What buffer space is related to "sendto failed: No buffer space available" log entries?

[nusenu]

Hi,

I'm trying to solve the following issue:

unbound: notice: sendto failed: No buffer space available
unbound: notice: remote address is .... port 53

OS: FreeBSD 12.0
unbound 1.8.3

The number next to "dropped due to full socket buffers"
in the output of "netstat -s -p udp"
does not increase when this happens, so I assume it is not related to UDP but about outbound TCP queries?

so far I tried:

1) so-sndbuf: 4m + related kern.ipc.maxsockbuf sysctl setting  (UDP)

2) net.inet.tcp.sendbuf_max=16777216
net.inet.tcp.sendbuf_inc=16384

3) outgoing-num-tcp: 20 (50 and 100)

but the issue persists.

What buffer is the log entry about and what setting could be changed to address this?

On FreeBSD 11.2 this issue did not occur (same amount of qps).

thanks,
nusenu

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