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Mac OS X 10.9 (Mavericks) Unicast ARP Cache Bug

It has been observed on the TCNJ network that the Mac OS X 10.9 (Mavericks) unicast ARP cache bug appears.

The original Apple discussion forum thread can be found at https://discussions.apple.com/thread/5483424.

This bug causes your network connection to stop working for a few seconds at fixed intervals (usually 30 or 60 seconds). This may be observed while using applications such as Facetime or other video streaming applications that may momentarily pause during your session.

This issue affects Macs running OS X 10.9 while using the internal gigabit Ethernet adapter or internal wireless. If you are using an external USB 100Mbps Ethernet adapter, the problem may not occur. To verify you are having this issue, run the “ping” command to see if you get 4-5 “Request timeout” messages at regular intervals.

Open a Terminal (located in the Applications folder > Utilities folder) and type the following command:

testmac:~ testuser$:~ testuser$ ping beast.tcnj.edu
PING beauty.tcnj.edu (159.91.14.68): 56 data bytes
64 bytes from 159.91.14.68: icmp_seq=0 ttl=63 time=1.026 ms
64 bytes from 159.91.14.68: icmp_seq=1 ttl=63 time=3.680 ms
64 bytes from 159.91.14.68: icmp_seq=2 ttl=63 time=1.308 ms
(...)
64 bytes from 159.91.14.68: icmp_seq=37 ttl=63 time=1.443 ms
64 bytes from 159.91.14.68: icmp_seq=38 ttl=63 time=1.397 ms
64 bytes from 159.91.14.68: icmp_seq=39 ttl=63 time=5.447 ms
Request timeout for icmp_seq 40
Request timeout for icmp_seq 41
Request timeout for icmp_seq 42
Request timeout for icmp_seq 43
Request timeout for icmp_seq 44
64 bytes from 159.91.14.68: icmp_seq=45 ttl=63 time=2.347 ms
64 bytes from 159.91.14.68: icmp_seq=46 ttl=63 time=1.330 ms
64 bytes from 159.91.14.68: icmp_seq=47 ttl=63 time=1.690 ms
(...)
64 bytes from 159.91.14.68: icmp_seq=54 ttl=63 time=1.321 ms
64 bytes from 159.91.14.68: icmp_seq=55 ttl=63 time=1.387 ms
64 bytes from 159.91.14.68: icmp_seq=56 ttl=63 time=1.573 ms
^C
--- beauty.tcnj.edu ping statistics ---
57 packets transmitted, 52 packets received, 8.8% packet loss
round-trip min/avg/max/stddev = 1.026/2.498/5.447/1.323 ms
testmac:~ testuser$

Once you verify that you are seeing the bug, perform the following commands below to apply a temporary fix by disabling the unicast ARP function.

Type the following two lines. The first one may give you an error message if the /etc/sysctl.conf file doesn’t already exist. This is okay.

sudo cp /etc/sysctl.conf /etc/sysctl.conf.bak
echo "net.link.ether.inet.arp_unicast_lim=0" | sudo tee -a /etc/sysctl.conf

Reboot your Mac and re-run the “ping” command test. You should no longer see the “Request timeout” messages appearing regularly.

NOTE: This bug appears to be fixed in Mac OS X 10.10 (Yosemite), however Yosemite has other known wireless bugs separate from this issue which will require fixes from Apple.

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