tcpdump：snaplen 设置为 0 但仍然得到“捕获期间数据包大小受限”？

Question

由于这个问题，我将在 MySQL 服务器后端嗅探一些数据包，看看会发生什么：

# tcpdump -vv -s0 -c 100 -i bond0 tcp port 3306 and host 192.168.3.87 -w /home/quanta/3.87_aborted.pcap

如您所见，我将 snaplen 设置为 0，但仍然收到消息“捕获期间数据包大小受限”：

MySQL Protocol
    Packet Length: 579
    Packet Number: 96
    text: 336
    text: 1004352
    text: 147619
    text: 336
[Packet size limited during capture: MySQL truncated]

我的界面上的卸载设置：

# ethtool -k bond0
Offload parameters for bond0:
Cannot get device rx csum settings: Operation not supported
rx-checksumming: off
tx-checksumming: on
scatter-gather: on
tcp segmentation offload: on
udp fragmentation offload: off
generic segmentation offload: off
generic-receive-offload: off

---

/proc/net/bonding/bond0：

Ethernet Channel Bonding Driver: v3.4.0-1 (October 7, 2008)

Bonding Mode: adaptive load balancing
Primary Slave: None
Currently Active Slave: eth1
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: eth1
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:30:48:c3:20:be

---

回复@HeatfanJohn：

# ethtool -k eth1
Offload parameters for eth1:
Cannot get device udp large send offload settings: Operation not supported
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp segmentation offload: on
udp fragmentation offload: off
generic segmentation offload: off
generic-receive-offload: on

---

回复@Doon:

bond0     UP BROADCAST RUNNING MASTER MULTICAST  MTU:1500  Metric:1

嗅探-s 1500还显示：

 41  11.371783  192.168.6.7 -> 192.168.3.87 MySQL Response OK
 42  11.371974 192.168.3.87 -> 192.168.6.7  MySQL Request Ping
 43  11.371996  192.168.6.7 -> 192.168.3.87 MySQL Response OK
 44  11.378562 192.168.3.87 -> 192.168.6.7  MySQL Request Query
 45  11.379604  192.168.6.7 -> 192.168.3.87 MySQL Response
 46  11.402431 192.168.3.87 -> 192.168.6.7  MySQL Request Query
 47  11.402992  192.168.6.7 -> 192.168.3.87 MySQL Response[Packet size limited during capture]

Answer 1

这是答案形式，措辞正确:)我会在禁用分段卸载的情况下尝试此操作。启用它后，tcpdump 很可能无法获得它所期望的结果。

——马特