A lot of NIC manufactures provide either some hooks into something like DPDK or simple LD_PRELOADable libraries that offload the network syscalls (socket, poll,

A lot of NIC manufactures provide either some hooks into something like DPDK or simple LD_PRELOADable libraries that offload the network syscalls (socket, poll, recv, etc.) into a library that talks to the NIC directly using mmap'd IO I presume.