使用libmilter和opendkim生成DKIM-Signature

1，下载libmilter

wget ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.14.5.tar.gz

2，编译libmilter

[root@localhost dkim]# mv sendmail.8.14.5.tar.gz sendmail-8.14.5.tar.gz
[root@localhost dkim]# tar zxf sendmail-8.14.5.tar.gz
[root@localhost dkim]# cd sendmail-8.14.5/libmilter/
[root@localhost libmilter]# ./Build
....
[root@localhost libmilter]# ./Build install
Configuration: pfx=, os=Linux, rel=2.6.18-194.el5, rbase=2, rroot=2.6.18-194, arch=x86_64, sfx=, variant=optimized
Making in /home/iedm/dkim/sendmail-8.14.5/obj.Linux.2.6.18-194.el5.x86_64/libmilter
if [ ! -d /usr/include/libmilter ]; then mkdir -p /usr/include/libmilter; else :; fi
install -c -o root -g bin -m 0444 ../../include/libmilter/mfapi.h /usr/include/libmilter/mfapi.h
install -c -o root -g bin -m 0444 ../../include/libmilter/mfdef.h /usr/include/libmilter/mfdef.h
install -c -o root -g bin -m 0444 libmilter.a /usr/lib
[root@localhost libmilter]# 

注：在编译opendkim前要编译libmilter，否则在opendkim执行configure会出错
checking for milter library and includes... configure: error: milter not found

3，下载opendkim

wget http://sourceforge.net/projects/opendkim/files/opendkim-2.6.2.tar.gz

4，编译opendkim

[root@localhost dkim]# tar zxf opendkim-2.6.2.tar.gz 
[root@localhost dkim]# cd opendkim-2.6.2
[root@localhost opendkim-2.6.2]# ./configure
....
[root@localhost opendkim-2.6.2]# make
....
[root@localhost opendkim-2.6.2]# make install
....
[root@localhost opendkim-2.6.2]#

5，使用openssl生成公钥和私钥

[root@localhost dkim]# openssl genrsa -out rsa.private 1024
Generating RSA private key, 1024 bit long modulus
...............................++++++
..................++++++
e is 65537 (0x10001)
[root@localhost dkim]# openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
writing RSA key
[root@localhost dkim]# ls -l rsa*
-rw-r--r-- 1 root root 887 07-04 10:53 rsa.private
-rw-r--r-- 1 root root 272 07-04 10:53 rsa.public
[root@localhost dkim]# cat rsa.public 
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5/9JMdcOkRvhfNRWXzKUuWypJ
oaLsL1jhZzZ535NYDEZTyUu8SUaZenY8+j84yzf8D/CiaLa6fQIE3ORD8rttdQAH
0P4Zvztak7k6UptojT/lFqEVAEgAcYrKbB4EGM0df1N7coSGDe6FBshRzgW4lI75
fThJnSxKbe5KrVyKUQIDAQAB
-----END PUBLIC KEY-----
[root@localhost dkim]# 

6，通过dns txt记录设置公钥

[root@localhost dkim]# host -t txt s120701._domainkey.iyoutui.com
s120701._domainkey.iyoutui.com descriptive text "k=rsa\; t=y\; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5/9JMdcOkRvhfNRWXzKUuWypJoaLsL1jhZzZ535NYDEZTyUu8SUaZenY8+j84yzf8D/CiaLa6fQIE3ORD8rttdQAH0P4Zvztak7k6UptojT/lFqEVAEgAcYrKbB4EGM0df1N7coSGDe6FBshRzgW4lI75fThJnSxKbe5KrVyKUQIDAQAB"
[root@localhost dkim]#

其中s120701是selector，_domainkey固定，iyoutui.com是发信域名。比如gmail发出邮件的DKIM-Signature如下

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=20120113;
        h=mime-version:date:message-id:subject:from:to:cc:content-type;
        bh=hDX73a4kPsHa/h6++RPoq1865EevSf3TwGuZJ9ZDZdU=;
        b=X5GwgZLp5AEeSkswVMgPhpE0/f4r/+vzq/b4WK6ppNcE4VfvPk1aGNHWp/5tknMpGM
         hK80iNSl+IqyDWL5vEr9sUfCXOHpRas10X2jHeK+SPQS86Lq6qB2W2M9enrKFYRovuwk
         ZZ3Gv2w8GLIcRcvZ7GTuNem8Jkr3Vou6vAgg5zSpFFtsI/gOSsnmZcg0kUq+/bPTb8rg
         JM23yjvFkWWLJkxx5SuItBnJmWL9//yhRFuRKAs5iA3mgGu6JyP4XMTeWRP/kNi7d8Vo
         Jzmtz2mrJVfi3r2wyYws+4//C3uOCoBzKhR+i4WpXJH9ho554Tmhk6gnaI+eFSjAyU7P
         /bwQ==

