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dhcp4: bind listeners to 0.0.0.0 and filter based on received interface.
Some DHCP messages are broadcast to 255.255.255.255, and binding to a specific address filters those packets out. Instead, we have to translate the "listen on ip:port" intent to "listen on interface:port", and filter based on the interface that received the packet. Fixes #27.
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441857b277
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@ -16,6 +16,7 @@ package dhcp4
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import (
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import (
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"errors"
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"errors"
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"fmt"
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"io"
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"io"
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"net"
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"net"
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"time"
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"time"
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@ -62,16 +63,66 @@ type conn interface {
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//
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//
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// Multiple goroutines may invoke methods on a Conn simultaneously.
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// Multiple goroutines may invoke methods on a Conn simultaneously.
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type Conn struct {
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type Conn struct {
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conn conn
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conn conn
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ifIndex int
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}
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}
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// NewConn creates a Conn bound to the given UDP ip:port.
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// NewConn creates a Conn bound to the given UDP ip:port.
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func NewConn(addr string) (*Conn, error) {
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func NewConn(addr string) (*Conn, error) {
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c, err := newPortableConn(addr)
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return newConn(addr, newPortableConn)
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}
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func newConn(addr string, n func(int) (conn, error)) (*Conn, error) {
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if addr == "" {
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addr = "0.0.0.0:67"
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}
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ifIndex := 0
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udpAddr, err := net.ResolveUDPAddr("udp4", addr)
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if err != nil {
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if err != nil {
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return nil, err
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return nil, err
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}
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}
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return &Conn{c}, nil
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if !udpAddr.IP.To4().Equal(net.IPv4zero) {
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// Caller wants to listen only on one address. However, DHCP
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// packets are frequently broadcast, so we can't just listen
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// on the given address. Instead, we need to translate it to
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// an interface, and then filter incoming packets based on
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// their received interface.
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ifIndex, err = ipToIfindex(udpAddr.IP)
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if err != nil {
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return nil, err
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}
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}
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c, err := n(udpAddr.Port)
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if err != nil {
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return nil, err
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}
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return &Conn{
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conn: c,
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ifIndex: ifIndex,
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}, nil
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}
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func ipToIfindex(ip net.IP) (int, error) {
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intfs, err := net.Interfaces()
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if err != nil {
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return 0, err
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}
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for _, intf := range intfs {
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addrs, err := intf.Addrs()
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if err != nil {
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return 0, err
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}
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for _, addr := range addrs {
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if ipnet, ok := addr.(*net.IPNet); ok {
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if ipnet.IP.Equal(ip) {
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return intf.Index, nil
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}
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}
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}
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}
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return 0, fmt.Errorf("IP %s not found on any local interface", ip)
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}
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}
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// Close closes the DHCP socket.
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// Close closes the DHCP socket.
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@ -89,6 +140,9 @@ func (c *Conn) RecvDHCP() (*Packet, *net.Interface, error) {
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if err != nil {
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if err != nil {
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return nil, nil, err
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return nil, nil, err
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}
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}
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if c.ifIndex != 0 && ifidx != c.ifIndex {
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continue
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}
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pkt, err := Unmarshal(b)
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pkt, err := Unmarshal(b)
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if err != nil {
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if err != nil {
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continue
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continue
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@ -97,6 +151,7 @@ func (c *Conn) RecvDHCP() (*Packet, *net.Interface, error) {
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if err != nil {
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if err != nil {
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return nil, nil, err
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return nil, nil, err
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}
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}
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// TODO: possibly more validation that the source lines up
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// TODO: possibly more validation that the source lines up
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// with what the packet says.
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// with what the packet says.
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return pkt, intf, nil
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return pkt, intf, nil
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@ -157,11 +212,8 @@ type portableConn struct {
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conn *ipv4.PacketConn
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conn *ipv4.PacketConn
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}
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}
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func newPortableConn(addr string) (conn, error) {
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func newPortableConn(port int) (conn, error) {
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if addr == "" {
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c, err := net.ListenPacket("udp4", fmt.Sprintf(":%d", port))
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addr = ":67"
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}
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c, err := net.ListenPacket("udp4", addr)
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if err != nil {
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if err != nil {
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return nil, err
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return nil, err
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}
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}
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@ -41,27 +41,12 @@ type linuxConn struct {
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// Unlike NewConn, NewSnooperConn does not bind to the ip:port,
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// Unlike NewConn, NewSnooperConn does not bind to the ip:port,
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// enabling the Conn to coexist with other services on the machine.
