Table of Contents
This chapter explains how to configure network interface cards to work with tcp/ip.
Recent Linux distributions often include a graphical application to configure the network. Some people complain that these applications mess networking configurations up when used simultaneously with command line configurations. Notably Network Manager (often replaced by wicd) and yast are known to not care about configuration changes via the command line.
Since the goal of this course is server administration, we will assume our Linux servers are always administered through the command line.
This chapter only focuses on using the command line for network interface configuration!
Unfortunately there is no single combination of Linux commands and /etc files that works on all Linux distributions. We discuss networking on two (large but distinct) Linux distribution families.
We start with Debian (this should also work on Ubuntu and Mint), then continue with RHEL (which is identical to CentOS and Fedora).
The /etc/network/interfaces file is a core network interface card configuration file on debian.
The screenshot below shows that our computer is configured for dhcp on eth0 (the first network interface card or nic).
paul@debian8:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcpConfiguring network cards for dhcp is good practice for clients, but servers usually require a fixed ip address.
The screenshot below shows /etc/network/interfaces configured with a fixed ip address.
root@debian7~# cat /etc/network/interfaces
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 10.42.189.198
broadcast 10.42.189.207
netmask 255.255.255.240
gateway 10.42.189.193The screenshot above also shows that you can provide more configuration than just the ip address. See interfaces(5) for help on setting a gateway, netmask or any of the other options.
It is adviced (but not mandatory) to down an interface before changing its configuration. This can be done with the ifdown command.
The command will not give any output when downing an interface with a fixed ip address. However ifconfig will no longer show the interface.
root@ubu1104srv:~# ifdown eth0 root@ubu1104srv:~# ifconfig lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:106 errors:0 dropped:0 overruns:0 frame:0 TX packets:106 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:11162 (11.1 KB) TX bytes:11162 (11.1 KB)
An interface that is down cannot be used to connect to the network.
Below a screenshot of ifup bringing the eth0 ethernet interface up using dhcp. (Note that this is a Ubuntu 10.10 screenshot, Ubuntu 11.04 omits ifup output by default.)
root@ubu1010srv:/etc/network# ifup eth0 Internet Systems Consortium DHCP Client V3.1.3 Copyright 2004-2009 Internet Systems Consortium. All rights reserved. For info, please visit https://www.isc.org/software/dhcp/ Listening on LPF/eth0/08:00:27:cd:7f:fc Sending on LPF/eth0/08:00:27:cd:7f:fc Sending on Socket/fallback DHCPREQUEST of 192.168.1.34 on eth0 to 255.255.255.255 port 67 DHCPNAK from 192.168.33.100 DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 DHCPOFFER of 192.168.33.77 from 192.168.33.100 DHCPREQUEST of 192.168.33.77 on eth0 to 255.255.255.255 port 67 DHCPACK of 192.168.33.77 from 192.168.33.100 bound to 192.168.33.77 -- renewal in 95 seconds. ssh stop/waiting ssh start/running, process 1301 root@ubu1010srv:/etc/network#
The details of dhcp are covered in a separate chapter in the Linux Servers course.
The /etc/sysconfig/network file is a global (across all network cards) configuration file. It allows us to define whether we want networking (NETWORKING=yes|no), what the hostname should be (HOSTNAME=) and which gateway to use (GATEWAY=).
[root@rhel6 ~]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=rhel6
GATEWAY=192.168.1.1There are a dozen more options settable in this file, details can be found in /usr/share/doc/initscripts-*/sysconfig.txt.
Note that this file contains no settings at all in a default RHEL7 install (with networking enabled).
[root@rhel71 ~]# cat /etc/sysconfig/network
# Created by anacondaEach network card can be configured individually using the /etc/sysconfig/network-scripts/ifcfg-* files. When you have only one network card, then this will probably be /etc/sysconfig/network-scripts/ifcfg-eth0.
Below a screenshot of /etc/sysconfig/network-scripts/ifcfg-eth0 configured for dhcp (BOOTPROTO="dhcp"). Note also the NM_CONTROLLED paramater to disable control of this nic by Network Manager. This parameter is not explained (not even mentioned) in /usr/share/doc/initscripts-*/sysconfig.txt, but many others are.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="dhcp"
ONBOOT="yes"The BOOTPROTO variable can be set to either dhcp or bootp, anything else will be considered static meaning there should be no protocol used at boot time to set the interface values.
RHEL7 adds ipv6 variables to this file.
