The document discusses Linux network configuration files and how to configure network interfaces. It describes key configuration files such as /etc/resolv.conf for DNS resolution, /etc/hosts for local host name resolution, and /etc/nsswitch.conf for the name service switch. It also covers how to configure static and dynamic IP addresses via command line, configuration files, and GUI tools for different Linux distributions like Red Hat, Fedora, Ubuntu and Debian.

1. Linux Network Configuration http://www.yolinux.com/TUTORIALS/LinuxTutorialNetworking.html
Linux TCP/IP Network Configuration Files:
File Description
/etc/resolve.conf List DNS servers for internet domain name resolution.
Manual page for: /etc/resolv.conf
/etc/hosts Lists hosts to be resolved locally (not by DNS).
Manual page for: /etc/hosts
/etc/nsswitch.conf List order of host name search. Typically look at local files, then
NIS server, then DNS server.
Manual page for: /etc/nsswitch.conf
Red Hat/Fedora/CentOS: /etc/sysconfig/network Specify network configuration. eg. Static IP, DHCP, NIS, etc.
Red Hat/Fedora/CentOS: /etc/sysconfig/network- Specify TCP network information.
scripts/ifcfg-device
Ubuntu/Debian: /etc/network/interfaces Specify network configuration and devices. eg. Static IP and info,
DHCP, etc.
Domain Resolution Configuration Files:
File: /etc/resolv.conf - host name resolver configuration file
search name-of-domain.com - Name of your domain or ISP's domain if using their name server
nameserver XXX.XXX.XXX.XXX - IP address of primary name server
nameserver XXX.XXX.XXX.XXX - IP address of secondary name server
This configures Linux so that it knows which DNS server will be resolving domain names into IP addresses. If using DHCP
client, this will automatically be sent to you by the ISP and loaded into this file as part of the DHCP protocol. If using a
static IP address, ask the ISP or check another machine on your network.
Red Hat/Fedora GUI: /usr/sbin/system-config-network (select tab "DNS").
File: /etc/hosts - locally resolve node names to IP addresses
127.0.0.1 your-node-name.your-domain.com localhost.localdomain localhost
XXX.XXX.XXX.XXX node-name
Note when adding hosts to this file, place the fully qualified name first. (It helps sendmail identify your server correctly) i.e.:
XXX.XXX.XXX.XXX superserver.yolinux.com superserver
This informs Linux of local systems on the network which are not handled by the DNS server. (or for all systems in your
LAN if you are not using DNS or NIS)
The file format for the hosts file is specified by RFC 952.
Red Hat/Fedora configuration GUI: /usr/sbin/system-config-network (select tab "Hosts").
File: /etc/nsswitch.conf - System Databases and Name Service Switch configuration file
hosts: files dns nisplus nis
This example tells Linux to first resolve a host name by looking at the local hosts file(/etc/hosts), then if the name is not
found look to your DNS server as defined by /etc/resolv.conf and if not found there look to your NIS server.
In the past this file has had the following names: /etc/nsswitch.conf, /etc/svc.conf, /etc/netsvc.conf, ... depending on the
distribution.
Fedora / Red Hat Network Configuration Files:
/etc/sysconfig/network
Red Hat network configuration file used by the system during the boot process.
File: /etc/sysconfig/network-scripts/ifcfg-eth0
Configuration settings for your first ethernet port (0). Your second port is eth1.
File:
/etc/modprobe.conf (kernel 2.6)
/etc/modules.conf (kernel 2.4)
(or for older systems: /etc/conf.modules)
Example statement for Intel ethernet card:
alias eth0 eepro100
Modules for other devices on the system will also be listed. This tells the kernel which device driver to use if configured as
a loadable module. (default for Red Hat)
Fedora / Red Hat Network GUI Configuration Tools:
The following GUI tools edit the system configuration files. There is no difference in the configuration developed with the GUI
tools and that developed by editing system configuration files directly.
TCP/IP ethernet configuration:
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TCP/IP ethernet configuration:
Network configuration:
/usr/sbin/system-config-network (FC-2/3) GUI shown here
--->
/usr/bin/redhat-config-network (/usr/bin/neat) (RH 7.2+
FC-1)
Text console configuration tool:
/usr/sbin/system-config-network-tui (Text User Interface
(TUI) for Fedora Core 2/3)
/usr/bin/redhat-config-network-tui (RH 9.0 - FC-1)
Text console network configuration tool.
First interface only - eth0: /usr/sbin/netconfig
/usr/bin/netcfg (GUI) (last available with RH 7.1)
Gnome Desktop:
Gnome Desktop Network Configuration
/usr/bin/gnome-network-preferences (RH 9.0 - FC-3)
Proxy configuration. Choose one of three options:
1. Direct internet connection
2. Manual proxy configuration (specify proxy and port)
3. Automatic proxy configuration (give URL)
Assigning an IP address:
Computers may be assiged a static IP address or assigned one dynamically. Typically a server will require a static IP while a
workstation will use DHCP (dynamic IP assignment). The Linux server requires a static IP so that those who wish to use its
resources can find the system. It is more easily found if the IP address does not change and is static. This is not important for
the Linux client workstation and thus it is easier to use an automated Dynamic Host Configuration Protocol (DHCP) for IP
address assignment.
Static IP address assignment:
Choose one of the following methods:
Command Line:
/sbin/ifconfig eth0 192.168.10.12 netmask 255.255.255.0 broadcast 192.168.10.255
Network address by convention would be the lowest: 192.168.10.0
Broadcast address by convention would be the highest: 192.168.10.255
The gateway can be anything, but following convention: 192.168.10.1
Note: the highest and lowest addresses are based on the netmask. The previous example is based on a netmask of
255.255.255.0
Red Hat / Fedora GUI tools:
/usr/bin/neat Gnome GUI network administration tool. Handles all interfaces. Configure for Static IP or DHCP
client.
(First available with Red Hat 7.2.)
/usr/bin/netcfg (Handles all interfaces) (last available in Red Hat 7.1)
Red Hat / Fedora Console tools:
/usr/sbin/system-config-network-tui (Text User Interface)
/usr/sbin/netconfig (Only seems to work for the first network interface eth0 but not eth1,...)
Directly edit configuration files/scripts. See format below.
The ifconfig command does NOT store this information permanently. Upon reboot this information is lost. Manually add the
network configuration to /etc/sysconfig/network-scripts/ifcfg-eth0 (Red Hat/Fedora/CentOS) for the first NIC, ifcfg-eth1 for
the second, etc, or /etc/network/interfaces (Ubuntu) as shown below. Any other commands you may want to add to the
system boot sequence can be added to the end of the file /etc/rc.d/rc.local. The commands netcfg and netconfig make
permanent changes to system network configuration files located in /etc/sysconfig/network-scripts/, so that this information
is retained and used upon system boot.
The IANA has allocated IP addresses in the range of 192.168.0.0 to 192.168.255.255 for private networks.
Helpful tools:
Network Calculators: Subnet mask calculator, node calculator, mask inverter, ...
IP subnet calculator
Command line IP Configuration: ifconfig
ifconfig interface [aftype] options | address ...
where:
interface: eth0, eth1, eth2 represent the computer ethernet interfaces
aftype: inet (TCP/IP, default), inet6 (IPv6), ax25 (AMPR Packet Radio), ddp (Appletalk Phase 2), ipx (Novell IPX) or netrom
(AMPR Packet radio)
Options:
Option Description
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Option Description
up Activate the interface. Implied if IP addresses are specified.
down Shut down interface
Enable ARP protocol on this interface. Allow ARP to detect the addresses of computer hosts
arp
attached to the network.
