For a file-based HTTP server's weak ETags I've used a concatenation of st_dev, st_ino, and inode generation number. For strong ETags I've used a SHA-512 hash.

For a file-based HTTP server's weak ETags I've used a concatenation of st_dev, st_ino, and inode generation number. For strong ETags I've used a SHA-512 hash. In combination with If-Match: and If-None-Match: this is very powerful.