Use IP_BIND_ADDRESS_NO_PORT when dialing the ORPort on Linux.

When the orport-srcaddr option is set, we bind to a source IP address
before dialing the ORPort/ExtORPort. tor similarly binds to a source IP
address when OutboundBindAddress is set in torrc. Since tor 0.4.7.13,
tor sets IP_BIND_ADDRESS_NO_PORT, and because problems arise when some
programs use IP_BIND_ADDRESS_NO_PORT and some do not, we also have to
start using IP_BIND_ADDRESS_NO_PORT when we upgrade tor
(tpo/anti-censorship/pluggable-transports/snowflake#40270).

Related: tpo/anti-censorship/pluggable-transports/snowflake#40198

go.mod

@@ -20,5 +20,6 @@ require (
 	gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/goptlib v1.4.0
 	golang.org/x/crypto v0.6.0
 	golang.org/x/net v0.7.0
+	golang.org/x/sys v0.5.0
 	google.golang.org/protobuf v1.26.0
 )

server/dial.go

@@ -0,0 +1,14 @@
+//go:build !linux
+// +build !linux
+
+package main
+
+import "syscall"
+
+// dialerControl does nothing.
+//
+// On Linux, this function would set the IP_BIND_ADDRESS_NO_PORT socket option
+// in preparation for a future bind-before-connect.
+func dialerControl(network, address string, c syscall.RawConn) error {
+	return nil
+}

server/dial_linux.go

@@ -0,0 +1,45 @@
+//go:build linux
+// +build linux
+
+package main
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// dialerControl prepares a syscall.RawConn for a future bind-before-connect by
+// setting the IP_BIND_ADDRESS_NO_PORT socket option.
+//
+// On Linux, setting the IP_BIND_ADDRESS_NO_PORT socket option helps conserve
+// ephemeral ports when binding to a specific IP addresses before connecting
+// (bind before connect), by not assigning the port number when bind is called,
+// but waiting until connect. But problems arise if there are multiple processes
+// doing bind-before-connect, and some of them use IP_BIND_ADDRESS_NO_PORT and
+// some of them do not. When there is a mix, the ones that do will have their
+// ephemeral ports reserved by the ones that do not, leading to EADDRNOTAVAIL
+// errors.
+//
+// tor does bind-before-connect when the OutboundBindAddress option is set in
+// torrc. Since version 0.4.7.13 (January 2023), tor sets
+// IP_BIND_ADDRESS_NO_PORT unconditionally on platforms that support it, and
+// therefore we must do the same, to avoid EADDRNOTAVAIL errors.
+//
+// # References
+//
+// https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/40201#note_2839472
+// https://forum.torproject.net/t/tor-relays-inet-csk-bind-conflict/5757/10
+// https://blog.cloudflare.com/how-to-stop-running-out-of-ephemeral-ports-and-start-to-love-long-lived-connections/
+// https://blog.cloudflare.com/the-quantum-state-of-a-tcp-port/
+// https://forum.torproject.net/t/stable-release-0-4-5-16-and-0-4-7-13/6216
+func dialerControl(network, address string, c syscall.RawConn) error {
+	var sockErr error
+	err := c.Control(func(fd uintptr) {
+		sockErr = syscall.SetsockoptInt(int(fd), unix.SOL_IP, unix.IP_BIND_ADDRESS_NO_PORT, 1)
+	})
+	if err == nil {
+		err = sockErr
+	}
+	return err
+}

server/server.go

@@ -76,7 +76,9 @@ func handleConn(conn net.Conn, orPortSrcAddr *net.IPNet) error {
 	addr := conn.RemoteAddr().String()
 	statsChannel <- addr != ""
 
-	dialer := net.Dialer{}
+	dialer := net.Dialer{
+		Control: dialerControl,
+	}
 	if orPortSrcAddr != nil {
 		// Use a random source IP address in the given range.
 		ip, err := randIPAddr(orPortSrcAddr)