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// enabling the Conn to coexist with other services on the machine.
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func NewSnooperConn(addr string) (*Conn, error) {
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func NewSnooperConn(addr string) (*Conn, error) {
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c, err := newLinuxConn(addr)
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return newConn(addr, newLinuxConn)
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if err != nil {
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return nil, err
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}
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return &Conn{c}, nil
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}
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}
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func newLinuxConn(addr string) (conn, error) {
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func newLinuxConn(port int) (conn, error) {
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if addr == "" {
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if port == 0 {
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addr = "0.0.0.0:67"
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return nil, errors.New("must specify a listen port")
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}
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udpAddr, err := net.ResolveUDPAddr("udp4", addr)
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if err != nil {
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return nil, err
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}
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if udpAddr.IP != nil && udpAddr.IP.To4() == nil {
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return nil, fmt.Errorf("%s is not an IPv4 address", addr)
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}
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udpAddr.IP = udpAddr.IP.To4()
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if udpAddr.Port == 0 {
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return nil, fmt.Errorf("%s must specify a listen port", addr)
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}
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}
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filter, err := bpf.Assemble([]bpf.Instruction{
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filter, err := bpf.Assemble([]bpf.Instruction{
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@ -70,7 +55,7 @@ func newLinuxConn(addr string) (conn, error) {
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// Get UDP dport
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// Get UDP dport
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bpf.LoadIndirect{Off: 2, Size: 2},
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bpf.LoadIndirect{Off: 2, Size: 2},
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// Correct dport?
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// Correct dport?
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bpf.JumpIf{Cond: bpf.JumpEqual, Val: uint32(udpAddr.Port), SkipFalse: 1},
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bpf.JumpIf{Cond: bpf.JumpEqual, Val: uint32(port), SkipFalse: 1},
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// Accept
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// Accept
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bpf.RetConstant{Val: 1500},
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bpf.RetConstant{Val: 1500},
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// Ignore
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// Ignore
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@ -80,7 +65,7 @@ func newLinuxConn(addr string) (conn, error) {
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return nil, err
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return nil, err
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}
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}
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c, err := net.ListenPacket("ip4:17", udpAddr.IP.String())
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c, err := net.ListenPacket("ip4:17", "0.0.0.0")
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if err != nil {
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if err != nil {
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return nil, err
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return nil, err
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}
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}
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@ -99,7 +84,7 @@ func newLinuxConn(addr string) (conn, error) {
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}
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}
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ret := &linuxConn{
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ret := &linuxConn{
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port: uint16(udpAddr.Port),
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port: uint16(port),
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conn: r,
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conn: r,
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}
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}
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return ret, nil
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return ret, nil
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@ -24,7 +24,7 @@ import (
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)
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)
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func testConn(t *testing.T, impl conn, addr string) {
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func testConn(t *testing.T, impl conn, addr string) {
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c := &Conn{impl}
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c := &Conn{impl, 0}
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s, err := net.Dial("udp4", addr)
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s, err := net.Dial("udp4", addr)
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if err != nil {
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if err != nil {
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@ -121,10 +121,11 @@ func TestPortableConn(t *testing.T) {
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if err != nil {
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if err != nil {
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t.Fatal(err)
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t.Fatal(err)
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}
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}
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port := l.LocalAddr().(*net.UDPAddr).Port
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addr := l.LocalAddr().String()
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addr := l.LocalAddr().String()
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l.Close()
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l.Close()
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c, err := newPortableConn(addr)
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c, err := newPortableConn(port)
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if err != nil {
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if err != nil {
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t.Fatalf("creating the conn: %s", err)
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t.Fatalf("creating the conn: %s", err)
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}
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}
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@ -146,7 +147,7 @@ func TestLinuxConn(t *testing.T) {
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if err != nil {
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if err != nil {
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t.Fatal(err)
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t.Fatal(err)
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}
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}
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c, err := newLinuxConn(l.LocalAddr().String())
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c, err := newLinuxConn(l.LocalAddr().(*net.UDPAddr).Port)
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if err != nil {
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if err != nil {
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t.Fatalf("creating the linuxconn: %s", err)
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t.Fatalf("creating the linuxconn: %s", err)
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}
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}
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