[root@rhel71 network-scripts]# cat ifcfg-enp0s3
TYPE="Ethernet"
BOOTPROTO="dhcp"
DEFROUTE="yes"
PEERDNS="yes"
PEERROUTES="yes"
IPV4_FAILURE_FATAL="no"
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_PEERDNS="yes"
IPV6_PEERROUTES="yes"
IPV6_FAILURE_FATAL="no"
NAME="enp0s3"
UUID="9fa6a83a-2f8e-4ecc-962c-5f614605f4ee"
DEVICE="enp0s3"
ONBOOT="yes"
[root@rhel71 network-scripts]#Below a screenshot of a fixed ip configuration in /etc/sysconfig/network-scripts/ifcfg-eth0.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="none"
IPADDR="192.168.1.99"
NETMASK="255.255.255.0"
GATEWAY="192.168.1.1"
ONBOOT="yes"The HWADDR can be used to make sure that each network card gets the correct name when multiple network cards are present in the computer. It can not be used to assign a mac address to a network card. For this, you need to specify the MACADDR variable. Do not use HWADDR and MACADDR in the same ifcfg-ethx file.
The BROADCAST= and NETWORK= parameters from previous RHEL/Fedora versions are obsoleted.
On RHEL7 you should run nmcli connection reload if you changed configuration files in /etc/sysconfig/ to enable your changes.
The nmcli tool has many options to configure networking on the command line in RHEL7/CentOS7
man nmcli
Another recommendation for RHEL7/CentOS7 is to use nmtui. This tool will use a 'windowed' interface in command line to manage network interfaces.
nmtui
The ifup and ifdown commands will set an interface up or down, using the configuration discussed above. This is identical to their behaviour in Debian and Ubuntu.
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2452 errors:0 dropped:0 overruns:0 frame:0
TX packets:1881 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:257036 (251.0 KiB) TX bytes:184767 (180.4 KiB)The use of /sbin/ifconfig without any arguments will present you with a list of all active network interface cards, including wireless and the loopback interface. In the screenshot below eth0 has no ip address.
root@ubu1010:~# ifconfig
eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Interrupt:43 Base address:0xe000
eth1 Link encap:Ethernet HWaddr 00:26:bb:12:7a:5e
inet addr:192.168.1.30 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::226:bbff:fe12:7a5e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:11141791 errors:202 dropped:0 overruns:0 frame:11580126
TX packets:6473056 errors:3860 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3476531617 (3.4 GB) TX bytes:2114919475 (2.1 GB)
Interrupt:23
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:2879 errors:0 dropped:0 overruns:0 frame:0
TX packets:2879 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:486510 (486.5 KB) TX bytes:486510 (486.5 KB)You can also use ifconfig to obtain information about just one network card.
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2969 errors:0 dropped:0 overruns:0 frame:0
TX packets:1918 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:335942 (328.0 KiB) TX bytes:190157 (185.7 KiB)When /sbin is not in the $PATH of a normal user you will have to type the full path, as seen here on Debian.
paul@debian5:~$ /sbin/ifconfig eth3
eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30
inet addr:192.168.1.29 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:27155 errors:0 dropped:0 overruns:0 frame:0
TX packets:30527 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:13095386 (12.4 MiB) TX bytes:25767221 (24.5 MiB)You can also use ifconfig to bring an interface up or down. The difference with ifup is that ifconfig eth0 up will re-activate the nic keeping its existing (current) configuration, whereas ifup will read the correct file that contains a (possibly new) configuration and use this config file to bring the interface up.
[root@rhel6 ~]# ifconfig eth0 down
[root@rhel6 ~]# ifconfig eth0 up
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2995 errors:0 dropped:0 overruns:0 frame:0
TX packets:1927 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:339030 (331.0 KiB) TX bytes:191583 (187.0 KiB)You can temporary set an ip address with ifconfig. This ip address is only valid until the next ifup/ifdown cycle or until the next reboot.
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
[root@rhel6 ~]# ifconfig eth0 192.168.33.42 netmask 255.255.0.0
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.33.42 Bcast:192.168.255.255 Mask:255.255.0.0
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0You can also use ifconfig to set another mac address than the one hard coded in the network card. This screenshot shows you how.
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C [root@rhel6 ~]# ifconfig eth0 hw ether 00:42:42:42:42:42 [root@rhel6 ~]# ifconfig eth0 | grep HWaddr eth0 Link encap:Ethernet HWaddr 00:42:42:42:42:42
The ifconfig tool is deprecated on some systems. Use the ip tool instead.
To see ip addresses on RHEL7 for example, use this command:
[root@rhel71 ~]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 08:00:27:89:22:33 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.135/24 brd 192.168.1.255 scope global dynamic enp0s3
valid_lft 6173sec preferred_lft 6173sec
inet6 fe80::a00:27ff:fe89:2233/64 scope link
valid_lft forever preferred_lft forever
[root@rhel71 ~]#Home and client Linux desktops often have /sbin/dhclient running. This is a daemon that enables a network interface to lease an ip configuration from a dhcp server. When your adapter is configured for dhcp or bootp, then /sbin/ifup will start the dhclient daemon.
When a lease is renewed, dhclient will override your ifconfig set ip address!
