Add support for setting do-not-fragment bit (linux) (#39)

Signed-off-by: Jeremiah Millay <jmillay@fastly.com>

README.md

@@ -110,6 +110,9 @@ This library supports setting the `SO_MARK` socket option which is equivalent to
 flag in standard ping binaries on linux. Setting this option requires the `CAP_NET_ADMIN` capability
 (via `setcap` or elevated privileges). You can set a mark (ex: 100) with `pinger.SetMark(100)` in your code.
 
+Setting the "Don't Fragment" bit is supported under Linux which is equivalent to `ping -Mdo`.
+You can enable this with `pinger.SetDoNotFragment(true)`.
+
 ### Windows
 
 You must use `pinger.SetPrivileged(true)`, otherwise you will receive

packetconn.go

@@ -19,6 +19,7 @@ type packetConn interface {
 	WriteTo(b []byte, dst net.Addr) (int, error)
 	SetTTL(ttl int)
 	SetMark(m uint) error
+	SetDoNotFragment() error
 }
 
 type icmpConn struct {

ping.go

@@ -85,6 +85,7 @@ var (
 	ipv6Proto = map[string]string{"icmp": "ip6:ipv6-icmp", "udp": "udp6"}
 
 	ErrMarkNotSupported = errors.New("setting SO_MARK socket option is not supported on this platform")
+	ErrDFNotSupported   = errors.New("setting do-not-fragment bit is not supported on this platform")
 )
 
 // New returns a new Pinger struct pointer.
@@ -195,6 +196,9 @@ type Pinger struct {
 	// mark is a SO_MARK (fwmark) set on outgoing icmp packets
 	mark uint
 
+	// df when true sets the do-not-fragment bit in the outer IP or IPv6 header
+	df bool
+
 	// trackerUUIDs is the list of UUIDs being used for sending packets.
 	trackerUUIDs []uuid.UUID
 
@@ -414,6 +418,11 @@ func (p *Pinger) Mark() uint {
 	return p.mark
 }
 
+// SetDoNotFragment sets the do-not-fragment bit in the outer IP header to the desired value.
+func (p *Pinger) SetDoNotFragment(df bool) {
+	p.df = df
+}
+
 // Run runs the pinger. This is a blocking function that will exit when it's
 // done. If Count or Interval are not specified, it will run continuously until
 // it is interrupted.
@@ -447,6 +456,12 @@ func (p *Pinger) RunWithContext(ctx context.Context) error {
 		}
 	}
 
+	if p.df {
+		if err := conn.SetDoNotFragment(); err != nil {
+			return fmt.Errorf("error setting do-not-fragment: %v", err)
+		}
+	}
+
 	conn.SetTTL(p.TTL)
 	return p.run(ctx, conn)
 }

ping_test.go

@@ -644,6 +644,7 @@ func (c testPacketConn) SetFlagTTL() error                 { return nil }
 func (c testPacketConn) SetReadDeadline(t time.Time) error { return nil }
 func (c testPacketConn) SetTTL(t int)                      {}
 func (c testPacketConn) SetMark(m uint) error              { return nil }
+func (c testPacketConn) SetDoNotFragment() error           { return nil }
 
 func (c testPacketConn) ReadFrom(b []byte) (n int, ttl int, src net.Addr, err error) {
 	return 0, 0, nil, nil

utils_linux.go

@@ -65,6 +65,42 @@ func (c *icmpV6Conn) SetMark(mark uint) error {
 	)
 }
 
+// SetDoNotFragment sets the do-not-fragment bit in the IP header of outgoing ICMP packets.
+func (c *icmpConn) SetDoNotFragment() error {
+	fd, err := getFD(c.c)
+	if err != nil {
+		return err
+	}
+	return os.NewSyscallError(
+		"setsockopt",
+		syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO),
+	)
+}
+
+// SetDoNotFragment sets the do-not-fragment bit in the IP header of outgoing ICMP packets.
+func (c *icmpv4Conn) SetDoNotFragment() error {
+	fd, err := getFD(c.icmpConn.c)
+	if err != nil {
+		return err
+	}
+	return os.NewSyscallError(
+		"setsockopt",
+		syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO),
+	)
+}
+
+// SetDoNotFragment sets the do-not-fragment bit in the IPv6 header of outgoing ICMPv6 packets.
+func (c *icmpV6Conn) SetDoNotFragment() error {
+	fd, err := getFD(c.icmpConn.c)
+	if err != nil {
+		return err
+	}
+	return os.NewSyscallError(
+		"setsockopt",
+		syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_MTU_DISCOVER, syscall.IP_PMTUDISC_DO),
+	)
+}
+
 // getFD gets the system file descriptor for an icmp.PacketConn
 func getFD(c *icmp.PacketConn) (uintptr, error) {
 	v := reflect.ValueOf(c).Elem().FieldByName("c").Elem()

utils_other.go

@@ -30,3 +30,18 @@ func (c *icmpv4Conn) SetMark(mark uint) error {
 func (c *icmpV6Conn) SetMark(mark uint) error {
 	return ErrMarkNotSupported
 }
+
+// SetDoNotFragment sets the do-not-fragment bit in the IP header of outgoing ICMP packets.
+func (c *icmpConn) SetDoNotFragment() error {
+	return ErrDFNotSupported
+}
+
+// SetDoNotFragment sets the do-not-fragment bit in the IP header of outgoing ICMP packets.
+func (c *icmpv4Conn) SetDoNotFragment() error {
+	return ErrDFNotSupported
+}
+
+// SetDoNotFragment sets the do-not-fragment bit in the IPv6 header of outgoing ICMPv6 packets.
+func (c *icmpV6Conn) SetDoNotFragment() error {
+	return ErrDFNotSupported
+}

utils_windows.go

@@ -41,3 +41,18 @@ func (c *icmpv4Conn) SetMark(mark uint) error {
 func (c *icmpV6Conn) SetMark(mark uint) error {
 	return ErrMarkNotSupported
 }
+
+// SetDoNotFragment sets the do-not-fragment bit in the IP header of outgoing ICMP packets.
+func (c *icmpConn) SetDoNotFragment() error {
+	return ErrDFNotSupported
+}
+
+// SetDoNotFragment sets the do-not-fragment bit in the IP header of outgoing ICMP packets.
+func (c *icmpv4Conn) SetDoNotFragment() error {
+	return ErrDFNotSupported
+}
+
+// SetDoNotFragment sets the do-not-fragment bit in the IPv6 header of outgoing ICMPv6 packets.
+func (c *icmpV6Conn) SetDoNotFragment() error {
+	return ErrDFNotSupported
+}