CEL Language Features

CEL, the Common Expression Language, is a small expression language designed for fast and safe evaluation of user-defined rules. openvpn-auth-oauth2 uses CEL for configuration values that need logic, such as identity validation, deriving the OpenVPN username, and resolving client configuration names. CEL expressions can inspect the variables provided by openvpn-auth-oauth2, call the documented string and list helpers, and return the type required by the specific setting. They cannot access files, make network calls, or execute arbitrary code.

CEL supports many standard operations:

Comparison Operators

• == (equals)
• != (not equals)
• <, <=, >, >= (numeric comparisons)

Logical Operators

• && (AND)
• || (OR)
• ! (NOT)

Available context

The same namespaced context is available to oauth2.openvpn-username, oauth2.validate.expression, and openvpn.client-config.expression:

Field	Type	Description
auth.mode	string	interactive for the initial login or non-interactive for token refresh
openvpn.commonName	string	Common name supplied by OpenVPN
openvpn.ip	string	OpenVPN client IP address
openvpn.sessionState	string	Current OpenVPN session state
token.claims	map<string, dynamic>	Raw OAuth2 ID token claims; an empty map when no parsed ID token is available
token.ip	string	IP address claim from the OAuth2 ID token; an empty string when unavailable
user.subject	string	Provider-independent user subject
user.email	string	Provider-independent email address
user.username	string	Provider-independent username
user.groups	list<string>	Provider-independent list of groups
user.roles	list<string>	Provider-independent list of roles

Provider data is normalized before CEL runs. For a generic OIDC provider, user.groups comes from oauth2.groups-claim and user.roles comes from the roles claim. For GitHub, organizations populate user.groups and teams populate user.roles in org:slug format when those fields are used. Google group validation populates user.groups with the configured groups that match the user.

During oauth2.openvpn-username, user.username is the provider's username candidate, such as preferred_username from an ID token or UserInfo response, or the GitHub login. The expression result becomes the resolved username. oauth2.validate.expression and openvpn.client-config.expression receive that resolved value in user.username. An empty expression or an empty expression result falls back to openvpn.commonName.

Prefer user.* for portable expressions. Use token.claims.* when a rule depends on a provider-specific raw claim. Raw claims may be absent during a refresh or when identity data comes from UserInfo.

Extension Libraries

All CEL expressions load the CEL strings extension and CEL lists extension.

String Functions

The following string functions are available through the CEL strings extension:

Basic String Operations

• startsWith(<string>) - Check if string starts with prefix
• endsWith(<string>) - Check if string ends with suffix
• contains(<string>) - Check if string contains substring
• matches(<string>) - Check if string matches a regular expression pattern

Case Conversion

• lowerAscii() - Convert ASCII characters to lowercase
  • Example: 'TacoCat'.lowerAscii() returns 'tacocat'
• upperAscii() - Convert ASCII characters to uppercase
  • Example: 'TacoCat'.upperAscii() returns 'TACOCAT'

String Searching

• indexOf(<string>) - Returns the index of the first occurrence of a substring (or -1 if not found)
  • Example: 'hello mellow'.indexOf('ello') returns 1
• indexOf(<string>, <int>) - Search starting from a specific position
  • Example: 'hello mellow'.indexOf('ello', 2) returns 7
• lastIndexOf(<string>) - Returns the index of the last occurrence of a substring
  • Example: 'hello mellow'.lastIndexOf('ello') returns 7
• lastIndexOf(<string>, <int>) - Search up to a specific position

String Manipulation

• substring(<int>) - Extract substring from position to end
  • Example: 'tacocat'.substring(4) returns 'cat'
• substring(<int>, <int>) - Extract substring from start (inclusive) to end (exclusive)
  • Example: 'tacocat'.substring(0, 4) returns 'taco'
• trim() - Remove leading and trailing whitespace
  • Example: ' \ttrim\n '.trim() returns 'trim'
• replace(<string>, <string>) - Replace all occurrences of a substring
  • Example: 'hello hello'.replace('he', 'we') returns 'wello wello'
• replace(<string>, <string>, <int>) - Replace with a limit on number of replacements
  • Example: 'hello hello'.replace('he', 'we', 1) returns 'wello hello'
• reverse() - Reverse the string
  • Example: 'gums'.reverse() returns 'smug'

String Splitting and Joining

• split(<string>) - Split string by separator into a list
  • Example: 'hello hello hello'.split(' ') returns ['hello', 'hello', 'hello']
• split(<string>, <int>) - Split with a limit on number of substrings
  • Example: 'hello hello hello'.split(' ', 2) returns ['hello', 'hello hello']
• join() - Join list of strings (on a list, not a string)
  • Example: ['hello', 'mellow'].join() returns 'hellomellow'
• join(<string>) - Join list of strings with separator
  • Example: ['hello', 'mellow'].join(' ') returns 'hello mellow'

String Formatting

• format(<list>) - Format string with printf-style substitutions
  • Supports: %s (string), %d (integer), %f (float), %e (scientific), %b (binary), %x/%X (hex), %o (octal)
  • Example: "Hello %s, you have %d messages".format(['Alice', 5]) returns 'Hello Alice, you have 5 messages'

