Linux Networking

TCP/IP Network 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         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)
    Red Hat/Fedora 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:

  • 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.

Static IP address assignment:

Choose one of the following methods:

  • Command Line:
        /sbin/ifconfig eth0 netmask broadcast

    Network address by convention would be the lowest:
    Broadcast address by convention would be the highest:
    The gateway can be anything, but following convention: Note: the highest and lowest addresses are based on the netmask. The previous example is based on a netmask of

  • 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 commands to the end of the file /etc/rc.d/rc.local to execute them upon boot.) 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.

The IANA has allocated IP addresses in the range of to for private networks.

Helpful tools:

Ubuntu / Debian IP Configuration Files:

File: /etc/network/interfaces

Static IP example:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static

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


  • 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

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)
    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:

    HOSTNAME=my-hostname      - Hostname is defined here and by command hostname

    (Gateway is assigned by DHCP server.)

    OR for NIS client configuration:

    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:

    ONBOOT=yes                       - Will activate upon system boot

    RHEL4/FC3 additions:

    • TYPE=Ethernet

    OR for DHCP client configuration:


    RHEL4/FC3 additions:

    • IPV6INIT=no
    • USERCTL=no
    • PEERDNS=yes
    • TYPE=Ethernet

(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)

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"

Network IP aliasing:

Assign more than one IP address to one ethernet card:

    ifconfig eth0   XXX.XXX.XXX.XXX netmask broadcast XXX.XXX.XXX.255
    ifconfig eth0:0   netmask broadcast
    ifconfig eth0:1   netmask broadcast

    route add -host XXX.XXX.XXX.XXX dev eth0
    route add -host dev eth0
    route add -host 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:
          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:  Bcast:  Mask:
          Interrupt:5 Base address:0xe400 

eth0:1    Link encap:Ethernet  HWaddr 00:10:4C:25:7A:3F
          inet addr:  Bcast:  Mask:
          Interrupt:5 Base address:0xe400

Config file: /etc/sysconfig/network-scripts/ifcfg-eth0:0


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
	Broadcast: 4.XXX.XXX.255
	Network: 4.XXX.XXX.0
	Boot server 131.XXX.XXX.4
	Next server
	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

Activating and De-Activating your NIC:
Commands for starting and stopping TCP/IP network services on an interface:

  • 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)

255 1
/32 16,777,214 65,534 254 Invalid
1 address
254 128 /31 33,554,430 131,070 510 Invalid
2 addresses
252 64 /30 67,108,862 262,142 1,022 2 hosts
4 addresses
248 32 /29 134,217,726 524,286 2,046 6 hosts
8 addresses
240 16 /28 268,435,454 1,048,574 4,094 14 hosts
16 addresses
224 8 /27 536,870,910 2,097,150 8,190 30 hosts
32 addresses
192 4 /26 1,073,741,822 4,194,302 16,382 62 hosts
64 addresses
128 2 /25 2,147,483,646 8,388,606 32,766 126 hosts
128 addresses
Binary position 8 7 6 5 4 3 2 1
Value 128 64 32 16 8 4 2 1
Example: 192 1 1 0 0 0 0 0 0

Example 192=128+64

Some addresses are reserved and outside this scope. Loopback (, 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 for your office.

  • Network Base address
  • Computer 1
  • Computer 2
  • Computer 3
  • Computer 4
  • Computer 5
  • DSL router/Gateway
  • Broadcast address

Of the eight addresses, there are six assigned to hardware systems and ultimately only five usable addresses.


Network Classes:

The concept of network classes is a little obsolete as subnets are now used to define smaller networks. 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: –
  • 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: –
  • 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: –
  • Class D: Defined by the first 8 bits with a range from 224 – 239
    This is reserved for multicast networks (RFC988)
    Range: –
  • Class E: Defined by the first 8 bits with a range from 240 – 255
    This is reserved for experimental use.
    Range: –

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

    See file /etc/sysconfig/network for storing this configuration.


