Asset canister architecture reference

Reference

An asset canister, also referred to as a 'frontend canister' is a type of canister deployed on ICP that can be used to store and retrieve an application's frontend, typically used to serve static HTML, JS, and CSS assets.

Architecture reference

The asset canister created by dfx is written in Rust and exposes the following methods:

#[update]
async fn authorize(other: Principal)

#[update]
async fn grant_permission(arg: GrantPermissionArguments)

#[update]
async fn validate_grant_permission(arg: GrantPermissionArguments) -> Result<String, String>

#[update]
async fn deauthorize(other: Principal)

#[update]
async fn revoke_permission(arg: RevokePermissionArguments)

#[update]
async fn validate_revoke_permission(arg: RevokePermissionArguments) -> Result<String, String>

#[update]
fn list_authorized() -> Vec<Principal>

#[query(manual_reply = true)]
fn list_permitted(arg: ListPermittedArguments) -> ManualReply<Vec<Principal>>

#[update]
async fn take_ownership()

#[query]
fn retrieve(key: Key) -> RcBytes

#[update(guard = "can_commit")]
fn store(arg: StoreArg)

#[update(guard = "can_prepare")]
fn create_batch() -> CreateBatchResponse

#[update(guard = "can_prepare")]
fn create_chunk(arg: CreateChunkArg) -> CreateChunkResponse

#[update(guard = "can_commit")]
fn create_asset(arg: CreateAssetArguments)

#[update(guard = "can_commit")]
fn set_asset_content(arg: SetAssetContentArguments)

#[update(guard = "can_commit")]
fn unset_asset_content(arg: UnsetAssetContentArguments)

#[update(guard = "can_commit")]
fn delete_asset(arg: DeleteAssetArguments)

#[update(guard = "can_commit")]
fn clear()

#[update(guard = "can_commit")]
fn commit_batch(arg: CommitBatchArguments)

#[query]
fn get(arg: GetArg) -> EncodedAsset

#[query]
fn get_chunk(arg: GetChunkArg) -> GetChunkResponse

#[query]
fn list() -> Vec<AssetDetails>

#[query]
fn certified_tree() -> CertifiedTree

#[query]
fn http_request(req: HttpRequest) -> HttpResponse

#[query]
fn http_request_streaming_callback(token: StreamingCallbackToken) -> StreamingCallbackHttpResponse

#[query]
fn get_asset_properties(key: Key) -> AssetProperties

#[update(guard = "can_commit")]
fn set_asset_properties(arg: SetAssetPropertiesArguments)

Default Content Security Policy (CSP)

[
    {
        "match": "**/*",
        "headers": {
            // Security: The Content Security Policy (CSP) given below aims at working with many apps rather than providing maximal security.
            // We recommend tightening the CSP for your specific application. Some recommendations are as follows:
            // - Use the CSP Evaluator (https://csp-evaluator.withgoogle.com/) to validate the CSP you define.
            // - Follow the “Strict CSP” recommendations (https://csp.withgoogle.com/docs/strict-csp.html). However, note that in the context of the IC,
            //   nonces cannot be used because the response bodies must be static to work well with HTTP asset certification.
            //   Thus, we recommend to include script hashes (in combination with strict-dynamic) in the CSP as described
            //   in https://csp.withgoogle.com/docs/faq.html in section “What if my site is static and I can't add nonces to scripts?”.
            //   See for example the II CSP (https://github.com/dfinity/internet-identity/blob/main/src/internet_identity/src/http.rs).
            // - It is recommended to tighten the connect-src directive. With the current CSP configuration the browser can
            //   make requests to https://*.icp0.io, hence being able to call any canister via https://icp0.io/api/v2/canister/{canister-ID}.
            //   This could potentially be used in combination with another vulnerability (e.g. XSS) to exfiltrate private data.
            //   The developer can configure this policy to only allow requests to their specific canisters,
            //   e.g: connect-src 'self' https://icp-api.io/api/v2/canister/{my-canister-ID}, where {my-canister-ID} has the following format: aaaaa-aaaaa-aaaaa-aaaaa-aaa
            // - It is recommended to configure style-src, style-src-elem and font-src directives with the resources your canister is going to use
            //   instead of using the wild card (*) option. Normally this will include 'self' but also other third party styles or fonts resources (e.g: https://fonts.googleapis.com or other CDNs)

