Commit cbe7d8db authored by Russ Cox's avatar Russ Cox

net: avoid TCP self-connect

Fixes #2690.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5650071
parent f7a36839
......@@ -84,3 +84,34 @@ func TestDialTimeout(t *testing.T) {
}
}
}
func TestSelfConnect(t *testing.T) {
// Test that Dial does not honor self-connects.
// See the comment in DialTCP.
// Find a port that would be used as a local address.
l, err := Listen("tcp", "127.0.0.1:0")
if err != nil {
t.Fatal(err)
}
c, err := Dial("tcp", l.Addr().String())
if err != nil {
t.Fatal(err)
}
addr := c.LocalAddr().String()
c.Close()
l.Close()
// Try to connect to that address repeatedly.
n := 100000
if testing.Short() {
n = 1000
}
for i := 0; i < n; i++ {
c, err := Dial("tcp", addr)
if err == nil {
c.Close()
t.Errorf("#%d: Dial %q succeeded", i, addr)
}
}
}
......@@ -227,13 +227,43 @@ func DialTCP(net string, laddr, raddr *TCPAddr) (*TCPConn, error) {
if raddr == nil {
return nil, &OpError{"dial", net, nil, errMissingAddress}
}
fd, err := internetSocket(net, laddr.toAddr(), raddr.toAddr(), syscall.SOCK_STREAM, 0, "dial", sockaddrToTCP)
// TCP has a rarely used mechanism called a 'simultaneous connection' in
// which Dial("tcp", addr1, addr2) run on the machine at addr1 can
// connect to a simultaneous Dial("tcp", addr2, addr1) run on the machine
// at addr2, without either machine executing Listen. If laddr == nil,
// it means we want the kernel to pick an appropriate originating local
// address. Some Linux kernels cycle blindly through a fixed range of
// local ports, regardless of destination port. If a kernel happens to
// pick local port 50001 as the source for a Dial("tcp", "", "localhost:50001"),
// then the Dial will succeed, having simultaneously connected to itself.
// This can only happen when we are letting the kernel pick a port (laddr == nil)
// and when there is no listener for the destination address.
// It's hard to argue this is anything other than a kernel bug. If we
// see this happen, rather than expose the buggy effect to users, we
// close the fd and try again. If it happens twice more, we relent and
// use the result. See also:
// http://golang.org/issue/2690
// http://stackoverflow.com/questions/4949858/
for i := 0; i < 2 && err == nil && laddr == nil && selfConnect(fd); i++ {
fd.Close()
fd, err = internetSocket(net, laddr.toAddr(), raddr.toAddr(), syscall.SOCK_STREAM, 0, "dial", sockaddrToTCP)
}
if err != nil {
return nil, err
}
return newTCPConn(fd), nil
}
func selfConnect(fd *netFD) bool {
l := fd.laddr.(*TCPAddr)
r := fd.raddr.(*TCPAddr)
return l.Port == r.Port && l.IP.Equal(r.IP)
}
// TCPListener is a TCP network listener.
// Clients should typically use variables of type Listener
// instead of assuming TCP.
......
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