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Request and Response Behavior for Custom Origins

Topics

How CloudFront Processes and Forwards Requests to Your Custom Origin Server

This topic contains information about how CloudFront processes viewer requests and forwards the requests to your custom origin.

Topics

Authentication

For DELETE, GET, HEAD, PATCH, POST, and PUT requests, if you configure CloudFront to forward the Authorization header to your origin, you can configure your origin server to request client authentication.

For OPTIONS requests, you can configure your origin server to request client authentication only if you use the following CloudFront settings:

  • Configure CloudFront to forward the Authorization header to your origin
  • Configure CloudFront to not cache the response to OPTIONS requests

You can configure CloudFront to forward requests to your origin using either HTTP or HTTPS; for more information, see Using HTTPS with CloudFront.

Caching Duration and Minimum TTL

For web distributions, to control how long your objects stay in a CloudFront cache before CloudFront forwards another request to your origin, you can:

  • Configure your origin to add a Cache-Control or an Expires header field to each object.
  • Specify a value for Minimum TTL in CloudFront cache behaviors.
  • Use the default value of 24 hours.

For more information, see Specifying How Long Objects Stay in a CloudFront Edge Cache (Expiration).

Client IP Addresses

If a viewer sends a request to CloudFront and does not include an X-Forwarded-For request header, CloudFront gets the IP address of the viewer from the TCP connection, adds an X-Forwarded-For header that includes the IP address, and forwards the request to the origin. For example, if CloudFront gets the IP address 192.0.2.2 from the TCP connection, it forwards the following header to the origin:

X-Forwarded-For: 192.0.2.2

If a viewer sends a request to CloudFront and includes an X-Forwarded-For request header, CloudFront gets the IP address of the viewer from the TCP connection, appends it to the end of the X-Forwarded-For header, and forwards the request to the origin. For example, if the viewer request includes X-Forwarded-For: 192.0.2.4,192.0.2.3 and CloudFront gets the IP address 192.0.2.2 from the TCP connection, it forwards the following header to the origin:

X-Forwarded-For: 192.0.2.4,192.0.2.3,192.0.2.2

Some applications, such as load balancers (including Elastic Load Balancing), web application firewalls, reverse proxies, intrusion prevention systems, and API Gateway, append the IP address of the CloudFront edge server that forwarded the request onto the end of the X-Forwarded-For header. For example, if CloudFront includes X-Forwarded-For: 192.0.2.2 in a request that it forwards to ELB and if the IP address of the CloudFront edge server is 192.0.2.199, the request that your EC2 instance receives contains the following header:

X-Forwarded-For: 192.0.2.2,192.0.2.199

Note
The X-Forwarded-For header contains IPv4 addresses (such as 192.0.2.44) and IPv6 addresses (such as 2001:0db8:85a3:0000:0000:8a2e:0370:7334), as applicable.

Client-Side SSL Authentication

CloudFront does not support client authentication with client-side SSL certificates. If an origin requests a client-side certificate, CloudFront drops the request.

Compression

CloudFront forwards requests that have the Accept-Encoding field values "identity" and "gzip". For more information, see Serving Compressed Files.

Conditional Requests

When CloudFront receives a request for an object that has expired from an edge cache, it forwards the request to the origin either to get the latest version of the object or to get confirmation from the origin that the CloudFront edge cache already has the latest version. Typically, when the origin last sent the object to CloudFront, it included an ETag value, a LastModified value, or both values in the response. In the new request that CloudFront forwards to the origin, CloudFront adds one or both of the following:

  • An If-Match or If-None-Match header that contains the ETag value for the expired version of the object.
  • An If-Modified-Since header that contains the LastModified value for the expired version of the object.

The origin uses this information to determine whether the object has been updated and, therefore, whether to return the entire object to CloudFront or to return only an HTTP 304 status code (not modified).

Cookies

You can configure CloudFront to forward cookies to your origin. For more information, see Configuring CloudFront to Cache Objects Based on Cookies.

