---
name: "x509v3.cnf"
section: "5"
platform: "openbsd"
package: "man"
version: "8.0-current-20261101"
release: "OpenBSD 8.0-current (2026-11-01)"
platforms: ["openbsd"]
url: "https://new.carta.tech/x509v3.cnf"
---

# x509v3.cnf(5)

X.509 V3 certificate extension configuration format

## NAME

**x509v3.cnf** – X.509 V3 certificate extension configuration format

## DESCRIPTION

Several of the OpenSSL utilities can add extensions to a certificate or certificate request based on the contents of a configuration file. The file format is based on the [openssl.cnf(5)](/openssl.cnf/5.md) format.

Typically the application will contain an option to point to an extension section. Each line of the extension section takes the form:

*extension\_name*=\[**critical**,\] *extension\_options*

If **critical** is present, then the extension will be critical.

The format of *extension\_options* depends on the value of *extension\_name*.

There are four main types of extension: string extensions, multi-valued extensions, raw extensions, and arbitrary extensions.

String extensions simply have a string which contains either the value itself or how it is obtained. For example:

```
nsComment="This is a Comment"
```

Multi-valued extensions have a short form and a long form. The short form is a list of names and values:

```
basicConstraints=critical,CA:true,pathlen:1
```

The long form allows the values to be placed in a separate section:

```
basicConstraints=critical,@bs_section
[bs_section]
CA=true
pathlen=1
```

Both forms are equivalent.

The syntax of raw extensions is governed by the extension code: it can for example contain data in multiple sections. The correct syntax to use is defined by the extension code itself: check out the certificate policies extension for an example.

If an extension type is unsupported, then the arbitrary extension syntax must be used; see the ARBITRARY EXTENSIONS section for more details.

## STANDARD EXTENSIONS

The following sections describe each supported extension in detail.

### Basic constraints

This is a multi-valued extension which indicates whether a certificate is a CA certificate. The first (mandatory) name is **CA** followed by **TRUE** or **FALSE**. If **CA** is **TRUE**, then an optional **pathlen** name followed by a non-negative value can be included. For example:

```
basicConstraints=CA:TRUE
basicConstraints=CA:FALSE
basicConstraints=critical,CA:TRUE, pathlen:0
```

A CA certificate must include the **basicConstraints** value with the **CA** field set to **TRUE**. An end user certificate must either set **CA** to **FALSE** or exclude the extension entirely. Some software may require the inclusion of **basicConstraints** with **CA** set to **FALSE** for end entity certificates.

The **pathlen** parameter indicates the maximum number of CAs that can appear below this one in a chain. So if you have a CA with a **pathlen** of zero, it can only be used to sign end user certificates and not further CAs.

### Key usage

Key usage is a multi-valued extension consisting of a list of names of the permitted key usages.

The supported names are: **digitalSignature**, **nonRepudiation**, **keyEncipherment**, **dataEncipherment**, **keyAgreement**, **keyCertSign**, **cRLSign**, **encipherOnly**, and **decipherOnly**. Examples:

```
keyUsage=digitalSignature, nonRepudiation
keyUsage=critical, keyCertSign
```

### Extended key usage

This extension consists of a list of purposes for which the certificate public key can be used.

These can either be object short names or the dotted numerical form of OIDs. While any OID can be used, only certain values make sense. In particular the following PKIX, NS and MS values are meaningful:

| | |
| --- | --- |
| *value* | *meaning* |
| **serverAuth** | TLS server authentication |
| **clientAuth** | TLS client authentication |
| **codeSigning** | code signing |
| **emailProtection** | E-mail protection (S/MIME) |
| **timeStamping** | trusted timestamping |
| **OCSPSigning** | OCSP signing |
| **ipsecIKE** | IPsec internet key exchange |
| **msCodeInd** | Microsoft individual code signing (authenticode) |
| **msCodeCom** | Microsoft commercial code signing (authenticode) |
| **msCTLSign** | Microsoft trust list signing |
| **msEFS** | Microsoft encrypted file system |

Examples:

```
extendedKeyUsage=critical,codeSigning,1.2.3.4
extendedKeyUsage=serverAuth,clientAuth
```

### Subject key identifier

This is really a string extension and can take two possible values. Either the word **hash** which will automatically follow the guidelines in RFC 3280 or a hex string giving the extension value to include. The use of the hex string is strongly discouraged. Example:

```
subjectKeyIdentifier=hash
```

### Authority key identifier

The authority key identifier extension permits two options, **keyid** and **issuer**: both can take the optional value **always**.

If the **keyid** option is present, an attempt is made to copy the subject key identifier from the parent certificate. If the value **always** is present, then an error is returned if the option fails.

The **issuer** option copies the issuer and serial number from the issuer certificate. This will only be done if the **keyid** option fails or is not included unless the **always** flag will always include the value. Example:

```
authorityKeyIdentifier=keyid,issuer
```

### Subject alternative name

The subject alternative name extension allows various literal values to be included in the configuration file. These include **email** (an email address), **URI** (a uniform resource indicator), **DNS** (a DNS domain name), **RID** (a registered ID: OBJECT IDENTIFIER), **IP** (an IP address), **dirName** (a distinguished name), and **otherName**.

