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This is more a sanity check than anything...

Using RHEL 6.6

So, we had a machine that was taken off the network during some upgrades.

In that time I must have decided to switch it from using NetworkManager to going completely by the old timey way of modifying ifcfg-eth# files, as I generally prefer that over dealing with NM. (it was offline for a long period, so I don't specifically remember doing this, but I must have)

When we plugged it back into the switch it could communicate with all of our local subnets (we use about 5 different subnet's in our building), BUT could not communicate with the subnetts outside of our building, these subnets are the ones that lead out to our DNS servers, and our gateway's to the internet.

Anyway, switching back to NetworkManager being enabled, and redoing the exact same config with NM makes everything work perfect... And I have no Idea why.

Can anyone think of a reason why NetworkManager configuration would work differently than without it? but still allow basic network communication?

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    Do you have the output of "ip route" or "netstat -nrv" with and without NetworkManager? Are you using dhcp in your ifcfg-ethx files?
    – Aaron
    Aug 5, 2015 at 22:09
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    @Aaron ... Damn, I feel like an idiot, i can't believe I didn't check those before... Anyway, As it turns out, one of the other admins in our group decided to setup eth3 as an access point on another management network, we've done it on most of our servers, but It hadn't been done on this server yet... As it turns out that was throwing off the routing tables, I haven't fully sorted out why, but that was it. Then switching to NM, eth3 wasn't configured so it didn't interfere. Thanks for the Help!
    – Gravy
    Aug 5, 2015 at 22:38
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    No problem. I will add it below so you can mark it as answered.
    – Aaron
    Aug 5, 2015 at 22:44

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Sometimes when folks move from NetworkManager to static network configuration files, they miss routes that were being provided by DHCP and handled by the NetworkManager. Comparing the output of "ip route" or "netstat -nrv" before and after your configuration changes may highlight the differences that need to be manually defined in a route-(interface) file.

NetworkManager is also sometimes used for 802.1x or VPN support, though this does not appear to be the case for you.

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