The following should work for you:
cp /etc/ssl/openssl.cnf /tmp/
echo '[ subject_alt_name ]' >> /tmp/openssl.cnf
echo 'subjectAltName = DNS:www.example.com, DNS:site1.example.com, DNS:site2.example.com' >> /tmp/openssl.cnf
openssl req -x509 -nodes -newkey rsa:2048 \
-config /tmp/openssl.cnf \
-extensions subject_alt_name \
-keyout www.example.com.key \
-out www.example.com.pem \
-subj '/C=XX/ST=XXXX/L=XXXX/O=XXXX/OU=XXXX/CN=www.example.com/emailAddress=postmaster@example.com'
Result:
$ openssl x509 -in www.example.com.pem -text -noout
Certificate:
Data:
Version: 3 (0x2)
Serial Number: 17965603478303142689 (0xf952a52d7bc7f321)
Signature Algorithm: sha256WithRSAEncryption
Issuer: C=XX, ST=XXXX, L=XXXX, O=XXXX, OU=XXXX, CN=www.example.com/emailAddress=flb.helpdesk@morpho.com
Validity
Not Before: Apr 3 15:34:27 2015 GMT
Not After : May 3 15:34:27 2015 GMT
Subject: C=XX, ST=XXXX, L=XXXX, O=XXXX, OU=XXXX, CN=www.example.com/emailAddress=flb.helpdesk@morpho.com
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:c6:e8:17:93:a4:8f:08:11:61:11:1b:cc:17:52:
b6:33:39:33:0e:94:3f:ac:ae:dd:70:4b:e6:d6:b0:
11:f1:93:b4:3d:dc:34:99:27:b6:45:4b:13:0c:69:
1e:11:d2:b9:38:5f:e0:d1:b0:58:e4:3c:c0:9f:58:
3d:5d:fa:67:3e:3c:05:1b:e3:86:20:18:d5:d7:83:
77:b5:0c:1d:9a:26:96:10:3f:2c:e5:ce:ed:6e:99:
5a:35:3e:06:f0:52:aa:72:5e:c0:33:7c:c8:16:f9:
6b:3e:7d:7e:5a:1f:cf:11:63:4d:ad:bf:77:bd:e3:
0f:8f:24:1d:f5:c8:06:ab:d9:62:8d:13:56:62:a9:
b8:77:c0:11:b6:ff:a7:63:93:a7:22:c2:41:48:6f:
bd:42:10:00:33:14:da:3b:ca:e0:07:c2:b6:50:55:
f0:4d:6b:0d:eb:87:a8:bd:4d:c6:1b:20:d8:27:68:
d0:e2:3b:32:91:b8:8e:cf:25:06:bf:43:fd:8f:96:
fa:eb:af:0f:e1:5c:47:06:84:8b:f4:35:0a:a8:f3:
7e:af:34:50:7f:62:bc:5e:53:09:90:97:27:cf:9a:
56:d7:f6:af:32:92:c4:c9:ab:90:6e:a6:09:20:0b:
46:28:22:0b:45:71:b9:17:77:d8:da:63:24:27:5c:
60:a5
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Subject Alternative Name:
DNS:www.example.com, DNS:site1.example.com, DNS:site2.example.com
Signature Algorithm: sha256WithRSAEncryption
83:ce:18:74:f9:17:92:91:bd:82:13:ac:64:e7:de:9e:38:d7:
26:0f:2d:3e:8f:f2:17:c5:fc:17:06:fb:27:e7:0a:b7:f2:8e:
bb:18:6e:f4:4c:17:e8:8d:dc:51:d5:d2:e6:1c:72:e4:28:75:
a2:79:ae:b6:b6:8c:ec:08:08:4d:73:79:b7:22:26:40:ac:38:
5c:f2:2b:4f:44:60:55:db:90:b3:63:74:ad:e7:26:12:ad:0d:
ec:4a:cd:4f:7a:a0:54:62:f5:a7:bc:80:c2:fa:34:43:3c:f1:
aa:f7:6e:4e:e1:80:fb:c7:cc:44:3a:2b:a4:6c:5b:0f:3f:83:
6e:8d:d5:28:cf:6c:f0:c6:40:4d:c4:d4:3f:9c:9d:a4:47:a7:
27:d1:5b:2b:5f:0d:bd:3f:7c:2e:19:fa:bc:24:bd:1f:64:81:
8b:a4:e3:33:10:35:55:f9:73:2d:8b:e8:b8:d7:e3:49:0c:35:
af:53:df:48:d8:df:ce:b1:5f:6c:74:1c:74:89:45:2e:28:2c:
1f:fe:d8:a4:44:9c:c7:bc:d8:6a:46:38:df:e3:d0:05:37:27:
d0:08:e5:93:b8:0e:d9:d9:dd:7c:28:75:18:27:be:4e:72:47:
13:b9:a2:93:0e:83:e9:b8:49:f4:75:ad:e0:0f:9b:e5:96:4f:
33:33:f1:27
If you want request instead of self-signed just replace -x509 with -new and -extensions with -reqexts.
Update
Subject Alternative Name can be specified directly in command line with the recent versions of openssl:
openssl req -x509 -nodes -newkey rsa:2048 \
-subj '/C=XX/ST=XXXX/L=XXXX/O=XXXX/OU=XXXX/CN=www.example.com/emailAddress=postmaster@example.com' \
-addext 'subjectAltName = DNS:www.example.com, DNS:site1.example.com, DNS:site2.example.com' \
-keyout www.example.com.key \
-out www.example.com.pem
See also https://security.stackexchange.com/questions/74345/provide-subjectaltname-to-openssl-directly-on-the-command-line/183973#183973
both domain names will point to the same IP
If you want to avoid certificate warnings with 2 different sites sharing the same IP:port, you will need to use a SAN/UCC certificate that supports multiple/alternate names in one cert.
Take a look at this thread:
Apache SSL VirtualHosts on a single IP using UCC/SAN certificate
If ordering a SAN/UCC cert is not an option, you will need to use multiple IP addresses.
Best Answer
Historically, your first statement is accurate. Now, there are multiple options:
This has been answered numerous times on ServerFault by myself and others. I'd suggest searching for further details unless you have a specific question.