Multi-NIC discovery

A laptop with Wi-Fi + Ethernet both up has more than one path to broadcast on. The kernel's default-route NIC wins by default — every other interface stays silent unless you explicitly tell go-udap otherwise. This page explains the wire-level details.

The problem

Default go-udap discover binds 0.0.0.0:17784 and writes to 255.255.255.255:17784. The kernel chooses which interface to emit the packet on based on its routing table — typically the default-route NIC.

If the device is on the other NIC's subnet (say, a Squeezebox on your wired LAN while your Wi-Fi default route points at a tethered phone), the broadcast never reaches it.

--interface NAME

Forces a specific egress interface. The mechanism is platform-specific:

• macOS: IP_BOUND_IF socket option (IPPROTO_IP option 25). Takes the interface index from EnumerateInterfaces(). The kernel routes all output through that NIC regardless of route-table preference.
• Linux: SO_BINDTODEVICE socket option (SOL_SOCKET). Takes the interface name string. Same effect as IP_BOUND_IF. Requires CAP_NET_RAW on most distros — usually granted to the binary or the user.
• Windows: Not yet supported. IP_UNICAST_IF exists on Windows but its broadcast semantics are undocumented; investigation is tracked as Task #29. Until then, the flag is hidden from --help on Windows.

The local socket is bound to 0.0.0.0:17784 regardless of the egress interface — not to the interface's specific IP. Binding to the specific IP would prevent the socket from receiving limited-broadcast replies (those only reach 0.0.0.0-bound sockets). This was a real bug caught by Phase 6 hardware testing.

--all-interfaces

Fans out across every usable interface. Implementation: one UDPTransport per interface, composed into a MultiTransport.

The catch: multiple sockets all binding to 0.0.0.0:17784 simultaneously requires SO_REUSEPORT (in addition to SO_REUSEADDR), set before bind(). go-udap uses net.ListenConfig.Control to set both options pre-bind via a platform-specific helper (setReusePortPreBind).

On Windows, SO_REUSEPORT doesn't exist; --all-interfaces is therefore not supported there.

What you'd see in tcpdump

./go-udap --all-interfaces discover on a Mac with en0 (Wi-Fi, 192.168.1.118) and en7 (Ethernet, 192.168.1.226) both on the same subnet:

16:58:38.746037 IP 192.168.1.118.17784 > 255.255.255.255.17784: UDP, length 27
16:58:38.746183 IP 192.168.1.226.17784 > 255.255.255.255.17784: UDP, length 27
16:58:38.746539 IP   0.0.0.0.17784 > 255.255.255.255.17784: UDP, length 61

Two source IPs on the outbound — confirming fan-out really did go out both NICs. The third line is the device reply.

Why not just set IP_MULTICAST_IF?

UDAP discovery uses broadcast, not multicast. The IP_MULTICAST_IF socket option doesn't affect broadcast routing on any of the three target platforms.

See also

• How UDAP discovery works — the broader discovery story
• How to discover on a multi-NIC laptop — the practical recipe
• Global flags reference — flag documentation