Linux Networking Setup

Linux Networking Setup Required Information To enable networking, you must configure your network interface card or cards with an IP address and netmask. The kernel must have support for your cards compiled in, either as modular support or direct support. If you don't have kernel support read the sections about the kernel and how to compile it. To set your cards up, do the following. In my example my network is 192.168.1.0, IP=192.168.1.100, broadcast=192.168.1.255, netmask=255.255.255.0, gateway-192.168.1.1, nameserver=192.168.1.10. 1. Determine your machines IP address from your network administrator 2. Your network mask. This determines which portion of the IP address specifies the subnetwork number and which portion specifies the host. Class C (most networks) 255.255.255.0 Class B 255.255.0.0 3. Your network address which is your IP address bit wise anded with the network mask. 4. Your broadcast address. Used to broadcast packets to every machine on your subnet. 5. Your gateway address. The address of the machine that is your gateway to the outside

world. In many cases: Ex: IP: 192.168.1.100 Gateway: 192.168.1.1 6. Your nameserver address. Translates host names into IP addresses. 192.168.1.10 Ex: IP: 192.168.1.100

Mask: 255.255.255.0

Net Addr:: 192.168.1.0

7. Your broadcast address. Used to broadcast packets to every machine on your subnet. 8. Your gateway address. The address of the machine that is your gateway to the outside

world. In many cases: Ex: IP: 192.168.1.100 Gateway: 192.168.1.1 9. Your nameserver address. Translates host names into IP addresses. 192.168.1.10 Ex: IP: 192.168.1.100

Mask: 255.255.255.0

Net Addr: 192.168.1.255

10. Your gateway address. The address of the machine that is your gateway to the outside

world. In many cases: Ex: IP: 192.168.1.100 Gateway: 192.168.1.1 11. Your nameserver address. Translates host names into IP addresses. 192.168.1.10

Configuration tools There are many network configuration tools today. They are: netconf linuxconf netconfig ifconfig

A GUI interactive interface available on Redhat 6.1 A GUI interactive interface available on Redhat 6.1 which includes netconf configuration. A GUI step by step interface A text based program to configure the network interface. Type "man ifconfig" for info.

These programs will modify values in the following files: •

/etc/sysconfig/network - Defines your network and some of its characteristics.

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/etc/HOSTNAME - Shows the host name of this host. IF your name is "myhost" then that is exactly the text this file will contain.

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/etc/resolv.conf - Specifies the domain to be searched for host names to connect to, the nameserver address, and the search order for the nameservers.

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/etc/host.conf - Specifies the order nameservice looks to resolve names.

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/etc/hosts - Shows addresses and names of local hosts.

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/etc/networks - Provides a database of network names with network addresses similar to the /etc/hosts file. This file is not required for operation.

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/etc/sysconfig/network-scripts/ifcfg-eth* - There is a file for each network interface. This file contains the IP address of the interface and many other setup variables.

Analysis Tools •

netstat - Displays information about the systems network connections, including port connections, routing tables, and more. The command "netstar -r" will display the routing table.

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traceroute - This command can be used to determine the network route from your computer to some other computer on your network or the internet. To use it you can type "route IPaddress" of the computer you want to see the route to.

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nslookup - Used to query DNS servers for information about hosts.

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arp - This program lets the user read or modify their arp cache.

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tcpdump - This program allows the user to see TCP traffic on their network.

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dig(1) - Send domain name query packets to name servers for debugging or testing.

