Network/Troubleshooting

Occasionally network related issues may arise. The following outlines basic troubleshooting methods.

Check Network Connection

• Check that the ethernet cable is plugged in or that you are connected to a wireless network.
• Check that you have an IP address.
  • On Windows: type 'ipconfig /all' into the command prompt
  • Linux/UNIX based hosts: type 'ifconfig' into a terminal

PING

PING is a network utility that sends ICMP packets to a specified host to test network connectivity.

• Open up a command prompt. Type 'ping' followed by the hostname or IP address of the host you want to test connectivity to.

bash$ ping google.com
PING google.com (74.125.228.34) 56(84) bytes of data.
64 bytes from iad23s06-in-f2.1e100.net (74.125.228.34): icmp_seq=1 ttl=52 time=4.52 ms
64 bytes from iad23s06-in-f2.1e100.net (74.125.228.34): icmp_seq=2 ttl=52 time=4.17 ms
64 bytes from iad23s06-in-f2.1e100.net (74.125.228.34): icmp_seq=3 ttl=52 time=4.41 ms
64 bytes from iad23s06-in-f2.1e100.net (74.125.228.34): icmp_seq=4 ttl=52 time=4.04 ms

• Linux/UNIXS may want to add the '-c' flag followed by a number to specify how many times to ping the destination host.

Trace Route

The traceroute tool utilizes the IP protocol to track the route one's packets take through a network. Traceroute is useful when trying to figure out why a host is unreachable because it shows where the connection failed.

• Windows:
  • In a command prompt type "tracert" followed by the hostname or IP address of the host you want to test the route to.

C:\>tracert google.com
Tracing route to google.com [74.125.228.38]
over a maximum of 30 hops:
  1    <1 ms    <1 ms    <1 ms  gw-486.umiacs.umd.edu [192.168.86.1] 
  2    <1 ms    <1 ms    <1 ms  umfirewall00.umiacs.umd.edu [128.8.120.17] 
  3    13 ms    <1 ms    <1 ms  avw1hub-gw.umiacs.umd.edu [128.8.120.1] 
  4     1 ms    <1 ms    <1 ms  vlan8.css-priv-r1.net.umd.edu [128.8.6.129] 
  5    <1 ms    <1 ms    <1 ms  gi6-1.css-core-r1.net.umd.edu [128.8.0.117] 
  6    <1 ms    <1 ms    <1 ms  gi3-2.css-fw-r1.net.umd.edu [128.8.0.82] 
  7     1 ms     2 ms     2 ms  128.8.0.226 
  8     2 ms     2 ms     2 ms  107-0-84-29-static.hfc.comcastbusiness.net [107.0.84.29] 
  9     3 ms     2 ms     2 ms  xe-3-1-2-0-ar04.capitolhghts.md.bad.comcast.net [68.85.114.113] 
 10     5 ms     6 ms     4 ms  pos-5-7-0-0-cr01.ashburn.va.ibone.comcast.net [68.86.90.85] 
 11     4 ms     4 ms     5 ms  pos-0-2-0-0-pe01.ashburn.va.ibone.comcast.net [68.86.86.70] 
 12     4 ms     6 ms     4 ms  75.149.231.62 
 13    42 ms     4 ms     4 ms  209.85.252.80 
 14     5 ms     5 ms     5 ms  72.14.238.175 
 15     4 ms     4 ms     5 ms  iad23s06-in-f6.1e100.net [74.125.228.38] 
Trace complete.

• Linux/UNIX:
  • In a terminal type 'traceroute' followed by the hostname or IP address of the host you want to test the route to.

bash$ traceroute google.com
traceroute: Warning: google.com has multiple addresses; using 74.125.228.72
traceroute to google.com (74.125.228.72), 64 hops max, 52 byte packets
 1  10.109.160.1 (10.109.160.1)  1.329 ms  1.086 ms  1.232 ms
 2  129-2-129-129.wireless.umd.edu (129.2.129.129)  2.780 ms  1.924 ms  2.127 ms
 3  te1-6.css-core-r1.net.umd.edu (128.8.0.121)  2.514 ms  2.293 ms  1.863 ms
 4  gi3-2.css-fw-r1.net.umd.edu (128.8.0.82)  2.429 ms  2.548 ms  2.284 ms
 5  128.8.0.226 (128.8.0.226)  3.838 ms  3.465 ms  3.635 ms
 6  107-0-84-29-static.hfc.comcastbusiness.net (107.0.84.29)  3.941 ms  3.445 ms  3.303 ms
 7  xe-3-1-2-0-ar04.capitolhghts.md.bad.comcast.net (68.85.114.113)  3.522 ms  4.333 ms  3.441 ms
 8  pos-5-1-0-0-cr01.ashburn.va.ibone.comcast.net (68.86.90.241)  7.269 ms
    pos-4-12-0-0-cr01.newyork.ny.ibone.comcast.net (68.86.90.173)  9.025 ms  7.192 ms
 9  pos-0-4-0-0-pe01.ashburn.va.ibone.comcast.net (68.86.86.146)  5.236 ms  5.636 ms  6.229 ms
10  75.149.231.62 (75.149.231.62)  5.186 ms  5.113 ms  6.725 ms
11  209.85.252.46 (209.85.252.46)  6.836 ms  6.352 ms  78.371 ms
12  72.14.238.247 (72.14.238.247)  7.823 ms  7.616 ms  8.331 ms
13  iad23s07-in-f8.1e100.net (74.125.228.72)  10.798 ms  13.383 ms  7.540 ms

Other Things to consider

• Is your IP address manually set?
  • In order for networking to function properly a machine must have a unique IP address on its network. In many cases IP address are assigned automatically via the DHCP protocol, but it is also possible to manually set an IP address. If your machine happens to have a manually set IP address that conflicts with that of another host, networking issues may arise.
• Are you behind a firewall?
  • Some establishments use firewalls to restrict certain types of traffic both in and out of their networks. If you are having trouble accessing a specific service, this might be the case. To get around this you can connect to the VPN, or use SSH tunneling.
• Other useful commands:
  • dig -performs DNS lookups and displays the answers that are returned from the name server(s) that were queried
  • host -simple utility for performing DNS loookups
  • netstat (linux/UNIX) -shows network status
  • route print (Windows) -shows network routing tables

Change your default DNS server

Please see DNS to change your default DNS server.