-arp Disable ARP protocol on this interface
Enable promiscuous mode. Receive all packets on the network not just those destined for this
promisc
interface.
-promisc Disable promiscuous mode.
Specify the Maximum Transfer Unit (MTU) of the interface. The MTU is the maximum number of
mtu ##
octets the interface is able to handle in a single transaction. Defaults: Ethernet: 1500 SLIP: 296
broadcast
Set the network broadcast address for this interface.
XXX.XXX.XXX.XXX
netmask
Set the IP network mask for this interface.
XXX.XXX.XXX.XXX
Man page: ifconfig
Ubuntu / Debian IP Configuration Files:
File: /etc/network/interfaces
Static IP example:
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 208.88.34.106
netmask 255.255.255.248
broadcast 208.88.34.111
network 208.88.34.104
gateway 208.88.34.110
Dynamic IP (DHCP) example:
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
auto eth1
iface eth1 inet dhcp
auto eth2
iface eth2 inet dhcp
auto ath0
iface ath0 inet dhcp
auto wlan0
iface wlan0 inet dhcp
Interfaces:
lo: Loopback interface (network within your system without slowing down for the real ethernet based network)
eth0: First ethernet interface card
wlan0: First wireless network interface
Also see " man interfaces"
Ubuntu GUI Network Tools:
/usr/bin/gnome-nettool (apt-get install gnome-nettool)
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/usr/bin/network-admin (apt-get install gnome-network-admin)
Red Hat / Fedora Core IP Configuration Files:
The Red Hat configuration tools store the configuration information in the file /etc/sysconfig/network.
They will also allow one to configure routing information.
File: /etc/sysconfig/network
Static IP address Configuration: (Configure gateway address)
NETWORKING=yes
HOSTNAME=my-hostname - Hostname is defined here and by command hostname
FORWARD_IPV4=true - True for NAT firewall gateways and linux routers.
False for everyone else - desktops and servers.
GATEWAY="XXX.XXX.XXX.YYY" - Used if your network is connected to another network or the internet.
Static IP configuration. Gateway not defined here for DHCP client.
OR for DHCP client configuration:
NETWORKING=yes
HOSTNAME=my-hostname - Hostname is defined here and by command hostname
(Gateway is assigned by DHCP server.)
OR for NIS client configuration:
NETWORKING=yes
HOSTNAME=my-hostname - Hostname is defined here and by command hostname
NISDOMAIN=NISProject1 - NIS domain to attach
File (Red Hat/Fedora): /etc/sysconfig/network-scripts/ifcfg-eth0
(S.u.s.e.: /etc/sysconfig/network/ifcfg-eth-id-XX:XX:XX:XX:XX)
This file used by the command scripts ifup and ifdown
Static IP address configuration:
DEVICE=eth0
BOOTPROTO=static
BROADCAST=XXX.XXX.XXX.255
IPADDR=XXX.XXX.XXX.XXX
NETMASK=255.255.255.0
NETWORK=XXX.XXX.XXX.0
ONBOOT=yes - Will activate upon system boot
RHEL4/FC3 additions:
TYPE=Ethernet
HWADDR=XX:XX:XX:XX:XX:XX
GATEWAY=XXX.XXX.XXX.XXX
OR for DHCP client configuration:
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
RHEL4/FC3 additions:
IPV6INIT=no
USERCTL=no
PEERDNS=yes
TYPE=Ethernet
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TYPE=Ethernet
HWADDR=XX:XX:XX:XX:XX:XX
(Used by script /etc/sysconfig/network-scripts/ifup to bring the various network interfaces on-line)
To disable DHCP change BOOTPROTO=dhcp to BOOTPROTO=none
In order for updated information in any of these files to take effect, one must issue the command: service network restart (or:
/etc/init.d/network restart)
Network IP aliasing:
Assign more than one IP address to one ethernet card:
ifconfig eth0 XXX.XXX.XXX.XXX netmask 255.255.255.0 broadcast XXX.XXX.XXX.255
ifconfig eth0:0 192.168.10.12 netmask 255.255.255.0 broadcast 192.168.10.255
ifconfig eth0:1 192.168.10.14 netmask 255.255.255.0 broadcast 192.168.10.255
route add -host XXX.XXX.XXX.XXX dev eth0
route add -host 192.168.10.12 dev eth0
route add -host 192.168.10.14 dev eth0
In this example 0 and 1 are aliases in addition to the regular eth0. The result of the ifconfig command:
eth0 Link encap:Ethernet HWaddr 00:10:4C:25:7A:3F
inet addr:XXX.XXX.XXX.XXX Bcast:XXX.XXX.XXX.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:14218 errors:0 dropped:0 overruns:0 frame:0
TX packets:1362 errors:0 dropped:0 overruns:0 carrier:0
collisions:1 txqueuelen:100
Interrupt:5 Base address:0xe400
eth0:0 Link encap:Ethernet HWaddr 00:10:4C:25:7A:3F
inet addr:192.168.10.12 Bcast:192.168.10.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:5 Base address:0xe400
eth0:1 Link encap:Ethernet HWaddr 00:10:4C:25:7A:3F
inet addr:192.168.10.14 Bcast:192.168.10.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
Interrupt:5 Base address:0xe400
Config file: /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE=eth0:0
ONBOOT=yes
BOOTPROTO=static
BROADCAST=192.168.10.255
IPADDR=192.168.10.12
NETMASK=255.255.255.0
NETWORK=192.168.10.0
ONBOOT=yes
Aliases can also be shut down independently. i.e.: ifdown eth0:0
The option during kernel compile is: CONFIG_IP_ALIAS=y (Enabled by default in Redhat)
Note: The Apache web server can be configured so that different IP addresses can be assigned to specific domains being
hosted. See Apache configuration and "configuring an IP based virtual host" in the YoLinux Web site configuration tutorial.
DHCP Linux Client: get connection info: /sbin/pump -i eth0 --status
(Red Hat Linux 7.1 and older)
Device eth0
IP: 4.XXX.XXX.XXX
Netmask: 255.255.252.0
Broadcast: 4.XXX.XXX.255
Network: 4.XXX.XXX.0
Boot server 131.XXX.XXX.4
Next server 0.0.0.0
Gateway: 4.XXX.XXX.1
Domain: vz.dsl.genuity.net
Nameservers: 4.XXX.XXX.1 4.XXX.XXX.2 4.XXX.XXX.3
Renewal time: Sat Aug 11 08:28:55 2001
Expiration time: Sat Aug 11 11:28:55 2001
Changing the host name:
This is a three step process:
1. Issue the command: hostname new-host-name
2. Change network configuration file: /etc/sysconfig/network
Edit entry: HOSTNAME=new-host-name
3. Restart systems which relied on the hostname (or reboot):
Restart network services: service network restart
(or: /etc/init.d/network restart)
Restart desktop:
Bring down system to console mode: init 3
Bring up X-Windows: init 5
One may also want to check the file /etc/hosts for an entry using the system name which allows the system to be self aware.
The hostname may be changed at runtime using the command: sysctl -w kernel.hostname="superserver"
Note that hostnames may only contain alphanumeric characters, minus signs ("-"), and periods ("."). They must begin with an
alphabetic character and end with an alphanumeric character.
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alphabetic character and end with an alphanumeric character.
