Public bug reported:

Test setup:
OS: Ubuntu 16.04
Hardware: D15_v2 VM on Azure

Steps to reproduce:
1) sudo apt-get install dnsmasq
2) sudo sysctl -w net.ipv4.neigh.default.gc_thresh1=1
3) sudo sysctl -w net.ipv4.neigh.default.gc_thresh2=1
4) sudo sysctl -w net.ipv4.neigh.default.gc_thresh3=1
5) dig @127.0.0.1 google.com

Result: 
~$ dig @127.0.0.1 google.com
../../../../lib/isc/unix/socket.c:2104: internal_send: 127.0.0.1#53: Invalid 
argument
../../../../lib/isc/unix/socket.c:2104: internal_send: 127.0.0.1#53: Invalid 
argument
../../../../lib/isc/unix/socket.c:2104: internal_send: 127.0.0.1#53: Invalid 
argument

; <<>> DiG 9.10.3-P4-Ubuntu <<>> @127.0.0.1 google.com
; (1 server found)
;; global options: +cmd
;; connection timed out; no servers could be reached 

However, an external DNS server still works fine (dig @8.8.8.8
google.com, for example).

We discovered this as the default max ARP cache size is 1024, and we're
running a large cluster with a lot of intra-cluster network traffic.
Increasing the size of the ARP cache solves this problem, but it seems
like dnsmasq should still work and just be slow, like other applications
(curl for example just takes longer to connect)

** Affects: dnsmasq (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1702726

Title:
  dnsmasq fails when the ARP cache is full

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