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SSH passphrase en Cron mediante Expect

 ·  🎃 kr0m

Si tenemos la key Ssh protegida mediante passphrase puede resultar un problema al realizar tareas desde Cron, por ejemplo al ejecutar Rsyncs. Una solución rápida a esta problemática es ejecutar un script en Expect al que se le indica la passphrase y ejecuta el Rsync.

La instalación de Expect variará según el sistema operativo utilizado, en FreeBSD tendremos que instalar el paquete binario y generar un enlace simbólico genérico al binario:

pkg install expect  
which tclsh8.6  
ln -s /usr/local/bin/tclsh8.6 /usr/local/bin/tclsh  
echo "dev-tcltk/expect threads -debug doc" > /etc/portage/package.use/expect  
emerge -av dev-tcltk/expect  
bzip2 -d /usr/share/doc/expect-5.45.4/examples/autoexpect.bz2  
cp /usr/share/doc/expect-5.45.4/examples/autoexpect /usr/local/bin/  
chmod 700 /usr/local/bin/autoexpect 

Ahora como usuario regular lanzamos expect_autoexpect/autoexpect con el comando que queramos automatizar:

expect_autoexpect /usr/local/bin/rsync -avz --del --rsh=’\ssh -p22' /home/kr0m/data/ kr0m@ :/home/kr0m/data/

Podemos ver el contenido del script:

cat script.exp

#!/usr/local/bin/expect -f
# This Expect script was generated by autoexpect on Tue Sep 22 18:03:18 2020
# Expect and autoexpect were both written by Don Libes, NIST.
# Note that autoexpect does not guarantee a working script.  It
# necessarily has to guess about certain things.  Two reasons a script
# might fail are:
# 1) timing - A surprising number of programs (rn, ksh, zsh, telnet,
# etc.) and devices discard or ignore keystrokes that arrive "too
# quickly" after prompts.  If you find your new script hanging up at
# one spot, try adding a short sleep just before the previous send.
# Setting "force_conservative" to 1 (see below) makes Expect do this
# automatically - pausing briefly before sending each character.  This
# pacifies every program I know of.  The -c flag makes the script do
# this in the first place.  The -C flag allows you to define a
# character to toggle this mode off and on.

set force_conservative 0  ;# set to 1 to force conservative mode even if
     ;# script wasn't run conservatively originally
if {$force_conservative} {
 set send_slow {1 .1}
 proc send {ignore arg} {
  sleep .1
  exp_send -s -- $arg

# 2) differing output - Some programs produce different output each time
# they run.  The "date" command is an obvious example.  Another is
# ftp, if it produces throughput statistics at the end of a file
# transfer.  If this causes a problem, delete these patterns or replace
# them with wildcards.  An alternative is to use the -p flag (for
# "prompt") which makes Expect only look for the last line of output
# (i.e., the prompt).  The -P flag allows you to define a character to
# toggle this mode off and on.
# Read the man page for more info.
# -Don

set timeout -1
spawn /usr/local/bin/rsync -avz --del {--rsh=ssh -p22} /home/kr0m/data/ kr0m@
match_max 100000
expect -exact "Enter passphrase for key '/home/kr0m/.ssh/id_rsa': "
send -- "PASSWORD\r"
expect eof

En este caso se trata de un script de backup, este script será muy sencillo, tan solo llamará al script en Expect:

vi /home/kr0m/


Asignamos los permisos necesarios al script:

chmod 700 /home/kr0m/

Este script se podrá crontabear sin que pida ninguna passphrase:

crontab -e

* 10 * * * /home/kr0m/
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