com.amazonaws.services.securitytoken

Interface AWSSecurityTokenService

All Known Subinterfaces:

AWSSecurityTokenServiceAsync

All Known Implementing Classes:

AbstractAWSSecurityTokenService, AbstractAWSSecurityTokenServiceAsync, AWSSecurityTokenServiceAsyncClient, AWSSecurityTokenServiceClient

public interface AWSSecurityTokenService

Interface for accessing AWS STS.

AWS Security Token Service

The AWS Security Token Service (STS) is a web service that enables you to request temporary, limited-privilege credentials for AWS Identity and Access Management (IAM) users or for users that you authenticate (federated users). This guide provides descriptions of the STS API. For more detailed information about using this service, go to Temporary Security Credentials.

As an alternative to using the API, you can use one of the AWS SDKs, which consist of libraries and sample code for various programming languages and platforms (Java, Ruby, .NET, iOS, Android, etc.). The SDKs provide a convenient way to create programmatic access to STS. For example, the SDKs take care of cryptographically signing requests, managing errors, and retrying requests automatically. For information about the AWS SDKs, including how to download and install them, see the Tools for Amazon Web Services page.

For information about setting up signatures and authorization through the API, go to Signing AWS API Requests in the AWS General Reference. For general information about the Query API, go to Making Query Requests in Using IAM. For information about using security tokens with other AWS products, go to AWS Services That Work with IAM in the Using IAM.

If you're new to AWS and need additional technical information about a specific AWS product, you can find the product's technical documentation at http://aws.amazon.com/documentation/.

Endpoints

The AWS Security Token Service (STS) has a default endpoint of https://sts.amazonaws.com that maps to the US East (N. Virginia) region. Additional regions are available, but must first be activated in the AWS Management Console before you can use a different region's endpoint. For more information about activating a region for STS see Activating STS in a New Region in the Using IAM.

For information about STS endpoints, see Regions and Endpoints in the AWS General Reference.

Recording API requests

STS supports AWS CloudTrail, which is a service that records AWS calls for your AWS account and delivers log files to an Amazon S3 bucket. By using information collected by CloudTrail, you can determine what requests were successfully made to STS, who made the request, when it was made, and so on. To learn more about CloudTrail, including how to turn it on and find your log files, see the AWS CloudTrail User Guide.

Method Summary

Methods

Modifier and Type	Method and Description
AssumeRoleResult	assumeRole(AssumeRoleRequest assumeRoleRequest) Returns a set of temporary security credentials (consisting of an access key ID, a secret access key, and a security token) that you can use to access AWS resources that you might not normally have access to.
AssumeRoleWithSAMLResult	assumeRoleWithSAML(AssumeRoleWithSAMLRequest assumeRoleWithSAMLRequest) Returns a set of temporary security credentials for users who have been authenticated via a SAML authentication response.
AssumeRoleWithWebIdentityResult	assumeRoleWithWebIdentity(AssumeRoleWithWebIdentityRequest assumeRoleWithWebIdentityRequest) Returns a set of temporary security credentials for users who have been authenticated in a mobile or web application with a web identity provider, such as Amazon Cognito, Login with Amazon, Facebook, Google, or any OpenID Connect-compatible identity provider.
DecodeAuthorizationMessageResult	decodeAuthorizationMessage(DecodeAuthorizationMessageRequest decodeAuthorizationMessageRequest) Decodes additional information about the authorization status of a request from an encoded message returned in response to an AWS request.
ResponseMetadata	getCachedResponseMetadata(AmazonWebServiceRequest request) Returns additional metadata for a previously executed successful request, typically used for debugging issues where a service isn't acting as expected.
GetFederationTokenResult	getFederationToken(GetFederationTokenRequest getFederationTokenRequest) Returns a set of temporary security credentials (consisting of an access key ID, a secret access key, and a security token) for a federated user.
GetSessionTokenResult	getSessionToken() Simplified method form for invoking the GetSessionToken operation.
GetSessionTokenResult	getSessionToken(GetSessionTokenRequest getSessionTokenRequest) Returns a set of temporary credentials for an AWS account or IAM user.
void	setEndpoint(String endpoint) Overrides the default endpoint for this client ("sts.amazonaws.com").
void	setRegion(Region region) An alternative to setEndpoint(String), sets the regional endpoint for this client's service calls.
void	shutdown() Shuts down this client object, releasing any resources that might be held open.