            // Notes about the CSP below:
            //   There is currently no other way to allow execution of WebAssembly modules with CSP.
            //   See: https://github.com/WebAssembly/content-security-policy/blob/main/proposals/CSP.md.
            // - We added img-src data: because data: images are used often.
            // - frame-ancestors: none mitigates clickjacking attacks. See https://owasp.org/www-community/attacks/Clickjacking.
            "Content-Security-Policy": "default-src 'self';script-src 'self';connect-src 'self' http://localhost:* https://icp0.io https://*.icp0.io https://icp-api.io;img-src 'self' data:;style-src * 'unsafe-inline';style-src-elem * 'unsafe-inline';font-src *;object-src 'none';base-uri 'self';frame-ancestors 'none';form-action 'self';upgrade-insecure-requests;",

            // Security: The permissions policy disables all features for security reasons. If your site needs such permissions, activate them.
            // To configure permissions go here https://www.permissionspolicy.com/
            "Permissions-Policy": "accelerometer=(), ambient-light-sensor=(), autoplay=(), battery=(), camera=(), cross-origin-isolated=(), display-capture=(), document-domain=(), encrypted-media=(), execution-while-not-rendered=(), execution-while-out-of-viewport=(), fullscreen=(), geolocation=(), gyroscope=(), keyboard-map=(), magnetometer=(), microphone=(), midi=(), navigation-override=(), payment=(), picture-in-picture=(), publickey-credentials-get=(), screen-wake-lock=(), sync-xhr=(), usb=(), web-share=(), xr-spatial-tracking=(), clipboard-read=(), clipboard-write=(), gamepad=(), speaker-selection=(), conversion-measurement=(), focus-without-user-activation=(), hid=(), idle-detection=(), interest-cohort=(), serial=(), sync-script=(), trust-token-redemption=(), window-placement=(), vertical-scroll=()",

            // Security: Mitigates clickjacking attacks.
            // See: https://owasp.org/www-community/attacks/Clickjacking.
            "X-Frame-Options": "DENY",

            // Security: Avoids forwarding referrer information to other origins.
            // See: https://owasp.org/www-project-secure-headers/#referrer-policy.
            "Referrer-Policy": "same-origin",

            // Security: Tells the user’s browser that it must always use HTTPS with your site.
            // See: https://owasp.org/www-project-secure-headers/#http-strict-transport-security
            "Strict-Transport-Security": "max-age=31536000; includeSubDomains",

            // Security: Prevents the browser from interpreting files as a different MIME type to what is specified in the Content-Type header.
            // See: https://owasp.org/www-project-secure-headers/#x-content-type-options
            "X-Content-Type-Options": "nosniff",

            // Security: Enables browser features to mitigate some of the XSS attacks. Note that it has to be in mode=block.
            // See: https://owasp.org/www-community/attacks/xss/
            "X-XSS-Protection": "1; mode=block"
        },
        // Uncomment to redirect all requests from .raw.icp0.io to .icp0.io
        // "allow_raw_access": false
    },
]

Troubleshooting

Here are some common resolutions for issues encountered with the asset canister.

Asset canister fails to upgrade

It is likely that the total size of all the static files stored in your asset canister exceed the recommended 1GB limit. The resolution is to uninstall and redeploy your canister code.

dfx canister uninstall-code <CANISTER NAME or ID>

dfx deploy --upgrade-unchanged <CANISTER NAME>

This will incur downtime. This first uninstalls the Wasm module from your canister. During this period, your canister will not be reachable. The second command redeploys with fresh state, after which point, your canister will be available to serve assets again.