feat: support setting ICMP source address for outgoing packets

Previously, the source IP for ICMP echo requests was determined by
system routing. This change applies the Pinger.Source value—already
used when binding the socket—to outgoing control messages as well,
allowing users to specify which local address ICMP packets originate
from.

This is useful on multi-interface systems or when testing routing
behavior from specific local addresses.

Signed-off-by: Steven Normore <[email protected]>

Author: snormore

packetconn.go

@@ -22,6 +22,7 @@ type packetConn interface {
 	SetDoNotFragment() error
 	SetBroadcastFlag() error
 	SetIfIndex(ifIndex int)
+	SetSource(source net.IP)
 	SetTrafficClass(uint8) error
 	InstallICMPIDFilter(id int) error
 }
@@ -30,6 +31,7 @@ type icmpConn struct {
 	c       *icmp.PacketConn
 	ttl     int
 	ifIndex int
+	source  net.IP
 }
 
 func (c *icmpConn) Close() error {
@@ -44,6 +46,10 @@ func (c *icmpConn) SetIfIndex(ifIndex int) {
 	c.ifIndex = ifIndex
 }
 
+func (c *icmpConn) SetSource(source net.IP) {
+	c.source = source
+}
+
 func (c *icmpConn) SetReadDeadline(t time.Time) error {
 	return c.c.SetReadDeadline(t)
 }
@@ -86,6 +92,13 @@ func (c *icmpv4Conn) WriteTo(b []byte, dst net.Addr) (int, error) {
 		cm = &ipv4.ControlMessage{IfIndex: c.ifIndex}
 	}
 
+	if c.source != nil {
+		if cm == nil {
+			cm = &ipv4.ControlMessage{}
+		}
+		cm.Src = c.source
+	}
+
 	return c.c.IPv4PacketConn().WriteTo(b, cm, dst)
 }
 
@@ -131,6 +144,13 @@ func (c *icmpV6Conn) WriteTo(b []byte, dst net.Addr) (int, error) {
 		cm = &ipv6.ControlMessage{IfIndex: c.ifIndex}
 	}
 
+	if c.source != nil {
+		if cm == nil {
+			cm = &ipv6.ControlMessage{}
+		}
+		cm.Src = c.source
+	}
+
 	return c.c.IPv6PacketConn().WriteTo(b, cm, dst)
 }
 

ping.go

@@ -565,6 +565,15 @@ func (p *Pinger) RunWithContext(ctx context.Context) error {
 		}
 		conn.SetIfIndex(iface.Index)
 	}
+
+	if p.Source != "" {
+		ip := net.ParseIP(p.Source)
+		if ip == nil {
+			return fmt.Errorf("invalid source address: %s", p.Source)
+		}
+		conn.SetSource(ip)
+	}
+
 	return p.run(ctx, conn)
 }
 

ping_test.go

@@ -498,6 +498,22 @@ func TestSetInterfaceName(t *testing.T) {
 	AssertError(t, err, "device not found")
 }
 
+func TestSetSource(t *testing.T) {
+	pinger := New("localhost")
+	pinger.Count = 1
+	pinger.Timeout = time.Second
+
+	// Set source address
+	pinger.Source = "127.0.0.1"
+	err := pinger.Run()
+	AssertNoError(t, err)
+
+	// Set source to invalid IP
+	pinger.Source = "invalid-ip"
+	err = pinger.Run()
+	AssertError(t, err, "invalid source address: invalid-ip")
+}
+
 // Test helpers
 func makeTestPinger() *Pinger {
 	pinger := New("127.0.0.1")
@@ -679,6 +695,7 @@ func (c testPacketConn) SetDoNotFragment() error           { return nil }
 func (c testPacketConn) SetBroadcastFlag() error           { return nil }
 func (c testPacketConn) InstallICMPIDFilter(id int) error  { return nil }
 func (c testPacketConn) SetIfIndex(ifIndex int)            {}
+func (c testPacketConn) SetSource(source net.IP)           {}
 func (c testPacketConn) SetTrafficClass(uint8) error       { return nil }
 
 func (c testPacketConn) ReadFrom(b []byte) (n int, ttl int, src net.Addr, err error) {