The **email** option can include a special **copy** value. This will automatically include any email addresses contained in the certificate subject name in the extension.

The IP address used in the **IP** options can be in either IPv4 or IPv6 format.

The value of **dirName** should point to a section containing the distinguished name to use as a set of name value pairs. Multi values AVAs can be formed by prefacing the name with a ‘+’ character.

**otherName** can include arbitrary data associated with an OID: the value should be the OID followed by a semicolon and the content in standard ASN1\_generate\_nconf(3) format. Examples:

```
subjectAltName=email:copy,email:my@other.address,URI:http://my.url.here/
subjectAltName=IP:192.168.7.1
subjectAltName=IP:13::17
subjectAltName=email:my@other.address,RID:1.2.3.4
subjectAltName=otherName:1.2.3.4;UTF8:some other identifier
subjectAltName=dirName:dir_sect
[dir_sect]
C=UK
O=My Organization
OU=My Unit
CN=My Name
```

### Issuer alternative name

The issuer alternative name option supports all the literal options of subject alternative name. It does not support the **email**:**copy** option because that would not make sense. It does support an additional **issuer**:**copy** option that will copy all the subject alternative name values from the issuer certificate (if possible). Example:

```
issuerAltName = issuer:copy
```

### Authority info access

The authority information access extension gives details about how to access certain information relating to the CA. Its syntax is *accessOID*; *location* where *location* has the same syntax as subject alternative name (except that **email**:**copy** is not supported). *accessOID* can be any valid OID but only certain values are meaningful, for example **OCSP** and **caIssuers**. Example:

```
authorityInfoAccess = OCSP;URI:http://ocsp.my.host/
authorityInfoAccess = caIssuers;URI:http://my.ca/ca.html
```

### CRL distribution points

This is a multi-valued extension whose options can be either in *name*:*value* pair form using the same form as subject alternative name or a single value representing a section name containing all the distribution point fields.

For a *name*:*value* pair a new DistributionPoint with the fullName field set to the given value, both the cRLissuer and reasons fields are omitted in this case.

In the single option case, the section indicated contains values for each field. In this section:

If the name is **fullname**, the value field should contain the full name of the distribution point in the same format as subject alternative name.

The name **CRLIssuer**, if present, should contain a value for this field in subject alternative name format.

If the name is **reasons**, the value field should consist of a comma separated field containing the reasons. Valid reasons are: **keyCompromise**, **CACompromise**, **affiliationChanged**, **superseded**, **cessationOfOperation**, **certificateHold**, **privilegeWithdrawn**, and **AACompromise**.

Simple examples:

```
crlDistributionPoints=URI:http://myhost.com/myca.crl
crlDistributionPoints=URI:http://my.com/my.crl,URI:http://oth.com/my.crl
```

Full distribution point example:

```
crlDistributionPoints=crldp1_section
[crldp1_section]
fullname=URI:http://myhost.com/myca.crl
CRLissuer=dirName:issuer_sect
reasons=keyCompromise, CACompromise
[issuer_sect]
C=UK
O=Organisation
CN=Some Name
```

### Issuing distribution point

This extension should only appear in CRLs. It is a multi-valued extension whose syntax is similar to the "section" pointed to by the CRL distribution points extension with a few differences.

The names **reasons** and **CRLissuer** are not recognized.

The name **onlysomereasons** is accepted, which sets this field. The value is in the same format as the CRL distribution point **reasons** field.

The names **onlyuser**, **onlyCA**, **onlyAA**, and **indirectCRL** are also accepted. The values should be a boolean values (**TRUE** or **FALSE**) to indicate the value of the corresponding field. Example:

```
issuingDistributionPoint=critical, @idp_section
[idp_section]
fullname=URI:http://myhost.com/myca.crl
indirectCRL=TRUE
onlysomereasons=keyCompromise, CACompromise
[issuer_sect]
C=UK
O=Organisation
CN=Some Name
```

### Certificate policies

This is a raw extension. All the fields of this extension can be set by using the appropriate syntax.

If you follow the PKIX recommendations and just use one OID, then you just include the value of that OID. Multiple OIDs can be set separated by commas, for example:

```
certificatePolicies= 1.2.4.5, 1.1.3.4
```

If you wish to include qualifiers, then the policy OID and qualifiers need to be specified in a separate section: this is done by using the @*section* syntax instead of a literal OID value.

The section referred to must include the policy OID using the name **policyIdentifier**. **CPSuri** qualifiers can be included using the syntax:

**CPS**.*nnn*=*value*

**userNotice** qualifiers can be set using the syntax:

**userNotice**.*nnn*=@*notice*

The value of the **userNotice** qualifier is specified in the relevant section. This section can include **explicitText**, **organization**, and **noticeNumbers** options. **explicitText** and **organization** are text strings, and **noticeNumbers** is a comma separated list of numbers. The **organization** and **noticeNumbers** options (if included) must *both* be present. If you use the **userNotice** option with IE5 then you need the **ia5org** option at the top level to modify the encoding: otherwise it will not be interpreted properly. Example:

```
certificatePolicies=ia5org,1.2.3.4,1.5.6.7.8,@polsect
[polsect]
policyIdentifier = 1.3.5.8
CPS.1="http://my.host.name/"
CPS.2="http://my.your.name/"
userNotice.1=@notice
[notice]
explicitText="Explicit Text Here"
organization="Organisation Name"
noticeNumbers=1,2,3,4
```

The **ia5org** option changes the type of the **organization** field. In RFC 2459, it can only be of type *DisplayText*. In RFC 3280, *IA5String* is also permissible. Some software (for example some versions of MSIE) may require **ia5org**.

