NAME¶

tcpdrop - Trace kernel-based TCP packet drops with details. Uses Linux eBPF/bcc.

SYNOPSIS¶

tcpdrop [-4 | -6] [-h]

DESCRIPTION¶

This tool traces TCP packets or segments that were dropped by the kernel, and shows details from the IP and TCP headers, the socket state, and the kernel stack trace. This is useful for debugging cases of high kernel drops, which can cause timer-based retransmits and performance issues.

This tool works using dynamic tracing of the tcp_drop() kernel function, which requires a recent kernel version.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS¶

CONFIG_BPF and bcc.

OPTIONS¶

-4

Trace IPv4 family only.

-6

Trace IPv6 family only.

-h

Print usage message.

EXAMPLES¶

Trace kernel-based TCP packet drops with details:

# tcpdrop

Trace IPv4 family only:

# tcpdrop -4

Trace IPv6 family only:

# tcpdrop -6

FIELDS¶

TIME

Time of the drop, in HH:MM:SS format.

PID

Process ID that was on-CPU during the drop. This may be unrelated, as drops can occur on the receive interrupt and be unrelated to the PID that was interrupted.

IP

IP address family (4 or 6)

SADDR

Source IP address.

SPORT

Source TCP port.

DADDR

Destination IP address.

DPORT

Destionation TCP port.

STATE

TCP session state ("ESTABLISHED", etc).

FLAGS

TCP flags ("SYN", etc).

OVERHEAD¶

This traces the kernel tcp_drop() function, which should be low frequency, and therefore the overhead of this tool should be negligible.

As always, test and understand this tools overhead for your types of workloads before production use.

SOURCE¶

This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS¶

Linux

AUTHOR¶

Brendan Gregg

SEE ALSO¶

tcplife(8), tcpaccept(8), tcpconnect(8), tcptop(8)