Manual Configuration You can use one of the above tools or configure the network the old fashioned way as follows: 1. First to use networking on any permanent basis you should setup the file /etc/sysconfig/network similar to the example shown below. 2. Assign an ip address with "ifconfig eth0 192.168.1.100 netmask 255.255.255.0 up". 3. Tell your machine that a hub is ready for information with the command "route add -net 192.168.0.0 netmask 255.255.255.0 eth0" 4. To contact hosts outside your network if a machine with IP address 192.168.1.1 is the gateway use the command "route add default gw 192.168.1.1 eth0" 5. If using a dialup connection use the command "route add default ppp0" The word default says if the packet is not for a machine on your local network, send it to the default device. These settings are not permanent, but go away the next time you boot. They are normally set up in the directory /etc/sysconfig/network-scripts. Add the network interface to the file /etc/sysconfig/network-scripts/ifcfg-eth*. For example the file ifcfg-eth0 if for the first ethernet

interface, ifcfg-eth1 for the second, ifcfg-lo is for the local interface. An example file from my system is: DEVICE="eth0" IPADDR="192.168.1.100" NETMASK="255.255.0.0" ONBOOT="yes" BOOTPROTO="none" IPXNETNUM_802_2="" IPXPRIMARY_802_2="no" IPXACTIVE_802_2="no" IPXNETNUM_802_3="" IPXPRIMARY_802_3="no" IPXACTIVE_802_3="no" IPXNETNUM_ETHERII="" IPXPRIMARY_ETHERII="no" IPXACTIVE_ETHERII="no" IPXNETNUM_SNAP="" IPXPRIMARY_SNAP="no" IPXACTIVE_SNAP="no"

Unless you know what you're doing it is best to use a network configuration tool. I cannot guarantee the accurateness of how to set these files up on your system.

Configuring an interface for multiple IP addresses If you want to configure your network card to act as more than one IP address, issue the following command: ifconfig dummy 192.168.1.102 netmask 255.255.255.0 This uses the dummy system interface capability supported in the kernel to setup another virtual interface which operates at IP address 192.168.1.102. Substitute the IP address that you want your virtual interface to be with an appropriate netmask for your network. To disable this, issue the following command. ifconfig dummy down Another way to use multiple IP addresses on one ethernet card is to set up a new file in your /etc/sysconfig/network-scripts directory. Copy your ifcfg-eth0 role to ifcfg-eth0:0. Edit that file and rename the device to "eth0:0" and the IP address to the desired IP address. You may also want to modify BROADCAST, NETWORK, or NETMASK. You can continue adding IP addresses by using :1, :2, etc such as ifcfg-eth0:2. To make it effective, you must reboot your system or issue the command "/etc/rc.d/init.d/network restart" as root.

Dynamically allocated IP addresses To get the IP address of a dynamically allocated network interface in a script file enter the following: dynip=`/sbin/ifconfig | grep -A 4 ppp0 | awk '/inet/ { print $2 } ' | sed -e s/addr://` Substitute the correct interface that you get your dynamic IP address in for ppp0 in the example above. This script line gets your dynamic IP address for use in a masquerade script. You can use the variable $dynip as in any other configuration. The next time you make a new connection you will need to extract the dynip value again and re-run the masquerade script.

Networking file formats, examples and considerations Below are listed some more in depth information about the networking files. •

/etc/sysconfig/network The /etc/inittab file contains the entry "si::sysinit:/etc/rc.d/rc.sysinit" which causes the system at startup to run the rc.sysinit script. The rc.sysinit file expects to find the file /etc/sysconfig/network if networking is to be enabled. The network file looks like this: NETWORKING=yes FORWARD_IPV4=false HOSTNAME=mymachine.mycompany.com DOMAINNAME=mycompany.com GATEWAY=192.168.1.1 GATEWAYDEV=eth0 Where GATEWAYDEV is the network interface card that is attached to the network the gateway machine is on. The GATEWAY is the actual IP address of the gateway machine.

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/etc/hosts - Defines local hosts.

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127.0.0.1 localhost localhost.localdomain 192.168.1.100 mymachine.mycompany.com

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/etc/services - Internet network services list. It associates port numbers with names of services. The file contains three fields which are name, port/protocol, and aliases with an optional comment.

