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How can one differentiate between the start of a network connect and the end of one, from the logs listed in /var/log/messages?

All the logs look identical.

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What version of Ubuntu are you running? A few log files, including this one, are named differently in different versions. (Please edit your post to include this information...then you can flag this comment as obsolete.) – Eliah Kagan May 27 '12 at 18:33
@EliahKagan @Ian Or to phrase it another way, in Ubuntu 12.04 desktop I get the following error message: ls: cannot access /var/logs/messages: No such file or directory. Please provide more information about what, exactly, you are asking about. – irrational John May 27 '12 at 18:39
@irrationalJohn It should actually be /var/log/messages, I neglected to notice that error in the question when I edited it originally. Anyway, /var/log/dmesg used to be called /var/log/messages, on older versions of Ubuntu (e.g., 10.04 LTS). I think it's still useful to have the release version in the question, though. – Eliah Kagan May 27 '12 at 18:51
@EliahKagan Definitely need more information. There is no way to provide an "answer" to the question as it is currently asked. – irrational John May 27 '12 at 18:57
Indeed. Ian, please also add specific lines taken from your /var/log/messages file to your question, so it's clear exactly what you're asking and exactly why it's unclear if they are describing connection or disconnection. – Eliah Kagan May 27 '12 at 18:59

On Ubuntu Server 12.04, that information usually goes to /var/log/syslog.
I have intentionally unplugged a ethernet card on a server and connected it back, I got this:

Jun  6 20:16:34 tagua kernel: [ 4416.930972] r8169 0000:01:06.0: eth0: link down
Jun  6 20:16:34 tagua kernel: [ 4416.932817] ADDRCONF(NETDEV_UP): eth0: link is not ready
Jun  6 20:19:09 tagua kernel: [ 4571.990507] r8169 0000:01:06.0: eth0: link up
Jun  6 20:19:09 tagua kernel: [ 4571.991172] ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready

If I run sudo ifdown eth0 the output is (among other lines):

Jun  6 20:28:05 tagua NetworkManager[1295]:  (eth0): carrier now OFF (device state 10)

When I reactivate it with sudo ifup eth0

Jun  6 20:28:23 tagua NetworkManager[1295]:  (eth0): carrier now ON (device state 10)
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