Networking on Red Hat Enterprise Linux

The following post is an attempt at covering Linux Network Configuration end-to-end to a “bit better than reasonable level”.  The brevity of the post is by design since it is the sort of post that is mostly referred to as a reference or quick lookup guide to remind me, and others, of the name of that file, or that command that does…

As much as I love UNIX and Linux, since everything is a command or a file, the downside of that is the requirement of the knowledge up front to a certain extent (largely alleviated by Google these days) and in terms of the command line, is not that intuitive, even with the help of man pages.

Sometimes you just need to look something up that you know you’ve done before, but it was a few months ago or a year or two ago and you just need that post to point you back in the right direction.


You can configure a NIC on the fly with

ifconfig eth0 ip-address netmask subnet-mask

The permanent configuration that will be read at boot time or when the /etc/init.d/network restart occurs is held in /etc/sysconfig/network-scripts/ifcfg-eth0 etc

If you need to write a config file from scratch, use this as a template/guide











When you’re done, restart networking

/etc/init.d/network restart

and check they all come up.  If not, recheck the ifcfg-eth files in /etc/sysconfig/network-scripts, paying attention to the ONBOOT=yes line.

To test which of your physical nics corresponds to the linux os network device, disconnect a cable and use

ethtool eth0

paying attention to the bottom line which reads “link detected – YES” or “link detected – NO”

If there is a PCI NIC in the system, RHEL may assign it’s ports eth0 and eth1 taking priority over the embedded nics on the system board.  This is generally not an expected behaviour if you’re new to it.

check all network configurations with

ifconfig -a | less

check the DNS addresses are populated in /etc/resolv.conf and perform an nslookup to verify network connectivity as ping packets are often dropped by firewalls.

Setting a default gateway

You can configure a default gateway in /etc/sysconfig/network

e.g. Add the line


Speed and Duplex setting can be viewed using

ethtool eth1


dmesg | grep -i duplex

or using mii-tool

Display all active TCP ports along with process ID and name using the port

netstat -atp

Display routing table in numeric form

netstat -r -nr

Display all netstat statistics

netstat -as

List open files that are network related

lsof -i

MAC Address to Device listing

arp -v

Look for connected interfaces “link detected  -yes”

ethtool eth0

Display run levels where networking starts

chkconfig network –list

Display network status

/etc/init.d/network status   or  /sbin/service/network status

Display all network device configuration

ifconfig -a

Useful files where networking configuration is stored

    /etc/hosts       -will overrride other forms of name resolution contained in /etc/nsswitch.conf

/etc/resolv.conf       -contains the IP addresses of DNS servers used for name resolution in TCP/IP networks.

/etc/nsswitch.conf       -controls the order that names are resolved to IP addresses, i.e. files, nis, dns


Display interfaces and metrics

netstat -i

Create an SSH tunnel of port 2381 (hpsmh) on remote host to local port (use 1025 up)

ssh -f username@ip_address -L 1025:ip_address:2381 -N

i.e. browsing to http://localhost:1025 is the same as http://remotehost:2381

 Troubleshooting a NIC

Below is an example of a busy backup network interface on a backup server.  Note how its dropping packets etc.

eth4      Link encap:Ethernet  HWaddr 10:1F:74:8B:8F:8X


          RX packets:22053199483 errors:40041 dropped:18775 overruns:46 frame:0

          TX packets:8811133044 errors:0 dropped:0 overruns:0 carrier:0

          collisions:0 txqueuelen:1000

          RX bytes:31314447740529 (28.4 TiB)  TX bytes:6356693939792 (5.7 TiB)



Possible Causes of Ethernet Errors

Collisions: Signifies when the NIC card detects itself and another server on the LAN attempting data transmissions at the same time. Collisions can be expected as a normal part of Ethernet operation and are typically below 0.1% of all frames sent. Higher error rates are likely to be caused by faulty NIC cards or poorly terminated cables.

Single Collisions: The Ethernet frame went through after only one collision

Multiple Collisions: The NIC had to attempt multiple times before successfully sending the frame due to collisions.

CRC Errors: Frames were sent but were corrupted in transit. The presence of CRC errors, but not many collisions usually is an indication of electrical noise. Make sure that you are using the correct type of cable, that the cabling is undamaged and that the connectors are securely fastened.

Frame Errors: An incorrect CRC and a non-integer number of bytes are received. This is usually the result of collisions or a bad Ethernet device.

FIFO and Overrun Errors: The number of times that the NIC was unable of handing data to its memory buffers because the data rate the capabilities of the hardware. This is usually a sign of excessive traffic.

Length Errors: The received frame length was less than or exceeded the Ethernet standard. This is most frequently due to incompatible duplex settings.

Carrier Errors: Errors are caused by the NIC card losing its link connection to the hub or switch. Check for faulty cabling or faulty interfaces on the NIC and networking equipment.


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