Cross-Origin Resource Sharing (CORS)

If you want CloudFront to respect cross-origin resource sharing settings, configure CloudFront to forward the Origin header to your origin. For more information, see Configuring CloudFront to Cache Objects Based on Request Headers.

Encryption

You can require viewers to use HTTPS to send requests to CloudFront and require CloudFront to forward requests to your custom origin by using the protocol that is used by the viewer. For more information, see the following distribution settings:

CloudFront forwards HTTPS requests to the origin server using the SSLv3, TLSv1.0, TLSv1.1, and TLSv1.2 protocols. For custom origins, you can choose the SSL protocols that you want CloudFront to use when communicating with your origin:

  • If you're using the CloudFront console, choose protocols by using the Origin SSL Protocols check boxes. For more information, see Creating a Web Distribution.
  • If you're using the CloudFront API, specify protocols by using the OriginSslProtocols element. For more information, see OriginSslProtocols and DistributionConfig in the Amazon CloudFront API Reference.

If the origin is an Amazon S3 bucket, CloudFront always uses TLSv1.2.

Important
Other versions of SSL and TLS are not supported.

For more information about using HTTPS with CloudFront, see Using HTTPS with CloudFront. For lists of the ciphers that CloudFront supports for HTTPS communication between viewers and CloudFront, and between CloudFront and your origin, see Supported SSL/TLS Protocols and Ciphers for Communication Between Viewers and CloudFront.

GET Requests That Include a Body

If a viewer GET request includes a body, CloudFront returns an HTTP status code 403 (Forbidden) to the viewer.

HTTP Methods

If you configure CloudFront to process all of the HTTP methods that it supports, CloudFront accepts the following requests from viewers and forwards them to your custom origin:

  • DELETE
  • GET
  • HEAD
  • OPTIONS
  • PATCH
  • POST
  • PUT

CloudFront always caches responses to GET and HEAD requests. You can also configure CloudFront to cache responses to OPTIONS requests. CloudFront does not cache responses to requests that use the other methods.

For information about configuring whether your custom origin processes these methods, see the documentation for your origin.

Important
If you configure CloudFront to accept and forward to your origin all of the HTTP methods that CloudFront supports, configure your origin server to handle all methods. For example, if you configure CloudFront to accept and forward these methods because you want to use POST, you must configure your origin server to handle DELETE requests appropriately so viewers can't delete resources that you don't want them to. For more information, see the documentation for your HTTP server.

HTTP Request Headers and CloudFront Behavior (Custom and S3 Origins)

The following table lists HTTP request headers that you can forward to both custom and Amazon S3 origins (with the exceptions that are noted). For each header, the table includes information about the following:

  • CloudFront behavior if you don't configure CloudFront to forward the header to your origin, which causes CloudFront to cache your objects based on header values.

  • Whether you can configure CloudFront to cache objects based on header values for that header.

    You can configure CloudFront to cache objects based on values in the Date and User-Agent headers, but we don't recommend it. These headers have a lot of possible values, and caching based on their values would cause CloudFront to forward significantly more requests to your origin.

For more information about caching based on header values, see Configuring CloudFront to Cache Objects Based on Request Headers.