Method Detail

setEndpoint

void setEndpoint(String endpoint)

Overrides the default endpoint for this client ("sts.amazonaws.com"). Callers can use this method to control which AWS region they want to work with.

Callers can pass in just the endpoint (ex: "sts.amazonaws.com") or a full URL, including the protocol (ex: "sts.amazonaws.com"). If the protocol is not specified here, the default protocol from this client's ClientConfiguration will be used, which by default is HTTPS.

For more information on using AWS regions with the AWS SDK for Java, and a complete list of all available endpoints for all AWS services, see: http://developer.amazonwebservices.com/connect/entry.jspa?externalID= 3912

This method is not threadsafe. An endpoint should be configured when the client is created and before any service requests are made. Changing it afterwards creates inevitable race conditions for any service requests in transit or retrying.

Parameters:

endpoint - The endpoint (ex: "sts.amazonaws.com") or a full URL, including the protocol (ex: "sts.amazonaws.com") of the region specific AWS endpoint this client will communicate with.

setRegion

void setRegion(Region region)

An alternative to setEndpoint(String), sets the regional endpoint for this client's service calls. Callers can use this method to control which AWS region they want to work with.

By default, all service endpoints in all regions use the https protocol. To use http instead, specify it in the ClientConfiguration supplied at construction.

This method is not threadsafe. A region should be configured when the client is created and before any service requests are made. Changing it afterwards creates inevitable race conditions for any service requests in transit or retrying.

Parameters:

region - The region this client will communicate with. See Region.getRegion(com.amazonaws.regions.Regions) for accessing a given region. Must not be null and must be a region where the service is available.

See Also:

Region.getRegion(com.amazonaws.regions.Regions), Region.createClient(Class, com.amazonaws.auth.AWSCredentialsProvider, ClientConfiguration), Region.isServiceSupported(String)

assumeRole

AssumeRoleResult assumeRole(AssumeRoleRequest assumeRoleRequest)

Returns a set of temporary security credentials (consisting of an access key ID, a secret access key, and a security token) that you can use to access AWS resources that you might not normally have access to. Typically, you use AssumeRole for cross-account access or federation.

Important: You cannot call AssumeRole by using AWS account credentials; access will be denied. You must use IAM user credentials or temporary security credentials to call AssumeRole.

For cross-account access, imagine that you own multiple accounts and need to access resources in each account. You could create long-term credentials in each account to access those resources. However, managing all those credentials and remembering which one can access which account can be time consuming. Instead, you can create one set of long-term credentials in one account and then use temporary security credentials to access all the other accounts by assuming roles in those accounts. For more information about roles, see IAM Roles (Delegation and Federation) in the Using IAM.

For federation, you can, for example, grant single sign-on access to the AWS Management Console. If you already have an identity and authentication system in your corporate network, you don't have to recreate user identities in AWS in order to grant those user identities access to AWS. Instead, after a user has been authenticated, you call AssumeRole (and specify the role with the appropriate permissions) to get temporary security credentials for that user. With those temporary security credentials, you construct a sign-in URL that users can use to access the console. For more information, see Common Scenarios for Temporary Credentials in the Using IAM.

The temporary security credentials are valid for the duration that you specified when calling AssumeRole, which can be from 900 seconds (15 minutes) to 3600 seconds (1 hour). The default is 1 hour.

Optionally, you can pass an IAM access policy to this operation. If you choose not to pass a policy, the temporary security credentials that are returned by the operation have the permissions that are defined in the access policy of the role that is being assumed. If you pass a policy to this operation, the temporary security credentials that are returned by the operation have the permissions that are allowed by both the access policy of the role that is being assumed, and the policy that you pass. This gives you a way to further restrict the permissions for the resulting temporary security credentials. You cannot use the passed policy to grant permissions that are in excess of those allowed by the access policy of the role that is being assumed. For more information, see Permissions for AssumeRole, AssumeRoleWithSAML, and AssumeRoleWithWebIdentity in the Using IAM.

To assume a role, your AWS account must be trusted by the role. The trust relationship is defined in the role's trust policy when the role is created. You must also have a policy that allows you to call sts:AssumeRole.

