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kenmorgan
 Posted: Jan 8 2016, 12:57 PM
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It was a close call between using "Installation and Setup Support" or "Networking." I picked this one because my problem does involve "setup."

I did have a router with two computers networked, but it seems to be having a problem--losing the Internet. But just to make sure it was not something in the computers or their settings, I tried hooking one computer at a time directly to the cable modem. As far as I could tell, all the settings in the two computers were the same. Here's what I found after some fiddling:

(1) Unplugging a computer's ethernet cable from the modem, immediately wipes out the nameservers in /etc/resolv.conf. I assume we have Network Manager to thank for this great wisdom.

(2) The namesservers are rewritten in that file ONLY IF both the modem and the computer plugged into it are rebooted. Then the nameservers are mysteriously restored, presumably again by Network Manager.

So the necessary procedure to enable a computer to access the Internet directly from the modem would seem to be:

(1) Plug the computer's ethernet cable into the modem.
(2) Reboot the modem.
(3) Reboot the computer.

This procedure works for computer 1 but not for computer 2.

So...there seems to be something strange going on with computer 2.

Any ideas would be greatly appreciated.

Excursus: When Network Manager was first introduced, I was still using Fedora. However, I remember reading a number of complaints about it on the Fedora forum. I suspect the complaints are still well taken. One reason is that with the introduction of Network Manager, something else disappeared. What became of the GUI entitled "Network Configuration"? It had four tabs: Devices, Hardward, DNS, and Hosts. This is not coming from my memory; when the GUI was still available, I had printed several of these windows. Anyway, I suspect that with the GUI allowing manual input of all that data, there were far fewer problems. My big question is this: does this GUI still exist somewhere so that a package with this GUI could be installed? Is "network-manager-applet" available? It would certainly make my day! laugh.gif

Ken

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tux99
 Posted: Jan 8 2016, 10:32 PM
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Have you tried disabling IPv6? It's always a good idea (unless of course your Internet connection is actually IPv6 based)

http://scientificlinuxforum.org/index.php?showtopic=1376

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kenmorgan
 Posted: Jan 9 2016, 12:36 AM
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Hi Tux. It's been awhile. Glad to "see" you again. I still love the VLC player!

I disabled ipv6 the way you described in the link you provided--putting ipv6.disable=1 in the kernel command line in /boot/grub/grub.conf.

No, still no Internet access. By the way when I ping yahoo.com, the response is "unknown host yahoo.com.


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kenmorgan
 Posted: Jan 9 2016, 02:18 AM
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I put the router back in. Both computers are connected, but still computer 1 can access the Internet, computer 2 cannot.

Doing some Google searching, I discovered the route command. This is the first difference I've found between the two computers.

Computer 1 (which can access the Internet):


CODE
[kmorgan@localhost1 ~]$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
link-local      *               255.255.0.0     U     1002   0        0 eth0
default         192.168.1.1     0.0.0.0         UG    0      0        0 eth0


NOTE: 192.168.1.1 is the router IP.

Computer 2 (which CANNOT access the Internet):

CODE

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     1      0        0 eth0


Could this difference be THE difference?

I don't know where this file is (if it's a file), nor do I know how to modify it.

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tux99
 Posted: Jan 9 2016, 08:25 PM
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QUOTE (kenmorgan @ Jan 9 2016, 03:18 AM)

Could this difference be THE difference?


Well yes of course, there might be additional issues but you certainly need to first set a default gateway otherwise you won't be able to access the internet.

Try adding a line like this:
GATEWAY=192.168.1.1

in /etc/sysconfig/network

This post has been edited by tux99: Jan 9 2016, 08:28 PM

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helikaon
 Posted: Jan 11 2016, 10:12 AM
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Hi guys,

Ken, just couple of points:
- you didnt say (or i overlooked it) how the network is defined at your NIC on both of your linux boxes - so the q. stands - is it static? or is it dhcp?

- if it is dhcp then no surprise the DNS changes, because dhcp client read the whole network config from dhcp server incl. the DNS IPs - so if your router runs dhcp server, then the 2 computers being behind router read DNS from router, then, if you plug them directly, then dhcp read DNS config from dhcp server of your ISP - so it might change

- if these 2 computers are workstations, just turn off the NetworkManager and use only standard 'network' startup script - NM is not needed and frankly, i dont even know why the Red Hat is stuck up on using this 'infant terrible' for server platform, while it's good enough only for laptops / wifi configurations etc


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kenmorgan
 Posted: Jan 11 2016, 06:28 PM
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Tux: Thanks so much for the suggestion. I tried adding that line to /etc/sysconfig/network, but it didn't help. I also looked at /etc/sysconfig/network on computer 1, which did have access the Internet, and it didn't have that line. Evidently, the line's not needed.

Helikaon: Thanks for jumping in! To answer your question, when I started this thread, the two computers had static IPs I assigned (my traditional way of setting up the network), and I did not use the router's DHCP service. However, I had earlier tried it the with DHCP; computer 2 could not access the Internet that way either. I even tried connecting computer 2 directly to the modem. Still it couldn't access the Internet.

After Tux's suggestion didn't solve the problem, and I'd been struggling with the problem for over a day, I decided to reinstall CentOS 6 from scratch on computer 2. So I guess the mystery will forever go unsolved! ohmy.gif

I set up the network the way I always have--with the router's DHCP service turned off. I assign the computers my usual static IPs:

192.168.1.100 = localhost1.localdomain
192.168.1.200 = localhost2.localdomain (which now has the reinstalled OS)

Everything is now working as before (including the router). Well, almost everything. But since I've gotten your kind attention, I'll mention it. Maybe I'm getting to old, and I'm just forgetting some simple step after a reinstallation. I can ssh in both directions, and I can mount computer 1's directories onto computer 2 (which has the reinstalled OS), but not the reverse. The mount command goes through with no error messages. But when I look at the directory in computer 1 where the mount should have occurred, it is still empty. http://dl.dropbox.com/u/2835777/BangHead1.gif

What am I forgetting!?!?

Thanks so much.

Ken


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kenmorgan
 Posted: Jan 11 2016, 09:24 PM
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Success! http://dl.dropbox.com/u/2835777/egyptian.gif

It was just as I said--getting too old! sad.gif

After installing CentOS 6 on computer 2, I had to create manually the file /etc/exports. Which I did. But a typo occurred. The file had these two lines:

/ 192.168.1.100(rw,sync)
/home 192.168.1.200(rw,sync)

The second line of course should have been

/home 192.168.1.100(rw,sync)

After making that change, the mount worked. Odd, though, that the unsuccessful mounts didn't give me an error message that could have led me quickly to the solution. mad.gif

Oh, well. All's well that ends well. smile.gif

Ken

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helikaon
 Posted: Jan 19 2016, 01:11 PM
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http://th166.photobucket.com/albums/u117/rdshear/Smiley%20Faces/th_smiley-face-thumbs-up.gif

congratz, i'm glad you solved it.

Networking troubles can be challenging, since, as you experienced first hand, there is many factors that can influence the outcome.
Starting with cable ran over with a office chair wheel (seen that many time in different offices, LOL), going through typos in network config files, iptables dropping connection, bad routing tables etc etc etc ...

But you came through victorious, at that's what matters biggrin.gif

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