We present a static analysis of endian portability for C programs. Our analysis can infer that a given program, or two syntactically close versions thereof, compute the same outputs, when run with the same inputs on platforms with different byte-orders, a.k.a. endiannesses. We target low-level C programs that abuse C pointers and unions, hence rely on implementation-specific behaviors undefined in the C standard. Our method is based on abstract interpretation, and parametric in the choice of a numerical abstract domain. We first present a novel concrete collecting semantics, relating the behaviors of two versions of a program, running on platforms with different endiannesses. We propose a joint memory abstraction, able to infer equivalence relations between little- and big-endian memories. We introduce a novel symbolic predicate domain, to infer relations between individual bytes of the variables in the two programs, which has near-linear cost, and the right amount of relationality to express (bitwise) arithmetic properties relevant to endian portability. We implemented a prototype static analyzer, able to scale to large real-world industrial software, with zero false alarms.