Header Behavior If You Don't Configure CloudFront to Cache Based on Header Values Caching Based on Header Values Is Supported
Other-defined headers CloudFront forwards the headers to your origin. Yes
Accept CloudFront removes the header. Yes
Accept-Charset CloudFront removes the header. Yes
Accept-Encoding If the value contains gzip, CloudFront forwards Accept-Encoding: gzip to your origin. If the value does not contain gzip, CloudFront removes the Accept-Encoding header field before forwarding the request to your origin. No
Accept-Language CloudFront removes the header. Yes
Authorization [See the AWS documentation website for more details] Yes
Cache-Control CloudFront forwards the header to your origin. No
CloudFront-Forwarded-Proto CloudFront does not add the header before forwarding the request to your origin. For more information, see Configuring CloudFront to Cache Objects Based on the Protocol of the Request. Yes
CloudFront-Is-Desktop-Viewer CloudFront does not add the header before forwarding the request to your origin. For more information, see Configuring CloudFront to Cache Objects Based on the Device Type. Yes
CloudFront-Is-Mobile-Viewer CloudFront does not add the header before forwarding the request to your origin. For more information, see Configuring CloudFront to Cache Objects Based on the Device Type. Yes
CloudFront-Is-Tablet-Viewer CloudFront does not add the header before forwarding the request to your origin. For more information, see Configuring CloudFront to Cache Objects Based on the Device Type. Yes
CloudFront-Viewer-Country CloudFront does not add the header before forwarding the request to your origin. Yes
Connection CloudFront replaces this header with Connection: Keep-Alive before forwarding the request to your origin. No
Content-Length CloudFront forwards the header to your origin. No
Content-MD5 CloudFront forwards the header to your origin. Yes
Content-Type CloudFront forwards the header to your origin. Yes
Cookie If you configure CloudFront to forward cookies, it will forward the Cookie header field to your origin. If you don't, CloudFront removes the Cookie header field. For more information, see Configuring CloudFront to Cache Objects Based on Cookies. No
Date CloudFront forwards the header to your origin. Yes, but not recommended
Expect CloudFront removes the header. Yes
From CloudFront forwards the header to your origin. Yes
Host CloudFront sets the value to the domain name of the origin that is associated with the requested object. You can't cache based on the Host header for Amazon S3 origins. Yes (custom) No (S3)
If-Match CloudFront forwards the header to your origin. Yes
If-Modified-Since CloudFront forwards the header to your origin. Yes
If-None-Match CloudFront forwards the header to your origin. Yes
If-Range CloudFront forwards the header to your origin. Yes
If-Unmodified-Since CloudFront forwards the header to your origin. Yes
Max-Forwards CloudFront forwards the header to your origin. No
Origin CloudFront forwards the header to your origin. Yes
Pragma CloudFront forwards the header to your origin. No
Proxy-Authenticate CloudFront removes the header. No
Proxy-Authorization CloudFront removes the header. No
Proxy-Connection CloudFront removes the header. No
Range CloudFront forwards the header to your origin. For more information, see How CloudFront Processes Partial Requests for an Object (Range GETs). Yes, by default
Referer CloudFront removes the header. Yes
Request-Range CloudFront forwards the header to your origin. No
TE CloudFront removes the header. No
Trailer CloudFront removes the header. No
Transfer-Encoding CloudFront forwards the header to your origin. No
Upgrade CloudFront removes the header. No
User-Agent CloudFront replaces the value of this header field with Amazon CloudFront. If you want CloudFront to cache your content based on the device the user is using, see Configuring CloudFront to Cache Objects Based on the Device Type. Yes, but not recommended
Via CloudFront forwards the header to your origin. Yes
Warning CloudFront forwards the header to your origin. Yes
X-Amz-Cf-Id CloudFront adds the header to the viewer request before forwarding the request to your origin. The header value contains an encrypted string that uniquely identifies the request. No
X-Edge-* CloudFront removes all X-Edge-* headers. No
X-Forwarded-For CloudFront forwards the header to your origin. For more information, see Client IP Addresses. Yes
X-Forwarded-Proto CloudFront removes the header. Yes
X-Real-IP CloudFront removes the header. No

HTTP Version

CloudFront forwards requests to your custom origin using HTTP/1.1.

Maximum Length of a Request and Maximum Length of a URL

The maximum length of a request, including the path, the query string (if any), and headers, is 20,480 bytes.

CloudFront constructs a URL from the request. The maximum length of this URL is 8192 bytes.

If a request or a URL exceeds these limits, CloudFront returns HTTP status code 413, Request Header Fields Too Large, to the viewer, and then terminates the TCP connection to the viewer.

OCSP Stapling

When a viewer submits an HTTPS request for an object, either CloudFront or the viewer must confirm with the certificate authority (CA) that the SSL certificate for the domain has not been revoked. OCSP stapling speeds up certificate validation by allowing CloudFront to validate the certificate and to cache the response from the CA, so the client doesn't need to validate the certificate directly with the CA.