Change the host name using GUI tool: /usr/sbin/system-config-network
(Red Hat / Fedora / CentOS)
Hostname entries are made in two places:
Select the "DNS" tab. Select the "Devices" tab + "Edit" + the "General" tab.
Activating and De-Activating your NIC:
Commands for starting and stopping TCP/IP network
services on a Network Interface Card (NIC):
Activate: /sbin/ifup eth0
(Also: ifconfig eth0 up - Note: Even if no IP
address is assigned you can listen.)
De-Activate: /sbin/ifdown eth0
(Also: ifconfig eth0 down)
These scripts use the scripts and NIC config
files in /etc/sysconfig/network-scripts/
GUI Interface control/configuration:
Start/Stop network interfaces
/usr/bin/system-control-network (Fedora Core 2/3)
/usr/bin/redhat-control-network (RH 9.0 - FC-1)
Configure Ethernet, ISDN, modem, token Ring,
Wireless or DSL network connection:
/usr/sbin/system-config-network-druid (FC2/3)
/usr/sbin/redhat-config-network-druid (RH 9 -
FC-1)
Subnets:
M # OF CLASS C
CLASS CLASS
A SUB Slash CLASS A CLASS B CLASS B CLASS C SUB CLASS C SUB
A C
S NETS Fmt HOSTS HOSTS MASK MASK HOSTS MASK
MASK HOSTS
K
1
Invalid
255 or /32 16,777,214 255.0.0.0 65,534 255.255.0.0 254 255.255.255.0 255.255.255.255
1 address
256
Invalid
254 128 /31 33,554,430 254.0.0.0 131,070 255.254.0.0 510 255.255.254.0 2 255.255.255.254
addresses
2 hosts
252 64 /30 67,108,862 252.0.0.0 262,142 255.252.0.0 1,022 255.255.252.0 4 255.255.255.252
addresses
6 hosts
248 32 /29 134,217,726 248.0.0.0 524,286 255.248.0.0 2,046 255.255.248.0 8 255.255.255.248
addresses
14 hosts
240 16 /28 268,435,454 240.0.0.0 1,048,574 255.240.0.0 4,094 255.255.240.0 16 255.255.255.240
addresses
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addresses
30 hosts
224 8 /27 536,870,910 224.0.0.0 2,097,150 255.224.0.0 8,190 255.255.224.0 32 255.255.255.224
addresses
62 hosts
192 4 /26 1,073,741,822 192.0.0.0 4,194,302 255.192.0.0 16,382 255.255.192.0 64 255.255.255.192
addresses
126 hosts
128 2 /25 2,147,483,646 128.0.0.0 8,388,606 255.128.0.0 32,766 255.255.128.0 128 255.255.255.128
addresses
Binary position 8 7 6 5 4321
Value 128 64 32 16 8 4 2 1
Example: 192 1 1 0 0 0000
Example 192=128+64
Some addresses are reserved and outside this scope. Loopback (127.0.0.1), reserved class C 192.168.XXX.XXX, reserved
class B 172.31.XXX.XXX and reserved class A 10.XXX.XXX.XXX.
Subnet Example:
Your ISP assigns you a subnet mask of 255.255.255.248 for your office.
208.88.34.104 Network Base address
208.88.34.105 Computer 1
208.88.34.106 Computer 2
208.88.34.107 Computer 3
208.88.34.108 Computer 4
208.88.34.109 Computer 5
208.88.34.110 DSL router/Gateway
208.88.34.111 Broadcast address
Of the eight addresses, there are six assigned to hardware systems and ultimately only five usable addresses.
Links:
Subnet calculator
Table of subnets
IP Subnetting, Variable Subnetting, and CIDR (Supernetting)
CISCO.com: Subnet Masking and Addressing
Network Classes:
The concept of network classes is a little obsolete as subnets are now used to define smaller networks using CIDR (Classless
Inter-Domain Routing) as detailed above. These subnets may be part of a class A, B, C, etc network. For historical reference
the network classes are defined as follows:
Class A: Defined by the first 8 bits with a range of 0 - 127.
First number (8 bits) is defined by Internic i.e. 77.XXX.XXX.XXX
One class A network can define 16,777,214 hosts.
Range: 0.0.0.0 - 127.255.255.255
Class B: Defined by the first 8 bits with a range from 128 - 191
First two numbers (16 bits) are defined by Internic i.e. 182.56.XXX.XXX
One class B network can define 65,534 hosts.
Range: 128.0.0.0 - 191.255.255.255
Class C: Defined by the first 8 bits with a range from 192 - 223
First three numbers (24 bits) are defined by Internic i.e. 220.56.222.XXX
One class B network can define 254 hosts.
Range: 192.0.0.0 - 223.255.255.255
Class D: Defined by the first 8 bits with a range from 224 - 239
This is reserved for multicast networks (RFC988)
Range: 224.0.0.0 - 239.255.255.255
Class E: Defined by the first 8 bits with a range from 240 - 255
This is reserved for experimental use.
Range: 240.0.0.0 - 247.255.255.255
Enable Forwarding:
Forwarding allows the network packets on one network interface (i.e. eth0) to be forwarded to another network interface (i.e.
eth1). This will allow the Linux computer to conect ("ethernet bridge") or route network traffic.
The bridge configuration will merge two (or several) networks into one single network topology. IpTables firewall rules can be
used to filter traffic.
A router configuration can support multicast and basic IP routing using the "route" command. IP masquerading (NAT) can be
used to connect private local area networks (LAN) to the internet or load balance servers.
Turn on IP forwarding to allow Linux computer to act as a gateway or router.
echo 1 > /proc/sys/net/ipv4/ip_forward
Default is 0. One can add firewall rules by using ipchains.
Another method is to alter the Linux kernel config file: /etc/sysctl.conf Set the following value:
net.ipv4.ip_forward = 1
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See file /etc/sysconfig/network for storing this configuration.
FORWARD_IPV4=true
Change the default "false" to "true".
All methods will result in a proc file value of "1". Test: cat /proc/sys/net/ipv4/ip_forward
The TCP Man page - Linux Programmer's Manual and /usr/src/linux/Documentation/proc.txt (Kernel 2.2 RH 7.0-) cover
/proc/sys/net/ipv4/* file descriptions.
Also see: (YoLinux tutorials)
Configure Linux as an internet gateway router: Using Linux and iptables/ipchains to set up an internet gateway for home or
office (iptables)
Load balancing servers using LVS (Linux Virtual Server) (ipvsadm)
Adding a network interface card (NIC):
Manual method: This does not alter the permanent configuration and will only configure support until the next reboot.
cd /lib/modules/2.2.5-15/net/ - Use kernel version for your system. This example uses 2.2.5-15
(Fedora Core 3: /lib/modules/2.6.12-1.1381_FC3/kernel/net/)
Here you will find the modules supported by your system.
It can be permanently added to:
/etc/modprobe.conf (kernel 2.6)
/etc/modules.conf (kernel 2.4)
(or for older systems: /etc/conf.modules)
Example:
alias eth0 3c59x
/sbin/insmod 3c59x (For a 3Com ethernet card)
This inserts the specified module into the kernel.
/sbin/modprobe 3c59x
This also loads a module into the system kernel.
Modprobe command line options:
-r : to unload the module.
/sbin/modprobe -l * : list all modules.
/sbin/modprobe -lt net * : List only network modules
/sbin/modprobe -t net * : Try loading all network modules and see what sticks. (act of desperation)
ifconfig ...
