FreeBSD 使用手冊
前一頁 | 章 14. 系統安全 | 下一頁

# 14.9. OpenSSL

Written by: Tom Rhodes.

One feature that many users overlook is the OpenSSL toolkit included in FreeBSD. OpenSSL provides an encryption transport layer on top of the normal communications layer; thus allowing it to be intertwined with many network applications and services.

Some uses of OpenSSL may include encrypted authentication of mail clients, web based transactions such as credit card payments and more. Many ports such as `www/apache13-ssl`, and `mail/sylpheed-claws` will offer compilation support for building with OpenSSL.

> 注: In most cases the Ports Collection will attempt to build the `security/openssl` port unless the `WITH_OPENSSL_BASE` make variable is explicitly set to “yes”.

The version of OpenSSL included in FreeBSD supports Secure Sockets Layer v2/v3 (SSLv2/SSLv3), Transport Layer Security v1 (TLSv1) network security protocols and can be used as a general cryptographic library.

> 注: While OpenSSL supports the IDEA algorithm, it is disabled by default due to United States patents. To use it, the license should be reviewed and, if the restrictions are acceptable, the `MAKE_IDEA` variable must be set in `make.conf`.

One of the most common uses of OpenSSL is to provide certificates for use with software applications. These certificates ensure that the credentials of the company or individual are valid and not fraudulent. If the certificate in question has not been verified by one of the several “Certificate Authorities”, or CAs, a warning is usually produced. A Certificate Authority is a company, such as VeriSign, which will sign certificates in order to validate credentials of individuals or companies. This process has a cost associated with it and is definitely not a requirement for using certificates; however, it can put some of the more paranoid users at ease.

## 14.9.1. Generating Certificates

To generate a certificate, the following command is available:

    # openssl req -new -nodes -out req.pem -keyout cert.pem
    Generating a 1024 bit RSA private key
    ................++++++
    .......................................++++++
    writing new private key to 'cert.pem'
    -----
    You are about to be asked to enter information that will be incorporated
    into your certificate request.
    What you are about to enter is what is called a Distinguished Name or a DN.
    There are quite a few fields but you can leave some blank
    For some fields there will be a default value,
    If you enter '.', the field will be left blank.
    -----
    Country Name (2 letter code) [AU]:US
    State or Province Name (full name) [Some-State]:PA
    Locality Name (eg, city) []:Pittsburgh
    Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Company
    Organizational Unit Name (eg, section) []:Systems Administrator
    Common Name (eg, YOUR name) []:localhost.example.org
    Email Address []:trhodes@FreeBSD.org

    Please enter the following 'extra' attributes
    to be sent with your certificate request
    A challenge password []:SOME PASSWORD
    An optional company name []:Another Name

Notice the response directly after the “Common Name” prompt shows a domain name. This prompt requires a server name to be entered for verification purposes; placing anything but a domain name would yield a useless certificate. Other options, for instance expire time, alternate encryption algorithms, etc. are available. A complete list may be obtained by viewing the openssl(1) manual page.

Two files should now exist in the directory in which the aforementioned command was issued. The certificate request, `req.pem`, may be sent to a certificate authority who will validate the credentials that you entered, sign the request and return the certificate to you. The second file created will be named `cert.pem` and is the private key for the certificate and should be protected at all costs; if this falls in the hands of others it can be used to impersonate you (or your server).

In cases where a signature from a CA is not required, a self signed certificate can be created. First, generate the RSA key:

    # openssl dsaparam -rand -genkey -out myRSA.key 1024

Next, generate the CA key:

    # openssl gendsa -des3 -out myca.key myRSA.key

Use this key to create the certificate:

    # openssl req -new -x509 -days 365 -key myca.key -out new.crt

Two new files should appear in the directory: a certificate authority signature file, `myca.key` and the certificate itself, `new.crt`. These should be placed in a directory, preferably under `/etc`, which is readable only by `root`. Permissions of 0700 should be fine for this and they can be set with the `chmod` utility.

## 14.9.2. Using Certificates, an Example

So what can these files do? A good use would be to encrypt connections to the Sendmail MTA. This would dissolve the use of clear text authentication for users who send mail via the local MTA.

> 注: This is not the best use in the world as some MUAs will present the user with an error if they have not installed the certificate locally. Refer to the documentation included with the software for more information on certificate installation.

The following lines should be placed inside the local `.mc` file:

    dnl SSL Options
    define(`confCACERT_PATH',`/etc/certs')dnl
    define(`confCACERT',`/etc/certs/new.crt')dnl
    define(`confSERVER_CERT',`/etc/certs/new.crt')dnl
    define(`confSERVER_KEY',`/etc/certs/myca.key')dnl
    define(`confTLS_SRV_OPTIONS', `V')dnl

Where `/etc/certs/` is the directory to be used for storing the certificate and key files locally. The last few requirements are a rebuild of the local `.cf` file. This is easily achieved by typing `make` `install` within the `/etc/mail` directory. Follow that up with `make` `restart` which should start the Sendmail daemon.

If all went well there will be no error messages in the `/var/log/maillog` file and Sendmail will show up in the process list.

For a simple test, simply connect to the mail server using the telnet(1) utility:

    # telnet example.com 25
    Trying 192.0.34.166...
    Connected to example.com.
    Escape character is '^]'.
    220 example.com ESMTP Sendmail 8.12.10/8.12.10; Tue, 31 Aug 2004 03:41:22 -0400 (EDT)
    ehlo example.com
    250-example.com Hello example.com [192.0.34.166], pleased to meet you
    250-ENHANCEDSTATUSCODES
    250-PIPELINING
    250-8BITMIME
    250-SIZE
    250-DSN
    250-ETRN
    250-AUTH LOGIN PLAIN
    250-STARTTLS
    250-DELIVERBY
    250 HELP
    quit
    221 2.0.0 example.com closing connection
    Connection closed by foreign host.

If the “STARTTLS” line appears in the output then everything is working correctly.

前一頁 | 內容 | 下一頁
Kerberos5 | 上一層 | VPN over IPsec

本文及其他文件，可由此下載：ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/。

若有 FreeBSD 方面疑問，請先閱讀 FreeBSD 相關文件，如不能解決的話，再洽詢 <questions@FreeBSD.org>。
關於本文件的問題，請洽詢 <doc@FreeBSD.org>。