Dial the (Ext)ORPort with a random srcaddr in 127.0.5.0/24.

This is a hack to attempt to conserve emphemeral port numbers.
snowflake#40201 (comment 2839394)

go.mod

@@ -3,3 +3,5 @@ module gitlab.torproject.org/dcf/extor-static-cookie
 go 1.15
 
 require git.torproject.org/pluggable-transports/goptlib.git v1.2.0
+
+replace git.torproject.org/pluggable-transports/goptlib.git v1.2.0 => gitlab.torproject.org/dcf/goptlib v0.0.0-20220930193603-cd79bbc900ad

go.sum

-git.torproject.org/pluggable-transports/goptlib.git v1.2.0 h1:0qRF7Dw5qXd0FtZkjWUiAh5GTutRtDGL4GXUDJ4qMHs=
-git.torproject.org/pluggable-transports/goptlib.git v1.2.0/go.mod h1:4PBMl1dg7/3vMWSoWb46eGWlrxkUyn/CAJmxhDLAlDs=
+gitlab.torproject.org/dcf/goptlib v0.0.0-20220930193603-cd79bbc900ad h1:n3z4mhfahnulwrQRoKR0vJHFdxHSgztu+YVNRFJCYp8=
+gitlab.torproject.org/dcf/goptlib v0.0.0-20220930193603-cd79bbc900ad/go.mod h1:4PBMl1dg7/3vMWSoWb46eGWlrxkUyn/CAJmxhDLAlDs=

main.go

@@ -25,6 +25,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"net"
 	"os"
 	"os/signal"
@@ -160,13 +161,29 @@ func handler(conn *net.TCPConn, ptInfo *pt.ServerInfo, cookie authCookie) error
 		return err
 	}
 
-	or, err := pt.DialOr(ptInfo, metadata.Useraddr, metadata.Transport)
+	or, err := pt.DialOrWithDialer(&net.Dialer{
+		LocalAddr: localAddr(),
+	}, ptInfo, metadata.Useraddr, metadata.Transport)
 	if err != nil {
 		return err
 	}
 	defer or.Close()
 
-	return proxy(conn, or)
+	return proxy(conn, or.(*net.TCPConn))
+}
+
+// localAddr returns a random localhost IP address, suitable to be used as the
+// LocalAddr in a net.Dialer in a call to pt.DialOrWithDialer.
+//
+// The reason for using multiple source addresses when dialing the ORPort is to
+// conserve ephemeral ports:
+// https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/40198
+func localAddr() net.Addr {
+	var b byte
+	for b == 0 {
+		b = byte(rand.Uint32())
+	}
+	return &net.TCPAddr{IP: net.IPv4(127, 0, 5, b)}
 }
 
 // proxy copies from a to b and from b to a, persisting until both copies have