The performance improvement of OCSP stapling is more pronounced when CloudFront receives a lot of HTTPS requests for objects in the same domain. Each server in a CloudFront edge location must submit a separate validation request. When CloudFront receives a lot of HTTPS requests for the same domain, every server in the edge location soon has a response from the CA that it can "staple" to a packet in the SSL handshake; when the viewer is satisfied that the certificate is valid, CloudFront can serve the requested object. If your distribution doesn't get much traffic in a CloudFront edge location, new requests are more likely to be directed to a server that hasn't validated the certificate with the CA yet. In that case, the viewer separately performs the validation step and the CloudFront server serves the object. That CloudFront server also submits a validation request to the CA, so the next time it receives a request that includes the same domain name, it has a validation response from the CA.

Persistent Connections

When CloudFront gets a response from your origin, it tries to maintain the connection for several seconds in case another request arrives during that period. Maintaining a persistent connection saves the time required to re-establish the TCP connection and perform another TLS handshake for subsequent requests.

For more information, including how to configure the duration of persistent connections, see Origin Keep-alive Timeout (Amazon EC2, Elastic Load Balancing, and Other Custom Origins Only) in the section Values That You Specify When You Create or Update a Web Distribution.

Protocols

CloudFront forwards HTTP or HTTPS requests to the origin server based on the following:

  • The protocol of the request that the viewer sends to CloudFront, either HTTP or HTTPS.
  • The value of the Origin Protocol Policy field in the CloudFront console or, if you're using the CloudFront API, the OriginProtocolPolicy element in the DistributionConfig complex type. In the CloudFront console, the options are HTTP Only, HTTPS Only, and Match Viewer.

If you specify HTTP Only or HTTPS Only, CloudFront forwards requests to the origin server using the specified protocol, regardless of the protocol in the viewer request.

If you specify Match Viewer, CloudFront forwards requests to the origin server using the protocol in the viewer request. Note that CloudFront caches the object only once even if viewers make requests using both HTTP and HTTPS protocols.

Important
If CloudFront forwards a request to the origin using the HTTPS protocol, and if the origin server returns an invalid certificate or a self-signed certificate, CloudFront drops the TCP connection.

For information about how to update a distribution using the CloudFront console, see Viewing and Updating CloudFront Distributions. For information about how to update a distribution using the CloudFront API, go to UpdateDistribution in the Amazon CloudFront API Reference.

Query Strings

You can configure whether CloudFront forwards query string parameters to your origin. For more information, see Configuring CloudFront to Cache Based on Query String Parameters.

Origin Response Timeout

The origin response timeout, also known as the origin request timeout or origin read timeout, applies to both of the following values:

  • The amount of time, in seconds, that CloudFront waits for a response after forwarding a request to a custom origin
  • The amount of time, in seconds, that CloudFront waits after receiving a packet of a response from the origin and before receiving the next packet

For more information, including how to configure the origin response timeout, see Origin Response Timeout (Amazon EC2, Elastic Load Balancing, and Other Custom Origins Only) in the section Values That You Specify When You Create or Update a Web Distribution.

Simultaneous Requests for the Same Object (Traffic Spikes)

When a CloudFront edge location receives a request for an object and either the object isn't currently in the cache or the object has expired, CloudFront immediately sends the request to your origin. If there's a traffic spike—if additional requests for the same object arrive at the edge location before your origin responds to the first request—CloudFront pauses briefly before forwarding additional requests for the object to your origin. Typically, the response to the first request will arrive at the CloudFront edge location before the response to subsequent requests. This brief pause helps to reduce unnecessary load on your origin server. If additional requests are not identical because, for example, you configured CloudFront to cache based on request headers or cookies, CloudFront forwards all of the unique requests to your origin.

