Fix ICMPv4 EchoReply packet checksum

The checksum was not being reset before being re-calculated and sent out.
This caused the sent checksum to always be `0x0800`.

Fixes #605.

PiperOrigin-RevId: 260965059

2 files changed, 3 insertions, 3 deletions

diff --git a/pkg/tcpip/network/ipv4/icmp.go b/pkg/tcpip/network/ipv4/icmp.go
index fbef6947d..497164cbb 100644
--- a/pkg/tcpip/network/ipv4/icmp.go
+++ b/pkg/tcpip/network/ipv4/icmp.go
@@ -94,6 +94,7 @@ func (e *endpoint) handleICMP(r *stack.Route, netHeader buffer.View, vv buffer.V
 		pkt := header.ICMPv4(hdr.Prepend(header.ICMPv4MinimumSize))
 		copy(pkt, h)
 		pkt.SetType(header.ICMPv4EchoReply)
+		pkt.SetChecksum(0)
 		pkt.SetChecksum(^header.Checksum(pkt, header.ChecksumVV(vv, 0)))
 		sent := stats.ICMP.V4PacketsSent
 		if err := r.WritePacket(nil /* gso */, hdr, vv, header.ICMPv4ProtocolNumber, r.DefaultTTL()); err != nil {
diff --git a/test/syscalls/linux/raw_socket_icmp.cc b/test/syscalls/linux/raw_socket_icmp.cc
index ad19120d5..1c07bacc2 100644
--- a/test/syscalls/linux/raw_socket_icmp.cc
+++ b/test/syscalls/linux/raw_socket_icmp.cc
@@ -470,9 +470,8 @@ void RawSocketICMPTest::ExpectICMPSuccess(const struct icmphdr& icmp) {
         EXPECT_EQ(recvd_icmp->un.echo.id, icmp.un.echo.id);
         // A couple are different.
         EXPECT_EQ(recvd_icmp->type, ICMP_ECHOREPLY);
-        // The checksum is computed in such a way that it is guaranteed to have
-        // changed.
-        EXPECT_NE(recvd_icmp->checksum, icmp.checksum);
+        // The checksum computed over the reply should still be valid.
+        EXPECT_EQ(Checksum(recvd_icmp), 0);
         break;
     }
   }