First make certain your network is properly
configured. If your Ethernet device is up and running, running ifconfig should display
something like:
student:/tmp> /sbin/ifconfig
eno167777 Link encap:Ethernet HWaddr 00:0C:29:BB:92:C2
inet addr:192.168.1.14 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:febb:92c2/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:3244 errors:0 dropped:0 overruns:0 frame:0
TX packets:2006 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:4343606 (4.1 Mb) TX bytes:169082 (165.1 Kb)
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:65536 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:0
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
On older systems you probably will see a less cryptic
name than eno167777, like eth0, or for a
wireless connection, you might see something like
wlan0 or wlp3s0.
You can also show your IP address with:
student:/tmp> ip addr show
1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default
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: eno16777736: mtu 1500 qdisc pfifo_fast state \
UP group default qlen 1000
link/ether 00:0c:29:bb:92:c2 brd ff:ff:ff:ff:ff:ff
p inet 192.168.1.14/24 brd 192.168.1.255 scope global dynamic eno16777736
valid_lft 84941sec preferred_lft 84941sec
inet 192.168.1.15/24 brd 192.168.1.255 scope global secondary dynamic eno16777736
valid_lft 85909sec preferred_lft 85909sec
inet6 fe80::20c:29ff:febb:92c2/64 scope link
valid_lft forever preferred_lft forever
Does the IP address look valid? Depending on where
you are using this from, it is most likely a Class C
IP address; in the above this is 192.168.1.14
If it does not show a device with an IP
address, you may need to start or restart the network
and/or NetworkManager. Exactly how you do
this depends on your system. For most distributions
one of these commands will accomplish this:
student:/tmp> sudo systemctl restart NetworkManager
student:/tmp> sudo systemctl restart network
student:/tmp> sudo service NetworkManager restart
student:/tmp> sudo service network restart
If your device was up but had no IP address,
the above should have helped fix it, but you can try to
get a fresh address with:
student:/tmp> sudo dhclient eth0
substituting the right name for the Ethernet device.
When you type in a name of a site such as
google.com,
that name needs to be connected to
a known IP address. This is usually done employing
the DNS sever (Domain
Name System)
First, see if the site is up and reachable with ping:
student:/tmp> sudo ping -c 3 google.com
PING google.com (216.58.216.238) 56(84) bytes of data.
64 bytes from ord31s22-in-f14.1e100.net (216.58.216.238): icmp_seq=1 ttl=51 time=21.7 ms
64 bytes from ord31s22-in-f14.1e100.net (216.58.216.238): icmp_seq=2 ttl=51 time=23.8 ms
64 bytes from ord31s22-in-f14.1e100.net (216.58.216.238): icmp_seq=3 ttl=51 time=21.3 ms
--- google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 21.388/22.331/23.813/1.074 ms
Note:
- We have used sudo for
ping; recent Linux
distributions have required this to avoid
clueless or malicious users from flooding
systems with such queries.
- We have
used -c 3 to limit to 3 packets;
otherwise ping would run forever until
forcibly terminated, say with CTRL-C.
If the result was:
ping: unknown host google.com
It is likely that something is wrong with your DNS
set-up. (Note on some systems you will never see the
unknown host message, but you will get a suspicious
result like:
student:/tmp> sudo ping l89xl28vkjs.com
PING l89xl28vkjs.com.site (127.0.53.53) 56(84) bytes of data.
64 bytes from 127.0.53.53: icmp_seq=1 ttl=64 time=0.016 ms
...
where the 127.0.x.x address is a loop
feeding back to the host machine you are on. You can
eliminate this as being a valid address by doing:
student:/tmp> host l89xl28vkjs.com
Host l89xl28vkjs.com not found: 3(NXDOMAIN)
whereas a correct result would look like:
student:/tmp> host google.com
google.com has address 216.58.216.206
google.com has IPv6 address 2607:f8b0:4009:80b::200e
google.com mail is handled by 20 alt1.aspmx.l.google.com.
google.com mail is handled by 10 aspmx.l.google.com.
google.com mail is handled by 30 alt2.aspmx.l.google.com.
google.com mail is handled by 40 alt3.aspmx.l.google.com.
google.com mail is handled by 50 alt4.aspmx.l.google.com.
The above command utilizes the DNS server
configured in /etc/resolv.conf on your
machine. If you wanted to override that you could
do:
host 8.8.8.8
8.8.8.8.in-addr.arpa domain name pointer google-public-dns-a.google.com.
student@linux:~> host google.com 8.8.8.8
Using domain server:
Name: 8.8.8.8
Address: 8.8.8.8#53
Aliases:
google.com has address 216.58.216.110
google.com has IPv6 address 2607:f8b0:4009:804::1002
...\
where we have used the publicly available
DNS server provided by Google
itself. (Using this or another public server can be
a good trick sometimes if your network is up but
DNS is ill; in that case you can also enter
it in resolv.conf.)
Note that there is another file,
/etc/hosts, where you can associate names
with IP addresses, which is used
before the DNS server is
consulted. This is most useful for specifying nodes
on your local network.
