dokumen.tips_linux-networking-commands.ppt

Commands Reviewed
Ifconfig
dmesg
netstat
ping
route
traceroute
nslookup
arp
dig

ifconfig
• ifconfig is used to assign an address to
a network interface and/or configure
network interface parameters. ifconfig
must be used at boot time to define the
network address of each interface
present on a machine; it may also be
used at a later time to redefine an
interface's address or other operating
parameters.

ifconfig eth0
• eth0 Link encap:Ethernet HWaddr 00:04:AC:16:54:4C
• inet addr:192.168.2.1 Bcast:192.168.2.255
Mask:255.255.255.0
• inet6 addr: fe80::204:acff:fe16:544c/10 Scope:Link
• UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
• RX packets:116242 errors:0 dropped:0 overruns:0 frame:0
• TX packets:123380 errors:0 dropped:0 overruns:0 carrier:0
• collisions:9052
• RX bytes:39103579 (37.2 Mb) TX bytes:106270739 (101.3
Mb)

When would you use ifconfig?
• To determine if an interface has been
recognized and configured on a system
• To initially assign an IP address to an
interface
• to bring an interface up or down

What should you do if the
interface isn’t found?
• This kind of problem usually appears on
initial setup of a machine
• [root@localhost root]# ifconfig eth3 up
• eth3: unknown interface: No such device

See if the device has been
found - use dmesg
• The command 'dmesg', which is used to print kernel messages, is
very useful in determining if a piece of hardware has been found,
and if so, what the system is referring to it as.
• Don’t forger to monitor the /var/log/messages file for any strange
activity.
TIP
pipe the dmesg to the less command
or the reply will scroll off the screen
dmesg | less

Before you have a useable
interface
• Determine if you can ping the localhost
• ping localhost or
• ping 127.0.0.1

Does the localhost respond?
• [root@localhost root]# ping localhost
• PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
• 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255
time=1.968 msec
time=113 usec
time=178 usec
time=116 usec

Confirm you have set up the
interface correctly.
• ping the address returned by the ifconfig
command “Ping 192.168.2.1”
eth0 Link encap:Ethernet HWaddr 00:04:AC:16:54:4C
inet addr:192.168.2.1 Bcast:192.168.2.255 Mask:255.255.255.0
inet6 addr: fe80::204:acff:fe16:544c/10 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:116242 errors:0 dropped:0 overruns:0 frame:0
TX packets:123380 errors:0 dropped:0 overruns:0 carrier:0
collisions:9052
RX bytes:39103579 (37.2 Mb) TX bytes:106270739 (101.3 Mb)

ping
• System administration command.
Confirm that a remote host is online and
responding. ping is intended for use in
network testing, measurement, and
management. Because of the load it can
impose on the network, it is unwise to
use ping during normal operations or
from automated scripts.

route
• Gateways route data between networks
• All devices make routing decisions
– local network? Deliver to destination
– remote network? Forward to local gateway

route -n
• -n prevents route from converting IP
addresses to hostnames
• the 0.0.0.0 entry is the default gateway
[root@localhost root]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth1

route -Cn (display cache
table)
• [root@localhost root]# route -Cn
• Kernel IP routing cache
• Source Destination Gateway Flags Metric Ref Use
Iface
• 192.168.1.101 192.168.1.55 192.168.1.55 il 0 0 65 lo
• 192.168.2.1 192.168.2.75 192.168.2.75 0 1 0 eth0
• 192.168.1.55 192.175.48.42 192.168.1.1 0 0 0 eth1
• 192.168.1.55 192.168.1.101 192.168.1.101 0 0 1 eth1
• 192.168.1.55 192.168.1.101 192.168.1.101 0 1 0 eth1
• 127.0.0.1 127.0.0.1 127.0.0.1 l 0 0 3 lo
• 192.168.1.101 192.168.1.255 192.168.1.255 ibl 0 0 4 lo
• 127.0.0.1 127.0.0.1 127.0.0.1 l 0 0 1 lo

traceroute
• Trace route taken by packets to reach
network host. traceroute attempts
tracing by launching UDP probe
packets with a small TTL (time to live),
then listening for an ICMP "time
exceeded" reply from a gateway. host
is the destination hostname or the IP
number of host to reach. packetsize is
the packet size in bytes of the probe
datagram. Default is 38 bytes.
Note
Traceroute has lost some of
its effectiveness since most
ISP’s disallow it from
running on their networks

nslookup
• Query Internet domain name servers. nslookup has two modes:
interactive and noninteractive. Interactive mode allows the user to
query name servers for information about various hosts and domains
or to print a list of hosts in a domain. It is entered either when no
arguments are given (default name server will be used) or when the
first argument is a hyphen and the second argument is the hostname
or Internet address of a name server. Noninteractive mode is used to
print just the name and requested information for a host or domain.
It is used when the name of the host to be looked up is given as the
first argument. Any of the keyword=value pairs listed under the
interactive set command can be used as an option on the command
line by prefacing the keyword with a -. The optional second
argument specifies a name server.