User-Agent Header

If you want CloudFront to cache different versions of your objects based on the device a user is using to view your content, we recommend that you configure CloudFront to forward the applicable headers to your custom origin:

  • CloudFront-Is-Desktop-Viewer
  • CloudFront-Is-Mobile-Viewer
  • CloudFront-Is-SmartTV-Viewer
  • CloudFront-Is-Tablet-Viewer

Based on the value of the User-Agent header, CloudFront sets the value of these headers to true or false before forwarding the request to your origin. If a device falls into more than one category, more than one value might be true. For example, for some tablet devices, CloudFront might set both CloudFront-Is-Mobile-Viewer and CloudFront-Is-Tablet-Viewer to true. For more information about configuring CloudFront to cache based on request headers, see Configuring CloudFront to Cache Objects Based on Request Headers.

You can configure CloudFront to cache objects based on values in the User-Agent header, but we don't recommend it. The User-Agent header has a lot of possible values, and caching based on those values would cause CloudFront to forward significantly more requests to your origin.

If you do not configure CloudFront to cache objects based on values in the User-Agent header, CloudFront CloudFront adds a User-Agent header with the following value before it forwards a request to your origin:

User-Agent = Amazon CloudFront

CloudFront adds this header regardless of whether the request from the viewer includes a User-Agent header. If the request from the viewer includes a User-Agent header, CloudFront removes it.

How CloudFront Processes Responses from Your Custom Origin Server

This topic contains information about how CloudFront processes responses from your custom origin.

Topics

100-Continue Responses

Your origin cannot send more than one 100-Continue response to CloudFront. After the first 100-Continue response, CloudFront expects an HTTP 200 OK response. If your origin sends another 100-Continue response after the first one, CloudFront will return an error.

Caching

  • Ensure that the origin server sets valid and accurate values for the Date and Last-Modified header fields.
  • If requests from viewers include the If-Match or If-None-Match request header fields, set the ETag response header field. If you do not specify an ETag value, CloudFront ignores subsequent If-Match or If-None-Match headers.
  • CloudFront normally respects a Cache-Control: no-cache header in the response from the origin. For an exception, see Simultaneous Requests for the Same Object (Traffic Spikes).

Canceled Requests

If an object is not in the edge cache, and if a viewer terminates a session (for example, closes a browser) after CloudFront gets the object from your origin but before it can deliver the requested object, CloudFront does not cache the object in the edge location.

Content Negotiation

If your origin returns Vary:* in the response, and if the value of Minimum TTL for the corresponding cache behavior is 0, CloudFront caches the object but still forwards every subsequent request for the object to the origin to confirm that the cache contains the latest version of the object. CloudFront doesn't include any conditional headers, such as If-None-Match or If-Modified-Since. As a result, your origin returns the object to CloudFront in response to every request.

If your origin returns Vary:* in the response, and if the value of Minimum TTL for the corresponding cache behavior is any other value, CloudFront processes the Vary header as described in HTTP Response Headers that CloudFront Removes or Updates.

Cookies

If you enable cookies for a cache behavior, and if the origin returns cookies with an object, CloudFront caches both the object and the cookies. Note that this reduces cacheability for an object. For more information, see Configuring CloudFront to Cache Objects Based on Cookies.

Dropped TCP Connections

If the TCP connection between CloudFront and your origin drops while your origin is returning an object to CloudFront, CloudFront behavior depends on whether your origin included a Content-Length header in the response:

  • Content-Length header – CloudFront returns the object to the viewer as it gets the object from your origin. However, if the value of the Content-Length header doesn't match the size of the object, CloudFront doesn't cache the object.
  • Transfer-Encoding: Chunked – CloudFront returns the object to the viewer as it gets the object from your origin. However, if the chunked response is not complete, CloudFront does not cache the object.
  • No Content-Length header – CloudFront returns the object to the viewer and caches it, but the object may not be complete. Without a Content-Length header, CloudFront cannot determine whether the TCP connection was dropped accidentally or on purpose.

We recommend that you configure your HTTP server to add a Content-Length header to prevent CloudFront from caching partial objects.

