wireshark抓包大于1500字节和提示checksum offload的原因

Posted 2020-10-21

问题：

1. wireshark抓包大于1500字节（如下图所示）
2. wireshark抓包提示：[incorrect, should be xxxx (maybe caused by "TCP checksum offload"?)]

原因：
wireshark是在数据包经过cpu，送到网卡处理之前抓取的。现在操作系统上的网卡大多有一个功能，帮助cpu减轻负担，网卡承担了将segment分段和对数据包进行checksum的工作。
wireshark抓取的数据包还需要网卡进行下一步的分段和checksum，所以我们看到数据包有2000多字节。

解决办法：
disable操作系统网卡的large sender offload 和 check sum offload功能。

1. windows操作。在控制面板，网络连接，网卡属性，配置，高级设置，windows 10 截图如下：

2. Linux操作。

          查看：ethtool --show-offload  ethX

+++++++++++++++++++++++++++++++++++++++++++++++
[[email protected] ~]# ethtool --show-offload ens33
Features for ens33:
rx-checksumming: off
tx-checksumming: on
tx-checksum-ipv4: off [fixed]
tx-checksum-ip-generic: on
tx-checksum-ipv6: off [fixed]
tx-checksum-fcoe-crc: off [fixed]
tx-checksum-sctp: off [fixed]
scatter-gather: on
tx-scatter-gather: on
tx-scatter-gather-fraglist: off [fixed]
tcp-segmentation-offload: on
tx-tcp-segmentation: on
tx-tcp-ecn-segmentation: off [fixed]
tx-tcp6-segmentation: off [fixed]
tx-tcp-mangleid-segmentation: off
+++++++++++++++++++++++++++++++++++++++++++++++

     配置取消网卡分段功能:  ethtool -K ens33 tso off

+++++++++++++++++++++++++++++++++++++++++++++++
[[email protected] ~]# ethtool -K ens33 tso off
[[email protected] ~]# ethtool --show-offload ens33
Features for ens33:
rx-checksumming: off
tx-checksumming: on
tx-checksum-ipv4: off [fixed]
tx-checksum-ip-generic: on
tx-checksum-ipv6: off [fixed]
tx-checksum-fcoe-crc: off [fixed]
tx-checksum-sctp: off [fixed]
scatter-gather: on
tx-scatter-gather: on
tx-scatter-gather-fraglist: off [fixed]
tcp-segmentation-offload: off
tx-tcp-segmentation: off
tx-tcp-ecn-segmentation: off [fixed]
tx-tcp6-segmentation: off [fixed]
tx-tcp-mangleid-segmentation: off
+++++++++++++++++++++++++++++++++++++++++++++++

参考wireshark wiki : https://wiki.wireshark.org/CaptureSetup/Offloading