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Connectivity testing creates lots of failed neighbor entries

On a fresh installation of Fedora 30 on my laptop, I get lots of failed neighbor entries for libvirt's bridge (virbr0) after only a few minutes. On my desktop (also Fedora 30), I get them for my docker bridges, my libvirt bridge, and my disconnected ethernet interface (I have two ethernet interfaces). The addresses all reverse-resolve to Fedora Project domains.

If I recall correctly, this is because Linux considers all IP addresses to be on-link when an interface has no configured routes. So, when NM comes around to check for connectivity on those interfaces (my inactive ethernet NIC is unplugged, with state DOWN, so there shouldn't be any reason to test it for connectivity as far as I can tell), Linux adds the IPs to the neighbor table and marks them failed.

This is benign enough on my laptop, but after a long time, this overflows the neighbor table on my desktop, which has many interfaces, and causes dmesg to start complaining of neighbor table overflows constantly. I discovered this when I was debugging horrible network performance on my desktop. I'm still not sure if this is related to that performance issue, but I figured I'd report it anyway.

Here's an example of the output of ip -r -4 neigh after about an hour of time logged in on my desktop, with all the bridges disabled (still shows my disconnected interface):

proxy01.fedoraproject.org dev enp8s0  FAILED
_gateway dev eno1 lladdr f0:9f:c2:68:58:7d REACHABLE
proxy10.fedoraproject.org dev enp8s0  FAILED
vm3.fedora.ibiblio.org dev enp8s0  FAILED
67.203.2.67.rdns.colocationamerica.com dev enp8s0  FAILED
proxy06.fedoraproject.org dev enp8s0  FAILED
67.219.144.68 dev enp8s0  FAILED
proxy14.fedoraproject.org dev enp8s0  FAILED
proxy09.fedoraproject.org dev enp8s0  FAILED
proxy13-rdu02.fedoraproject.org dev enp8s0  FAILED
vm18.fedora.ibiblio.org dev enp8s0  FAILED

And this is the output of ip link (eno1 is the interface that I am actually using, and enp8s0 is unplugged):

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enp8s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN mode DEFAULT group default qlen 1000
    link/ether 78:24:af:... brd ff:ff:ff:ff:ff:ff
3: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 78:24:af:... brd ff:ff:ff:ff:ff:ff
Edited by William Temple
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