The easy way: Red Hat versions 6.2 and later, ship with Kudzu, a device detection program which runs during system
initialization. (/etc/rc.d/init.d/kudzu) This can detect a newly installed NIC and load the appropriate driver. Then use /usr/sbin
/netconfig to configure the IP address and network settings. The configuration will be stored so that it will be utilized upon
system boot.
Systems with two NIC cards: Typically two cards are used when connecting to two networks. In this case the device must be
defined using one of three methods:
1. Use the Red Hat GUI tool /usr/bin/netcfg
OR
2. Define network parameters in configuration files:
Define new device in file (Red Hat/Fedora) /etc/sysconfig/network-scripts/ifcfg-eth1
(S.u.s.e 9.2: /etc/sysconfig/network/ifcfg-eth-id-XX:XX:XX:XX:XX)
DEVICE=eth1
BOOTPROTO=static
IPADDR=192.168.10.12
NETMASK=255.255.255.0
GATEWAY=XXX.XXX.XXX.XXX
HOSTNAME=node-name.name-of-domain.com
DOMAIN=name-of-domain.com
Special routing information may be specified, if necessary, in the file
(Red Hat/Fedora): /etc/sysconfig/static-routes
(S.u.s.e. 9.2: /etc/sysconfig/network/routes)
Example:
eth1 net XXX.XXX.XXX.0 netmask 255.255.255.0 gw XXX.XXX.XXX.XXX
OR
3. Define network parameters using Unix command line interface:
Define IP address:
ifconfig eth0 XXX.XXX.XXX.XXX netmask 255.255.255.0 broadcast XXX.XXX.XXX.255
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ifconfig eth0 XXX.XXX.XXX.XXX netmask 255.255.255.0 broadcast XXX.XXX.XXX.255
ifconfig eth1 192.168.10.12 netmask 255.255.255.0 broadcast 192.168.10.255
If necessary, define route with with the route command:
Examples:
route add default gw XXX.XXX.XXX.XXX dev eth0
route add -net XXX.XXX.XXX.0 netmask 255.255.255.0 gw XXX.XXX.XXX.XXX dev eth0
Where XXX.XXX.XXX.XXX is the gateway to the internet as defined by your ISP or network operator.
If a mistake is made just repeat the route command substituting "del" in place of "add".
Configuring your NIC: Speed and Duplex settings:
This is usually not necessary because most ethernet adapters can auto-negotiate link speed and duplex setting.
List NIC speed and configuration: mii-tool
eth0: negotiated 100baseTx-FD flow-control, link ok
Verbose mode: mii-tool -v
eth0: negotiated 100baseTx-FD flow-control, link ok
product info: Intel 82555 rev 4
basic mode: autonegotiation enabled
basic status: autonegotiation complete, link ok
capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
Set NIC configuration: mii-tool -F option
Option Parameters
-F 100baseTx-FD
100baseTx-HD
10baseT-FD
10baseT-HD
-A 100baseT4
100baseTx-FD
100baseTx-HD
10baseT-FD
10baseT-HD
Query NIC with ethtool:
Command Description
ethtool -g eth0 Queries ethernet device for rx/tx ring parameter information.
ethtool -a eth0 Queries ethernet device for pause parameter information.
ethtool -c eth0 Queries ethernet device for coalescing information.
ethtool -i eth0 Queries ethernet device for associated driver information.
ethtool -d eth0 Prints a register dump for the specified ethernet device.
ethtool -k eth0 Queries ethernet device for offload information.
ethtool -S eth0 Queries ethernet device for NIC and driver statistics.
Man Pages:
mii-tool - view, manipulate media-independent interface status
ethtool - Display or change ethernet card settings
Route:
Static routes: IP (Internet Protocol) uses a routing table to determine where packets should be sent. First the packet is
examined to see if its' destination is for the local or remote network. If it is to be sent to a remote network, the routing table is
consulted to determine the path. If there is no information in the routing table then the packet is sent to the default gateway.
Static routes are set with the route command and with the configuration file
(Red Hat/Fedora): /etc/sysconfig/network-scripts/route-eth0
or
(Red Hat 7: /etc/sysconfig/static-routes)
(S.u.s.e. 9.2: /etc/sysconfig/network/routes):
10.2.3.0/16 via 192.168.10.254
See command: /etc/sysconfig/network-scripts/ifup-routes eth0
Dynamic routes: RIP (Routing Information Protocol) is used to define dynamic routes. If multiple routes are possible, RIP will
choose the shortest route. (Fewest hops between routers not physical distance.) Routers use RIP to broadcast the routing table
over UDP port 520. The routers would then add new or improved routes to their routing tables.
Man pages:
route - show / manipulate the IP routing table (Static route)
Show routes:
Option Description
-n display IP addresses. Do not resolve host names for faster results.
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-e Print more extensive information about routes.
-v Verbose.
--help Route command information.
Manipulate routes:
Option Description
add or del or neither Add or delete route information. If not specified then print route table information.
-host XXX.XXX.XXX.XXX Add a single computer host identified by the IP address.
-net XXX.XXX.XXX.XXX Add a network identified by the network address, to the route.
gw XXX.XXX.XXX.XXX Specify the network gateway.
netmask XXX.XXX.XXX.XXX Specify the network netmask.
default Of all the routes specified, identify one as the default network route.
(typically the gateway is specified as the default route)
Examples:
Show routing table: route -e
Access individual computer host specified via network interface card eth1:
route add -host 123.213.221.231 eth1
Access ISP network identified by the network address and netmask using network interface card eth0:
route add -net 10.13.21.0 netmask 255.255.255.0 gw 192.168.10.254 eth0
Conversly: route del -net 10.13.21.0 netmask 255.255.255.0 gw 192.168.10.254 eth0
Specify default gateway to use to access remote network via network interface card eth0:
route add default gw 201.51.31.1 eth0
(Gateway can also be defined in /etc/sysconfig/network)
Specify two gateways for two network destinations: (i.e. one external, one internal private network. Two
routers/gateways will be specified.)
Add internet gateway as before: route add default gw 201.51.31.1 eth0
Add second private network: route add -net 10.0.0.0 netmask 255.0.0.0 gw 192.168.10.254 eth0
routed - network routing daemon. Uses RIP protocol to update routing table.
ipx_route - show / manipulate the IPX routing table - IPX is the Novell networking protocol (Not typically used unless your
office has Novell servers)
ifuser - Identify destinations routed to a particular network interface.
VPN, Tunneling:
Commercial VPN Linux software solutions - YoLinux
OpenSWAN.org - IPSec VPN for Linux
FreeSWAN.org - IPSec VPN for Linux
FreeSWAN tutorial - howto
OpenVPN - SSL VPN solution for site to site, WiFi security, and enterprise-scale remote access with load balancing,
failover, and fine-grained access-controls.
SSL-Explorer - Java SLL based VPN
Quagga dynamic routing suite VLAN
n2n pier to pier within a private fabric
CIPE: Crypto IP Encapsulation (Easiest way to configure two Linux gateways connecting two private networks over the
internet with encryption.)
CIPE Home page - CIPE is a simple encapsulation system that securely connects two subnets.
GRE Tunneling - Generic Routing Encapsulation - Hugo Samayoa
VPN HowTo - Matthew D. Wilson
Installing and Running PPTP on Linux
L2TP Extensions (l2tpext) Internet Drafts.
Usefull Linux networking commands:
/etc/rc.d/init.d/network start - command to start, restart or stop the network
netstat - Display connections, routing tables, stats etc
List externally connected processes: netstat -punta
-a: Show both listening and non-listening sockets.
