have 2 parameters left, if so the first is the new ID file
-  if [ -n "$2" ]; then
-    if expr "$1" : ".*\.pub" > /dev/null ; then
-      ID_FILE="$1"
-    else
-      ID_FILE="$1.pub"
-    fi
-    shift         # and this should leave $1 as the target name
+# Copyright (c) 1999-2013 Philip Hands <phil@hands.com>
+#               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
+
+# 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" "$0"
+    exit 1
   fi
-else
-  if [ x$SSH_AUTH_SOCK != x ] && ssh-add -L >/dev/null 2>&1; then
-    GET_ID="$GET_ID ssh-add -L"
+fi
+
+DEFAULT_PUB_ID_FILE=$(ls -t ${HOME}/.ssh/id*.pub 2>/dev/null | grep -v -- '-cert.pub$' | head -n 1)
+
+usage () {
+  printf 'Usage: %s [-h|-?|-n] [-i [identity_file]] [-p port] [[-o <ssh -o options>] ...] [user@]hostname\n' "$0" >&2
+  exit 1
+}
+
+# escape any single quotes in an argument
+quote() {
+  printf "%s\n" "$1" | sed -e "s/'/'\\\\''/g"
+}
+
+use_id_file() {
+  local L_ID_FILE="$1"
+
+  if expr "$L_ID_FILE" : ".*\.pub$" >/dev/null ; then
+    PUB_ID_FILE="$L_ID_FILE"
+  else
+    PUB_ID_FILE="$L_ID_FILE.pub"
   fi
+
+  PRIV_ID_FILE=$(dirname "$PUB_ID_FILE")/$(basename "$PUB_ID_FILE" .pub)
+
+  # check that the files are readable
+  for f in $PUB_ID_FILE $PRIV_ID_FILE ; do
+    ErrMSG=$( { : < $f ; } 2>&1 ) || {
+      printf "\n%s: ERROR: failed to open ID file '%s': %s\n\n" "$0" "$f" "$(printf "%s\n" "$ErrMSG" | sed -e 's/.*: *//')"
+      exit 1
+    }
+  done
+  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
 
-if [ -z "`eval $GET_ID`" ] && [ -r "${ID_FILE}" ] ; then
-  GET_ID="cat \"${ID_FILE}\""
+while test "$#" -gt 0
+do
+  [ "${SEEN_OPT_I}" ] && expr "$1" : "[-]i" >/dev/null && {
+        printf "\n%s: ERROR: -i option must not be specified more than once\n\n" "$0"
+        usage
+  }
+
+  OPT= OPTARG=
+  # implement something like getopt to avoid Solaris pain
+  case "$1" in
+    -i?*|-o?*|-p?*)
+      OPT="$(printf -- "$1"|cut -c1-2)"
+      OPTARG="$(printf -- "$1"|cut -c3-)"
+      shift
+      ;;
+    -o|-p)
+      OPT="$1"
+      OPTARG="$2"
+      shift 2
+      ;;
+    -i)
+      OPT="$1"
+      test "$#" -le 2 || expr "$2" : "[-]" >/dev/null || {
+        OPTARG="$2"
+        shift
+      }
+      shift
+      ;;
+    -n|-h|-\?)
+      OPT="$1"
+      OPTARG=
+      shift
+      ;;
+    --)
+      shift
+      while test "$#" -gt 0
+      do
+        SAVEARGS="${SAVEARGS:+$SAVEARGS }'$(quote "$1")'"
+        shift
+      done
+      break
+      ;;
+    -*)
+      printf "\n%s: ERROR: invalid option (%s)\n\n" "$0" "$1"
+      usage
+      ;;
+    *)
+      SAVEARGS="${SAVEARGS:+$SAVEARGS }'$(quote "$1")'"
+      shift
+      continue
+      ;;
+  esac
+
+  case "$OPT" in
+    -i)
+      SEEN_OPT_I="yes"
+      use_id_file "${OPTARG:-$DEFAULT_PUB_ID_FILE}"
+      ;;
+    -o|-p)
+      SSH_OPTS="${SSH_OPTS:+$SSH_OPTS }$OPT '$(quote "$OPTARG")'"
+      ;;
+    -n)
+      DRY_RUN=1
+      ;;
+    -h|-\?)
+      usage
+      ;;
+  esac
+done 
+
+eval set -- "$SAVEARGS"
+
+if [ $# != 1 ] ; then
+  printf '%s: ERROR: Too many arguments.  Expecting a target hostname, got: %s\n\n' "$0" "$SAVEARGS" >&2
+  usage
 fi
 
-if [ -z "`eval $GET_ID`" ]; then
-  echo "$0: ERROR: No identities found" >&2
-  exit 1
+# drop trailing colon
+USER_HOST=$(printf "%s\n" "$1" | sed 's/:$//')
+# 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"
+
+if [ -z "$(eval $GET_ID)" ] && [ -r "${PUB_ID_FILE:=$DEFAULT_PUB_ID_FILE}" ] ; then
+  use_id_file "$PUB_ID_FILE"
 fi
 
-if [ "$#" -lt 1 ] || [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
-  echo "Usage: $0 [-i [identity_file]] [user@]machine" >&2
+if [ -z "$(eval $GET_ID)" ] ; then
+  printf '%s: ERROR: No identities found\n' "$0" >&2
   exit 1
 fi
 
-# strip any trailing colon
-host=`echo $1 | sed 's/:$//'`
+# populate_new_ids() uses several global variables ($USER_HOST, $SSH_OPTS ...)
+# and has the side effect of setting $NEW_IDS
+populate_new_ids() {
+  local L_SUCCESS="$1"
 
