4

After restarting a headless CentOS 6.3 machine, it lost outbound internet connectivity, i.e. I can still connect to the server via SSH (ssh root@**.126.18.56), but stuff such as ping google.com gives google.com: unknown host, and yum list some_package gives a lot of network errors.

This is what ifconfig gives:

eth0      Link encap:Ethernet  HWaddr 00:25:90:78:2D:5D  
          inet addr:**.126.18.56  Bcast:**.126.18.255  Mask:255.255.255.0
          inet6 addr: fe80::225:90ff:fe78:2d5d/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:75594 errors:0 dropped:0 overruns:0 frame:0
          TX packets:787 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:7074741 (6.7 MiB)  TX bytes:144391 (141.0 KiB)
          Interrupt:20 Memory:f7a00000-f7a20000 

eth1      Link encap:Ethernet  HWaddr 00:25:90:78:2D:5C  
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Interrupt:16 Memory:f7900000-f7920000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:6 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:504 (504.0 b)  TX bytes:504 (504.0 b)

I have absolutely no clue how to debug this, and I find it very strange since I can still connect via ssh.

EDIT: Weirdly, /etc/resolv.conf does not contain any entries, or none that I can make sense of: 

# Generated by NetworkManager
search sui-inter.net


# No nameservers found; try putting DNS servers into your
# ifcfg files in /etc/sysconfig/network-scripts like so:
#
# DNS1=xxx.xxx.xxx.xxx
# DNS2=xxx.xxx.xxx.xxx
# DOMAIN=lab.foo.com bar.foo.com

So is it possible that rebooting the server erased that file? It worked before at least! And how do I solve this?

By the way, pinging an IP address works.

Improve this question
4
  • Can you check the DNS settings? (Note that if you connect to the server via ssh then you use your own (local) DNS)
    – Hennes
    Dec 19, 2012 at 9:48
  • Sure, but how do I check the DNS settings of the server machine? Networking is not my favourite hobby...
    – grssnbchr
    Dec 19, 2012 at 9:50
  • 1
    ssh to the server, then check /etc/resolv.conf for nameserver entries. (e.g. it may contain a line such as nameserver 8.8.8.8.). Then try pinging that IP (it should be reachable). If it is not then the external nameserver is down or otherwise unreachable. -- Note that if you can ping a server on the net by IP (but not by name) then you have not lost all internet connectivity (in which case: edit the post). Also, this might fit better on another site.
    – Hennes
    Dec 19, 2012 at 9:54
  • I edited my post.
    – grssnbchr
    Dec 19, 2012 at 10:17

4 Answers 4

Reset to default
2

Ok You can edit the below file to prevent from overwriting resolv.conf

file name is /etc/sysconfig/network-scripts/ifcfg-eth0

Value to set is PEERDNS=no

Improve this answer
1
  • This is the accepted answer because editing that file prevented NetworkManager or whatever program from resetting resolv.conf at boot time. Now, DNS values specified in either resolv.conf or ifcfg-eth0 will stay in resolv.conf.
    – grssnbchr
    Dec 19, 2012 at 14:30
4

I think your file /etc/resolv.conf is not present or no entries in that. Generally it contains the nameserver address which is nothing but DNS server address. All domain names like google.com etc will be resolved by using that DNS server.

Incase of ssh root@**.126.18.56, it is working because here you are giving direct IP address instead of name.

Improve this answer
1
  • Yes, that is true. See my above updated post. But do not confuse my local machine which works fine with my remote server machine. ssh [email protected] works as well, so the hostname of the server gets resolved correctly.
    – grssnbchr
    Dec 19, 2012 at 10:19
1

I seems that CentOS rebuilds /etc/resolv.conf on boot, using values found in the scripts in /etc/sysconfig/network-scripts. Those are probably missing, simply not set or maybe corrupt.

So is it possible that rebooting the server erased that file? It worked before at least!

It recreated it with new, up to date values. Except that something went wrong. (e.g. because it had no values to set)

And how do I solve this?

Quick workaround: echo "nameservers 8.8.8.8" > /etc/resolv.conf

Proper solution: Configure the network properly. Start by looking at the Networkmanager. ( I can not go in detail here since I have no CentOS experience. )

Improve this answer
1
  • Thanks for your answer, it helped me understand a bit how CentOS handles resolv.conf. I accepted the other answer since it concretely specifies the solution to the problem.
    – grssnbchr
    Dec 19, 2012 at 14:29
1

Don't allow NetworkManager to manage your server's NIC.

Edit /etc/sysconfig/network-scripts/ifcfg-eth0. Find the line:

NM_CONTROLLED=yes

Change it to read:

NM_CONTROLLED=no

If the line doesn't exist, just add it.

Improve this answer
2
  • So should this also solve the above problems with resetting DNS settings? Is this an NM-related issue on CentOS?
    – grssnbchr
    Dec 20, 2012 at 5:10
  • And every other mystery problem you're having with the NIC. Using NetworkManager is not recommended for servers.
    – Michael Hampton
    Dec 20, 2012 at 5:45

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .