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+#!/usr/bin/perl
+
+# verify-cn -- a sample OpenVPN tls-verify script
+#
+# Return 0 if cn matches the common name component of
+# subject, 1 otherwise.
+#
+# For example in OpenVPN, you could use the directive:
+#
+# tls-verify "./verify-cn /etc/openvpn/allowed_clients"
+#
+# This would cause the connection to be dropped unless
+# the client common name is listed on a line in the
+# allowed_clients file.
+
+die "usage: verify-cn cnfile certificate_depth subject" if (@ARGV != 3);
+
+# Parse out arguments:
+# cnfile -- The file containing the list of common names, one per
+# line, which the client is required to have,
+# taken from the argument to the tls-verify directive
+# in the OpenVPN config file.
+# The file can have blank lines and comment lines that begin
+# with the # character.
+# depth -- The current certificate chain depth. In a typical
+# bi-level chain, the root certificate will be at level
+# 1 and the client certificate will be at level 0.
+# This script will be called separately for each level.
+# x509 -- the X509 subject string as extracted by OpenVPN from
+# the client's provided certificate.
+($cnfile, $depth, $x509) = @ARGV;
+
+if ($depth == 0) {
+ # If depth is zero, we know that this is the final
+ # certificate in the chain (i.e. the client certificate),
+ # and the one we are interested in examining.
+ # If so, parse out the common name substring in
+ # the X509 subject string.
+
+ if ($x509 =~ / CN=([^,]+)/) {
+ $cn = $1;
+ # Accept the connection if the X509 common name
+ # string matches the passed cn argument.
+ open(FH, '<', $cnfile) or exit 1; # can't open, nobody authenticates!
+ while (defined($line = <FH>)) {
+ if ($line !~ /^[[:space:]]*(#|$)/o) {
+ chop($line);
+ if ($line eq $cn) {
+ exit 0;
+ }
+ }
+ }
+ close(FH);
+ }
+
+ # Authentication failed -- Either we could not parse
+ # the X509 subject string, or the common name in the
+ # subject string didn't match the passed cn argument.
+ exit 1;
+}
+
+# If depth is nonzero, tell OpenVPN to continue processing
+# the certificate chain.
+exit 0;