-p: Show PID of process owning socket
-u: Show UDP
-t: Show TCP
-n: Show IP addresses only. Don't resolve host names
-g: Show multi-cast group membership info
-c: Continuous mode - update info every second
-v: Verbose
-e: Extended information
-o: show network timer information
List all connected processes: netstat -nap
Show network statistics: netstat -s
Display routing table info: netstat -rn
$ netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
Flags:
G: route uses gateway
U: Interface is "up"
H: Only a single host is accessible (eg. loopback)
D: Entry generated by ICMP redirect message
M: Modified by ICMP redirect message
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M: Modified by ICMP redirect message
Display interface statistics: netstat -i
$ netstat -i
Kernel Interface table
Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0 1500 0 2224 0 0 0 1969 0 0 0 BMRU
lo 16436 0 1428 0 0 0 1428 0 0 0 LRU
Where:
RX-OK/TX-OK: number of packets transmitted/received error free
RX-ERR/TX-ERR: number of dammaged/error packets transmitted/received
RX-DRP/TX-DRP: number of dropped packets
RX-OVR/TX-OVR: number of packets dropped because of a buffer overrun
Flags:
B: A broadcast address has been set
L: This interface is a loopback device
M: All packets are received
N: Trailers are avoided
O: ARP is turned off for this interface
P: Point-to-point connection
R: Interface is running
U: Interface is up
ping - send ICMP ECHO_REQUEST packets to network hosts. Use Cntl-C to stop ping.
traceroute - print the route packets take to network host.
(Ubuntu Note: Typically Ubuntu installs tracepath for IPv4 and traceroute6 for IPv6. One can install traceroute: apt-get
install traceroute)
traceroute IP-address-of-server
traceroute domain-name-of-server
mtr - a network diagnostic tool introduced in Fedora - Like traceroute except it gives more network quality and network
diagnostic info. Leave running to get real time stats. Reports best and worst round trip times in milliseconds.
mtr IP-address-of-server
mtr domain-name-of-server
whois - Lookup a domain name in the internic whois database.
finger - Display information on a system user. i.e. finger user@host Uses $HOME/.plan and $HOME/.project user files.
iptables - IP firewall administration (Linux kernel 2.6/2.4) See YoLinux firewall/gateway configuration.
ipchains - IP firewall administration (Linux kernel 2.2) See YoLinux firewall/gateway configuration.
socklist - Display list of open sockets, type, port, process id and the name of the process. Kill with fuser or kill.
host - Give a host name and the command will return IP address. Unlike nslookup, the host command will use both
/etc/hosts as well as DNS.
Example: host domain-name-of-server
nslookup - Give a host name and the command will return IP address. Also see Testing your DNS (YoLinux Tutorial) Note
that nslookup does not use the /etc/hosts file.
inetd/xinetd: Network Socket Listener Daemons:
The network listening daemons listen and respond to all network socket connections made on the TCP/IP ports assigned to it.
The ports are defined by the file /etc/services. When a connection is made, the listener will attempt to invoke the assigned
program and pipe the data to it. This simplified matters by allowing the assigned program to read from stdin instead of making
its own sockets connection. The listener hadles the network socket connection. Two network listening and management
daemons have been used in Red Hat Linux distributions:
inetd: Red Hat 6.x and older
xinetd: Red Hat 7.0-9.0, Fedora Core
inetd:
Configuration file: /etc/inetd.conf
Entries in this file consist of a single line made up of the following fields:
service socket-type protocol wait user server cmdline
service: The name assigned to the service. Matches the name given in the file /etc/services
socket-type:
stream: connection protocols (TCP)
dgram: datagram protocols (UDP)
raw
rdm
seqpacket
protocol: Transport protocol name which matches a name in the file /etc/protocols. i.e. udp, icmp, tcp, rpc/udp,
rpc/tcp, ip, ipv6
wait: Applies only to datagram protocols (UDP).
wait[.max]: One server for the specified port at any time (RPC)
nowait[.max]: Continue to listen and launch new services if a new connection is made. (multi-threaded)
Max refers to the maximum number of server instances spawned in 60 seconds. (default=40)
user[.group]: login id of the user the process is executed under. Often nobody, root or a special restricted id for that
service.
server: Full path name of the server program to be executed.
cmdline: Command line to be passed to the server. This includes argument 0 (argv[0]), that is the command name.
This field is empty for internal services. Example of internal TCP services: echo, discard, chargen (character
generator), daytime (human readable time), and time (machine readable time). (see RFC)
Sample File: /etc/inetd.conf
#echo stream tcp nowait root internal
#echo dgram udp wait root internal
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#echo dgram udp wait root internal
ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -a
#pop-3 stream tcp nowait root /usr/sbin/tcpd ipop3d
#swat stream tcp nowait.400 root /usr/sbin/swat swat
A line may be commented out by using a '#' as the first character in the line. This will turn the service off. The maximum
length of a line is 1022 characters.
The inet daemon must be restarted to pick up the changes made to the file:
/etc/rc.d/init.d/inetd restart
For more information see the man pages "inetd" and "inetd.conf".
xinetd: Extended Internet Services Daemon:
Xinetd has access control machanisms, logging capabilities, the ability to make services available based on time, and can
place limits on the number of servers that can be started, redirect services to different ports and network interfaces (NIC)
or even to a different server, chroot a service etc... and thus a worthy upgrade from inetd.
Use the command chkconfig --list to view all system services and their state. It will also list all network services
controlled by xinetd and their respective state under the title "xinetd based services". (Works for xinetd (RH7.0+) but not
inetd)
The xinetd network daemon uses PAM also called network wrappers which invoke the /etc/hosts.allow and
/etc/hosts.deny files.
Configuration file: /etc/xinetd.conf which in turn uses configuration files found in the directory /etc/xinetd.d/.
To turn a network service on or off:
Edit the file /etc/xinetd.d/service-name
Set the disable value:
disable = yes
or
disable = no
Restart the xinetd process using the signal:
SIGUSR1 (kill -SIGUSR1 process-id) - Soft reconfiguration does not terminate existing connections.
(Important if you are connected remotely)
SIGUSR2 - Hard reconfiguration stops and restarts the xinetd process.
(Note: Using the HUP signal will terminate the process.)
OR
Use the chkconfig command: chkconfig service-name on
(or off)
This command will also restart the xinetd process to pick up the new configuration.
The file contains entries of the form:
service service-name
{
attribute assignment-operator value value ...
...
{
Where:
attribute:
disable:
yes
no
type:
RPC
INTERNAL:
UNLISTED: Not found in /etc/rpc or /etc/services
id: By default the service id is the same as the service name.
socket_type:
stream: TCP
dgram: UDP
raw: Direct IP access
seqpacket: service that requires reliable sequential datagram transmission
flags: Combination of: REUSE, INTERCEPT, NORETRY, IDONLY, NAMEINARGS, NODELAY, DISABLE, KEEPALIVE,
NOLIBWRAP.
See the xinetd man page for details.
protocol: Transport protocol name which matches a name in the file /etc/protocols.
wait:
no: multi-threaded
yes: single-threaded - One server for the specified port at any time (RPC)
user: See file : /etc/passwd
group: See file : /etc/group
server: Program to execute and recieve data stream from socket. (Fully qualified name - full pathe name of
program)
server_args: Unlike inetd, arg[0] or the name of the service is not passed.
only_from: IP address, factorized address, netmask range, hostname or network name from file
/etc/networks.
no_access: Deny from ... (inverse of only_from)
access_times
port: See file /etc/services
Also: log_type, log_on_success, log_on_failure (Log options: += PID,HOST,USERID,EXIT,DURATION,ATTEMPT
and RECORD), rpc_version, rpc_number, env, passenv, redirect, bind, interface, banner, banner_success,
banner_fail, per_source, cps, max_load, groups, enabled, include, includedir, rlimit_as, rlimit_cpu, rlimit_data,
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banner_fail, per_source, cps, max_load, groups, enabled, include, includedir, rlimit_as, rlimit_cpu, rlimit_data,
rlimit_rss, rlimit_stack.
