Published on 2025-08-07T06:18:08Z

Slack-ImgProxy

Slack-ImgProxy is a specialized web crawler from the communication platform Slack. Its sole purpose is to handle image processing. When a user shares a link to an image in a Slack chat, this bot fetches, caches, and optimizes that image for display. It provides security and privacy benefits by serving all images over HTTPS and obscuring referrer data. A key characteristic is that it intentionally disregards robots.txt directives, as image proxying is considered an essential function.

What is Slack-ImgProxy?

Slack-ImgProxy is a web crawler from Slack that handles image processing for the messaging platform. It functions as an image proxy service that retrieves, caches, and optimizes images that are shared in Slack channels. The bot identifies itself in server logs with a user-agent string like Slack-ImgProxy 0.19 (+https://api.slack.com/robots). Unlike standard crawlers, Slack-ImgProxy is designed to ignore robots.txt directives, as image proxying is considered an essential function of the platform. It focuses exclusively on images and does not crawl or index any other content.

Why is Slack-ImgProxy crawling my site?

Slack-ImgProxy is visiting your website because a user has shared an image from your site in a Slack conversation. The bot is retrieving that specific image to display it within the Slack interface. It does not randomly crawl your site; its visits are triggered on-demand by user actions. The frequency of visits depends entirely on how often your images are shared in Slack. This is considered an authorized activity as part of the normal functioning of a social platform.

What is the purpose of Slack-ImgProxy?

Slack-ImgProxy serves three main functions. First, it enhances security by ensuring all images are served over HTTPS, even if the original source is not. Second, it protects user privacy by obscuring referrer data that might otherwise reveal sensitive information about a Slack workspace or channel. Third, it improves performance by resizing and reformatting images to reduce bandwidth and improve load times within the Slack client. For website owners, this means your images can reach a wide audience through Slack, though you will not receive direct referrer information.

How do I block Slack-ImgProxy?

Blocking Slack-ImgProxy is not possible through the robots.txt file, as the bot is designed to ignore these directives to ensure that images shared by users are always displayed. Preventing it from accessing your images would require more advanced methods, such as blocking its user-agent or IP ranges at the firewall level. However, this is generally not recommended as it could interfere with the legitimate sharing of your content.

How to verify the authenticity of the user-agent operated by Slack?

Reverse IP lookup technique

To verify user-agent authenticity, you can use host linux command two times with the IP address of the requester.
  1. > host IPAddressOfRequest
    This command returns the reverse lookup hostname (e.g., 4.4.8.8.in-addr.arpa.). 
  2. > host ReverseDNSFromTheOutputOfFirstRequest
If the output matches the original IP address and the domain is associated with a trusted operator (e.g., Slack), the user-agent can be considered legitimate.

IP list lookup technique

Some operators provide a public list of IP addresses used by their crawlers. This list can be cross-referenced to verify a user-agent's authenticity. However, both operators and website owners may find it challenging to maintain an up-to-date list, so use this method with caution and in conjunction with other verification techniques.