diff --git a/src/conn.go b/src/conn.go
index 7b35829..41a5b85 100644
--- a/src/conn.go
+++ b/src/conn.go
@@ -1,9 +1,31 @@
 package main
 
 import (
+	"errors"
 	"net"
 )
 
+func parseEndpoint(s string) (*net.UDPAddr, error) {
+
+	// ensure that the host is an IP address
+
+	host, _, err := net.SplitHostPort(s)
+	if err != nil {
+		return nil, err
+	}
+	if ip := net.ParseIP(host); ip == nil {
+		return nil, errors.New("Failed to parse IP address: " + host)
+	}
+
+	// parse address and port
+
+	addr, err := net.ResolveUDPAddr("udp", s)
+	if err != nil {
+		return nil, err
+	}
+	return addr, err
+}
+
 func updateUDPConn(device *Device) error {
 	netc := &device.net
 	netc.mutex.Lock()
diff --git a/src/conn_linux.go b/src/conn_linux.go
index e973b25..a349a9e 100644
--- a/src/conn_linux.go
+++ b/src/conn_linux.go
@@ -1,10 +1,253 @@
+/* Copyright 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ *
+ * This implements userspace semantics of "sticky sockets", modeled after
+ * WireGuard's kernelspace implementation.
+ */
+
 package main
 
 import (
+	"errors"
 	"golang.org/x/sys/unix"
 	"net"
+	"strconv"
+	"unsafe"
 )
 