-{ eval "$GET_ID" ; } | ssh $host "umask 077; test -d ~/.ssh || mkdir ~/.ssh ; cat >> ~/.ssh/authorized_keys" || exit 1
+  # repopulate "$@" inside this function 
+  eval set -- "$SSH_OPTS"
 
-cat <<EOF
-Now try logging into the machine, with "ssh '$host'", and check in:
+  umask 0177
+  local L_TMP_ID_FILE=$(mktemp ~/.ssh/ssh-copy-id_id.XXXXXXXXXX)
+  trap "rm -f $L_TMP_ID_FILE*" EXIT TERM INT QUIT
+  printf '%s: INFO: attempting to log in with the new key(s), to filter out any that are already installed\n' "$0" >&2
+  NEW_IDS=$(
+    eval $GET_ID | {
+      while read ID ; do
+        printf '%s\n' "$ID" > $L_TMP_ID_FILE
 
-  ~/.ssh/authorized_keys
+        # 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:-$L_TMP_ID_FILE}" \
+            -o PreferredAuthentications=publickey \
+            -o IdentitiesOnly=yes "$@" exit 2>$L_TMP_ID_FILE.stderr </dev/null
+        if [ "$?" = "$L_SUCCESS" ] ; then
+          : > $L_TMP_ID_FILE
+        else
+          grep 'Permission denied' $L_TMP_ID_FILE.stderr >/dev/null || {
+            sed -e 's/^/ERROR: /' <$L_TMP_ID_FILE.stderr >$L_TMP_ID_FILE
+            cat >/dev/null #consume the other keys, causing loop to end
+          }
+        fi
+
+        cat $L_TMP_ID_FILE
+      done
+    }
+  )
+  rm -f $L_TMP_ID_FILE* && trap - EXIT TERM INT QUIT
+
+  if expr "$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\n' "$0" >&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
+}
 
-to make sure we haven't added extra keys that you weren't expecting.
+REMOTE_VERSION=$(ssh -v -o PreferredAuthentications=',' "$@" 2>&1 |
+                 sed -ne 's/.*remote software version //p')
 
-EOF
+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 "$ADDED" ] ; then
+      exit 1
+    fi
+    ;;
+  *)
+    # Assuming that the remote host treats ~/.ssh/authorized_keys as one might expect
+    populate_new_ids 0
+    [ "$DRY_RUN" ] || printf '%s\n' "$NEW_IDS" | ssh "$@" "
+		umask 077 ;
+		mkdir -p .ssh && cat >> .ssh/authorized_keys || exit 1 ;
+		if type restorecon >/dev/null 2>&1 ; then restorecon -F .ssh .ssh/authorized_keys ; fi" \
+      || exit 1
+    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:   "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
index cb15ab24..67a59e49 100644
--- a/contrib/ssh-copy-id.1
+++ b/contrib/ssh-copy-id.1
@@ -1,75 +1,186 @@
 .ig \"  -*- nroff -*-
-Copyright (c) 1999 Philip Hands Computing <http://www.hands.com/>
+Copyright (c) 1999-2013 hands.com Ltd. <http://hands.com/>
 
-Permission is granted to make and distribute verbatim copies of
-this manual provided the copyright notice and this permission notice
-are preserved on all copies.
+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.
 
-Permission is granted to copy and distribute modified versions of this
-manual under the conditions for verbatim copying, provided that the
-entire resulting derived work is distributed under the terms of a
-permission notice identical to this one.
-
-Permission is granted to copy and distribute translations of this
-manual into another language, under the above conditions for modified
-versions, except that this permission notice may be included in
-translations approved by the Free Software Foundation instead of in
-the original English.
+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.
 ..
-.TH SSH-COPY-ID 1 "14 November 1999" "OpenSSH"
-.SH NAME
-ssh-copy-id \- install your public key in a remote machine's authorized_keys
-.SH SYNOPSIS
-.B ssh-copy-id [-i [identity_file]]
-.I "[user@]machine"
+.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 n
+.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
-.BR ssh-copy-id
-is a script that uses ssh to log into a remote machine and
-append the indicated identity file to that machine's
-.B ~/.ssh/authorized_keys
-file.
-.PP
-If the
-.B -i
-option is given then the identity file (defaults to
-.BR ~/.ssh/id_rsa.pub )
-is used, regardless of whether there are any keys in your
-.BR ssh-agent .
-Otherwise, if this:
-.PP
-.B "      ssh-add -L"
-.PP
-provides any output, it uses that in preference to the identity file.
-.PP
-If the
-.B -i
-option is used, or the
-.B ssh-add
-produced no output, then it uses the contents of the identity
-file.  Once it has one or more fingerprints (by whatever means) it
-uses ssh to append them to
-.B ~/.ssh/authorized_keys
-on the remote machine (creating the file, and directory, if necessary.)
-
-.SH NOTES
-This program does not modify the permissions of any
-pre-existing files or directories. Therefore, if the remote
-.B sshd
-has
-.B StrictModes
-set in its
-configuration, then the user's home,
-.B ~/.ssh
-folder, and
-.B ~/.ssh/authorized_keys
-file may need to have group writability disabled manually, e.g. via
-
-.B "      chmod go-w ~ ~/.ssh ~/.ssh/authorized_keys"
-
-on the remote machine.
-
-.SH "SEE ALSO"
-.BR ssh (1),
-.BR ssh-agent (1),
-.BR sshd (8)
+.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 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 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 you 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