homarr/packages/image-proxy/src/index.ts

import { createHmac, hkdfSync } from "node:crypto";

import { createId } from "@homarr/common";
import { env } from "@homarr/common/env";
import { decryptSecret, encryptSecret } from "@homarr/common/server";
import { fetchWithTrustedCertificatesAsync } from "@homarr/core/infrastructure/http";
import { createLogger } from "@homarr/core/infrastructure/logs";
import { ErrorWithMetadata } from "@homarr/core/infrastructure/logs/error";
import { createGetSetChannel } from "@homarr/redis";

const logger = createLogger({ module: "imageProxy" });

export const IMAGE_PROXY_REDIS_TTL_SECONDS = 86400;

const createHmacChannel = (hmac: `${string}.${string}`) => createGetSetChannel<string>(`image-proxy:hmac:${hmac}`);
const createUrlByIdChannel = (id: string) =>
  createGetSetChannel<{
    url: `${string}.${string}`;
    headers: `${string}.${string}`;
  }>(`image-proxy:url:${id}`);

/** The upstream body, forwarded as-is rather than buffered. */
type ProxiedImageBody = Awaited<ReturnType<typeof fetchWithTrustedCertificatesAsync>>["body"];

export type ForwardImageResult =
  | { image: ProxiedImageBody; contentType: string | null; contentLength: string | null }
  | { error: "not-found" }
  | { error: "upstream-error"; statusCode: number }
  | { error: "fetch-error" };

export class ImageProxy {
  private static hmacKey: Buffer<ArrayBuffer> | null = null;

  /**
   * Generate secure key for image-proxy from encryption key
   */
  private static getHmacKey(): Buffer<ArrayBuffer> {
    this.hmacKey ??= Buffer.from(hkdfSync("sha256", env.SECRET_ENCRYPTION_KEY, "", "image-proxy", 32));
    return this.hmacKey;
  }

  public async createImageAsync(url: string, headers?: Record<string, string>): Promise<string> {
    const existingId = await this.getExistingIdAsync(url, headers);
    if (existingId) {
      logger.debug("Image already exists in the proxy", {
        id: existingId,
        url: this.redactUrl(url),
        headers: this.redactHeaders(headers ?? null),
      });
      return this.createImageUrl(existingId);
    }

    const id = createId();
    await this.storeImageAsync(id, url, headers);

    return this.createImageUrl(id);
  }

  public async forwardImageAsync(id: string): Promise<ForwardImageResult> {
    const urlAndHeaders = await this.getImageUrlAndHeadersAsync(id);
    if (!urlAndHeaders) {
      return { error: "not-found" };
    }

    const proxyUrl = this.createImageUrl(id);
    let response: Awaited<ReturnType<typeof fetchWithTrustedCertificatesAsync>>;
    try {
      response = await fetchWithTrustedCertificatesAsync(urlAndHeaders.url, {
        headers: urlAndHeaders.headers ?? {},
      });
    } catch (error) {
      logger.error(
        new ErrorWithMetadata(
          "Failed to connect to image source",
          {
            id,
            url: this.redactUrl(urlAndHeaders.url),
            headers: this.redactHeaders(urlAndHeaders.headers),
            proxyUrl,
          },
          { cause: error },
        ),
      );
      return { error: "fetch-error" };
    }

    if (!response.ok) {
      logger.error(
        new ErrorWithMetadata("Failed to fetch image", {
          id,
          url: this.redactUrl(urlAndHeaders.url),
          headers: this.redactHeaders(urlAndHeaders.headers),
          proxyUrl,
          statusCode: response.status,
        }),
      );
      return { error: "upstream-error", statusCode: response.status };
    }

    const contentLength = response.headers.get("content-length");
    logger.debug("Forwarding image succeeded", {
      id,
      url: this.redactUrl(urlAndHeaders.url),
      headers: this.redactHeaders(urlAndHeaders.headers),
      proxyUrl,
      size: contentLength ? `${(Number(contentLength) / 1024).toFixed(1)}KB` : "unknown",
    });

    // The body is forwarded as a stream rather than buffered: a board can request dozens of posters
    // at once, and buffering held every one of them in memory in full before responding.
    return {
      image: response.body,
      contentType: response.headers.get("content-type"),
      contentLength,
    };
  }

  private createImageUrl(id: string): string {
    return `/api/image-proxy/${id}`;
  }

  private async getImageUrlAndHeadersAsync(id: string) {
    const urlHeaderChannel = createUrlByIdChannel(id);
    const urlHeader = await urlHeaderChannel.getAsync();
    if (!urlHeader) {
      logger.warn("Image not found in the proxy", { id });
      return null;
    }

    return {
      url: decryptSecret(urlHeader.url),
      headers: JSON.parse(decryptSecret(urlHeader.headers)) as Record<string, string> | null,
    };
  }

  private async getExistingIdAsync(url: string, headers: Record<string, string> | undefined): Promise<string | null> {
    const urlHash = this.hashSecret(url);
    const headerHash = this.hashSecret(JSON.stringify(headers ?? null));

    const channel = createHmacChannel(`${urlHash}.${headerHash}`);
    return await channel.getAsync();
  }

  private async storeImageAsync(id: string, url: string, headers: Record<string, string> | undefined): Promise<void> {
    const urlHash = this.hashSecret(url);
    const headerHash = this.hashSecret(JSON.stringify(headers ?? null));

    const hashChannel = createHmacChannel(`${urlHash}.${headerHash}`);
    const urlHeaderChannel = createUrlByIdChannel(id);
    await urlHeaderChannel.setAsync(
      {
        url: encryptSecret(url),
        headers: encryptSecret(JSON.stringify(headers ?? null)),
      },
      { ttlSeconds: IMAGE_PROXY_REDIS_TTL_SECONDS },
    );
    await hashChannel.setAsync(id, { ttlSeconds: IMAGE_PROXY_REDIS_TTL_SECONDS });

    logger.debug("Stored image in the proxy", {
      id,
      url: this.redactUrl(url),
      headers: this.redactHeaders(headers ?? null),
    });
  }

  private hashSecret(value: string): string {
    return createHmac("sha256", ImageProxy.getHmacKey()).update(value).digest("hex");
  }

  private redactUrl(url: string): string {
    const urlObject = new URL(url);

    const redactedSearch = [...urlObject.searchParams.keys()].map((key) => `${key}=REDACTED`).join("&");

    return `${urlObject.origin}${urlObject.pathname}${redactedSearch ? `?${redactedSearch}` : ""}`;
  }

  private redactHeaders(headers: Record<string, string> | null): string | null {
    if (!headers) return null;

    return Object.keys(headers).join(", ");
  }
}