- * Use this for IPv4 addresses. It's defined as an array of octets, so
- * that it's not aligned on its "natural" boundary, and it's defined as
- * a structure in the hopes that this makes it harder to naively use
- * EXTRACT_32BITS() to extract the value - in many cases you just want
- * to use UNALIGNED_MEMCPY() to copy its value, so that it remains in
- * network byte order.
+ * Use this for IPv4 addresses and netmasks.
+ *
+ * It's defined as an array of octets, so that it's not guaranteed to
+ * be aligned on its "natural" boundary (in some packet formats, it
+ * *isn't* so aligned), and it's defined as a structure in the hopes
+ * that this makes it harder to naively use EXTRACT_BE_U_4() to extract
+ * the value - in many cases you just want to use UNALIGNED_MEMCPY() to
+ * copy its value, so that it remains in network byte order.
+ *
+ * (Among other things, we don't want somebody thinking "IPv4 addresses,
+ * they're in network byte order, so we want EXTRACT_BE_U_4(), right?"
+ * and then handing the result to system APIs that expect network-order
+ * IPv4 addresses, such as inet_ntop(), on their little-endian PCs, getting
+ * the wrong behavior, and concluding "oh, it must be in *little*-endian
+ * order" and "fixing" it to use EXTRACT_LE_U_4(). Yes, people do this;
+ * that's why Wireshark has tvb_get_ipv4(), to extract an IPv4 address from
+ * a packet data buffer; it was introduced in reaction to somebody who
+ * *had* done that.)
projects / tcpdump / blobdiff