The best source of information is the man page and its many examples.
assignment-operator:
=
+=: add a value to the set of values
-=: delete a value from the set of values
Then restart the daemon: /etc/rc.d/init.d/xinetd restart
Example from man page: Limit telnet sessions to 8 Mbytes of memory and a total 20 CPU seconds for child processes.
service telnet
{
socket_type = stream
wait = no
nice = 10
user = root
server = /usr/etc/in.telnetd
rlimit_as = 8M
rlimit_cpu = 20
}
[Pitfall] Red Hat 7.1 with updates as of 07/06/2001 required that I restart the xinetd services before FTP would work
properly even though xinetd had started without failure during the boot sequence. I have no explanation as to why this
occurs or how to fix it other than to restart xinetd: /etc/rc.d/init.d/xinetd restart.
Man Pages:
xinetd
xinetd.conf
xinetd.log
tcpd
For more info see:
LinuxFocus.org: xinetd - Frederic Raynal
RedHat.com: Controlling Access to Services
http://www.xinetd.org
See RFC's: 862, 863, 864, 867, 868, 1413.
man page xinetd, xinetd.conf, xinetd.log
Remote commands: rcp, rsh, rlogin, rwho, ...
Most of the original Unix remote commands have been superceeded by secure shell equivalents. Instead of telnet, rsh or rlogin,
one should use the encrypted connection ssh.
telnet - user interface to the TELNET protocol
rlogin - remote login
rsh - remote shell to execute a command and return results
uux - Remote command execution over UUCP
rcp - remote file copy
uucp - Unix to Unix copy
uuxqt - UUCP execution daemon
uucico - UUCP file transfer daemon
cu - Call up another system (cu is an old legacy command which is reported to not work very well)
See the YoLinux.com secure shell tutorial for use of ssh, rssh, scp and sftp
RWHO: Remote Who daemon - rwhod
The "rwho" command is used to display users logged into computers on your LAN.
By default, Red Hat Linux has the network interface to the rwhod disabled. Thus if one issues the command " rwho", you will only
see who is logged into the system you are logged into and not remote systems on the network. This is a safe approach for
internet servers as it reduces the exposure of a service which could be exploited by hackers. If you wish to use rwhod on a local
private and firewall protected network, here is how:
Allow broacast capabilities. Edit /etc/init.d/rwhod
change from: daemon rwhod
to: daemon rwhod -b
Start service:
Set service to start with system boot: chkconfig --level 345 rwhod on
Start rwhod service: service rwhod start
(or: service rwhod restart)
Man pages:
rwho: who is logged in on local network machines
rwhod: system status server
who: show who is logged on to the same system
RPC: Remote Procedure Calls (Portmapper)
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Portmapper is a network service required to support RPC's. Many services such as NFS (file sharing services) and NIS
(Network Information Services) require portmapper.
An RPC server makes available a collection of procedures (programs) that a client system may call and then receive the
returned results. The list of services available is listed in /etc/rpc on the server. The message communication is in a machine
independent form called XDR (External Data Representation format).
List RPC services supported: [root]# rpcinfo -p localhost
Starting portmap server:
/etc/rc.d/init.d/portmap start
service portmap start (Red Hat/Fedora Core)
Man Pages:
portmap - DARPA port to RPC program number mapper
rpcinfo - report RPC information
pmap_dump - print a list of all registered RPC programs
pmap_set - set the list of registered RPC programs
/etc/rpc - rpc program number data base
PAM: Network Wrappers:
Pluggable Authentication Modules for Linux (TCP Wrappers)
This system allows or denies network access. One can reject or allow specific IP addresses or subnets to access your system.
File: /etc/hosts.allow
in.ftpd:208.188.34.105
This specifically allows the given IP address to ftp to your system. One can also specify an entire domain. i.e. .name-
of-domain.com
Note the beginning ".".
File: /etc/hosts.deny
ALL:ALL
This generally denies any access.
See the pam man page.
File: /etc/inetd.conf
ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -a
The inet daemon accepts the incoming network stream and assigns it to the PAM TCP wrapper, /usr/sbin/tcpd, which accepts
or denies the network connection as defined by /etc/hosts.allow and /etc/hosts.deny and then passes it along to ftp. This is
logged to /var/log/secure
Advanced PAM: More specific access can be assigned and controlled by controlling the level of authentication required for
access.
Files reflect the inet service name. Rules and modules are stacked to achieve the level of security desired.
See the files in /etc/pam.d/... (some systems use /etc/pam.conf)
The format: service type control module-path module-arguments
auth - (type) Password is required for the user
nullok - Null or non-existatant password is acceptable
shadow - encrypted passwords kept in /etc/shadow
account - (type) Verifies password. Can track and force password changes.
password - (type) Controls password update
retry=3 - Sets the number of login attempts
minlen=8 - Set minimum length of password
session - (type) Controls monitoring
Modules:
/lib/security/pam_pwdb.so - password database module
/lib/security/pam_shells.so -
/lib/security/pam_cracklib.so - checks is password is crackable
/lib/security/pam_listfile.so
After re-configuration, restart the inet daemon: killall -HUP inetd
For more info see:
Wietse's Papers
Pluggable Authentication Modules for Linux (PAM) Home Page
ICMP:
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ICMP is the network protocol used by the ping and traceroute commands.
ICMP redirect packets are sent from the router to the host to inform the host of a better route. To enable ICMP redirect, add the
following line to /etc/sysctl.conf :
net.ipv4.conf.all.accept_redirects = 1
Add the following to the file: /etc/rc.d/rc.local
for f in /proc/sys/net/ipv4/conf/*/accept_redirects
do
echo 1 > $f
done
Command to view Kernel IP routing cache: /sbin/route -Cn
NOTE: This may leave you vulnerable to hackers as attackers may alter your routes.
Blocking ICMP and look invisible to ping:
The following firewall rules will drop ICMP requests.
Iptables:
iptables -A OUTPUT -p icmp -d 0/0 -j DROP
Ipchains:
ipchains -A output -p icmp -d 0/0 -j DENY
OR drop all incomming pings:
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all
This is sometimes necessary to look invisible to DOS (Denial Of Service) attackers who use ping to watch your machine and
launch an attack when it's pressence is detected
Network Monitoring Tools:
tcpdump - dump traffic on a network. See discussion below.
Command line option Description
-c Exit after receiving count packets.
-C Specify size of output dump files.
-i Specify interface if multiple exist. Lowest used by default. i.e. eth0
-w file-name Write the raw packets to file rather than parsing and printing them out.
They can later be printed with the -r option.
-n Improve speed by not performing DNS lookups. Report IP addresses.
-t Don't print a timestamp on each dump line.
Filter expressions:
primitive Description
host host-name If host has multiple IP's, all will be checked.
net network-number Network number.
net network-number mask mask Network number and netmask specified.
port port-number Port number specified.
tcp Sniff TCP packets.
udp Sniff UDP packets.
icmp Sniff icmp packets.
