by Marko.Silokunnas:

Current implementation of ipv4.Header doesn't have an easy way to set flags
of the header. RFC791 defines that the flags field of the header is 3 bits 
wide and the fragment offset field is 13 bits wide. In the current implementation
of ipv4.Header fragment offset is defined as an int and marshaled into a 16 bit 
wide field in Marshal(). This allows the flags to be set by manually using bitwise 
operations, but it is rather ugly. 

I propose that a Flags field should be added to ipv4.Header and the marshalling
and unmarshalling functions should take care of the rest.