HMAP: A Technique and Tool For Remote Identification of HTTP Servers
Dustin William Lee · 2001
this document already (among other uses). For instance Apache/1.3.11 returns an ETag header with the following format: "0-574-38379154;3a5b7811". Jigsaw/2.1.2 server returns ETags like the following: "mvanct:s0jndthg". Since there are no official guidelines for how ETags should be constructed and presented these tend to be a good characteristic for fingerprinting