An IPv4 header is the leading part of an IPv4 packet. It contains information needed to deliver and process the packet, including source and destination addresses, packet length, protocol, and fragmentation details. Routers and operating systems use these fields to decide how to handle the packet.
In security, the header matters because network code may trust its fields when validating, routing, or modifying packet data. If a packet changes after an earlier check, later processing may rely on stale or inconsistent metadata. In Linux networking paths, checksum handling and IPv4 fragmentation can involve updates to the header; careful validation helps prevent malformed or altered packets from causing unsafe writes or crossing isolation boundaries. Keeping host kernels patched and treating guest-originated traffic as untrusted are important defenses.