About Etag

https://stackoverflow.com/questions/47512043/how-etags-are-generated-and-configured/62996104#62996104

Different webservers returns ETag like:

Nginx: "5e132e20-417" i.e. "hex(MTime)-hex(Size)". Not configurable.
BusyBox httpd the same as Nginx Apache/2.2: "42-417-59b782a99f493" i.e. "hex(INode)-hex(Size)-hex(MTime in nanoseconds)". Can be configured but MTime anyway will be in nanos
Apache/2.4: "417-59b782a99f493" i.e. "hex(Size)-hex(MTime in nanoseconds)" i.e. without INode which is friendly for load balancing when identical file have different INode on different servers.
OpenWrt uhttpd: "42-417-5e132e20" i.e. "hex(INode)-hex(Size)-hex(MTime)". Not configurable.
Tomcat 9: W/"1047-1578315296666" i.e. Weak"Size-MTime in milliseconds". This is incorrect ETag because it should be strong as for a static file i.e. octal compatibility.
LightHTTPD: most weird: "hashcode(42-1047-1578315296666771000)" i.e. INode-Size-MTime but then reduced to a simple integer by hashcode. Can be configured but you can only disable one part (etag.use-inode = "disabled")
MS IIS: it have a form Filetimestamp:ChangeNumber e.g. "53dbd5819f62d61:0". Not documented, not configurable but can be disabled.
Jetty: based on last mod, size and hashed. See Resource.getWeakETag()
Kitura (Swift): "W/hex(Size)-hex(MTime)" StaticFileServer.calculateETag