Lowe D. Networking For Dummies

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Book IX

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Commands for Administering Users

The passwd command

This command changes the password for a user account. Its syntax is

passwd [user]

If you don’t supply a user, the password for the current user is changed.

The passwd command prompts you to enter the new password twice in

order to prevent the possibility of mistyping the password.

The newusers command

The newusers command provides an easy way to create a group of new

user accounts. It reads a text file that contains one line for each new user,

listing the user’s name and password.

Here’s the syntax of the newusers command:

newusers [filename]

If you omit the filename, newusers accepts input from the console.

Suppose that you have a file named /root/island.users that contains

these lines:

gilligan ml9jiedr

skipper 1hiecr8u

professor dr0uxiaf

maryann choe7rlu

ginger jiuqled5

mrhowell j1emoaf1

lovie zo2priak

You can then create these seven stranded user accounts by issuing this

command:

$ newusers /root/island.users

Because the newusers file contains unencrypted passwords, you shouldn’t

leave it lying around. Require these new users to change their passwords

immediately and delete the file you used to create the users.

The groupadd command

The groupadd command creates a new group. It has the following syntax:

groupadd [options] group

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Although you have several possible options to use, the only one you’re likely

to need is -r, which creates a system group that has special privileges.

Here’s an example that creates a group named castaways:

$ groupadd castaways

That’s all you have to do to create a new group. To administer the group,

you use the gpasswd command.

The groupdel command

The groupdel command deletes a group. It has the following syntax:

groupdel group

Here’s an example that deletes a group named castaways:

$ groupdel castaways

Poof! The group is gone.

The gpasswd command

You use the gpasswd command to administer a group. This command has

several different syntax options.

To change the group password:

gpasswd [ -r | -R ] group

To add a user:

gpasswd -a user group

To remove a user:

gpasswd -d user group

To create group administrators and/or members:

gpasswd [-A administrators...] [-M members... ] group

The options are as follows:

✦ -r: Removes the password from the group.

✦ -R: Disables access to the group via the newgrp command.

✦ -a: Adds the specified user to the group.

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✦ -d: Deletes the specified user from the group.

✦ -A: Specifies one or more group administrators. Use commas with no

intervening spaces to separate the administrators from each other. Each

administrator must be an existing user.

✦ -M: Specifies one or more group members. Use commas with no inter-

vening spaces to separate the members from each other. Each member

must be an existing user.

The following example adds seven group members and one administrator to

a group called castaways:

$ gpasswd -A skipper -M skipper,gilligan,professor,maryann,

ginger,mrhowell,lovie castaways

If the rest of the group finally decides to throw Gilligan off the island, they

can remove him from the group with this command:

$ gpasswd -d gilligan castaways

Commands for Managing Ownership and Permissions

This section presents the details of the chown and chmod commands, which

are the essential tools for assigning file system rights in the Linux environment.

You can view the ownership and permissions for a file using the ls com-

mand with the -l option.

The chown command

The chown command changes the owner of a file. Normally, the user who

creates a file is the owner of the file. However, the owner can transfer the file

to someone else via this command. The basic syntax of this command is

chown user file

For example, to change the owner of a file named rescue.plans to user

professor, use this command:

$ chown professor rescue.plans

To change ownership of all the files in the directory named /home/island

to professor, use this command:

$ chown professor /home/island

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Issuing the following command would be a really bad idea:

$ chown gilligan rescue.plans

The chgrp command

Every file has not only an individual owner, but also a group owner. You can

change the group ownership using the chgrp command, which has the fol-

lowing basic syntax:

chgrp group file

For example, to grant the castaways group ownership of the file rescue.

plans, use this command:

$ chgrp castaways rescue.plans

To change group ownership of all the files in the directory named /home/

island to castaways, use this command:

$ chgrp castaways /home/island

The chmod command

The chmod command lets you change the permissions for a Linux file. Before

explaining the syntax of the chmod command, you need to look at the cryp-

tic way Linux reports file permissions. Linux grants three different types of

permissions — read, write, and execute — for three different scopes: owner,

group, and everyone. That’s a total of nine permissions.

When you use the ls command with the -l option, the permissions are

shown as a ten-character string that begins with a hyphen if the entry is for

a file or a d if the entry is for a directory. Then, the next nine letters are the

nine permissions, in this order:

✦ Read, write, execute for the owner

✦ Read, write, execute for the group

✦ Read, write, execute for everyone

The letters r, w, or x appear if the permission has been granted. If the per-

mission is denied, a hyphen appears.