### Policy constraints

This is a multi-valued extension which consists of the names **requireExplicitPolicy** or **inhibitPolicyMapping** and a non-negative integer value. At least one component must be present. Example:

```
policyConstraints = requireExplicitPolicy:3
```

### Inhibit any policy

This is a string extension whose value must be a non-negative integer. Example:

```
inhibitAnyPolicy = 2
```

### Name constraints

The name constraints extension is a multi-valued extension. The name should begin with the word **permitted** or **excluded**, followed by a semicolon. The rest of the name and the value follows the syntax of subjectAltName except **email**:**copy** is not supported and the **IP** form should consist of an IP addresses and subnet mask separated by a slash. Examples:

```
nameConstraints=permitted;IP:192.168.0.0/255.255.0.0
nameConstraints=permitted;email:.somedomain.com
nameConstraints=excluded;email:.com
```

### OCSP no check

The OCSP no check extension is a string extension, but its value is ignored. Example:

```
noCheck = ignored
```

### TLS Feature (aka must staple)

This is a multi-valued extension consisting of a list of TLS extension identifiers. Each identifier may be a number in the range from 0 to 65535 or a supported name. When a TLS client sends a listed extension, the TLS server is expected to include that extension in its reply.

The supported names are: **status\_request** and **status\_request\_v2**. Example:

```
tlsfeature = status_request
```

## DEPRECATED EXTENSIONS

The following extensions are non-standard, Netscape specific and largely obsolete. Their use in new applications is discouraged.

### Netscape string extensions

Netscape comment (**nsComment**) is a string extension containing a comment which will be displayed when the certificate is viewed in some browsers. Example:

```
nsComment = Some Random Comment
```

Other supported extensions in this category are: **nsBaseUrl**, **nsRevocationUrl**, **nsCaRevocationUrl**, **nsRenewalUrl**, **nsCaPolicyUrl**, and **nsSslServerName**.

### Netscape certificate type

This is a multi-valued extensions which consists of a list of flags to be included. It was used to indicate the purposes for which a certificate could be used. The **basicConstraints**, **keyUsage**, and extended key usage extensions are now used instead.

Acceptable values for **nsCertType** are: **client**, **server**, **email**, **objsign**, **reserved**, **sslCA**, **emailCA**, **objCA**.

## ARBITRARY EXTENSIONS

If an extension is not supported by the OpenSSL code, then it must be encoded using the arbitrary extension format. It is also possible to use the arbitrary format for supported extensions. Extreme care should be taken to ensure that the data is formatted correctly for the given extension type.

There are two ways to encode arbitrary extensions.

The first way is to use the word **ASN1** followed by the extension content using the same syntax as ASN1\_generate\_nconf(3). For example:

```
1.2.3.4=critical,ASN1:UTF8String:Some random data
1.2.3.4=ASN1:SEQUENCE:seq_sect
[seq_sect]
field1 = UTF8:field1
field2 = UTF8:field2
```

It is also possible to use the word **DER** to include the raw encoded data in any extension.

```
1.2.3.4=critical,DER:01:02:03:04
1.2.3.4=DER:01020304
```

The value following **DER** is a hex dump of the DER encoding of the extension. Any extension can be placed in this form to override the default behaviour. For example:

```
basicConstraints=critical,DER:00:01:02:03
```

## FILES

- */etc/ssl/x509v3.cnf*

  standard configuration file

## SEE ALSO

[openssl(1)](/openssl/1.md), ASN1\_generate\_nconf(3), OPENSSL\_config(3), [openssl.cnf(5)](/openssl.cnf/5.md)

## HISTORY

X509v3 extension code was first added to OpenSSL 0.9.2.

## CAVEATS

There is no guarantee that a specific implementation will process a given extension. It may therefore sometimes be possible to use certificates for purposes prohibited by their extensions because a specific application does not recognize or honour the values of the relevant extensions.

The **DER** and **ASN1** options should be used with caution. It is possible to create totally invalid extensions if they are not used carefully.

If an extension is multi-value and a field value must contain a comma, the long form must be used. Otherwise the comma would be misinterpreted as a field separator. For example,

```
subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
```

will produce an error, but the following form is valid:

```
subjectAltName=@subject_alt_section
[subject_alt_section]
subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
```

Due to the behaviour of the OpenSSL CONF library, the same field name can only occur once in a section. That means that

```
subjectAltName=@alt_section
[alt_section]
email=steve@here
email=steve@there
```

will only use the last value. This can be worked around by using the form:

```
[alt_section]
email.1=steve@here
email.2=steve@there
```