I was reading pages 18-21 in RFC 791 about the Internet Protocol and in the sections on Loose Source and Record Route (LSRR), and Strict Source and Record Route (SSRR), and Record Route (RR) it says that the smallest value for the pointer is 4. To confirm my understanding of how this pointer works - is it really 1-indexed?
Converting my comment to an answer, as requested :)
Based on the information in UNIX® Network Programming Volume 1, Third Edition: The Sockets Networking API, section 27.3 it certainly seems it is. I haven't verified this though.
Based on this diagram and this part of the description: "ptr is a pointer which contains the offset of the next IP address to be processed in the route, and we initialize it to 4, which points to the first IP address.", the
ptr field does indeed seem to be 1-indexed. Note that the NOP option isn't counted. The option, and therefore the counting, starts from