Examples:
tcpdump tcp port 80 and host server-1
tcpdump ip host server-1 and not server-2
iptraf - Interactive Colorful IP LAN Monitor
nmap - Network exploration tool and security scanner
List pingable nodes on network: nmap -sP 192.168.0.0/24
Scans network for IP addresses 192.168.0.0 to 192.168.0.255 using ping.
Ethereal - Network protocol analyzer. Examine data from a live network.
RPM's required:
ethereal-0.8.15-2.i386.rpm - Red Hat 7.1 Powertools CD RPM
ucd-snmp-4.2-12.i386.rpm - Red Hat 7.1 binary CD 1
ucd-snmp-utils-4.2-12.i386.rpm - Red Hat 7.1 binary CD 1
Also: gtk+, glib, glibc, XFree86-libs-4.0.3-5 (base install)
There is an error in the ethereal package because it does not show the snmp libraries as a dependancies, but you
can deduce this from the errors that you get if the ucd-snmp libraries are not installed.
EtherApe - Graphical network monitor for Unix modeled after etherman. This is a great network discovery program with
cool graphics. (Red Hat Powertools CD 7.1)
Gkrellm - Network and system monitor. Good for monitoring your workstation. (Red Hat Powertools CD)
IPTraf - ncurses-based IP LAN monitor. (Red Hat Powertools CD)
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IPTraf - ncurses-based IP LAN monitor. (Red Hat Powertools CD)
Cheops - Network discovery, location, diagnosis and management. Cheops can identify all of the computers that are on
your network, their IP address, their DNS name, the operating system they are running. Cheops can run a port scan on
any system on your network. (Red Hat Powertools CD)
ntop - Shows network usage in a way similar to what top does for processes. Monitors how much data is being sent and
received on your network. (Red Hat Powertools CD)
MRTG - Multi Router Traffic Grapher - Monitor network traffic load using SNMP and generate an HTML/GIF report. (See
sample output)
dnsad - IP traffic capture. Export to Cisco Netflow for network analysis reporting.
Big Brother - Monitoring ans services availablility.
OpenNMS.org - Network Management using SNMP.
Nagios - host, service and network monitoring
Angel network monitor
Using tcpdump to monitor the network:
[root]# ifconfig eth0 promisc - Put nic into promiscuous mode to sniff traffic.
[root]# tcpdump -n host not XXX.XXX.XXX.XXX | more - Sniff net but ignore IP which is your remote session.
[root]# ifconfig eth0 -promisc - Pull nic out of promiscuous mode.
Network Intrusion and Hacker Detection Systems:
SNORT: Monitor the network, performing real-time traffic analysis and packet logging on IP networks for the detection of an
attack or probe.
InterSect Alliance - Intrusiuon analysis. Identifies malicious or unauthorized access attempts.
ARP: Address Resolution Protocol
Ethernet hosts use the Address Resolution Protocol (ARP) to convert a 32-bit internet IP addresses into a 48-bit Ethernet MAC
address used by network hardware. (See: RFC 826) ARP broadcasts are sent to all hosts on the subnet by the data transmitting
host to see who replies. The broadcast is ignored by all except the intended receiver which recognizes the IP address as its
own. The MAC addresses are remembered (APR cache) for future network communications. Computers on the subnet typically
keep a cache of ARP responses. ARP broadcasts are passed on by hubs and switches but are blocked by routers.
Reverse ARP (See: RFC 903) is a bootstrap protocol which allows a client to broadcast requesting a server to reply with its IP
address.
View ARP tables:
Shows other systems on your network (including IP address conflicts): /sbin/arp -a
Show ARP table Linux style: /sbin/arp -e
List ARP table: cat /proc/net/arp
Note that the use of a switch instead of a hub will limit your view of other hosts. Typically all you will see in the arp table is your
router or gateway.
Set/Configure ARP tables:
Add a host's IP address: /sbin/arp -s hostname XX:XX:XX:XX:XX:XX pub
Delete a host from the table: /sbin/arp -d hostname
This can be used to remove a duplicate IP or force a new interface to provide info.
Man pages:
arp (8) man page - manipulate the system ARP cache
arpwatch (8) man page - keep track of ethernet/ip address pairings
arpsnmp (8) man page - keep track of ethernet/ip address pairings. Reads information generated by snmpwalk
arping (8) man page - send ARP REQUEST to a neighbor host
Print ARP reply (similar to arp -a): arping 192.168.10.99
ip (8) man page - show / manipulate routing, devices, policy routing and tunnels
View ARP table: ip neighbor
ARP is something that simply works. No Linux system configuration is necessary. It's all part of the ethernet and IP protocol. The
aforementioned information is just part of the Linux culture of full visibility into what is going on.
Configuring Linux For Network Multicast:
Regular network exchanges of data are peer to peer unicast transactions. An HTTP request to a web server (TCP/IP), email
SNMP (TCP/IP), DNS (UDP), FTP (TCP/IP), ... are all peer to peer unicast transactions. If one wants to transmit a video, audio
or data stream to multiple nodes with one transmission stream instead of multiple individual peer to peer connections, one for
each node, one may use multicasting to reduce network load. Note that multicast and a network broadcast are different.
Multicast messages are only "heard" by the nodes on the network that have "joined the multicast group" which are those that
are interested in the information.
The Linux kernel is Level-2 Multicast-Compliant. It meets all requirements to send, receive and act as a router for multicast
datagrams. For a process to receive multicast datagrams it has to request the kernel to join the multicast group and bind the
port receiving the datagrams. When a process is no longer interested in the multicast group, a request is made to the kernel to
leave the group. It is the kernel/host which joins the multicast group and not the process. Kernel configuration requires
"CONFIG_IP_MULTICAST=y". In order for the Linux kernel to support multicast routing, set the following in the kernel config:
CONFIG_IP_MULTICAST=y
CONFIG_IP_ROUTER=y
CONFIG_IP_MROUTE=y
CONFIG_NET_IPIP=y
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CONFIG_NET_IPIP=y
The default Red Hat / Fedora kernels are compiled to support multicast.
See the YoLinux tutorial on optimization and rebuilding the Linux kernal.
Note that on multihomed systems (more than one IP address/network card), only one device can be configured to handle
multicast.
Class D networks with a range of IP addresses from 224.0.0.0 to 239.255.255.255 (See Network Classes above) have typically
been reserved for multicast.
Usefull commands:
Command Description
List multicast group to which the host is subscribed. Use "Internet Group Management
cat /proc/net/igmp Protocol".
(See /usr/src/linux/net/core/igmp.c)
cat /proc/net List multicast interfaces.
/dev_mcast (See /usr/src/linux/net/core/dev_mcast.c)
ping 224.0.0.1 All hosts configured for multicast will respond with their IP addresses
ping 224.0.0.2 All routers configured for multicast will respond
ping 224.0.0.3 All PIM routers configured for multicast will respond
ping 224.0.0.4 All DVMRP routers configured for multicast will respond
ping 224.0.0.5 All OSPF routers configured for multicast will respond
Multicast transmissions are achieved through proper routing, router configuration (if communicating through subnets) and
programatically with the use of the following "C" function library calls:
Function Call Description
setsockopt() Pass information to the Kernel.
getsockopt() Retrieve information broadcast using multicast.
For more on multicast programming see: Multicast Howto.
The multicast application will specify the multicast loopback interface, TTL (network time to live), network interface and the
multicast group to add or drop.
Add route to support multicast:
route add 224.0.0.0 netmask 240.0.0.0 dev eth0
Note that if adding a route to forward packets through a router, that the router MUST be configured to forward multicast packets.
