Use a TUN/TAP Interface to Inject Your TCP Stack

TUN vs TAP

TUN and TAP are virtual kernel interfaces that let user-space programs inject and read raw packets. TUN works at layer 3 (IP packets to 10.0.0.1 with no Ethernet header). TAP works at layer 2 (full Ethernet frames with MAC addresses and EtherType 0x0800).

Creating a TUN Device

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Flags:

• IFF_TUN: layer 3, IP packets only
• IFF_TAP: layer 2, Ethernet frames
• IFF_NO_PI: skip the 4-byte packet info header

Read and write the fd like a file: read(fd, buf, 1500) returns an IP packet; write(fd, buf, len) injects one into the kernel routing table.

Common Uses

• WireGuard and OpenVPN tunnel encrypted IP through a TUN device
• QEMU and Docker use TAP for VM/container bridging
• User-space TCP/IP stack testing without kernel patches

Why This Exercise

This task asks you to set up a TUN interface and demonstrate packet I/O. This is the same kernel interface that lets you build and test a user-space TCP stack without touching kernel code. WireGuard creates a TUN device to intercept IP packets bound for 10.0.0.0/24 before encrypting and tunneling them over UDP to a peer endpoint.