HTTP Response Headers that CloudFront Removes or Updates

CloudFront removes or updates the following header fields before forwarding the response from your origin to the viewer:

  • Set-Cookie – If you configure CloudFront to forward cookies, it will forward the Set-Cookie header field to clients. For more information, see Configuring CloudFront to Cache Objects Based on Cookies.

  • Trailer

  • Transfer-Encoding – If your origin returns this header field, CloudFront sets the value to chunked before returning the response to the viewer.

  • Upgrade

  • Vary – Note the following:

    • If you configure CloudFront to forward any of the device-specific headers to your origin (CloudFront-Is-Desktop-Viewer, CloudFront-Is-Mobile-Viewer, CloudFront-Is-SmartTV-Viewer, CloudFront-Is-Tablet-Viewer) and you configure your origin to return Vary:User-Agent to CloudFront, CloudFront returns Vary:User-Agent to the viewer. For more information, see Configuring CloudFront to Cache Objects Based on the Device Type.
    • If you configure your origin to include either Accept-Encoding or Cookie in the Vary header, CloudFront includes the values in the response to the viewer.
    • If you configure CloudFront to forward a whitelist of headers to your origin, and if you configure your origin to return the header names to CloudFront in the Vary header (for example, Vary:Accept-Charset,Accept-Language), CloudFront returns the Vary header with those value to the viewer.
    • For information about how CloudFront processes a value of * in the Vary header, see Content Negotiation.
    • If you configure your origin to include any other values in the Vary header, CloudFront removes the values before returning the response to the viewer.

  • Via – Regardless of whether your origin returns this header field to CloudFront, CloudFront sets the value to:

    Via: 1.1 alphanumeric-string.cloudfront.net (CloudFront)

    before returning the response to the viewer. For example:

    Via: 1.1 1026589cc7887e7a0dc7827b4example.cloudfront.net (CloudFront)

Maximum File Size

The maximum size of a response body that CloudFront will return to the viewer is 20 GB. This includes chunked transfer responses that don't specify the Content-Length header value.

Origin Unavailable

If your origin server is unavailable and CloudFront gets a request for an object that is in the edge cache but that has expired (for example, because the period of time specified in the Cache-Control max-age directive has passed), CloudFront either serves the expired version of the object or serves a custom error page. For more information, see How CloudFront Processes and Caches HTTP 4xx and 5xx Status Codes from Your Origin.

In some cases, an object that is seldom requested is evicted and is no longer available in the edge cache. CloudFront can't serve an object that has been evicted.

Redirects

If you change the location of an object on the origin server, you can configure your web server to redirect requests to the new location. After you configure the redirect, the first time a viewer submits a request for the object, CloudFront Front sends the request to the origin, and the origin responds with a redirect (for example, 302 Moved Temporarily). CloudFront caches the redirect and returns it to the viewer. CloudFront does not follow the redirect.

You can configure your web server to redirect requests to one of the following locations:

  • The new URL of the object on the origin server. When the viewer follows the redirect to the new URL, the viewer bypasses CloudFront and goes straight to the origin. As a result, we recommend that you not redirect requests to the new URL of the object on the origin.
  • The new CloudFront URL for the object. When the viewer submits the request that contains the new CloudFront URL, CloudFront gets the object from the new location on your origin, caches it at the edge location, and returns the object to the viewer. Subsequent requests for the object will be served by the edge location. This avoids the latency and load associated with viewers requesting the object from the origin. However, every new request for the object will incur charges for two requests to CloudFront.

Transfer Encoding

CloudFront supports only the chunked value of the Transfer-Encoding header. If your origin returns Transfer-Encoding: chunked, CloudFront returns the object to the client as the object is received at the edge location, and caches the object in chunked format for subsequent requests.

If the viewer makes a Range GET request and the origin returns Transfer-Encoding: chunked, CloudFront returns the entire object to the viewer instead of the requested range.

We recommend that you use chunked encoding if the content length of your response cannot be predetermined. For more information, see Dropped TCP Connections.