+/* Supports source address caching
+ *
+ * It is important that the endpoint is only updated after the packet content has been authenticated.
+ *
+ * Currently there is no way to achieve this within the net package:
+ * See e.g. https://github.com/golang/go/issues/17930
+ */
+type Endpoint struct {
+	// source (selected based on dst type)
+	// (could use RawSockaddrAny and unsafe)
+	srcIPv6 unix.RawSockaddrInet6
+	srcIPv4 unix.RawSockaddrInet4
+	srcIf4  int32
+
+	dst unix.RawSockaddrAny
+}
+
+func zoneToUint32(zone string) (uint32, error) {
+	if zone == "" {
+		return 0, nil
+	}
+	if intr, err := net.InterfaceByName(zone); err == nil {
+		return uint32(intr.Index), nil
+	}
+	n, err := strconv.ParseUint(zone, 10, 32)
+	return uint32(n), err
+}
+
+func (end *Endpoint) ClearSrc() {
+	end.srcIf4 = 0
+	end.srcIPv4 = unix.RawSockaddrInet4{}
+	end.srcIPv6 = unix.RawSockaddrInet6{}
+}
+
+func (end *Endpoint) Set(s string) error {
+	addr, err := parseEndpoint(s)
+	if err != nil {
+		return err
+	}
+
+	ipv6 := addr.IP.To16()
+	if ipv6 != nil {
+		zone, err := zoneToUint32(addr.Zone)
+		if err != nil {
+			return err
+		}
+		ptr := (*unix.RawSockaddrInet6)(unsafe.Pointer(&end.dst))
+		ptr.Family = unix.AF_INET6
+		ptr.Port = uint16(addr.Port)
+		ptr.Flowinfo = 0
+		ptr.Scope_id = zone
+		copy(ptr.Addr[:], ipv6[:])
+		end.ClearSrc()
+		return nil
+	}
+
+	ipv4 := addr.IP.To4()
+	if ipv4 != nil {
+		ptr := (*unix.RawSockaddrInet4)(unsafe.Pointer(&end.dst))
+		ptr.Family = unix.AF_INET
+		ptr.Port = uint16(addr.Port)
+		ptr.Zero = [8]byte{}
+		copy(ptr.Addr[:], ipv4)
+		end.ClearSrc()
+		return nil
+	}
+
+	return errors.New("Failed to recognize IP address format")
+}
+
+func send6(sock uintptr, end *Endpoint, buff []byte) error {
+	var iovec unix.Iovec
+
+	iovec.Base = (*byte)(unsafe.Pointer(&buff[0]))
+	iovec.SetLen(len(buff))
+
+	cmsg := struct {
+		cmsghdr unix.Cmsghdr
+		pktinfo unix.Inet6Pktinfo
+	}{
+		unix.Cmsghdr{
+			Level: unix.IPPROTO_IPV6,
+			Type:  unix.IPV6_PKTINFO,
+			Len:   unix.SizeofInet6Pktinfo,
+		},
+		unix.Inet6Pktinfo{
+			Addr:    end.srcIPv6.Addr,
+			Ifindex: end.srcIPv6.Scope_id,
+		},
+	}
+
+	msghdr := unix.Msghdr{
+		Iov:     &iovec,
+		Iovlen:  1,
+		Name:    (*byte)(unsafe.Pointer(&end.dst)),
+		Namelen: unix.SizeofSockaddrInet6,
+		Control: (*byte)(unsafe.Pointer(&cmsg)),
+	}
+
+	msghdr.SetControllen(int(unsafe.Sizeof(cmsg)))
+
+	// sendmsg(sock, &msghdr, 0)
+
+	_, _, errno := unix.Syscall(
+		unix.SYS_SENDMSG,
+		sock,
+		uintptr(unsafe.Pointer(&msghdr)),
+		0,
+	)
+	if errno == unix.EINVAL {
+		end.ClearSrc()
+	}
+	return errno
+}
+
+func send4(sock uintptr, end *Endpoint, buff []byte) error {
+	var iovec unix.Iovec
+
+	iovec.Base = (*byte)(unsafe.Pointer(&buff[0]))
+	iovec.SetLen(len(buff))
+
+	cmsg := struct {
+		cmsghdr unix.Cmsghdr
+		pktinfo unix.Inet4Pktinfo
+	}{
+		unix.Cmsghdr{
+			Level: unix.IPPROTO_IP,
+			Type:  unix.IP_PKTINFO,
+			Len:   unix.SizeofInet6Pktinfo,
+		},
+		unix.Inet4Pktinfo{
+			Spec_dst: end.srcIPv4.Addr,
+			Ifindex:  end.srcIf4,
+		},
+	}
+
+	msghdr := unix.Msghdr{
+		Iov:     &iovec,
+		Iovlen:  1,
+		Name:    (*byte)(unsafe.Pointer(&end.dst)),
+		Namelen: unix.SizeofSockaddrInet4,
+		Control: (*byte)(unsafe.Pointer(&cmsg)),
+	}
+
+	msghdr.SetControllen(int(unsafe.Sizeof(cmsg)))
+
+	// sendmsg(sock, &msghdr, 0)
+
+	_, _, errno := unix.Syscall(
+		unix.SYS_SENDMSG,
+		sock,
+		uintptr(unsafe.Pointer(&msghdr)),
+		0,
+	)
+	if errno == unix.EINVAL {
+		end.ClearSrc()
+	}
+	return errno
+}
+
+func send(c *net.UDPConn, end *Endpoint, buff []byte) error {
+
+	// extract underlying file descriptor
+
+	file, err := c.File()
+	if err != nil {
+		return err
+	}
+	sock := file.Fd()
+
+	// send depending on address family of dst
+
+	family := *((*uint16)(unsafe.Pointer(&end.dst)))
+	if family == unix.AF_INET {
+		return send4(sock, end, buff)
+	} else if family == unix.AF_INET6 {
+		return send6(sock, end, buff)
+	}
+	return errors.New("Unknown address family of source")
+}
+
+func receiveIPv4(end *Endpoint, c *net.UDPConn, buff []byte) (error, *net.UDPAddr, *net.UDPAddr) {
+
+	file, err := c.File()
+	if err != nil {
+		return err, nil, nil
+	}
+
+	var iovec unix.Iovec
+	iovec.Base = (*byte)(unsafe.Pointer(&buff[0]))
+	iovec.SetLen(len(buff))
+
+	var cmsg struct {
+		cmsghdr unix.Cmsghdr
+		pktinfo unix.Inet6Pktinfo // big enough
+	}
+
+	var msg unix.Msghdr
+	msg.Iov = &iovec
+	msg.Iovlen = 1
+	msg.Name = (*byte)(unsafe.Pointer(&end.dst))
+	msg.Namelen = uint32(unix.SizeofSockaddrAny)
+	msg.Control = (*byte)(unsafe.Pointer(&cmsg))
+	msg.SetControllen(int(unsafe.Sizeof(cmsg)))
+
+	_, _, errno := unix.Syscall(
+		unix.SYS_RECVMSG,
+		file.Fd(),
+		uintptr(unsafe.Pointer(&msg)),
+		0,
+	)
+
+	if errno != 0 {
+		return errno, nil, nil
+	}
+
+	if cmsg.cmsghdr.Level == unix.IPPROTO_IPV6 &&
+		cmsg.cmsghdr.Type == unix.IPV6_PKTINFO &&
+		cmsg.cmsghdr.Len >= unix.SizeofInet6Pktinfo {
+
+	}
+
+	if cmsg.cmsghdr.Level == unix.IPPROTO_IP &&
+		cmsg.cmsghdr.Type == unix.IP_PKTINFO &&
+		cmsg.cmsghdr.Len >= unix.SizeofInet4Pktinfo {
+
+		info := (*unix.Inet4Pktinfo)(unsafe.Pointer(&cmsg.pktinfo))
+		println(info)
+
+	}
+
+	return nil, nil, nil
+}
+
 func setMark(conn *net.UDPConn, value uint32) error {
 	if conn == nil {
 		return nil
diff --git a/src/misc.go b/src/misc.go
index d93849e..bbe0d68 100644
--- a/src/misc.go
+++ b/src/misc.go
@@ -29,6 +29,11 @@ func (a *AtomicBool) Set(val bool) {
 	atomic.StoreInt32(&a.flag, flag)
 }
 