Using MFA with AssumeRole

You can optionally include multi-factor authentication (MFA) information when you call AssumeRole. This is useful for cross-account scenarios in which you want to make sure that the user who is assuming the role has been authenticated using an AWS MFA device. In that scenario, the trust policy of the role being assumed includes a condition that tests for MFA authentication; if the caller does not include valid MFA information, the request to assume the role is denied. The condition in a trust policy that tests for MFA authentication might look like the following example.

"Condition": {"Bool": {"aws:MultiFactorAuthPresent": true}}

For more information, see Configuring MFA-Protected API Access in the Using IAM guide.

To use MFA with AssumeRole, you pass values for the SerialNumber and TokenCode parameters. The SerialNumber value identifies the user's hardware or virtual MFA device. The TokenCode is the time-based one-time password (TOTP) that the MFA devices produces.

Parameters:

assumeRoleRequest -

Returns:

Result of the AssumeRole operation returned by the service.

Throws:

MalformedPolicyDocumentException - The request was rejected because the policy document was malformed. The error message describes the specific error.

PackedPolicyTooLargeException - The request was rejected because the policy document was too large. The error message describes how big the policy document is, in packed form, as a percentage of what the API allows.

RegionDisabledException - STS is not activated in the requested region for the account that is being asked to create temporary credentials. The account administrator must activate STS in that region using the IAM Console. For more information, see Activating and Deactivating AWS STS in an AWS Region in the Using IAM.

assumeRoleWithSAML

AssumeRoleWithSAMLResult assumeRoleWithSAML(AssumeRoleWithSAMLRequest assumeRoleWithSAMLRequest)

Returns a set of temporary security credentials for users who have been authenticated via a SAML authentication response. This operation provides a mechanism for tying an enterprise identity store or directory to role-based AWS access without user-specific credentials or configuration.

The temporary security credentials returned by this operation consist of an access key ID, a secret access key, and a security token. Applications can use these temporary security credentials to sign calls to AWS services. The credentials are valid for the duration that you specified when calling AssumeRoleWithSAML, which can be up to 3600 seconds (1 hour) or until the time specified in the SAML authentication response's SessionNotOnOrAfter value, whichever is shorter.

The maximum duration for a session is 1 hour, and the minimum duration is 15 minutes, even if values outside this range are specified.

Optionally, you can pass an IAM access policy to this operation. If you choose not to pass a policy, the temporary security credentials that are returned by the operation have the permissions that are defined in the access policy of the role that is being assumed. If you pass a policy to this operation, the temporary security credentials that are returned by the operation have the permissions that are allowed by both the access policy of the role that is being assumed, and the policy that you pass. This gives you a way to further restrict the permissions for the resulting temporary security credentials. You cannot use the passed policy to grant permissions that are in excess of those allowed by the access policy of the role that is being assumed. For more information, see Permissions for AssumeRole, AssumeRoleWithSAML, and AssumeRoleWithWebIdentity in the Using IAM.

Before your application can call AssumeRoleWithSAML, you must configure your SAML identity provider (IdP) to issue the claims required by AWS. Additionally, you must use AWS Identity and Access Management (IAM) to create a SAML provider entity in your AWS account that represents your identity provider, and create an IAM role that specifies this SAML provider in its trust policy.

Calling AssumeRoleWithSAML does not require the use of AWS security credentials. The identity of the caller is validated by using keys in the metadata document that is uploaded for the SAML provider entity for your identity provider.

For more information, see the following resources:

• About SAML 2.0-based Federation in the Using IAM.
• Creating SAML Identity Providers in the Using IAM.
• Configuring a Relying Party and Claims in the Using IAM.
• Creating a Role for SAML 2.0 Federation in the Using IAM.

Parameters:

assumeRoleWithSAMLRequest -

Returns:

Result of the AssumeRoleWithSAML operation returned by the service.

Throws:

MalformedPolicyDocumentException - The request was rejected because the policy document was malformed. The error message describes the specific error.

PackedPolicyTooLargeException - The request was rejected because the policy document was too large. The error message describes how big the policy document is, in packed form, as a percentage of what the API allows.

IDPRejectedClaimException - The identity provider (IdP) reported that authentication failed. This might be because the claim is invalid.

If this error is returned for the AssumeRoleWithWebIdentity operation, it can also mean that the claim has expired or has been explicitly revoked.

