When generating the OVPN configuration with easy_RSA, the following error occurred:

[ root: /usr/share/easy-rsa] #/usr/share/easy-rsa/build-key --batch zzzz.29761
Using Common Name: zzzz.29761
Generating a 2048 bit RSA private key
...............+++
.................................................................+++
writing new private key to 'zzzz.29761.key'
-----
Using configuration from /usr/share/easy-rsa/openssl-1.0.0.cnf
Error Loading extension section usr_cert
140516636624544:error:0E06D06C:configuration file routines:NCONF_get_string:no value:conf_lib.c:335:group=CA_default name=email_in_dn
140516636624544:error:2207507C:X509 V3 routines:v2i_GENERAL_NAME_ex:missing value:v3_alt.c:537:
140516636624544:error:22098080:X509 V3 routines:X509V3_EXT_nconf:error in extension:v3_conf.c:93:name=subjectAltName, value=zzzz.29761

View the version information of OpenVPN:

# openvpn --version
OpenVPN 2.3.2 x86_64-pc-linux-gnu [SSL (OpenSSL)] [LZO] [EPOLL] [PKCS11] [eurephia] [MH] [IPv6] built on Dec  1 2014
Originally developed by James Yonan
Copyright (C) 2002-2010 OpenVPN Technologies, Inc. <[email protected]>

in the configuration file openssl-1.0.0.cnf, the [usr_cert] section contains an additional parameter: subjectAltName=email:copy.
Openssl – 1.0.0. CNF content:

[root@control 2.0]# cat openssl-1.0.0.cnf
# For use with easy-rsa version 2.0 and OpenSSL 1.0.0*

# This definition stops the following lines choking if HOME isn't
# defined.
HOME                    = .
RANDFILE                = $ENV::HOME/.rnd
openssl_conf            = openssl_init

[ openssl_init ]
# Extra OBJECT IDENTIFIER info:
#oid_file               = $ENV::HOME/.oid
oid_section             = new_oids
engines                 = engine_section

# To use this configuration file with the "-extfile" option of the
# "openssl x509" utility, name here the section containing the
# X.509v3 extensions to use:
# extensions            =
# (Alternatively, use a configuration file that has only
# X.509v3 extensions in its main [= default] section.)

[ new_oids ]

# We can add new OIDs in here for use by 'ca' and 'req'.
# Add a simple OID like this:
# testoid1=1.2.3.4
# Or use config file substitution like this:
# testoid2=${testoid1}.5.6

####################################################################
[ ca ]
default_ca      = CA_default            # The default ca section

####################################################################
[ CA_default ]

dir             = $ENV::KEY_DIR         # Where everything is kept
certs           = $dir                  # Where the issued certs are kept
crl_dir         = $dir                  # Where the issued crl are kept
database        = $dir/index.txt        # database index file.
new_certs_dir   = $dir                  # default place for new certs.

certificate     = $dir/ca.crt           # The CA certificate
serial          = $dir/serial           # The current serial number
crl             = $dir/crl.pem          # The current CRL
private_key     = $dir/ca.key           # The private key
RANDFILE        = $dir/.rand            # private random number file

x509_extensions = usr_cert              # The extentions to add to the cert

# Extensions to add to a CRL. Note: Netscape communicator chokes on V2 CRLs
# so this is commented out by default to leave a V1 CRL.
# crl_extensions        = crl_ext

default_days    = 3650                  # how long to certify for
default_crl_days= 30                    # how long before next CRL
default_md      = sha256                # use public key default MD
preserve        = no                    # keep passed DN ordering

# A few difference way of specifying how similar the request should look
# For type CA, the listed attributes must be the same, and the optional
# and supplied fields are just that :-)
policy          = policy_anything

# For the CA policy
[ policy_match ]
countryName             = match
stateOrProvinceName     = match
organizationName        = match
organizationalUnitName  = optional
commonName              = supplied
name                    = optional
emailAddress            = optional

# For the 'anything' policy
# At this point in time, you must list all acceptable 'object'
# types.
[ policy_anything ]
countryName             = optional
stateOrProvinceName     = optional
localityName            = optional
organizationName        = optional
organizationalUnitName  = optional
commonName              = supplied
name                    = optional
emailAddress            = optional

####################################################################
[ req ]
default_bits            = $ENV::KEY_SIZE
default_keyfile         = privkey.pem
default_md              = sha256
distinguished_name      = req_distinguished_name
attributes              = req_attributes
x509_extensions = v3_ca # The extentions to add to the self signed cert

# Passwords for private keys if not present they will be prompted for
# input_password = secret
# output_password = secret

# This sets a mask for permitted string types. There are several options.
# default: PrintableString, T61String, BMPString.
# pkix   : PrintableString, BMPString (PKIX recommendation after 2004).
# utf8only: only UTF8Strings (PKIX recommendation after 2004).
# nombstr : PrintableString, T61String (no BMPStrings or UTF8Strings).
# MASK:XXXX a literal mask value.
string_mask = nombstr

