---
name: "tcpsynbl-bpfcc"
section: "8"
platform: "linux"
package: "bpfcc-tools"
version: "0.31.0+ds-7"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/tcpsynbl-bpfcc"
---

# tcpsynbl-bpfcc(8)

Show the TCP SYN backlog as a histogram. Uses BCC/eBPF.

## NAME

tcpsynbl - Show the TCP SYN backlog as a histogram. Uses BCC/eBPF.

## SYNOPSIS

**tcpsynbl \[-4 \| -6\]**

## DESCRIPTION

This tool shows the TCP SYN backlog size during SYN arrival as a histogram. This lets you see how close your applications are to hitting the backlog limit and dropping SYNs (causing performance issues with SYN retransmits), and is a measure of workload saturation. The histogram shown is measured at the time of SYN received, and a separate histogram is shown for each backlog limit.

This works by tracing the tcp\_v4\_syn\_recv\_sock() and tcp\_v6\_syn\_recv\_sock() kernel functions using dynamic instrumentation. Since these functions may change in future kernels, this tool may need maintenance to keep working.

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

## REQUIREMENTS

CONFIG\_BPF and BCC.

## OPTIONS

- \-h

  Print usage message.

- \-4

  Trace IPv4 family only.

- \-6

  Trace IPv6 family only.

## EXAMPLES

- Show the TCP SYN backlog as a histogram.

  \# **tcpsynbl**

- Trace IPv4 family only:

  \# **tcpsynbl -4**

- Trace IPv6 family only:

  \# **tcpsynbl -6**

## FIELDS

- backlog

  The backlog size when a SYN was received.

- count

  The number of times this backlog size was encountered.

- distribution

  An ASCII visualization of the count column.

## OVERHEAD

Inbound SYNs should be relatively low compared to packets and other events, so the overhead of this tool is expected to be negligible.

## SOURCE

This originated as a bpftrace tool from the book "BPF Performance Tools", published by Addison Wesley (2019):

-

  http://www.brendangregg.com/bpf-performance-tools-book.html

See the book for more documentation on this tool.

This version is in the BCC repository:

-

  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

## STABILITY

Unstable - in development.

## AUTHOR

Brendan Gregg

## SEE ALSO

tcptop(8)