+func toInt32(n uint32) int32 {
+	mask := uint32(1 << 31)
+	return int32(-(n & mask) + (n & ^mask))
+}
+
 func min(a uint, b uint) uint {
 	if a > b {
 		return b
diff --git a/src/tun_linux.go b/src/tun_linux.go
index 58a762a..accc6c6 100644
--- a/src/tun_linux.go
+++ b/src/tun_linux.go
@@ -120,14 +120,6 @@ func (tun *NativeTun) Name() string {
 	return tun.name
 }
 
-func toInt32(val []byte) int32 {
-	n := binary.LittleEndian.Uint32(val[:4])
-	if n >= (1 << 31) {
-		return -int32(^n) - 1
-	}
-	return int32(n)
-}
-
 func getDummySock() (int, error) {
 	return unix.Socket(
 		unix.AF_INET,
@@ -157,7 +149,8 @@ func getIFIndex(name string) (int32, error) {
 		return 0, errno
 	}
 
-	return toInt32(ifr[unix.IFNAMSIZ:]), nil
+	index := binary.LittleEndian.Uint32(ifr[unix.IFNAMSIZ:])
+	return toInt32(index), nil
 }
 
 func (tun *NativeTun) setMTU(n int) error {
diff --git a/src/uapi.go b/src/uapi.go
index 428b173..3a2f3f9 100644
--- a/src/uapi.go
+++ b/src/uapi.go
@@ -273,8 +273,7 @@ func ipcSetOperation(device *Device, socket *bufio.ReadWriter) *IPCError {
 				}
 
 			case "endpoint":
-				// TODO: Only IP and port
-				addr, err := net.ResolveUDPAddr("udp", value)
+				addr, err := parseEndpoint(value)
 				if err != nil {
 					logError.Println("Failed to set endpoint:", value)
 					return &IPCError{Code: ipcErrorInvalid}