For example, suppose the ls -l command lists these permissions:

-rw-r--r--

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You interpret this permission string like this:

✦ The first hyphen indicates that this is a file, not a directory.

✦ The next three positions are rw-. Therefore, the owner has read and

write permission on this file, but not execute permission.

✦ The next three positions are r--. That means the group owner has read

permissions but not write or execute permission.

✦ The last three positions are also r--. That means that everyone else has

read permission but not write or execute permission.

The full syntax of the chmod command is pretty complex. However, you can

do most of what you need to do with this form:

chmod specification file

Here, specification is in the form u=rwx, g=rwx, or o=rwx to set the permis-

sions for the user (owner), group, and others (everyone). You don’t have

to specify r, w, and x; you just list the permissions that you want to grant.

For example, to grant read and write permission for the user to a file named

rescue.plans, use this command:

$ chmod u=rw rescue.plans

You can also combine specifications, like this:

$ chmod u=rw,g=rw,o=r rescue.plans

To revoke all rights for the user, group, or others, don’t type anything after

the equal sign. For example, this command revokes all rights for others:

$ chmod o= rescue.plans

Networking Commands

The following sections present Linux commands that are used to display

information about the network or configure its settings.

You can find more detail about some of these commands in Book IV, Chapter 6.

The hostname command

The hostname command simply displays the computer’s host name. It has

the following syntax:

hostname [name]

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If you use this command without any parameters, the computer’s host name

is displayed. If you specify a name, the computer’s host name is changed to

the name you specify.

The ifconfig command

ifconfig displays and sets configuration options for network interfaces.

Although you can configure an Ethernet adapter using this command, you’ll

rarely have to. Linux does a pretty good job of automatically configuring

network adapters, and the GNOME-based Network Configuration tool sup-

plied with the Red Hat distribution should be able to handle most network

configuration chores. So you’ll use ifconfig mostly to display network

configuration settings.

The basic syntax for ifconfig is

ifconfig interface [address] [netmask mask]

[broadcast broadcast]

The following paragraphs describe the options that you can use on the

ifconfig command:

✦ Interface: The symbolic name for your network adapter. It’s typically

eth0 for the first Ethernet adapter or eth1 for the second adapter.

✦ Address: The IP address you want to assign to the interface, such as

192.168.1.100.

✦ netmask: The subnet mask to use, such as 255.255.255.0.

✦ broadcast: The broadcast, which should be the highest address on the

subnet. For example: 192.168.1.255.

If you enter ifconfig without any parameters, the ifconfig command

displays the current status of your network adapters, like this:

eth0 Link encap:Ethernet HWaddr 00:20:78:16:E0:6A

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:0 errors:0 dropped:0 overruns:0 frame:0

TX packets:11 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:100

RX bytes:0 (0.0 b) TX bytes:2916 (2.8 Kb)

Interrupt:11 Base address:0xd000

eth1 Link encap:Ethernet HWaddr 00:40:05:80:51:F3

inet addr:192.168.3.100 Bcast:192.168.3.255 Mask:255.255.255.0

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:2358 errors:0 dropped:0 overruns:0 frame:0

TX packets:1921 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:100

RX bytes:265194 (258.9 Kb) TX bytes:424467 (414.5 Kb)

Interrupt:3 Base address:0xc000

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lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX packets:93707 errors:0 dropped:0 overruns:0 frame:0

TX packets:93707 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

RX bytes:6393713 (6.0 Mb) TX bytes:6393713 (6.0 Mb)

To change the IP address of an adapter, use ifconfig like this:

$ ifconfig eth0 192.168.1.200

The netstat command

The netstat command lets you monitor just about every aspect of a Linux

server’s network functions. This command can generate page after page of

interesting information — if you know what it all means.

The two most common reasons to use netstat are to display the routing

table and to display open TCP/IP connections. The syntax for displaying the

routing table is

netstat -r

This results in a display similar to this:

Kernel IP routing table

Destination Gateway Genmask Flags MSS Window irtt Iface

192.168.1.0 * 255.255.255.0 U 0 0 0 eth1

192.168.1.0 * 255.255.255.0 U 0 0 0 eth0

127.0.0.0 * 255.0.0.0 U 0 0 0 lo

To display TCP/IP connections, use this syntax:

netstat -l

This results in a display similar to the following:

Active Internet connections (only servers)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 *:1024 *:* LISTEN

tcp 0 0 LSERVER:1025 *:* LISTEN

tcp 0 0 *:netbios-ssn *:* LISTEN

tcp 0 0 *:sunrpc *:* LISTEN

tcp 0 0 *:http *:* LISTEN

tcp 0 0 *:x11 *:* LISTEN

tcp 0 0 *:ssh *:* LISTEN

tcp 0 0 LSERVER:ipp *:* LISTEN

tcp 0 0 LSERVER:smtp *:* LISTEN

tcp 0 0 *:https *:* LISTEN

udp 0 0 *:1024 *:*

udp 0 0 LSERVER:1026 *:*

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udp 0 0 192.168.1.20:netbios-ns *:*

udp 0 0 *:netbios-ns *:*

udp 0 0 192.168.1.2:netbios-dgm *:*

udp 0 0 *:netbios-dgm *:*

udp 0 0 *:940 *:*

udp 0 0 *:sunrpc *:*

udp 0 0 *:631 *:*

From this display, you can tell which Linux services are actively listening on

TCP/IP ports.

The ping command

The ping command is the basic troubleshooting tool for TCP/IP. You use

it to determine whether basic TCP/IP connectivity has been established

between two computers. If you’re having any kind of network trouble

between two computers, the first troubleshooting step is almost always to

see whether the computers can ping each other.

The basic syntax of ping is straightforward:

ping [options] address

The options can be

✦ -c: The number of packets to send. If you omit this, ping continues to

send packets until you interrupt it.

✦ -d: Floods the network with packets, as many as 100 per second. Use

with care!

✦ -i: Specifies how many seconds to wait between sending packets. The

default is one second. If you’re having intermittent connection problems,

you may try letting ping run for a while with this option set to a higher

value, such as 60, to send a packet every minute.

✦ -R: Displays the route the packets take to get to the destination

computer.

Ping will continue to ping the destination computer until you interrupt it by

pressing Ctrl+Z.

You can specify the host to ping using an IP address, as in this example:

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$ ping 192.168.1.100

PING 192.168.1.100 (192.168.1.100) 56(84) bytes of data.

64 bytes from 192.168.1.100: icmp_seq=1 ttl=128 time=0.382 ms

64 bytes from 192.168.1.100: icmp_seq=2 ttl=128 time=0.345 ms

64 bytes from 192.168.1.100: icmp_seq=3 ttl=128 time=0.320 ms

64 bytes from 192.168.1.100: icmp_seq=4 ttl=128 time=0.328 ms

You can also ping using a DNS name, as in this example:

$ ping www.lowewriter.com

PING www.lowewriter.com (209.68.34.15) 56(84) bytes of data.

64 bytes from www.lowewriter.com (209.68.34.15): icmp_seq=1 ttl=47 time=88.9 ms

64 bytes from www.lowewriter.com (209.68.34.15): icmp_seq=2 ttl=47 time=87.9 ms

64 bytes from www.lowewriter.com (209.68.34.15): icmp_seq=3 ttl=47 time=88.3 ms

64 bytes from www.lowewriter.com (209.68.34.15): icmp_seq=4 ttl=47 time=87.2 ms

The route command

The route command displays or modifies the computer’s routing table.

To display the routing table, use route without any parameters. To add an

entry to the routing table, use this syntax:

route add [ -net | -host ] address [options]

To delete an entry, use this syntax:

route del [ -net | -host ] address [options]

The available options are as follows:

✦ netmask mask: Specifies the subnet mask for this entry.

✦ gw address: Specifies the Gateway Address for this entry.

✦ dev if: Specifies an interface (such as eth0 or eth1) for this entry.

If you enter route by itself, with no parameters, you’ll see the routing table,

as in this example:

$ 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 eth1

169.254.0.0 * 255.255.0.0 U 0 0 0 eth1

127.0.0.0 * 255.0.0.0 U 0 0 0 lo

default 192.168.1.1 0.0.0.0 UG 0 0 0 eth1

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Suppose that your network has a second router that serves as a link to

another private subnet, 192.168.2.0 (subnet mask 255.255.255.0). The inter-

face on the local side of this router is at 192.168.1.200. To add a static route

entry that sends packets intended for the 192.168.2.0 subnet to this router,

use a command like this:

$ route add 192.168.2.0 netmask 255.255.255.0 gw

192.168.1.200

The traceroute command

The traceroute command displays a list of all the routers that a packet

must go through in order to get from the local computer to a destination

on the Internet. Each one of these routers is called a hop. If you’re unable

to connect to another computer, you can use Tracert to find out exactly

where the problem is occurring.

Here’s the syntax:

traceroute [-i interface] host

Although several options are available for the traceroute command, the

one you’re most likely to use is -i, which lets you specify an interface. This

is useful if your computer has more than one network adapter.

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