Fix ioctl(2) code for OpenBSD (#1175)

This cleans up the mess to configure an IP address on a tun(4) device.

Handrolling a hardcoded ioctl(2) request is far from perfect, but Go
(golang.org/sys/unix) is to blame here.

Tested on OpenBSD 7.6 -current where yggdrasil now drives the interface
would use of ifconfig or other helpers.

src/tun/tun_freebsd.go

@@ -1,5 +1,5 @@
-//go:build openbsd || freebsd
-// +build openbsd freebsd
+//go:build freebsd
+// +build freebsd
 
 package tun
 

src/tun/tun_openbsd.go

@@ -0,0 +1,122 @@
+//go:build openbsd
+// +build openbsd
+
+package tun
+
+import (
+	"fmt"
+	"net"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+
+	wgtun "golang.zx2c4.com/wireguard/tun"
+)
+
+const (
+	SIOCAIFADDR_IN6       = 0x8080691a
+	ND6_INFINITE_LIFETIME = 0xffffffff
+)
+
+type in6_addrlifetime struct {
+	ia6t_expire    int64
+	ia6t_preferred int64
+	ia6t_vltime    uint32
+	ia6t_pltime    uint32
+}
+
+// Match types from the net package, effectively being [16]byte for IPv6 addresses.
+type in6_addr [16]uint8
+
+type sockaddr_in6 struct {
+	sin6_len      uint8
+	sin6_family   uint8
+	sin6_port     uint16
+	sin6_flowinfo uint32
+	sin6_addr     in6_addr
+	sin6_scope_id uint32
+}
+
+func (sa6 *sockaddr_in6) setSockaddr(addr [/*16*/]byte /* net.IP or net.IPMask */) {
+	sa6.sin6_len    = uint8(unsafe.Sizeof(*sa6))
+	sa6.sin6_family = unix.AF_INET6
+
+	for i := range sa6.sin6_addr {
+		sa6.sin6_addr[i] = addr[i]
+	}
+}
+
+type in6_aliasreq struct {
+	ifra_name       [syscall.IFNAMSIZ]byte
+	ifra_addr       sockaddr_in6
+	ifra_dstaddr    sockaddr_in6
+	ifra_prefixmask sockaddr_in6
+	ifra_flags      int32
+	ifra_lifetime   in6_addrlifetime
+}
+
+// Configures the TUN adapter with the correct IPv6 address and MTU.
+func (tun *TunAdapter) setup(ifname string, addr string, mtu uint64) error {
+	iface, err := wgtun.CreateTUN(ifname, int(mtu))
+	if err != nil {
+		return fmt.Errorf("failed to create TUN: %w", err)
+	}
+	tun.iface = iface
+	if mtu, err := iface.MTU(); err == nil {
+		tun.mtu = getSupportedMTU(uint64(mtu))
+	} else {
+		tun.mtu = 0
+	}
+	if addr != "" {
+		return tun.setupAddress(addr)
+	}
+	return nil
+}
+
+// Configures the "utun" adapter from an existing file descriptor.
+func (tun *TunAdapter) setupFD(fd int32, addr string, mtu uint64) error {
+	return fmt.Errorf("setup via FD not supported on this platform")
+}
+
+func (tun *TunAdapter) setupAddress(addr string) error {
+	var sfd int
+	var err error
+
+	ip, prefix, err := net.ParseCIDR(addr)
+	if err != nil {
+		tun.log.Errorf("Error in ParseCIDR: %v", err)
+		return err
+	}
+
+	// Create system socket
+	if sfd, err = unix.Socket(unix.AF_INET6, unix.SOCK_DGRAM, 0); err != nil {
+		tun.log.Printf("Create AF_INET6 socket failed: %v", err)
+		return err
+	}
+
+	// Friendly output
+	tun.log.Infof("Interface name: %s", tun.Name())
+	tun.log.Infof("Interface IPv6: %s", addr)
+	tun.log.Infof("Interface MTU: %d", tun.mtu)
+
+	// Create the address request
+	var ar in6_aliasreq
+	copy(ar.ifra_name[:], tun.Name())
+
+	ar.ifra_addr.setSockaddr(ip)
+
+	prefixmask := net.CIDRMask(prefix.Mask.Size())
+	ar.ifra_prefixmask.setSockaddr(prefixmask)
+
+	ar.ifra_lifetime.ia6t_vltime = ND6_INFINITE_LIFETIME
+	ar.ifra_lifetime.ia6t_pltime = ND6_INFINITE_LIFETIME
+
+	// Set the interface address
+	if err = unix.IoctlSetInt(sfd, SIOCAIFADDR_IN6, int(uintptr(unsafe.Pointer(&ar)))); err != nil {
+		tun.log.Errorf("Error in SIOCAIFADDR_IN6: %v", err)
+		return err
+	}
+
+	return nil
+}