feat: tlsmiddlebox experiment (#817)

See https://github.com/ooni/probe/issues/2124

internal/engine/experiment/tlsmiddlebox/syscall_unix.go

@@ -0,0 +1,53 @@
+//go:build aix || darwin || dragonfly || freebsd || (js && wasm) || linux || nacl || netbsd || openbsd || solaris
+
+package tlsmiddlebox
+
+//
+// syscall utilities for dialerTTLWrapperConn
+//
+
+import (
+	"net"
+	"syscall"
+)
+
+// SetTTL sets the IP TTL field for the underlying net.TCPConn
+func (c *dialerTTLWrapperConn) SetTTL(ttl int) error {
+	conn := c.Conn
+	tcpConn, ok := conn.(*net.TCPConn)
+	if !ok {
+		return errInvalidConnWrapper
+	}
+	rawConn, err := tcpConn.SyscallConn()
+	if err != nil {
+		return err
+	}
+	rawErr := rawConn.Control(func(fd uintptr) {
+		err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_TTL, ttl)
+	})
+	// The syscall err is given a higher priority and returned early if non-nil
+	if err != nil {
+		return err
+	}
+	return rawErr
+}
+
+// GetSoErr fetches the SO_ERROR value to look for soft ICMP errors in TCP
+func (c *dialerTTLWrapperConn) GetSoErr() (errno int, err error) {
+	conn := c.Conn
+	tcpConn, ok := conn.(*net.TCPConn)
+	if !ok {
+		return 0, errInvalidConnWrapper
+	}
+	rawConn, err := tcpConn.SyscallConn()
+	if err != nil {
+		return 0, errInvalidConnWrapper
+	}
+	rawErr := rawConn.Control(func(fd uintptr) {
+		errno, err = syscall.GetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_ERROR)
+	})
+	if rawErr != nil {
+		return 0, rawErr
+	}
+	return
+}