Quoting Thom van der Boon who wrote on Tue 2019-07-30 at 15:00:

> This looks like all the port 37 related traffic is coming from one provider 
> (Private customers of KPN) 

Another nl pool operator here.

Capturing incoming port 37 traffic for 60 seconds gave me:

# tcpdump -G 60 -W 1 -w zerofile -pni eth0 port 37
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size
262144 bytes
Maximum file limit reached: 1
66841 packets captured
68022 packets received by filter
0 packets dropped by kernel

Resolving the source IPs gives me

.speed.planet.nl
.direct-adsl.nl
.ip.telfort.nl
static.kpn.net

All related to KPN

And very rarely another reverse name.

One sampled IP does an UDP/37 request every 4 seconds, but no UDP/123
requests during a long sample.

                                             Koos

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