diff --git a/src/config.sub b/src/config.sub
new file mode 100755
index 0000000..325531a
--- /dev/null
+++ b/src/config.sub
@@ -0,0 +1,89 @@
+#!/bin/sh
+
+# config.sub: a neutral implementation for modern (cross-)systems.
+# any-target: all are assumed to be supported, no door-keeper failures.
+
+# this file is covered by COPYING.BAUTOMAKE.
+
+
+set -eu
+
+mb_script="$0"
+mb_status=1
+mb_dstamp='2020-01-18'
+
+mb_target="${1:-}"
+mb_extarg="${2:-}"
+
+export LC_ALL=C
+
+
+config_usage()
+{
+	printf 'usage:\n'                                   >&2
+	printf '\t%s [OPTION]\n' "$mb_script"               >&2
+	printf '\t%s (cross-)target-alias\n\n' "$mb_script" >&2
+
+	printf 'Options:\n' >&2
+	printf '\t%s\n'            \
+		'-h, --help'       \
+		'-t, --time-stamp' \
+		'-v, --version'    \
+		>&2
+
+	printf '\nThis is a neutral config.sub implementation for modern (cross-)systems.' >&2
+	printf '\nAll (cross-)targets are assumed to be supported, and there are no door-keeper failures.\n\n' >&2
+
+	printf 'pkgsite: https://git.foss21.org/bautomake\n' >&2
+	printf 'pkgbugs: bugs.automake@foss21.org\n'         >&2
+
+	exit ${mb_status}
+}
+
+
+for arg ; do
+	case "$arg" in
+		-h | --help)
+			mb_status=0
+			config_usage
+			;;
+
+		-t | --time-stamp)
+			printf '%s\n' "$mb_dstamp"
+			exit 0
+			;;
+
+		-v | --version)
+			printf 'foss21.org config.sub (%s)\n' "$mb_dstamp"
+			exit 0
+			;;
+
+		-*)
+			printf '%s: the argument `%s is not supported.\n\n' "$mb_script" "$arg'" >&2
+			exit 2
+	esac
+done
+
+
+# required target argument
+if [ -z "$mb_target" ]; then
+	config_usage
+fi
+
+# no unused arguments
+if [ -n "$mb_extarg" ]; then
+	mb_status=2
+	config_usage
+fi
+
+# validation
+if [ "$mb_target" != ${mb_target%% *} ]; then
+	printf '%s: input error: target alias may not contain spaces.\n\n' "$mb_script" >&2
+	exit 2
+fi
+
+# compatible output
+printf '%s\n' "$mb_target"
+
+# all done
+exit 0