可看到s=20120113;是selector，d=gmail.com;是域名，例如查询gmail.com的公钥方法如下

[root@localhost dkim]# host -t txt 20120113._domainkey.gmail.com
20120113._domainkey.gmail.com descriptive text "k=rsa\; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA1Kd87/UeJjenpabgbFwh+eBCsSTrqmwIYYvywlbhbqoo2DymndFkbjOVIPIldNs/m40KF+yzMn1skyoxcTUGCQs8g3FgD2Ap3ZB5DekAo5wMmk4wimDO+U8QzI3SD0" "7y2+07wlNWwIt8svnxgdxGkVbbhzY8i+RQ9DpSVpPbF7ykQxtKXkv/ahW3KjViiAH+ghvvIhkx4xYSIc9oSwVmAl5OctMEeWUwg8Istjqz8BZeTWbf41fbNhte7Y+YqZOwq1Sd0DbvYAD9NOZK9vlfuac0598HY+vtSBczUiKERHv1yRbcaQtZFh5wtiRrN04BLUTD21MycBX5jYchHjPY/wIDAQAB"
[root@localhost dkim]# 

7，通过dns txt记录设置验证失败的操作策略

[root@localhost dkim]# host -t txt _adsp._domainkey.iyoutui.com
_adsp._domainkey.iyoutui.com descriptive text "dkim=all"
[root@localhost dkim]# 

dkim配置的值有unknown/all/discardable，查看各大ESP都没有配置策略，所以这步可以省略。验证失败时，收件方按自己的策略来执行。测试了，gmail会丢弃邮件，qq、163会正常收下邮件。

8，修改t-test16.c扫描邮件内容生成DKIM-Signature

if ( argc < 2 )
	{
		printf("Usage: %s $eml_file\n", argv[0]);
		return 1;
	}

while ( fgets(pEmlLine, 1024, fEmlFile) != NULL )
{
	nLine++;
	if ( bHeader )
	{
		if ( strncmp(pEmlLine, "\r\n", 2) == 0 || strncmp(pEmlLine, "\n", 1) == 0 )
		{
			status = dkim_eoh(dkim);
			assert(status == DKIM_STAT_OK);
			bHeader = 0;
			continue;
		}

		if ( strncasecmp(pEmlLine, "From:", 5) != 0
			&& strncasecmp(pEmlLine, "To:", 3) != 0
			&& strncasecmp(pEmlLine, "Subject:", 8) != 0
			&& strncasecmp(pEmlLine, "Date:", 5) != 0
			&& strncasecmp(pEmlLine, "Reply-To:", 9) != 0
			&& strncasecmp(pEmlLine, "X-mailer:", 9) != 0
			&& strncasecmp(pEmlLine, "Message-ID:", 11) != 0 )
			continue;
		status = dkim_header(dkim, pEmlLine, strlen(pEmlLine));
		printf("num:%d, header:%d, [%s] %u\n", nLine, bHeader, pEmlLine, strlen(pEmlLine));
	}
	else
	{
		status = dkim_body(dkim, pEmlLine, strlen(pEmlLine));
	} // if ( bHeader )
	assert(status == DKIM_STAT_OK);
	memset(pEmlLine, '\0', 1024);
}

[root@localhost dkim]# cd opendkim-2.6.2/libopendkim/tests/
[root@localhost tests]# make t-test16
[root@localhost tests]# ./t-test16
Usage: ../../opendkim-2.6.2/libopendkim/tests/.libs/lt-t-test16 $eml_file
[root@localhost tests]# ../../libopendkim/tests/.libs/lt-t-test16 plain.eml
...
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=iyoutui.com;
        s=s120701; t=1341298992;
        bh=0iceU5a2cO3bQhL4Os527y4UIwNUmDEbsrqJ8a30EUI=;
        h=From:To:Subject:Date:Reply-To:X-mailer:Message-ID;
        b=cgopPO7K54jr4ezxTXpN0i6oCmyt3aPJgDT4vxcZDY3WDf0QfSIEOUa7bDf8W6PTN
         4Gw/GEXdLzxSLVArYTnZ64ij/LwALKvjF+oDPgBnHbC3xTODgEvIvtWe9OhcAcPOeV
         4WuZRZgYQjp4VpCs7GuAxSFBClCY2XUxpnbuowQM=

9，在邮件信头加上DKIM-Signature，发邮件到gmail通过签名验证

Received-SPF: pass (google.com: domain of [email protected] designates 173.252.205.131 as permitted sender) client-ip=173.252.205.131;
Authentication-Results: mx.google.com; spf=pass (google.com: domain of [email protected] designates 173.252.205.131 as permitted sender) [email protected]; dkim=pass (test mode) [email protected]
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=iyoutui.com;
        s=s120701; t=1341298992;
        bh=0iceU5a2cO3bQhL4Os527y4UIwNUmDEbsrqJ8a30EUI=;
        h=From:To:Subject:Date:Reply-To:X-mailer:Message-ID;
        b=cgopPO7K54jr4ezxTXpN0i6oCmyt3aPJgDT4vxcZDY3WDf0QfSIEOUa7bDf8W6PTN
         4Gw/GEXdLzxSLVArYTnZ64ij/LwALKvjF+oDPgBnHbC3xTODgEvIvtWe9OhcAcPOeV
         4WuZRZgYQjp4VpCs7GuAxSFBClCY2XUxpnbuowQM=

在gmail查看邮件原文看到如上信息，表示通过了dkim验证。
至此，使用libmilter和opendkim生成DKIM-Signature成功。