String Utilities

• strings.quote(<string>) - Make string safe to print by escaping special characters
  • Example: strings.quote('single-quote with "double quote"') returns '"single-quote with \"double quote\""'

List Functions

• in - Check if an element is in a list
• size() - Get the size of a list or map
• exists(var, predicate) - Check if any element matches
  • Example: user.groups.exists(g, g == 'vpn-users')
• all(var, predicate) - Check if every element matches
  • Example: user.groups.all(g, g.startsWith('vpn-'))
• exists_one(var, predicate) - Check if exactly one element matches
  • Example: user.groups.exists_one(g, g == 'vpn-admin')
• filter(var, predicate) - Keep matching elements
  • Example: user.groups.filter(g, g.startsWith('vpn-'))
• map(var, expression) - Transform elements
  • Example: user.groups.map(g, g.lowerAscii())
• map(var, predicate, expression) - Transform matching elements
  • Example: user.groups.map(g, g.startsWith('vpn-'), g.lowerAscii())

The CEL lists extension adds more list helpers:

• slice(<int>, <int>) - Return a sub-list
  • Example: ['base', 'admin', 'net'].slice(0, 2) returns ['base', 'admin']
• flatten() - Flatten one nested list level
  • Example: [['base'], ['admin', 'net']].flatten() returns ['base', 'admin', 'net']
• flatten(<int>) - Flatten a specific number of nested list levels
• sort() - Sort a list of comparable values
  • Example: ['net', 'base', 'admin'].sort() returns ['admin', 'base', 'net']
• sortBy(var, expression) - Sort a list by a derived key
  • Example: [{'name': 'admin', 'order': 2}, {'name': 'base', 'order': 1}].sortBy(p, p.order).map(p, p.name)
• reverse() - Reverse list order
  • Example: ['base', 'admin'].reverse() returns ['admin', 'base']
• distinct() - Remove duplicate list elements while keeping first occurrence
  • Example: ['base', 'admin', 'base'].distinct() returns ['base', 'admin']

Example for client configuration:

openvpn:
  client-config:
    expression: |
      (
        ['base'] +
        user.groups
          .filter(g, g.startsWith('vpn-'))
          .map(g, g.replace('vpn-', ''))
          .sort() +
        [user.username]
      ).distinct()

Example that maps each group to one or more shared config files:

openvpn:
  client-config:
    expression: |
      user.groups
        .map(g, {
          'GRP-VPN': ['base-vpn'],
          'GRP-ADMIN': ['base-vpn', 'admin-routes'],
          'GRP-NETWORK': ['base-vpn', 'network-routes']
        }[g])
        .flatten()
        .distinct()

Map/Object Functions

• has() - Check if a key exists in a map

Type Conversion

• string() - Convert value to string (useful for casting claim values)

For more details, see:

• CEL Specification
• CEL Strings Extension
• cel-go Strings Documentation
• cel-go Lists Documentation

Error Handling

Missing Claims

If you try to access a claim that doesn't exist in the ID token without checking first, the validation will fail:

---
# ❌ Bad - will fail if 'department' claim doesn't exist
expression: 'token.claims.department == "engineering"'
---
# ✅ Good - safely checks for claim existence first
expression: 'has(token.claims.department) && token.claims.department == "engineering"'

Invalid Expressions

If your CEL expression has syntax errors, openvpn-auth-oauth2 will fail to start and log an error message indicating the compilation failure.

Non-Boolean Results

The expression must evaluate to a boolean. If it evaluates to another type (string, number, etc.), the validation will fail:

# ❌ Bad - evaluates to a string, not a boolean
expression: 'openvpn.commonName'
---
# ✅ Good - evaluates to a boolean
expression: 'openvpn.commonName != ""'

Best Practices

1. Always use has() to check for optional claims before accessing them to avoid validation failures
2. Keep expressions simple and readable - complex logic can be hard to debug
3. Test your CEL expressions with different token scenarios during development
4. Log validation failures to help troubleshoot issues
5. Document your validation rules in comments or documentation for team members.

Security Considerations

• CEL validation happens after OAuth2 authentication, so users must authenticate successfully before CEL rules are applied
• CEL expressions cannot access external resources or make network calls - they can only evaluate the provided variables
• CEL is sandboxed and safe - expressions cannot execute arbitrary code or affect the system
• Combining CEL with other validation options (groups, roles, common name) provides defense-in-depth

Debugging

If validation fails, check the openvpn-auth-oauth2 logs for error messages. The logs will indicate:

• CEL compilation errors (if the expression syntax is invalid)
• Evaluation errors (if the expression fails during evaluation)
• Which specific validation check failed

Example log messages:

failed to evaluate CEL expression: no such key: unknown
CEL validation failed
CEL expression did not evaluate to a boolean value

Performance

CEL expressions are compiled once at a startup and then evaluated efficiently for each authentication request. The performance impact is minimal, even for complex expressions.