DLT_DOCSIS on every Ethernet interface in Linux

[Denis Ovsienko <denis () ovsienko info>]

Hello all.

libpcap 0.9.1 (2005) introduced (see commit 619a9fe) a Linux-specific
feature to present an Ethernet interface as DLT_DOCSIS:

# tcpdump -L -i eth0
Data link types for eth0 (use option -y to set):
  EN10MB (Ethernet)
  DOCSIS (DOCSIS) (printing not supported)

As [1] and [2] explain it, the only hardware capable of producing such
frames is Cisco uBR7100, uBR7200 and uBR10000.  As far as I could
gather, that hardware went end-of-sale and end-of-support as follows:

* uBR7100: 2008-04-25 and 2013-04-26 respectively
* uBR10000: 2018-08-24 and 2023-08-31 respectively

For uBR7200 I could not find an announcement for the router, but its
various modules went end-of-support on various dates between 2014 and
2021.

Is anybody aware of any real-world use case that still depends on this
feature?  I suggest to "#if 0" it one of the next libpcap releases and
if nobody complains in a year, remove it.  Or perhaps it could become
an off-by-default build option first.

1: https://wiki.wireshark.org/CaptureSetup/DOCSIS
2:
https://www.cisco.com/c/en/us/tech/broadband-cable/cable-modem-termination-systems-cmts/index.html

-- 
    Denis Ovsienko
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