So here's something enlightening, perhaps.
I've picked some random offsite dual-stack site, and tried to just telnet to port 80 manually from my Mac desktop.
----------
First try:
And the associated DNS activity under the hood has the AAAA data arrive before the A data:
dhcp-152-78-94-254:~ root# dscacheutil -flushcache
dhcp-152-78-94-254:~ root# tcpdump -i en1 port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on en1, link-type EN10MB (Ethernet), capture size 65535 bytes
12:07:44.600614 IP dhcp-152-78-94-254.ecs.soton.ac.uk.54643 > pigeon.ecs.soton.ac.uk.domain: 2165+ A?
www.limelightnetworks.com. (43)
12:07:44.600779 IP dhcp-152-78-94-254.ecs.soton.ac.uk.59266 > pigeon.ecs.soton.ac.uk.domain: 60607+ AAAA?
www.limelightnetworks.com. (43)
12:07:44.805891 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.59266: 60607 3/2/2 CNAME
llnw.vo.llnwd.net., AAAA 2a02:3d0:600:217:230:48ff:fe8b:8930, AAAA 2a02:3d0:600:217:230:48ff:fe8c:48be (202)
12:07:44.806222 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.54643: 2165 3/2/2 CNAME
llnw.vo.llnwd.net., A 87.248.211.154, A 87.248.211.245 (178)
12:07:45.140637 IP dhcp-152-78-94-254.ecs.soton.ac.uk.61818 > pigeon.ecs.soton.ac.uk.domain: 56374+ A?
llnw.vo.llnwd.net. (35)
12:07:45.143345 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.61818: 56374 2/2/2 A 87.248.211.245, A 87.248.211.154 (139)
^C
6 packets captured
290 packets received by filter
0 packets dropped by kernel
---------
Second try:
Connection closed by foreign host.
dhcp-152-78-94-254:~ tjc$ telnet
www.limelightnetworks.com 80
Trying 87.248.211.154...
Connected to
llnw.vo.llnwd.net.
Escape character is '^]'.
with the DNS activity this time seeing the A data beating the AAAA data:
dhcp-152-78-94-254:~ root# dscacheutil -flushcache
dhcp-152-78-94-254:~ root# tcpdump -i en1 port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on en1, link-type EN10MB (Ethernet), capture size 65535 bytes
12:08:58.680754 IP dhcp-152-78-94-254.ecs.soton.ac.uk.49412 > pigeon.ecs.soton.ac.uk.domain: 21922+ A?
www.limelightnetworks.com. (43)
12:08:58.681486 IP dhcp-152-78-94-254.ecs.soton.ac.uk.61183 > pigeon.ecs.soton.ac.uk.domain: 59003+ AAAA?
www.limelightnetworks.com. (43)
12:08:58.685454 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.49412: 21922 3/2/2 CNAME
llnw.vo.llnwd.net., A 87.248.211.154, A 87.248.211.245 (178)
12:08:58.686662 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.61183: 59003 3/2/2 CNAME
llnw.vo.llnwd.net., AAAA 2a02:3d0:600:217:230:48ff:fe8c:48be, AAAA 2a02:3d0:600:217:230:48ff:fe8b:8930 (202)
12:08:59.662767 IP dhcp-152-78-94-254.ecs.soton.ac.uk.62086 > pigeon.ecs.soton.ac.uk.domain: 7050+ PTR? 1.68.78.152.in-addr.arpa. (42)
12:08:59.668051 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.62086: 7050* 1/5/8 PTR pigeon.ecs.soton.ac.uk. (339)
12:08:59.669329 IP dhcp-152-78-94-254.ecs.soton.ac.uk.53286 > pigeon.ecs.soton.ac.uk.domain: 28306+ PTR? 254.94.78.152.in-addr.arpa. (44)
12:08:59.672169 IP pigeon.ecs.soton.ac.uk.domain > dhcp-152-78-94-254.ecs.soton.ac.uk.53286: 28306* 1/5/8 PTR dhcp-152-78-94-254.ecs.soton.ac.uk. (353)
^C
8 packets captured
346 packets received by filter
0 packets dropped by kernel
--------
So on my first go, the DNS returned AAAA information first and telnet connected by IPv6.
On my second go, the DNS returned A information first and telnet connected by IPv4.
And it's a split second difference in DNS responses being received by my client.
Interested if others can reproduce...
Tim
