#! /bin/sh
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#
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# Preserve the random seed between reboots. See urandom(4).
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#
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# Quietly do nothing if /dev/urandom does not exist
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[ -c /dev/urandom ] || exit 0
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URANDOM_SEED="/var/lib/random-seed"
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# shellcheck source=/dev/null
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[ -r "/etc/default/urandom" ] && . "/etc/default/urandom"
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if pool_bits=$(cat /proc/sys/kernel/random/poolsize 2> /dev/null); then
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pool_size=$((pool_bits/8))
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else
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pool_size=512
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fi
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check_file_size() {
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[ -f "$URANDOM_SEED" ] || return 1
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# Try to read two blocks but exactly one will be read if the file has
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# the correct size.
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size=$(dd if="$URANDOM_SEED" bs="$pool_size" count=2 2> /dev/null | wc -c)
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test "$size" -eq "$pool_size"
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}
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init_rng() {
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if check_file_size; then
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printf 'Initializing random number generator: '
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dd if="$URANDOM_SEED" bs="$pool_size" of=/dev/urandom count=1 2> /dev/null
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status=$?
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if [ "$status" -eq 0 ]; then
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echo "OK"
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else
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echo "FAIL"
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fi
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return "$status"
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fi
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}
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save_random_seed() {
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printf 'Saving random seed: '
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if touch "$URANDOM_SEED" 2> /dev/null; then
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old_umask=$(umask)
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umask 077
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dd if=/dev/urandom of="$URANDOM_SEED" bs="$pool_size" count=1 2> /dev/null
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status=$?
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umask "$old_umask"
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if [ "$status" -eq 0 ]; then
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echo "OK"
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else
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echo "FAIL"
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fi
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else
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status=$?
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echo "SKIP (read-only file system detected)"
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fi
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return "$status"
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}
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case "$1" in
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start|restart|reload)
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# Carry a random seed from start-up to start-up
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# Load and then save the whole entropy pool
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init_rng && save_random_seed;;
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stop)
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# Carry a random seed from shut-down to start-up
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# Save the whole entropy pool
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save_random_seed;;
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*)
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echo "Usage: $0 {start|stop|restart|reload}"
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exit 1
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esac
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