You could also use the dig utility if you
prefer:
student:/tmp> dig google.com
; <<>> DiG 9.9.5-rpz2+rl.14038.05-P1 <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29613
;; flags: qr rd ra; QUERY: 1, ANSWER: 11, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; MBZ: 1c20 , udp: 1280
;; QUESTION SECTION:
;google.com. IN A
;; ANSWER SECTION:
google.com. 244 IN A 173.194.46.67
google.com. 244 IN A 173.194.46.65
google.com. 244 IN A 173.194.46.71
google.com. 244 IN A 173.194.46.73
google.com. 244 IN A 173.194.46.69
google.com. 244 IN A 173.194.46.68
google.com. 244 IN A 173.194.46.64
google.com. 244 IN A 173.194.46.72
google.com. 244 IN A 173.194.46.70
google.com. 244 IN A 173.194.46.66
google.com. 244 IN A 173.194.46.78
;; Query time: 22 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Mon Apr 20 08:58:58 CDT 2015
;; MSG SIZE rcvd: 215
Suppose host or dig fail to
connect the name to an IP address. There
are many reasons DNS can fail, some of
which are:
- The DNS server is down. In this case
try pinging it to see if it is alive (you
should have the IP address in
/etc/resolv.conf.
- The server can be up and running, but
DNS may not be currently available on the
machine.
- Your route to the DNS
server may not be correct.
How can we test the route? Tracing the route to one
of the public name server we mentioned before:
student@linux:~> sudo traceroute 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
1 192.168.1.1 (192.168.1.1) 0.405 ms 0.494 ms 0.556 ms
2 10.132.4.1 (10.132.4.1) 15.127 ms 15.107 ms 15.185 ms
3 dtr02ftbgwi-tge-0-6-0-3.ftbg.wi.charter.com (96.34.24.122)
15.243 ms 15.327 ms 17.878 ms
4 crr02ftbgwi-bue-3.ftbg.wi.charter.com (96.34.18.116) 17.667 ms 17.734 ms 20.016 ms
5 crr01ftbgwi-bue-4.ftbg.wi.charter.com (96.34.18.108) 22.017 ms 22.359 ms 22.052 ms
6 crr01euclwi-bue-1.eucl.wi.charter.com (96.34.16.77) 29.430 ms 22.705 ms 22.076 ms
7 bbr01euclwi-bue-4.eucl.wi.charter.com (96.34.2.4) 17.795 ms 25.542 ms 25.600 ms
8 bbr02euclwi-bue-5.eucl.wi.charter.com (96.34.0.7) 28.227 ms 28.270 ms 28.303 ms
9 bbr01chcgil-bue-1.chcg.il.charter.com (96.34.0.9) 33.114 ms 33.072 ms 33.175 ms
10 prr01chcgil-bue-2.chcg.il.charter.com (96.34.3.9) 36.882 ms 36.794 ms 36.895 ms
11 96-34-152-30.static.unas.mo.charter.com (96.34.152.30) 42.585 ms 42.326 ms 42.401 ms
12 216.239.43.111 (216.239.43.111) 28.737 ms 216.239.43.113 (216.239.43.113)
24.558 ms 23.941 ms
13 209.85.243.115 (209.85.243.115) 24.269 ms 209.85.247.17 (209.85.247.17)
25.758 ms 216.239.50.123 (216.239.50.123) 25.433 ms
14 google-public-dns-a.google.com (8.8.8.8) 25.239 ms 24.003 ms 23.795 ms
Again, this should likely work for you, but what if
you only got the first line in the
traceroute output?
If this happened, most likely your default route is
wrong. Try:
student:/tmp> ip route show
efault via 192.168.1.1 dev eno16777736 proto static metric 1024
192.168.1.0/24 dev eno16777736 proto kernel scope link src 192.168.1.14
Most likely this is set to your network interface
and the IP address of your router, DSL, or
Cable Modem. Let's say that it is blank or simply
points to your own machine. Here's your problem!
At this point, you would need to add a proper
default route and run some of the same tests we just
did.
Note, an enhanced version of traceroute is
supplied by mtr, which runs continuously
(like top). Running it with the
--report-cycles option to limit how long it
runs:
student:/tmp> sudo mtr --report-cycles 3 8.8.8.8
My traceroute [v0.85]
c7 (0.0.0.0) Mon Apr 20 09:30:41 2015
Unable to allocate IPv6 socket for nameserver communication: Address family not supported
by protocol Packets Pings
Host Loss% Snt Last Avg Best Wrst StDev
0.0% 3 0.3 0.3 0.2 0.3 0.0
2. 10.132.4.1 0.0% 3 6.3 7.1 6.3 8.4 0.7
3. dtr02ftbgwi-tge-0-6-0-3.ftbg.wi. 0.0% 3 6.2 7.5 6.2 10.0 2.1
4. dtr01ftbgwi-bue-1.ftbg.wi.charte 0.0% 3 8.9 8.5 6.2 10.4 2.0
5. crr01ftbgwi-bue-4.ftbg.wi.charte 0.0% 3 8.9 9.7 8.9 10.4 0.0
6. crr01euclwi-bue-1.eucl.wi.charte 0.0% 3 16.5 17.4 14.2 21.5 3.7
7. bbr01euclwi-bue-4.eucl.wi.charte 0.0% 3 23.5 22.0 18.2 24.2 3.2
8. bbr02euclwi-bue-5.eucl.wi.charte 0.0% 3 18.9 22.7 18.1 31.1 7.2
9. bbr01chcgil-bue-1.chcg.il.charte 0.0% 3 22.9 23.0 22.9 23.1 0.0
10. prr01chcgil-bue-2.chcg.il.charte 0.0% 3 21.4 24.1 20.8 30.2 5.2
11. 96-34-152-30.static.unas.mo.char 0.0% 3 22.6 21.9 20.0 23.3 1.6
12. 216.239.43.111 0.0% 3 21.2 21.7 21.2 22.0 0.0
13. 72.14.237.35 0.0% 3 21.2 21.0 19.8 21.9 1.0
14. google-public-dns-a.google.com 0.0% 3 26.7 23.0 21.0 26.7 3.2