CEL matcher language reference

Common Expression Language (CEL) is an open source non-Turing complete language that implements common semantics for expression evaluation. Secure Web Proxy uses a subset of CEL conditions to make boolean authorization decisions based on attribute data. In general, a condition expression consists of one or more statements that are joined by logical operators (&&, ||, or !). Each statement expresses an attribute-based control rule that applies to the role binding and ultimately determines whether access is allowed.

Attributes

Session attributes and application attributes are used when defining Secure Web Proxy policies to describe either the session attributes or the application attributes that a policy applies to.

Session attributes, described by SessionMatcher, apply to session-specific attributes, such as the source or destination IP address, hosts, or IP range. Application attributes, described by ApplicationMatcher, apply to application attributes, such as request headers, HTTP request method, or request path.

Available attributes in `SessionMatcher` and `ApplicationMatcher`

The following table describes attributes that apply to both SessionMatcher and ApplicationMatcher.

Attribute	Attribute type	Description
`source.ip`	string	The IP address of the client that sent the request.
`source.port`	integer	The client port that sent the request.
`destination.port`	integer	The upstream port that the Secure Web Proxy instance is sending traffic to.
`host()`	string	The host value used for DNS resolution and upstream connections. This does not include the port. The value is determined by the following: Raw HTTP requests: the Host header Proxy tunnel HTTP CONNECT requests: The CONNECT target
`source.matchTag(SECURE_TAG)`	boolean	`True` if the source is associated with `SECURE_TAG`. The argument is the permanent ID of the secure tag, such as `source.matchTag('tagValues/123456')`.
`source.matchServiceAccount(SERVICE_ACCOUNT)`	boolean	`True` if the source is associated with `SERVICE_ACCOUNT`, such as `source.matchServiceAccount('x@my-project.iam.gserviceaccount.com')`.
`inUrlList(HOST_OR_URL, NAMED_LIST)`	boolean	`True` if `HOST_OR_URL` is present in the provided named list `NAMED_LIST`. For example: `inUrlList(host(), 'projects/1234/locations/us-east1/urlLists/allowed-repos')` `inUrlList(request.url(), 'projects/1234/locations/us-east1/urlLists/allowed-repos')` When a `UrlList` pattern is matched against values without a forward-slash (`/`), such as in `host()`, only the domain portion of the pattern is matched. For more information about `UrlList` interpretation, see How `UrlList` interprets entries.
`inIpRange(IP_ADDRESS, IP_RANGE)`	boolean	`True` if `IP_ADDRESS` is contained within the `IP_RANGE` such as `inIpRange(source.ip, '1.2.3.0/24')`. Subnet masks for IPv6 addresses cannot be larger than /64.

Attributes available only to `ApplicationMatcher`

The following table describes attributes that apply only to ApplicationMatcher.

Attribute	Attribute type	Description
`request.headers`	map	A string-to-string map of the HTTP request headers. If a header contains multiple values, the value in this map is a comma-separated string of all of the values of the header. The keys in this map are all lowercase.
`request.method`	string	The HTTP Request method, such as GET or POST.
`request.host`	string	Convenience equivalent to `request.headers['host']`. We recommend using `host()` in most cases.
`request.path`	string	The requested HTTP URL path.
`request.query`	string	The HTTP URL query in the format of `name1=value&name2=value2`, as it appears in the first line of the HTTP request. No decoding is performed.
`request.scheme`	string	The HTTP URL scheme, such as HTTP or HTTPS. Values for this attribute are all lowercase.
`request.url()`	string	Convenience for `host() + request.path`. This doesn't include the port and uses a host value that might differ from the host header.
`request.useragent()`	string	Convenience equivalent to `request.headers['user-agent']`.

Operators

Secure Web Proxy supports several operators that can be used to build complex logic expressions from simple expression statements. Secure Web Proxy supports logical operators, such as &&, ||, and !, and string manipulation operators, such as x.contains('y').

The logical operators let you verify multiple variables in a conditional expression. For example, request.method == 'GET' && host().matches('.*\.example.com') joins two statements, and requires both statements to be True in order to produce an overall result of True.

The string manipulation operators match strings or substrings that you define, and let you develop rules to control access to resources without listing every possible combination.

Logical operators

The following table describes the logical operators that Secure Web Proxy supports.

Example expression	Description
`x == "foo"`	Returns `True` if `x` is equal to the constant string literal argument.
`x == R"fo'o"`	Returns `True` if `x` is equal to the given raw string literal that does not interpret escape sequences. Raw string literals are convenient for expressing strings that the code must use to escape sequence characters.
`x == y`	Returns `True` if `x` is equal to `y`.
`x != y`	Returns `True` if `x` is not equal to `y`.
`x && y`	Returns `True` if both `x` and `y` are `True`.
`x \|\| y`	Returns `True` if `x`, `y`, or both are `True`.
`!x`	Returns `True` if the boolean value `x` is `False`, or returns `False` if the boolean value `x` is `True`.
`m['k']`	If key `k` is present, returns the value at key `k` in the string-to-string map `m`. If key `k` is not present, returns an error that causes the rule under evaluation to not match.

String manipulation operators

The following table describes the string manipulation operators that Secure Web Proxy supports.

Expression	Description
`x.contains(y)`	Returns `True` if the string `x` contains the substring `y`.
`x.startsWith(y)`	Returns `True` if the string `x` begins with the substring `y`.
`x.endsWith(y)`	Returns `True` if the string `x` ends with the substring `y`.
`x.matches(y)`	Returns `True` if the string `x` matches the specified RE2 pattern `y`. The RE2 pattern is compiled using the RE2::Latin1 option that disables Unicode features.
`x.lower()`	Returns the lowercase value of the string `x`.
`x.upper()`	Returns the uppercase value of the string `x`.
`x + y`	Returns the concatenated string `xy`.
`int(x)`	Converts the string result of `x` to an `int` type. The converted string can be used for integer comparison with standard arithmetic operators such as `>` and `<=`. This works only for values that can be integers.