    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.

Alos see: (YoLinux tutorials)

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)


        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)


    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 gw XXX.XXX.XXX.XXX


  3. Define network parameters using Unix command line interface: Define IP address:
        ifconfig eth0  XXX.XXX.XXX.XXX netmask broadcast XXX.XXX.XXX.255
        ifconfig eth1   netmask broadcast

    If necessary, define route with with the route command:

      route add default gw XXX.XXX.XXX.XXX dev eth0
      route add  -net  XXX.XXX.XXX.0  netmask 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
    -A 100baseT4
  • 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


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
(Red Hat 7: /etc/sysconfig/static-routes)
(S.u.s.e. 9.2: /etc/sysconfig/network/routes): via

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 routing table: route -e
    • Access individual computer host specified via network interface card eth1:
      route add -host eth1
    • Access ISP network identified by the network address and netmask using network interface card eth0:
      route add -net netmask gw eth0
      Conversly: route del -net netmask gw eth0
    • Specify default gateway to use to access remote network via network interface card eth0:
      route add default gw 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 eth0
      Add second private network: route add -net netmask gw 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:

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
    • List all connected processes: netstat -nap
    • Show network statistics: netstat -s
    • Kernel interface table info: netstat -a -i eth0
  • ping – send ICMP ECHO_REQUEST packets to network hosts. Use Cntl-C to stop ping.
  • traceroute – print the route packets take to network host
    • 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. Often used by game developers. See http://finger.planetquake.com/
  • 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

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

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
    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.)

  • 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 ...


  • 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
      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, 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:

For more info see:

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)

Portmpper is a network service required to support RPC’s. Many services such as NFS (file sharing services) require portmapper.

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:

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


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


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


  • /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:


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
   echo 1 > $f

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 -A OUTPUT -p icmp -d 0/0 -j DROP


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.


    • 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
      Scans network for IP addresses to 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)
  • 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.
  • scotty – Obtain status and configuration information about your network. Supports SNMP, ICMP, DNS, HTTP, SUN RPC, NTP, & UDP. (Red Hat Powertools CD)
  • 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.

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.

  • arp (8) man page – manipulate the system ARP cache
  • Shows other systems on your network (including IP address conflicts): arp -a
  • Show ARP table Linux style: arp -e
  • 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
  • List ARP table: cat /proc/net/arp
  • 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:


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 to (See Network Classes above) have typically been reserved for multicast.

Usefull commands:

Command Description
cat /proc/net/igmp List multicast group to which the host is subscribed. Use “Internet Group Management Protocol”.
(See /usr/src/linux/net/core/igmp.c)
cat /proc/net/dev_mcast List multicast interfaces.
(See /usr/src/linux/net/core/dev_mcast.c)
ping All hosts configured for multicast will respond with their IP addresses
ping All routers configured for multicast will respond
ping All PIM routers configured for multicast will respond
ping All DVMRP routers configured for multicast will respond
ping 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 netmask 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.

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.

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 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 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.
    The top layer for communications applications like email and the web.
    telnet, web browser, sendmail
    6 Presentation Layer.
    Syntax and format of data transfer.
    SMTP, http
    5 Session Layer.  
    4 Transport Layer.
    Connection, acknowledgement and data packet transmission.
    3 Network Layer. IP
    2 Data Link Layer.
    Error control, timing
    1 Physical Layer.
    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.

0 Responses to “Linux Networking”

  1. Leave a Comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s


This blog consists of external links only. I'm not responsible of any file and content as they are not under my control. Links and contents are not checked. All downloads are under your responsability.


anda pengunjung ke

  • 259,012 pengunjung
February 2008
« Jan   Mar »
My Popularity (by popuri.us) KampungBlog.com - Kumpulan Blog-Blog Indonesia


Earn $$ with WidgetBucks!



%d bloggers like this: