キャプチャ時にでる、UDP bad checksum エラー ( UDP checksum offload ) について

最近のNIC はNICでチェックサムを行っており、system(OS)パケットが送られた後にcheksum　を行う

    ここでキャプチャ

system(OS) -----------------> NIC ---------->

        dummy checksum           本当の checksum

dummy checksum のときにキャプチャしているので、cheksum エラーがでる

送信で bad checksum が出ていて、受信で bad checksum でなかったら、

NIC の問題ではなく、上記が原因であることがほとんど。

受信は checksum 後にキャプチャしているから。

この場合は無視してOK

- 送信　( Bad checksum )

    Source: 192.168.1.1 (192.168.1.1)

    Destination: 128.63.2.53 (128.63.2.53)

User Datagram Protocol, Src Port: 33978 (33978), Dst Port: domain (53)

    Source port: 33978 (33978)

    Destination port: domain (53)

    Length: 53

    Checksum: 0x8975 [incorrect, should be 0x29bb (maybe caused by "UDP checksum offload"?)]

        [Good Checksum: False]

        [Bad Checksum: True]

- 受信 ( Good checksum )

    Source: 192.5.5.241 (192.5.5.241)

    Destination: 192.168.1.1 (192.168.1.1)

User Datagram Protocol, Src Port: domain (53), Dst Port: 38562 (38562)

    Source port: domain (53)

    Destination port: 38562 (38562)

    Length: 837

    Checksum: 0xceb4 [correct]

        [Good Checksum: True]

        [Bad Checksum: False]

エラーはカウントされていない

root@arizona:~# LANG=C netstat -s

Ip:

    17708 total packets received

    0 forwarded

    0 incoming packets discarded

    17454 incoming packets delivered

    12100 requests sent out

Icmp:

    9 ICMP messages received

    0 input ICMP message failed.

    ICMP input histogram:

        destination unreachable: 9

    9 ICMP messages sent

    0 ICMP messages failed

    ICMP output histogram:

        destination unreachable: 9

IcmpMsg:

        InType3: 9

        OutType3: 9

Tcp:

    23 active connections openings

    3 passive connection openings

    10 failed connection attempts

    2 connection resets received

    2 connections established

    17115 segments received

    11879 segments send out

    0 segments retransmited

    0 bad segments received.

    165 resets sent

Udp:

    226 packets received

    9 packets to unknown port received.

    0 packet receive errors

    216 packets sent

UdpLite:

TcpExt:

    10 TCP sockets finished time wait in fast timer

    105 delayed acks sent

    2 packets directly queued to recvmsg prequeue.

    13717 packet headers predicted

    1499 acknowledgments not containing data payload received

    477 predicted acknowledgments

    1 connections reset due to early user close

IpExt:

    InMcastPkts: 52

    OutMcastPkts: 37

    InBcastPkts: 103

    InOctets: 16864327

    OutOctets: 10701647

    InMcastOctets: 9908

    OutMcastOctets: 7260

    InBcastOctets: 12159

root@arizona:~#