InvalidIdentityTokenException - The web identity token that was passed could not be validated by AWS. Get a new identity token from the identity provider and then retry the request.

ExpiredTokenException - The web identity token that was passed is expired or is not valid. Get a new identity token from the identity provider and then retry the request.

RegionDisabledException - STS is not activated in the requested region for the account that is being asked to create temporary credentials. The account administrator must activate STS in that region using the IAM Console. For more information, see Activating and Deactivating AWS STS in an AWS Region in the Using IAM.

assumeRoleWithWebIdentity

AssumeRoleWithWebIdentityResult assumeRoleWithWebIdentity(AssumeRoleWithWebIdentityRequest assumeRoleWithWebIdentityRequest)

Returns a set of temporary security credentials for users who have been authenticated in a mobile or web application with a web identity provider, such as Amazon Cognito, Login with Amazon, Facebook, Google, or any OpenID Connect-compatible identity provider.

For mobile applications, we recommend that you use Amazon Cognito. You can use Amazon Cognito with the AWS SDK for iOS and the AWS SDK for Android to uniquely identify a user and supply the user with a consistent identity throughout the lifetime of an application.

To learn more about Amazon Cognito, see Amazon Cognito Overview in the AWS SDK for Android Developer Guide guide and Amazon Cognito Overview in the AWS SDK for iOS Developer Guide.

Calling AssumeRoleWithWebIdentity does not require the use of AWS security credentials. Therefore, you can distribute an application (for example, on mobile devices) that requests temporary security credentials without including long-term AWS credentials in the application, and without deploying server-based proxy services that use long-term AWS credentials. Instead, the identity of the caller is validated by using a token from the web identity provider.

The temporary security credentials returned by this API consist of an access key ID, a secret access key, and a security token. Applications can use these temporary security credentials to sign calls to AWS service APIs. The credentials are valid for the duration that you specified when calling AssumeRoleWithWebIdentity, which can be from 900 seconds (15 minutes) to 3600 seconds (1 hour). By default, the temporary security credentials are valid for 1 hour.

Optionally, you can pass an IAM access policy to this operation. If you choose not to pass a policy, the temporary security credentials that are returned by the operation have the permissions that are defined in the access policy of the role that is being assumed. If you pass a policy to this operation, the temporary security credentials that are returned by the operation have the permissions that are allowed by both the access policy of the role that is being assumed, and the policy that you pass. This gives you a way to further restrict the permissions for the resulting temporary security credentials. You cannot use the passed policy to grant permissions that are in excess of those allowed by the access policy of the role that is being assumed. For more information, see Permissions for AssumeRole, AssumeRoleWithSAML, and AssumeRoleWithWebIdentity in the Using IAM.

Before your application can call AssumeRoleWithWebIdentity, you must have an identity token from a supported identity provider and create a role that the application can assume. The role that your application assumes must trust the identity provider that is associated with the identity token. In other words, the identity provider must be specified in the role's trust policy.

For more information about how to use web identity federation and the AssumeRoleWithWebIdentity API, see the following resources:

• Using Web Identity Federation APIs for Mobile Apps and Federation Through a Web-based Identity Provider.
• Web Identity Federation Playground. This interactive website lets you walk through the process of authenticating via Login with Amazon, Facebook, or Google, getting temporary security credentials, and then using those credentials to make a request to AWS.
• AWS SDK for iOS and AWS SDK for Android. These toolkits contain sample apps that show how to invoke the identity providers, and then how to use the information from these providers to get and use temporary security credentials.
• Web Identity Federation with Mobile Applications. This article discusses web identity federation and shows an example of how to use web identity federation to get access to content in Amazon S3.

Parameters:

assumeRoleWithWebIdentityRequest -

Returns:

Result of the AssumeRoleWithWebIdentity operation returned by the service.

Throws:

MalformedPolicyDocumentException - The request was rejected because the policy document was malformed. The error message describes the specific error.

PackedPolicyTooLargeException - The request was rejected because the policy document was too large. The error message describes how big the policy document is, in packed form, as a percentage of what the API allows.

IDPRejectedClaimException - The identity provider (IdP) reported that authentication failed. This might be because the claim is invalid.

If this error is returned for the AssumeRoleWithWebIdentity operation, it can also mean that the claim has expired or has been explicitly revoked.