# req_extensions = v3_req # The extensions to add to a certificate request

[ req_distinguished_name ]
countryName                     = Country Name (2 letter code)
countryName_default             = $ENV::KEY_COUNTRY
countryName_min                 = 2
countryName_max                 = 2

stateOrProvinceName             = State or Province Name (full name)
stateOrProvinceName_default     = $ENV::KEY_PROVINCE

localityName                    = Locality Name (eg, city)
localityName_default            = $ENV::KEY_CITY

0.organizationName              = Organization Name (eg, company)
0.organizationName_default      = $ENV::KEY_ORG

# we can do this but it is not needed normally :-)
#1.organizationName             = Second Organization Name (eg, company)
#1.organizationName_default     = World Wide Web Pty Ltd

organizationalUnitName          = Organizational Unit Name (eg, section)
#organizationalUnitName_default =

commonName                      = Common Name (eg, your name or your server\'s hostname)
commonName_max                  = 64

name                            = Name
name_max                        = 64

emailAddress                    = Email Address
emailAddress_default            = $ENV::KEY_EMAIL
emailAddress_max                = 40

# JY -- added for batch mode
organizationalUnitName_default = $ENV::KEY_OU
commonName_default = $ENV::KEY_CN
name_default = $ENV::KEY_NAME


# SET-ex3                       = SET extension number 3

[ req_attributes ]
challengePassword               = A challenge password
challengePassword_min           = 4
challengePassword_max           = 20

unstructuredName                = An optional company name

[ usr_cert ]

# These extensions are added when 'ca' signs a request.

# This goes against PKIX guidelines but some CAs do it and some software
# requires this to avoid interpreting an end user certificate as a CA.

basicConstraints=CA:FALSE

# Here are some examples of the usage of nsCertType. If it is omitted
# the certificate can be used for anything *except* object signing.

# This is OK for an SSL server.
# nsCertType                    = server

# For an object signing certificate this would be used.
# nsCertType = objsign

# For normal client use this is typical
# nsCertType = client, email

# and for everything including object signing:
# nsCertType = client, email, objsign

# This is typical in keyUsage for a client certificate.
# keyUsage = nonRepudiation, digitalSignature, keyEncipherment

# This will be displayed in Netscape's comment listbox.
nsComment                       = "Easy-RSA Generated Certificate"

# PKIX recommendations harmless if included in all certificates.
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid,issuer:always
extendedKeyUsage=clientAuth
keyUsage = digitalSignature


# This stuff is for subjectAltName and issuerAltname.
# Import the email address.
# subjectAltName=email:copy

# Copy subject details
# issuerAltName=issuer:copy

#nsCaRevocationUrl              = http://www.domain.dom/ca-crl.pem
#nsBaseUrl
#nsRevocationUrl
#nsRenewalUrl
#nsCaPolicyUrl
#nsSslServerName

[ server ]

# JY ADDED -- Make a cert with nsCertType set to "server"
basicConstraints=CA:FALSE
nsCertType                     = server
nsComment                      = "Easy-RSA Generated Server Certificate"
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid,issuer:always
extendedKeyUsage=serverAuth
keyUsage = digitalSignature, keyEncipherment

[ v3_req ]

# Extensions to add to a certificate request

basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment

[ v3_ca ]


# Extensions for a typical CA


# PKIX recommendation.

subjectKeyIdentifier=hash

authorityKeyIdentifier=keyid:always,issuer:always

# This is what PKIX recommends but some broken software chokes on critical
# extensions.
#basicConstraints = critical,CA:true
# So we do this instead.
basicConstraints = CA:true

# Key usage: this is typical for a CA certificate. However since it will
# prevent it being used as an test self-signed certificate it is best
# left out by default.
# keyUsage = cRLSign, keyCertSign

# Some might want this also
# nsCertType = sslCA, emailCA

# Include email address in subject alt name: another PKIX recommendation
# subjectAltName=email:copy
# Copy issuer details
# issuerAltName=issuer:copy

# DER hex encoding of an extension: beware experts only!
# obj=DER:02:03
# Where 'obj' is a standard or added object
# You can even override a supported extension:
# basicConstraints= critical, DER:30:03:01:01:FF

[ crl_ext ]

# CRL extensions.
# Only issuerAltName and authorityKeyIdentifier make any sense in a CRL.

# issuerAltName=issuer:copy
authorityKeyIdentifier=keyid:always,issuer:always

[ engine_section ]
#
# If you are using PKCS#11
# Install engine_pkcs11 of opensc (www.opensc.org)
# And uncomment the following
# verify that dynamic_path points to the correct location
#
#pkcs11 = pkcs11_section

[ pkcs11_section ]
engine_id = pkcs11
dynamic_path = /usr/lib/engines/engine_pkcs11.so
MODULE_PATH = $ENV::PKCS11_MODULE_PATH
PIN = $ENV::PKCS11_PIN
init = 0

Reference:
1 Error Loading the extension section usr_cert:http://stackoverflow.com/questions/24255205/error-loading-extension-section-usr-cert openvpn configuration file
2 a: http://www-2w.blog.163.com/blog/static/9793151820111010253869/