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/etc/protocols - Describes DARPA internet protocols available from the TCP/IP subsystem. Maps protocol ID numbers to protocol names. It includes protocol name, number, and aliases. The protocol file on my system:

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# # # # # # # #

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mymachine

/etc/protocols: $Id: protocols,v 1.1 1995/02/24 01:09:41 imurdock Exp $ Internet (IP) protocols from: @(#)protocols

5.1 (Berkeley) 4/17/89

Updated for NetBSD based on RFC 1340, Assigned Numbers (July 1992).

ip number icmp igmp ggp ipencap ``IP'') st tcp egp pup

0

IP

# internet protocol, pseudo protocol

1 2 3 4

ICMP IGMP GGP IP-ENCAP

# # # #

internet control message protocol Internet Group Management gateway-gateway protocol IP encapsulated in IP (officially

5 6 8 12

ST TCP EGP PUP

# # # #

ST datagram mode transmission control protocol exterior gateway protocol PARC universal packet protocol

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udp 17 hmp 20 xns-idp 22 rdp 27 iso-tp4 29 xtp 36 ddp 37 idpr-cmtp 39 rspf 73 vmtp 81 ospf 89 ipip 94 encap 98

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/etc/named.conf - Used for domain name service to configure named. Other files used are dependent on this file. This file is explained further in the DNS section

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/etc/resolv.conf - Specifies the domain to be searched for host names to connect to, the nameserver address, and the search order for the nameservers.

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domain mycompany.com search mycompany.com mynet.net nameserver 192.168.1.100 nameserver 192.168.199.1 nameserver 192.168.1.10

UDP HMP XNS-IDP RDP ISO-TP4 XTP DDP IDPR-CMTP RSPF VMTP OSPFIGP IPIP ENCAP

# user datagram protocol # host monitoring protocol # Xerox NS IDP # "reliable datagram" protocol # ISO Transport Protocol class 4 # Xpress Tranfer Protocol # Datagram Delivery Protocol # IDPR Control Message Transport #Radio Shortest Path First. # Versatile Message Transport # Open Shortest Path First IGP # Yet Another IP encapsulation # Yet Another IP encapsulation

The third line specifies that DNS should be tried on my machine first then use the normal nameserver on the fifth line. The fourth line specifies that my machine is running nameservices on another network which is using interface 192.168.199.1. This assumes the nameserver is set up on my machine which is explained in another section. • • • •

/etc/host.conf - Specifies the order nameservice looks to resolve names. An example file: order hosts, bind multi on nospoof on

The order specifies that when resolving names to first look in the /etc/host file, then use BIND8 (DNS) to resolve the name. The line "multi on" specifies that all valid addresses for a host found in the hosts file should be returned. The files in /etc/sysconfig/network-scripts control your network interfaces. The network interface file is described above in the section "Manual Configuration". If you want or need more in depth knowledge about how these files are used, you will need to read the document "How Linux Works CTDP Guide" or "The CTDP Linux Startup Manual". Otherwise you will need to analyze the system startup scripts which is no small task.

Older X windows configuration In Xwindows a working configuration is set up as follows: NAMES: hostname: mymachine.mycompany.com Domain: mycompany.com

Nameservers: 192.168.1.10 HOSTS: IP - 192.168.1.100 Name - mymachine.mycompany.com INTERFACES: Interface - eth0 IP- 192.168.1.100 proto - none atboot - yes Netmask: 255.255.255.0 Network: 192.168.1.0 Broadcast: 192.168.1.255 ROUTING: Default gateway: 192.168.1.1 Default gateway device: eth0 Interface - 192.168.1.100 Network Address - 192.168.1.0 Network gateway 192.168.1.1 Netmask - 255.255.255.0

Routing Routing table information is used to route incoming and outgoing network diagrams to other machines. On most simple configurations, there are three routes. One for sending packets to your own machine, one for sending packets to other machines on your network and one for sending packets to other machines outside your network through the gateway. Two programs (ifconfig and route) are used to configure these parameters. They are described in more detail in the routing section.