IDPCommunicationErrorException - The request could not be fulfilled because the non-AWS identity provider (IDP) that was asked to verify the incoming identity token could not be reached. This is often a transient error caused by network conditions. Retry the request a limited number of times so that you don't exceed the request rate. If the error persists, the non-AWS identity provider might be down or not responding.

InvalidIdentityTokenException - The web identity token that was passed could not be validated by AWS. Get a new identity token from the identity provider and then retry the request.

ExpiredTokenException - The web identity token that was passed is expired or is not valid. Get a new identity token from the identity provider and then retry the request.

RegionDisabledException - STS is not activated in the requested region for the account that is being asked to create temporary credentials. The account administrator must activate STS in that region using the IAM Console. For more information, see Activating and Deactivating AWS STS in an AWS Region in the Using IAM.

decodeAuthorizationMessage

DecodeAuthorizationMessageResult decodeAuthorizationMessage(DecodeAuthorizationMessageRequest decodeAuthorizationMessageRequest)

Decodes additional information about the authorization status of a request from an encoded message returned in response to an AWS request.

For example, if a user is not authorized to perform an action that he or she has requested, the request returns a Client.UnauthorizedOperation response (an HTTP 403 response). Some AWS actions additionally return an encoded message that can provide details about this authorization failure.

Only certain AWS actions return an encoded authorization message. The documentation for an individual action indicates whether that action returns an encoded message in addition to returning an HTTP code.

The message is encoded because the details of the authorization status can constitute privileged information that the user who requested the action should not see. To decode an authorization status message, a user must be granted permissions via an IAM policy to request the DecodeAuthorizationMessage ( sts:DecodeAuthorizationMessage) action.

The decoded message includes the following type of information:

• Whether the request was denied due to an explicit deny or due to the absence of an explicit allow. For more information, see Determining Whether a Request is Allowed or Denied in the Using IAM.
• The principal who made the request.
• The requested action.
• The requested resource.
• The values of condition keys in the context of the user's request.

Parameters:

decodeAuthorizationMessageRequest -

Returns:

Result of the DecodeAuthorizationMessage operation returned by the service.

Throws:

InvalidAuthorizationMessageException - The error returned if the message passed to DecodeAuthorizationMessage was invalid. This can happen if the token contains invalid characters, such as linebreaks.

getFederationToken

GetFederationTokenResult getFederationToken(GetFederationTokenRequest getFederationTokenRequest)

Returns a set of temporary security credentials (consisting of an access key ID, a secret access key, and a security token) for a federated user. A typical use is in a proxy application that gets temporary security credentials on behalf of distributed applications inside a corporate network. Because you must call the GetFederationToken action using the long-term security credentials of an IAM user, this call is appropriate in contexts where those credentials can be safely stored, usually in a server-based application.

If you are creating a mobile-based or browser-based app that can authenticate users using a web identity provider like Login with Amazon, Facebook, Google, or an OpenID Connect-compatible identity provider, we recommend that you use Amazon Cognito or AssumeRoleWithWebIdentity. For more information, see Federation Through a Web-based Identity Provider.

The GetFederationToken action must be called by using the long-term AWS security credentials of an IAM user. You can also call GetFederationToken using the security credentials of an AWS account (root), but this is not recommended. Instead, we recommend that you create an IAM user for the purpose of the proxy application and then attach a policy to the IAM user that limits federated users to only the actions and resources they need access to. For more information, see IAM Best Practices in the Using IAM.

The temporary security credentials that are obtained by using the long-term credentials of an IAM user are valid for the specified duration, between 900 seconds (15 minutes) and 129600 seconds (36 hours). Temporary credentials that are obtained by using AWS account (root) credentials have a maximum duration of 3600 seconds (1 hour)

Permissions

The permissions for the temporary security credentials returned by GetFederationToken are determined by a combination of the following:

• The policy or policies that are attached to the IAM user whose credentials are used to call GetFederationToken.
• The policy that is passed as a parameter in the call.

The passed policy is attached to the temporary security credentials that result from the GetFederationToken API call--that is, to the federated user. When the federated user makes an AWS request, AWS evaluates the policy attached to the federated user in combination with the policy or policies attached to the IAM user whose credentials were used to call GetFederationToken. AWS allows the federated user's request only when both the federated user and the IAM user are explicitly allowed to perform the requested action. The passed policy cannot grant more permissions than those that are defined in the IAM user policy.