Every host receives a hostname, often placed in a DNS name space forming the fqdn or Fully Qualified Domain Name.
This screenshot shows the hostname command and the configuration of the hostname on Red Hat/Fedora.
[root@rhel6 ~]# grep HOSTNAME /etc/sysconfig/network HOSTNAME=rhel6 [root@rhel6 ~]# hostname rhel6
Starting with RHEL7/CentOS7 this file is empty. The hostname is configured in the standard /etc/hostname file.
[root@rhel71 ~]# cat /etc/hostname
rhel71.linux-training.be
[root@rhel71 ~]#Ubuntu/Debian uses the /etc/hostname file to configure the hostname.
paul@debian8:~$ cat /etc/hostname server42 paul@debian8:~$ hostname server42
On all Linux distributions you can change the hostname using the hostname $newname command. This is not a permanent change.
[root@rhel6 ~]# hostname server42 [root@rhel6 ~]# hostname server42
On any Linux you can use sysctl to display and set the hostname.
[root@rhel6 ~]# sysctl kernel.hostname kernel.hostname = server42 [root@rhel6 ~]# sysctl kernel.hostname=rhel6 kernel.hostname = rhel6 [root@rhel6 ~]# sysctl kernel.hostname kernel.hostname = rhel6 [root@rhel6 ~]# hostname rhel6
The ip to mac resolution is handled by the layer two broadcast protocol arp. The arp table can be displayed with the arp tool. The screenshot below shows the list of computers that this computer recently communicated with.
root@barry:~# arp -a ? (192.168.1.191) at 00:0C:29:3B:15:80 [ether] on eth1 agapi (192.168.1.73) at 00:03:BA:09:7F:D2 [ether] on eth1 anya (192.168.1.1) at 00:12:01:E2:87:FB [ether] on eth1 faith (192.168.1.41) at 00:0E:7F:41:0D:EB [ether] on eth1 kiss (192.168.1.49) at 00:D0:E0:91:79:95 [ether] on eth1 laika (192.168.1.40) at 00:90:F5:4E:AE:17 [ether] on eth1 pasha (192.168.1.71) at 00:03:BA:02:C3:82 [ether] on eth1 shaka (192.168.1.72) at 00:03:BA:09:7C:F9 [ether] on eth1 root@barry:~#
Anya is a Cisco Firewall, faith is a laser printer, kiss is a Kiss DP600, laika is a laptop and Agapi, Shaka and Pasha are SPARC servers. The question mark is a Red Hat Enterprise Linux server running on a virtual machine.
You can use arp -d to remove an entry from the arp table.
[root@rhel6 ~]# arp Address HWtype HWaddress Flags Mask Iface ubu1010 ether 00:26:bb:12:7a:5e C eth0 anya ether 00:02:cf:aa:68:f0 C eth0 [root@rhel6 ~]# arp -d anya [root@rhel6 ~]# arp Address HWtype HWaddress Flags Mask Iface ubu1010 ether 00:26:bb:12:7a:5e C eth0 anya (incomplete) eth0 [root@rhel6 ~]# ping anya PING anya (192.168.1.1) 56(84) bytes of data. 64 bytes from anya (192.168.1.1): icmp_seq=1 ttl=254 time=10.2 ms ... [root@rhel6 ~]# arp Address HWtype HWaddress Flags Mask Iface ubu1010 ether 00:26:bb:12:7a:5e C eth0 anya ether 00:02:cf:aa:68:f0 C eth0
You can see the computer's local routing table with the /sbin/route command (and also with netstat -r ).
root@RHEL4b ~]# netstat -r Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 [root@RHEL4b ~]# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 [root@RHEL4b ~]#
It appears this computer does not have a gateway configured, so we use route add default gw to add a default gateway on the fly.
[root@RHEL4b ~]# route add default gw 192.168.1.1 [root@RHEL4b ~]# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0 [root@RHEL4b ~]#
Unless you configure the gateway in one of the /etc/ file from the start of this chapter, your computer will forget this gateway after a reboot.
If you can ping to another host, then tcp/ip is configured.
[root@RHEL4b ~]# ping 192.168.1.5 PING 192.168.1.5 (192.168.1.5) 56(84) bytes of data. 64 bytes from 192.168.1.5: icmp_seq=0 ttl=64 time=1004 ms 64 bytes from 192.168.1.5: icmp_seq=1 ttl=64 time=1.19 ms 64 bytes from 192.168.1.5: icmp_seq=2 ttl=64 time=0.494 ms 64 bytes from 192.168.1.5: icmp_seq=3 ttl=64 time=0.419 ms --- 192.168.1.5 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3009ms rtt min/avg/max/mdev = 0.419/251.574/1004.186/434.520 ms, pipe 2 [root@RHEL4b ~]#
To display or change network card settings, use ethtool. The results depend on the capabilities of your network card. The example shows a network that auto-negotiates it's bandwidth.
