/usr/bin/opendkim-genkey is in opendkim-tools 2.9.1-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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##
## Copyright (c) 2005-2008 Sendmail, Inc. and its suppliers.
## All rights reserved.
##
## Copyright (c) 2009-2013, The Trusted Domain Project. All rights reserved.
##
## opendkim-genkey -- generate a key and/or TXT record for DKIM service
##
## The output of this script includes two files based on the
## selector name:
##
## <selector>.private PEM-formatted private key, for use by
## opendkim when signing messages
##
## <selector>.txt A DNS TXT record suitable for insertion
## into or inclusion by a DNS zone file
## in order to publish the public key
## for retrieval by verifiers
use strict;
use warnings;
use File::Basename;
use Getopt::Long;
##
## Set up defaults
##
my $bits = 1024;
my $domain = "example.com";
my $version = "example.com";
my $outdir = ".";
my $nosubdomains = 0;
my $subdomains = 1;
my $hashalgs;
my $note;
my $flags = "";
my $selector = "default";
my $restricted = 0;
my $testmode = 0;
my $verbose = 0;
my $append = 0;
my $showversion = 0;
my $keyin;
my $status;
my $txtout;
my $keydata;
my $helponly;
my $hashout;
my $noteout;
my $comment;
my $len;
my $cur;
my $domstr;
my $progname = basename($0);
##
## Usage message
##
sub usage
{
print STDERR "$progname: usage: $progname [options]\n";
print STDERR "\t--append-domain include domain name in zone file stub\n";
print STDERR "\t--bits=n use n bits to generate the key\n";
print STDERR "\t--directory=path leave output in the named directory\n";
print STDERR "\t--domain=name generate data for the named domain [$domain]\n";
print STDERR "\t--hash-algorithms=list limit to use of the named algorithm(s)\n";
print STDERR "\t--help print help and exit\n";
print STDERR "\t--note=string include specified note in zone data\n";
print STDERR "\t--restrict restrict key to email use only\n";
print STDERR "\t--selector=name selector name [$selector]\n";
print STDERR "\t--subdomains allow signing of subdomains\n";
print STDERR "\t--testmode indicate key is in test mode\n";
print STDERR "\t--verbose increased output\n";
print STDERR "\t--version print version and exit\n";
}
##
## Argument processing
##
# parse command line arguments
Getopt::Long::Configure("bundling");
my $opt_retval = &Getopt::Long::GetOptions('a|append-domain!' => \$append,
'b|bits=i' => \$bits,
'D|directory=s' => \$outdir,
'd|domain=s' => \$domain,
'h|hash-algorithms=s' => \$hashalgs,
'help!' => \$helponly,
'n|note=s' => \$note,
'r|restrict!' => \$restricted,
's|selector=s' => \$selector,
'S!' => \$nosubdomains,
'subdomains!' => \$subdomains,
't|testmode!' => \$testmode,
'v|verbose+' => \$verbose,
'V|version!' => \$showversion,
);
if (!$opt_retval || $helponly)
{
usage();
if ($helponly)
{
exit(0);
}
else
{
exit(1);
}
}
if ($showversion)
{
print STDOUT "$progname v2.9.1\n";
exit 0;
}
## do this securely and in the right place
chdir($outdir) or die "$progname: $outdir: chdir(): $!";;
umask(077);
## generate a private key
if ($verbose >= 1)
{
print STDERR "$progname: generating private key\n";
}
if ($bits < 1024)
{
print STDERR "$progname: WARNING: RFC6376 advises minimum 1024-bit keys\n";
}
$status = system("openssl genrsa -out " . $selector . ".private " . $bits . " > /dev/null 2>&1");
if ($status != 0)
{
if ($? & 127)
{
print STDERR "$progname: openssl died with signal %d\n",
($? & 127);
exit(1);
}
else
{
print STDERR "$progname: openssl exited with status %d\n",
($? >> 8);
exit(1);
}
}
if ($verbose)
{
print STDERR "$progname: private key written to " . $selector . ".private\n";
}
## generate a public key based on the private key
if ($verbose)
{
print STDERR "$progname: extracting public key\n";
}
$status = system("openssl rsa -in " . $selector . ".private -pubout -out " . $selector . ".public -outform PEM > /dev/null 2>&1");
if ($status != 0)
{
if ($? & 127)
{
print STDERR "$progname: openssl died with signal %d\n",
($? & 127);
exit(1);
}
else
{
print STDERR "$progname: openssl exited with status %d\n",
($? >> 8);
exit(1);
}
}
if (!open($keyin, "<", $selector . ".public"))
{
print STDERR "$progname: unable to read from " . $selector . ".public: $!\n";
exit(1);
}
while (<$keyin>)
{
chomp;
if ($_ =~ /^-/)
{
next;
}
$keydata .= $_;
}
close($keyin);
## output the record
if ($testmode)
{
$flags = "t=y;";
}
if ($nosubdomains)
{
$subdomains = 0;
}
if (!$subdomains)
{
if ($flags eq "t=y;")
{
$flags = "t=y:s;";
}
else
{
$flags = "t=s;";
}
}
if ($restricted)
{
if ($flags ne "")
{
$flags .= " ";
}
$flags .= "s=email;";
}
if ($flags ne "")
{
$flags .= " ";
}
$hashout = "";
if (defined($hashalgs))
{
$hashout = " h=$hashalgs;";
}
$noteout = "";
if (defined($note))
{
$noteout = " n=\\\"$note\\\";";
}
$domstr = "";
if ($append)
{
$domstr = "." . $domain . ".";
}
if ($domain ne "")
{
$comment = " ; ----- DKIM key $selector for $domain"
}
else
{
$comment = "";
}
if (!open($txtout, ">", $selector . ".txt"))
{
print STDERR "$progname: unable to write from " . $selector . ".txt: $!\n";
exit(1);
}
print $txtout $selector . "._domainkey" . ${domstr} . "\tIN\tTXT\t( \"v=DKIM1;" . $noteout . $hashout . " k=rsa; " . $flags . "\"\n\t \"p=";
$len = length($keydata);
$cur = 0;
while ($len > 0)
{
if ($len < 250)
{
print $txtout substr($keydata, $cur);
$len = 0;
}
else
{
print $txtout substr($keydata, $cur, 250);
print $txtout "\"\n\t \"";
$cur += 250;
$len -= 250;
}
}
print $txtout "\" ) " . $comment . "\n";
close($txtout);
if ($verbose)
{
print STDERR "$progname: DNS TXT record written to " . $selector . ".txt\n";
}
## all done!
unlink($selector . ".public");
exit(0);
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