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

Use session attributes and application attributes while 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 IP address, hosts, and IP range. Application attributes, described by ApplicationMatcher, apply to application attributes such as request headers, request method, and request path.

Attributes available to `SessionMatcher` and `ApplicationMatcher`

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

Attribute	Attribute type	Description
`source.ip`	string	IP address of the client that sent the request.
`source.port`	integer	Client port that sent the request.
`destination.port`	integer	Upstream port to which your Secure Web Proxy instance sends the traffic.
`host()`	string	Host value used for DNS resolution and upstream connections. This doesn't include the port. The value of host() is determined by the following: Raw HTTP requests: Host header Proxy tunnel HTTP CONNECT requests: 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('[email protected]')`.
`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, 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 can't 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	String-to-string map of the request headers. If a header contains multiple values, then the value in this map is a comma-separated string of all the values of the header. All keys in this map are in lowercase.
`request.method`	string	Request method, such as GET or POST.
`request.host`	string	Convenience equivalent to `request.headers['host']`. We recommend that you use `host()` in most cases.
`request.path`	string	Requested URL path.
`request.query`	string	URL query in the format of `name1=value&name2=value2`, as it appears in the first line of the request. No decoding is performed.
`request.scheme`	string	URL scheme, such as HTTP or HTTPS. All values of this attribute are in 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 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`. Otherwise, it 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, it 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 by 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. You can use the converted string to compare integers with standard arithmetic operators such as `>` and `<=`. This works only for integer values.

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