Many routers do not support forwarding of multicast packets or have a default configuration which does not. The internet by
default does not forward multicast packets.
Serial Line IP:
Linux can support IP protocol over serial device interfaces. Over long distances this is typically supported using a modem over
telephone lines (POTS: Plain Old Telephone Service) or satellite communications.
PPP: Point-to-Point Protocol
This is the most common form of IP over serial line and is the most common technique used by telephone dial-up ISPs. The
following tutorials use a Hayes command set compatible modem.
YoLinux Tutorial: Configuring PPP dial up connections to an ISP
YoLinux Tutorial: Dialing Compuserve
YoLinux Tutorial: Dialing AOL
YoLinux Tutorial: Configuring PPP dial-in connections
PPTP: Point-to-Point Tunneling Protocol
RFC 2637: Point-to-Point Tunneling Protocol (PPTP).
PPTP-Linux Client - A PPTP Linux client that allows a linux system to connect to a PPTP server. Developed by C. S.
Ananian.
Counterpane Systems FAQ on Microsoft's PPTP Implementation - FAQ on the security flaws in Microsoft's PPTP
Implementation.
SLIP: Serial Line IP (older than PPP and less capable)
Devices:
Interfaces Description
sl0 sl1 sl2 sl3 SLIP interfaces. Linux kernel supports up to four.
COM1 COM2 COM3 COM4 Serial Ports (RS-232 hardware)
Serial devices (dial in)
/dev/ttyS0 /dev/ttyS1 /dev/ttyS2 /dev/ttyS3
(virtual terminal consoles)
/dev/cua0 /dev/cua1 /dev/cua2 /dev/cua3 Serial devices (dial out)
4 4 4 4 Interface major numbers (dial in)
5 5 5 5 Interface major numbers (dial out)
64 65 66 67 Interface minor numbers
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64 65 66 67 Interface minor numbers
The command ls -l /dev/ttyS* /dev/cua* will show the device major and minor numbers.
The major and minor numbers are used when creating a SPLIP interface:
Example:
mknod -m 666 /dev/cua1 c 5 65
chown root.uucp /dev/cua1
SLIP configuration:
Configure /etc/resolve.conf
(See notes above in this tutorial)
Attach network interface to serial line on COM2: /sbin/slattach -p slip -s 19200 /dev/ttyS1 &
Assign local and remote IP: /sbin/ifconfig sl0 192.168.1.10 pointopoint 192.168.1.40 up
Assign local IP (192.168.1.10) and connect to remote server (192.168.1.40)
Alternate example: /sbin/route add plip1 192.168.1.10 pointopoint 192.168.1.40
Add route: /sbin/route add default dev sl0 &
Also see Dialup IP (DIP)
PLIP: Parallel Line IP
Point to point serial links (rather than broadcast networks line ethernet), can alsobe supported over parallel printer ports.
An IP network at 10 to 20 kBps over parallel printer ports lp0 or lp1 are much faster than serial. Linux supports mode 0 PLIP
transferring half bytes of data at a time. Requires "NULL Printer" or "Turbo Laplink" printer connection. See kernel source
drivers/net/Space.c.
Interface I/O Port IRQ
plip0 0x3BC 7
plip1 0x378 7
plip2 0x278 5
PLIP Configuration:
ifconfig plip1 192.168.1.10 pointopoint 192.168.1.40
connect host 192.168.1.10 to remote host 192.168.1.40
route add default gw 192.168.1.40
Specify remote host as the gateway.
On the remote host at the other end of the cable, the opposite must be specified:
ifconfig plip1 192.168.1.40 pointopoint 192.168.1.10
route add 192.168.1.10 gw 192.168.1.40
Serial port related man pages:
setserial - get/set Linux serial port information
Typical configuration:
Interrupt detection: /sbin/setserial -W /dev/cua*
Configuration: /sbin/setserial /dev/cua1 auto_irq skip_test autoconfig
or /sbin/setserial /dev/cua1 auto_irq skip_test autoconfig uart 16550
Display Configuration: /sbin/setserial -bg /dev/cua*
Enable hardware handshake: stty crtscts < /dev/cua1
(verify: stty -s < /dev/cua1)
stty - change and print terminal line settings
tty - print the file name of the terminal connected to standard input
pppd - Point-to-Point Protocol Daemon
slattach - attach a network interface to a serial line
mknod - make block or character special files
Living in a MS/Windows World:
SMB4k: My favorite MS/Windows file share browser.
In Nautilus use the URL "smb:" to view MS/Windows servers. [tutorial]
LinNeighborhood: Linux workstation gui tool.
Make your life simple and use the GUI/File Manager LinNeighborhood. It uses smbmount, samba and smbclient to give
you access to MS/Windows servers and printers.
LinNeighborhood Home Page
LinNeighborhood Screen Shot
See the YoLinux tutorial on integrating Linux into a Microsoft network.
Network Definitions:
IPv4: Most of the Internet servers and personal computers use Internet Protocol version 4 (IPv4). This uses 32 bits to
assign a network address as defined by the four octets of an IP address up to 255.255.255.255. Which is the
representation of four 8 bit numbers thus totaling 32 bits.
IPv6: Internet Protocol version 6 (IPv6) uses a 128 bit address and thus billions and billions of potential addresses. The
protocol has also been upgraded to include new quality of service features and security. Currently Linux supports IPv6 but
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protocol has also been upgraded to include new quality of service features and security. Currently Linux supports IPv6 but
IPv4 is used when connecting your computer to the internet.
TCP/IP: (Transmission Control Protocol/Internet Protocol) uses a client - server model for communications. The protocol
defines the data packets transmitted (packet header, data section), data integrity verification (error detection bytes),
connection and acknowledgement protocol, and re-transmission.
TCP/IP time to live (TTL): This is a counting mechanism to determine how long a packet is valid before it reaches its
destination. Each time a TCP/IP packet passes through a router it will decrement its TTL count. When the count reaches
zero the packet is dropped by the router. This ensures that errant routing and looping aimless packets will not flood the
network.
MAC Address: (media access control) is the network card address used for communication between other network
devices on the subnet. This info is not routable. The ARP table maps TCP/IP address (global internet) to the local
hardware on the local network. Use the command /sbin/ifconfig to view both the IP address and the MAC address. The
MAC address uniquely identifies each node of a network and is used by the Ethernet protocol.
Full Duplex: Allows the simultaneous sending and receiving of packets. Most modern modems support full duplex.
Half Duplex: Allows the sending and receiving of packets in one direction at a time only.
OSI 7 Layer Model: The ISO (International Standards Organization) has defined the OSI (Open Systems Interconnection)
model for current networking protocols.
OSI Layer Description Linux Networking Use
7 Application Layer. telnet, web browser, sendmail
The top layer for communications applications like email and the web.
6 Presentation Layer. SMTP, http
Syntax and format of data transfer.
5 Session Layer.
4 Transport Layer. TCP
Connection, acknowledgement and data packet transmission. UDP
3 Network Layer. IP
ARP
2 Data Link Layer. Ethernet
Error control, timing
1 Physical Layer. Ethernet
Electrical characteristics of signal and NIC
Network Hub: Hardware to connect network devices together. The devices will all be on the same network and/or subnet.
All network traffic is shared and can be sniffed by any other node connected to the same hub.
Network Switch: Like a hub but creates a private link between any two connected nodes when a network connection is
established. This reduces the amount of network collisions and thus improves speed. Broadcast messages are still sent to
all nodes.
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