arp
• Physical networks have their own
addressing scheme
• IP addresses are used to direct a
datagram to a specific physical network

arp table
• arp maintains a table of translations
between IP addresses and ethernet
addresses
• [root@localhost root]# arp
• Address HWtype HWaddress Flags Mask Iface
• 192.168.1.1 ether 00:04:5A:DB:A1:C5 C eth1
• 192.168.2.75 ether 00:10:E0:04:61:84 C eth0
• 192.168.1.101 ether 00:09:B7:13:AA:13 C eth1

/etc/hosts
• Setting the hosts file will resolve names
to addresses
[root@localhost root]# less /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1 localhost.localdomain localhost
192.168.1.1 gatewayout.tcp-ip.ca
192.168.2.75 basement.tcp-ip.ca
192.168.1.101 just_a_node.tcp-ip.ca

arp -a will then resolve
• Instead of just names, you also get IP addresses
• [root@localhost root]# arp -a
• gatewayout.tcp-ip.ca (192.168.1.1) at 00:04:5A:DB:A1:C5 [ether] on eth1
• basement.tcp-ip.ca (192.168.2.75) at 00:10:E0:04:61:84 [ether] on eth0
• just_a_node.tcp-ip.ca (192.168.1.101) at 00:09:B7:13:AA:13 [ether] on eth1

netstat
• The netstat command symbolically displays the contents of various
network-related data structures. There are a number of output
formats, de- pending on the options for the information presented.
The first form of the command displays a list of active sockets for
each protocol. The second form presents the contents of one of the
other network data structures according to the option selected.
Using the third form, with a wait interval specified, netstat will
continuously display the information regarding packet traffic on the
configured network interfaces. The fourth form displays statistics
about the named protocol.

netstat -nr
• [root@localhost root]# netstat -nr
• Kernel IP routing table
• Destination Gateway Genmask Flags MSS Window irtt Iface
• 192.168.2.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
• 192.168.1.0 0.0.0.0 255.255.255.0 U 40 0 0 eth1
• 127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
• 0.0.0.0 192.168.1.1 0.0.0.0 UG 40 0 0 eth1

dig - supercharged nslookup
• oroot@localhost root]# dig cs.senecac.on.ca
• ; <<>> DiG 9.1.3 <<>> cs.senecac.on.ca
• ;; global options: printcmd
• ;; Got answer:
• ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10483
• ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 6, ADDITIONAL: 4
• ;; QUESTION SECTION:
• ;cs.senecac.on.ca. IN A
• ;; ANSWER SECTION:
• cs.senecac.on.ca. 86400 IN A 142.204.57.48

dig - better than nslookup
• ;; AUTHORITY SECTION:
• senecac.on.ca. 86400 IN NS ns.onet.on.ca.
• senecac.on.ca. 86400 IN NS ns2.senecac.on.ca.
• senecac.on.ca. 86400 IN NS hades.senecac.on.ca.
• senecac.on.ca. 86400 IN NS ittads.senecac.on.ca.
• senecac.on.ca. 86400 IN NS pulsar.senecac.on.ca.
• senecac.on.ca. 86400 IN NS nsprime.senecac.on.ca.
• ;; ADDITIONAL SECTION:
• ns2.senecac.on.ca. 86400 IN A 142.204.10.100
• hades.senecac.on.ca. 86400 IN A 142.204.57.15
• ittads.senecac.on.ca. 86400 IN A 142.204.6.57
• pulsar.senecac.on.ca. 86400 IN A 142.204.119.97
• ;; Query time: 2202 msec
• ;; SERVER: 127.0.0.1#53(127.0.0.1)
• ;; WHEN: Sun Sep 29 16:38:20 2002
• ;; MSG SIZE rcvd: 238

References
• For detailed syntax on these and other
linux commands look at
• http://www.oreillynet.com/linux/cmd/

dokumen.tips_linux-networking-commands.ppt

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