A typical use case is that the permissions of the IAM user whose credentials are used to call GetFederationToken are designed to allow access to all the actions and resources that any federated user will need. Then, for individual users, you pass a policy to the operation that scopes down the permissions to a level that's appropriate to that individual user, using a policy that allows only a subset of permissions that are granted to the IAM user.

If you do not pass a policy, the resulting temporary security credentials have no effective permissions. The only exception is when the temporary security credentials are used to access a resource that has a resource-based policy that specifically allows the federated user to access the resource.

For more information about how permissions work, see Permissions for GetFederationToken. For information about using GetFederationToken to create temporary security credentials, see GetFederationToken—Federation Through a Custom Identity Broker.

Parameters:

getFederationTokenRequest -

Returns:

Result of the GetFederationToken operation returned by the service.

Throws:

MalformedPolicyDocumentException - The request was rejected because the policy document was malformed. The error message describes the specific error.

PackedPolicyTooLargeException - The request was rejected because the policy document was too large. The error message describes how big the policy document is, in packed form, as a percentage of what the API allows.

RegionDisabledException - STS is not activated in the requested region for the account that is being asked to create temporary credentials. The account administrator must activate STS in that region using the IAM Console. For more information, see Activating and Deactivating AWS STS in an AWS Region in the Using IAM.

getSessionToken

GetSessionTokenResult getSessionToken(GetSessionTokenRequest getSessionTokenRequest)

Returns a set of temporary credentials for an AWS account or IAM user. The credentials consist of an access key ID, a secret access key, and a security token. Typically, you use GetSessionToken if you want to use MFA to protect programmatic calls to specific AWS APIs like Amazon EC2 StopInstances. MFA-enabled IAM users would need to call GetSessionToken and submit an MFA code that is associated with their MFA device. Using the temporary security credentials that are returned from the call, IAM users can then make programmatic calls to APIs that require MFA authentication. If you do not supply a correct MFA code, then the API returns an access denied error.

The GetSessionToken action must be called by using the long-term AWS security credentials of the AWS account or an IAM user. Credentials that are created by IAM users are valid for the duration that you specify, between 900 seconds (15 minutes) and 129600 seconds (36 hours); credentials that are created by using account credentials have a maximum duration of 3600 seconds (1 hour).

We recommend that you do not call GetSessionToken with root account credentials. Instead, follow our best practices by creating one or more IAM users, giving them the necessary permissions, and using IAM users for everyday interaction with AWS.

The permissions associated with the temporary security credentials returned by GetSessionToken are based on the permissions associated with account or IAM user whose credentials are used to call the action. If GetSessionToken is called using root account credentials, the temporary credentials have root account permissions. Similarly, if GetSessionToken is called using the credentials of an IAM user, the temporary credentials have the same permissions as the IAM user.

For more information about using GetSessionToken to create temporary credentials, go to Temporary Credentials for Users in Untrusted Environments in the Using IAM.

Parameters:

getSessionTokenRequest -

Returns:

Result of the GetSessionToken operation returned by the service.

Throws:

RegionDisabledException - STS is not activated in the requested region for the account that is being asked to create temporary credentials. The account administrator must activate STS in that region using the IAM Console. For more information, see Activating and Deactivating AWS STS in an AWS Region in the Using IAM.

getSessionToken

GetSessionTokenResult getSessionToken()

Simplified method form for invoking the GetSessionToken operation.

See Also:

getSessionToken(GetSessionTokenRequest)

shutdown

void shutdown()

Shuts down this client object, releasing any resources that might be held open. This is an optional method, and callers are not expected to call it, but can if they want to explicitly release any open resources. Once a client has been shutdown, it should not be used to make any more requests.

getCachedResponseMetadata

ResponseMetadata getCachedResponseMetadata(AmazonWebServiceRequest request)

Returns additional metadata for a previously executed successful request, typically used for debugging issues where a service isn't acting as expected. This data isn't considered part of the result data returned by an operation, so it's available through this separate, diagnostic interface.

Response metadata is only cached for a limited period of time, so if you need to access this extra diagnostic information for an executed request, you should use this method to retrieve it as soon as possible after executing a request.

Parameters:

request - The originally executed request.

Returns:

The response metadata for the specified request, or null if none is available.