Openssl X.509证书：在Subject Alternative Name中添加localhost是好主意（还是坏主意）？

我们正在决定是否应将“localhost”（以及类似于“127.0.0.1”的地址）添加为证书中的主题备选名称之一。一个好处可能是方便本地测试。但是会有什么缺点吗

在Subject Alternative Name中添加localhost是个好主意（还是坏主意）

这取决于你所遵循的标准和你的安全姿态

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第一件事优先（以下讨论）。必须定义完全限定的域名（FQDN）。该定义取自W.Richard Steven（第189页）：

以句号结尾的域名称为绝对域名或完全限定域名。如果域名没有在一段时间内结束，则假定域名需要完成。名称的填写方式取决于所使用的DNS软件

这意味着我们可以通过添加句点将

localhost

更改为完全限定的域名：

localhost.

这里有一个小实验：

$ hostname
debian-q500

$ hostname --fqdn
debian-q500

$ dnsdomainname 
$

$ ping debian-q500.
ping: unknown host debian-q500.
$ ping debian-q500.local
PING debian-q500.local (172.16.1.26) 56(84) bytes of data.
64 bytes from debian-q500.home.pvt (172.16.1.26): icmp_req=1 ttl=64 time=0.040 ms
64 bytes from debian-q500.home.pvt (172.16.1.26): icmp_req=2 ttl=64 time=0.035 ms
...

$ ping localhost.
PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_req=1 ttl=64 time=0.033 ms
64 bytes from localhost (127.0.0.1): icmp_req=2 ttl=64 time=0.037 ms
...
$ ping localhost.local
ping: unknown host localhost.local
$ ping localhost.localdomain
ping: unknown host localhost.localdomain

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其次是标准。其中最流行的是CAs广告浏览器发布的指南。CAs广告浏览器在CA/B论坛上发布其操作指南。这两个指南是：

还有另一种流行的方法，但它通常会延迟证书中列出的主机中的CA/B指南。该标准是IETF的RFC 5280：

RFC 5280将调用其他项目，如如何验证证书链以及如何在subjectAltName中列出电子邮件地址

基线指南

《基线指南》对名称作了如下说明：

9.2.1 Subject Alternative Name Extension    
Certificate Field: extensions:subjectAltName
Required/Optional: Required

Contents: This extension MUST contain at least one entry. Each
entry MUST be either a dNSName containing the Fully-Qualified
Domain Name or an iPAddress containing the IP address of a
server. The CA MUST confirm that the Applicant controls the
Fully-Qualified Domain Name or IP address or has been granted
the right to use it by the Domain Name Registrant or IP address
assignee, as appropriate.

Wildcard FQDNs are permitted.
...

及

最后,

11.1.3 Wildcard Domain Validation   

Before issuing a certificate with a wildcard character (*) in a
CN or subjectAltName of type DNS-ID, the CA MUST establish and
follow a documented procedure† that determines if the wildcard
character occurs in the first label position to the left of a
“registry-controlled” label or “public suffix” (e.g. “*.com”,
“*.co.uk”, see RFC 6454 Section 8.2 for further explanation).

If a wildcard would fall within the label immediately to the left
of a registry-controlled† or public suffix, CAs MUST refuse
issuance unless the applicant proves its rightful control of the
entire Domain Namespace. (e.g. CAs MUST NOT issue “*.co.uk” or
“*.local”, but MAY issue “*.example.com” to Example Co.).

因此，只要是完全限定的域名，

localhost

就可以了。事实上，

localhost

在指南中甚至没有提到

扩展验证

9.2.2 Subject Alternative Name Extension
Certificate field: subjectAltName:dNSName
Required/Optional: Required

Contents: This extension MUST contain one or more host Domain
Name(s) owned or controlled by the Subject and to be associated
with the Subject’s server. Such server MAY be owned and operated
by the Subject or another entity (e.g., a hosting service).
Wildcard certificates are not allowed for EV Certificates.

9.2.3 Subject Common Name Field
Certificate field: subject:commonName (OID: 2.5.4.3)
Required/Optional: Deprecated (Discouraged, but not prohibited)

Contents: If present, this field MUST contain a single Domain
Name(s) owned or controlled by the Subject and to be associated
with the Subject’s server. Such server MAY be owned and operated
by the Subject or another entity (e.g., a hosting service).
Wildcard certificates are not allowed for EV Certificates.

因此，只要是完全限定的域名，

localhost

就可以了。事实上，

localhost

在指南中甚至没有提到

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微软鼓励在KB315588中进行实践：

• 在站点的公共名称页面上，键入localhost，然后单击Next

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是用于嵌入式设备的专用SSL/TLS和SSH密钥的数据库。它在

bin/

中附带了一个SQlite3数据库

证书采用PEM格式（即，

----begincertificate---

和friends）。您可以使用以下内容转储所有证书：

$ sqlite3 lbb.db 
SQLite version 3.8.3 2013-12-17 16:32:56
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> .mode line
sqlite> .out certificates.txt
sqlite> SELECT certificate FROM certificates;
sqlite> .q

接下来，从文件中删除

证书=

：

$ sed -e "s|certificate = ||g" certificates.txt > temp.txt
$ mv temp.txt certificates.txt

现在使用

nawk

和

openssl

对每个证书进行解码：

nawk '
v{v=v"\n"$0}
/----BEGIN/ {v=$0}
/----END/&&v{
  print v > "tmp.cert"
  close("tmp.cert")
  system("openssl x509 -in tmp.cert -inform PEM -text -noout")
  v=x}' certificates.txt

如果我们知道他们，坏人肯定知道他们

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最后，它的安全态势。鉴于以上所述，这就是为什么这是个坏主意。这就是安全姿态的用武之地。来自彼得·古特曼（第45页）：

这里的问题是，“它是我的本地主机，还是你的本地主机”。因此，与其说是为您的

localhost

颁发证书并信任它的问题，不如说是无意中信任一个外国

localhost

的问题

一旦您的软件（如浏览器）信任颁发给

localhost

的证书，游戏就结束了

nawk '
v{v=v"\n"$0}
/----BEGIN/ {v=$0}
/----END/&&v{
  print v > "tmp.cert"
  close("tmp.cert")
  system("openssl x509 -in tmp.cert -inform PEM -text -noout")
  v=x}' certificates.txt

In practice CAs seem to issue certificates under more or less any
name to pretty much anybody, ranging from small-scale issues like
users buying certificates for the wonderfully open-ended mail [237]
through to the six thousand sites that commercial CAs like Comodo,
Cybertrust, Digicert, Entrust, Equifax, GlobalSign, GoDaddy,
Microsoft, Starfield and Verisign have certified for localhost,
with no apparent limit on how many times a CA will issue a
certificate for the same name [238].