mDNS: Avoid listening on internal interfaces and reduce footprint

[mangelajo]

mDNS was listening on internal ovn interfaces (pods) and allocating way too much memory per server, regular mDNS queries fit on unfragmented regular 1500 MTU packets.

Closes: https://issues.redhat.com/browse/USHIFT-264

Which issue(s) this PR addresses:

Closes #<Issue Number>

[mangelajo]

I updated the patch according to comments

[mangelajo]

@zshi-redhat I verified and we don't send duplicate responses although we are listening on both br-ex and eth0:

[mangelajo]

updated with the additional comments.

[mangelajo]

@ggiguash @zshi-redhat does it look ok to lgtm ?

[ggiguash]

/lgtm

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[openshift-ci-robot]

/retest-required

Remaining retests: 2 against base HEAD 9466daf6475e3921ec59754d44226a509d03e6ec and 8 for PR HEAD f7542feb6cb63ce81827f146c846efed1491f48b in total

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[zshi-redhat]

@zshi-redhat I verified and we don't send duplicate responses although we are listening on both br-ex and eth0:

@mangelajo I observed two reply packets when tcpdump on the physical interface (e.g. enp1s0):

23:10:37.644628 52:54:00:c7:7e:34 > 01:00:5e:00:00:fb, ethertype IPv4 (0x0800), length 74: 192.168.122.250.mdns > 224.0.0.251.mdns: 0 A (QM)? my-nginx.local. (32)
23:10:37.644902 52:54:00:c7:7e:34 > 01:00:5e:00:00:fb, ethertype IPv4 (0x0800), length 84: 192.168.122.250.mdns > 224.0.0.251.mdns: 0*- [0q] 1/0/0 A 192.168.122.250 (42)
23:10:37.644988 52:54:00:c7:7e:34 > 01:00:5e:00:00:fb, ethertype IPv4 (0x0800), length 84: 192.168.122.250.mdns > 224.0.0.251.mdns: 0*- [0q] 1/0/0 A 192.168.122.250 (42)

Which interface did you capture the mdns packet?