root@laika:~# ethtool eth0 Settings for eth0: Supported ports: [ TP ] Supported link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full 1000baseT/Full Supports auto-negotiation: Yes Advertised link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full 1000baseT/Full Advertised auto-negotiation: Yes Speed: 1000Mb/s Duplex: Full Port: Twisted Pair PHYAD: 0 Transceiver: internal Auto-negotiation: on Supports Wake-on: pumbg Wake-on: g Current message level: 0x00000033 (51) Link detected: yes
This example shows how to use ethtool to switch the bandwidth from 1000Mbit to 100Mbit and back. Note that some time passes before the nic is back to 1000Mbit.
root@laika:~# ethtool eth0 | grep Speed Speed: 1000Mb/s root@laika:~# ethtool -s eth0 speed 100 root@laika:~# ethtool eth0 | grep Speed Speed: 100Mb/s root@laika:~# ethtool -s eth0 speed 1000 root@laika:~# ethtool eth0 | grep Speed Speed: 1000Mb/s
1. Verify whether dhclient is running.
2. Display your current ip address(es).
3. Display the configuration file where this ip address is defined.
4. Follow the nic configuration in the book to change your ip address from dhcp client to fixed. Keep the same ip address to avoid conflicts!
5. Did you also configure the correct gateway in the previous question ? If not, then do this now.
6. Verify that you have a gateway.
7. Verify that you can connect to the gateway, that it is alive.
8. Change the last two digits of your mac address.
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
10. Explain why e-mail and websites are sent over tcp and not udp.
11. Display the hostname of your computer.
12. Which ip-addresses did your computer recently have contact with ?
1. Verify whether dhclient is running.
paul@debian5:~$ ps fax | grep dhclient
2. Display your current ip address(es).
paul@debian5:~$ /sbin/ifconfig | grep 'inet '
inet addr:192.168.1.31 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:127.0.0.1 Mask:255.0.0.03. Display the configuration file where this ip address is defined.
Ubuntu/Debian: cat /etc/network/interfaces Redhat/Fedora: cat /etc/sysconfig/network-scripts/ifcfg-eth*
4. Follow the nic configuration in the book to change your ip address from dhcp client to fixed. Keep the same ip address to avoid conflicts!
Ubuntu/Debian: ifdown eth0 vi /etc/network/interfaces ifup eth0
Redhat/Fedora: ifdown eth0 vi /etc/sysconfig/network-scripts/ifcfg-eth0 ifup eth0
5. Did you also configure the correct gateway in the previous question ? If not, then do this now.
6. Verify that you have a gateway.
paul@debian5:~$ /sbin/route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
7. Verify that you can connect to the gateway, that it is alive.
paul@debian5:~$ ping -c3 192.168.1.1 PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data. 64 bytes from 192.168.1.1: icmp_seq=1 ttl=254 time=2.28 ms 64 bytes from 192.168.1.1: icmp_seq=2 ttl=254 time=2.94 ms 64 bytes from 192.168.1.1: icmp_seq=3 ttl=254 time=2.34 ms --- 192.168.1.1 ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2008ms rtt min/avg/max/mdev = 2.283/2.524/2.941/0.296 ms
8. Change the last two digits of your mac address.
[root@rhel6 ~]# ifconfig eth0 hw ether 08:00:27:ab:67:XX
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
root@rhel6 ~# grep ^'http ' /etc/services http 80/tcp www www-http # WorldWideWeb HTTP http 80/udp www www-http # HyperText Transfer Protocol root@rhel6 ~# grep ^'smtp ' /etc/services smtp 25/tcp mail smtp 25/udp mail root@rhel6 ~# grep ^'ssh ' /etc/services ssh 22/tcp # The Secure Shell (SSH) Protocol ssh 22/udp # The Secure Shell (SSH) Protocol root@rhel6 ~# grep ^'telnet ' /etc/services telnet 23/tcp telnet 23/udp root@rhel6 ~# grep ^'nntp ' /etc/services nntp 119/tcp readnews untp # USENET News Transfer Protocol nntp 119/udp readnews untp # USENET News Transfer Protocol root@rhel6 ~# grep ^'ftp ' /etc/services ftp 21/tcp ftp 21/udp fsp fspd
10. Explain why e-mail and websites are sent over tcp and not udp.
Because tcp is reliable and udp is not.
11. Display the hostname of your computer.
paul@debian5:~$ hostname debian5
12. Which ip-addresses did your computer recently have contact with ?
root@rhel6 ~# arp -a ? (192.168.1.1) at 00:02:cf:aa:68:f0 [ether] on eth2 ? (192.168.1.30) at 00:26:bb:12:7a:5e [ether] on eth2 ? (192.168.1.31) at 08:00:27:8e:8a:a8 [ether] on eth2