Networking in Red Hat

1. Networking
RED HAT COMMANDS – CHAPTER 5
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2. Setting Up Networking
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ifconfig Displays the IP address and other interface-
related information
route Enables you to view or change the routing
information on the system
system-config-
network-tui
Provides a menu-driven utility for network
configuration

3. Network Interfaces Files
/etc/sysconfig/network This file contains gateway and hostname
information.
/etc/sysconfig/network-scripts This directory contains all the interface
config files for your system.
◦ cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=08:00:27:30:74:AA
ONBOOT=yes
DHCP_HOSTNAME=RHEL01
TYPE=Ethernet
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4. Static IP Addresses
Launch the utility:
# system-config-network-tui
You need to restart the network service for the changes to take effect
Instead of bringing down all the interfaces on the system by restarting the
network service, you could also just bring down a single interface to which
you have made changes
◦ Ifdown
◦ ifup
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5. ifconfig command
Syntax: ifconfig [options] [interface]
Options
◦ netmask MASK Specifies the netmask for the interface
◦ hw ADDRESS Sets the MAC address of the interface
◦ Up Brings up the interface
◦ down Bring down the interface
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6. ifconfig eth1
To view the current IP address of the eth0 interface
◦ cat /etc/sysconfig/network-scripts/ifcfg-eth1
The ifconfig command displays information about the interface
along with its IP address.
ifconfig command to display all the interfaces on the system
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7. Any time you make a change to an interface’s settings, you
need to bring down that interface and then bring it back up
again.
If multiple interfaces have been edited, you could also just
restart the entire networking service.
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Restart the network service as follows:
• # service network restart

8. Set an IP Address
ifconfig eth0 172.168.1.1 netmask 255.255.255.0
◦ This command gets you the same results as using the menu-driven
option.
◦ Don’t forget to bring the network interface down and then bring
it back up again (or restart the network service).
◦ Make sure that you always use the ifconfig command to verify
that your IP addresses have been set properly.
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9. Routing
When you have a system that has two or more network
interfaces ,they are called dual-homed or multihomed
systems.
You need to make sure that each interface has a gateway
that it can route through.
Using the route command, you can display the current
routes the system has and add additional routes if you need
to.
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10. Routing
Options Description
add Adds a net route
del Deletes an existing route
flush flush Flushes any temporary routes
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Syntax: route [options]

11. Routing
If you don’t have a default gateway set, you can again use the route
command to choose a default gateway.
Step 1. Assuming that the default gateway has not yet been set, you can use
the following to set it for the RHEL01 host:
# route add default gw 192.168.1.1 eth0
Step 2. You can now verify the new default gateway you added by calling the
route command again:
# route
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12. Creating Static Routes
To create a static route, you add entries to the
/etc/sysconfig/network-scripts/route-<interface> file.
Let’s see what an example of this file might look like:
Default 192.168.1.1 dev eth0
172.168.1.0/24 via 172.168.1.1 eth1
This sets the default gateway to 192.168.1.1, and any traffic that is
destined for the 172.168.1.1/24 network will go over the eth1
interface (to the internal network).
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13. Troubleshooting Network Connections
ping Tests the connectivity between two
hosts
traceroute Looks for latency in the path from host to
host
netstat Shows information about connections
(open, closed, and listening)
route Shows routing information
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14. Networking Utilities
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Syntax: ping DESTINATION
ping
• ping –c 2 172.168.1.2

15. netstat command
You can view listening or established ports using the netstat
command.
You can also use the netstat command to obtain information
on routing tables, listening sockets, and established
connections.
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16. netstat command
Syntax: netstat [options]
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17. netstat command
As an example, suppose you want to check that the SSH
server is listening correctly on port 22. You could use netstat to
check that the connection is available for your clients:
# netstat -tuape | grep ssh
If nothing is returned, there is a good chance the service isn’t
running or listening correctly on that port. You can, however,
see here that the SSH service is listening correctly (as
denoted by *:ssh in the output).
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18. Network Monitoring and Analysis
you can use a packet capture utility to view all the raw data being
sent across the interfaces on your machine.
To help out, you can use the tcpdump utility.
Syntax: tcpdump [options]
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19. tcpdump utility
As an example, suppose you want to know whether your system is
leasing a DHCP address properly. You can monitor the connection on the
eth0 interface to make sure that the DHCP server is responding properly.
Step 1. Start the tcpdump program and specify the interface that you
want it to listen on as well as the file that you’d like it to dump the
information to so that you can review it:
# tcpdump –i eth0 –w pkt_capture
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20. tcpdump utility
Step 2. While the packet capture is running, you need to obtain access to
another console. Call the dhclient utility, which pulls down a new IP
address for you from the DHCP server:
# dhclient
Step 3. When the system finishes determining its IP address, return to
the first console and stop the tcpdump program by pressing Ctrl+C. To
review the information you’ve captured, call the tcpdump program again:
# tcpdump –r pkt_capture | less
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21. Advanced Networking
Ethernet Bonding
◦ Ethernet bonding is used to combine multiple interfaces into one, creating
an increase in available bandwidth and redundancy. This is done by
creating a special network interface file called a channel bonding
interface.
◦ Because you are in a lab environment and using virtual network
interfaces, there really isn’t any additional bandwidth benefit, but
redundancy is always good.
◦ You need to create a third network interface on your RHEL01 virtual
system before you can create your bonded interfaces
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22. Ethernet Bonding
Step 1. Enable the bonding kernel module:
# nano /etc/modprobe.d/bond.conf
Alias bond0 bonding
Step 2. Create the channel bond interface file in the /etc/sysconfig/networkscripts
directory:
# nano /etc/sysconfig/network-scripts/ifcfg-bond0
DEVICE=”eth0”
IPADDR=172.168.1.1
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=none
USERCTL=no
BONDING_OPTS=”primary=eth1”
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23. Ethernet Bonding
Step 3. Edit the eth1 interface to look like the following:
DEVICE=eth1
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SALVE=yes
USERCTL=no
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24. Ethernet Bonding
Step 4. Edit the eth2 interface to look like the following:
DEVICE=eth2
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SALVE=yes
USERCTL=no
Step 5. You can now bring up the bonded interface:
# ifconfig bond0 up
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