Answer:

Use the command `dig`, e.g.

dig +trace mit.edu

;; global options: +cmd
.			19927	IN	NS	d.root-servers.net.
.			19927	IN	NS	e.root-servers.net.
.			19927	IN	NS	m.root-servers.net.
.			19927	IN	NS	l.root-servers.net.
.			19927	IN	NS	a.root-servers.net.
.			19927	IN	NS	k.root-servers.net.
.			19927	IN	NS	f.root-servers.net.
.			19927	IN	NS	j.root-servers.net.
.			19927	IN	NS	b.root-servers.net.
.			19927	IN	NS	h.root-servers.net.
.			19927	IN	NS	g.root-servers.net.
.			19927	IN	NS	i.root-servers.net.
.			19927	IN	NS	c.root-servers.net.
;; Received 228 bytes from 8.8.8.8#53(8.8.8.8) in 12 ms

edu.			172800	IN	NS	a.edu-servers.net.
edu.			172800	IN	NS	c.edu-servers.net.
edu.			172800	IN	NS	d.edu-servers.net.
edu.			172800	IN	NS	f.edu-servers.net.
edu.			172800	IN	NS	g.edu-servers.net.
edu.			172800	IN	NS	l.edu-servers.net.
;; Received 260 bytes from 199.7.83.42#53(199.7.83.42) in 223 ms

mit.edu.		172800	IN	NS	usw2.akam.net.
mit.edu.		172800	IN	NS	asia1.akam.net.
mit.edu.		172800	IN	NS	asia2.akam.net.
mit.edu.		172800	IN	NS	use2.akam.net.
mit.edu.		172800	IN	NS	ns1-37.akam.net.
mit.edu.		172800	IN	NS	ns1-173.akam.net.
mit.edu.		172800	IN	NS	eur5.akam.net.
mit.edu.		172800	IN	NS	use5.akam.net.
;; Received 404 bytes from 192.31.80.30#53(192.31.80.30) in 548 ms

mit.edu.		20	IN	A	23.46.134.151
;; Received 41 bytes from 63.150.131.81#53(63.150.131.81) in 4 ms

Answer:

To check the network connectivity to a specific server, you can use the mtr command.

e.g.

# mtr --report google.com
HOST: localhost                   Loss%   Snt   Last   Avg  Best  Wrst StDev
  1. 106.187.33.2                  0.0%    10    1.3   5.3   0.6  21.4   6.7
  2. 124.215.199.121               0.0%    10    0.6   6.5   0.4  12.4   4.8
  3. otejbb206.int-gw.kddi.ne.jp   0.0%    10    2.1   3.7   2.1  12.5   3.3
  4. ix-ote210.int-gw.kddi.ne.jp   0.0%    10    2.1   6.4   2.1  15.2   4.5
  5. 203.181.102.94                0.0%    10    2.6   2.7   2.2   3.9   0.6
  6. 209.85.249.195                0.0%    10    3.2   5.3   2.3  22.8   6.6
  7. 64.233.175.0                  0.0%    10    7.0  13.5   6.8  49.8  13.7
  8. 64.233.174.0                  0.0%    10   20.0  12.7  11.1  20.0   3.0
  9. 216.239.43.145                0.0%    10   12.0  11.8  11.5  12.2   0.2
 10. kix01s01-in-f8.1e100.net      0.0%    10   11.2  11.4  11.1  11.9   0.3

Answer:

To provide username and password when curl a url, you should use the "-u" or "--user" argument

e.g.

# curl --user john:password http://www.example.com

Answer:

In the past we already showed that you can get the IP information using the command "ifconfig", actually the modern way to do the job is

# ip a

1: lo:  mtu 16436 qdisc noqueue state UNKNOWN 
  ..

2: eth0:  mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:26:5e:3e:42:d2 brd ff:ff:ff:ff:ff:ff
 ..

Answer:

To add a static route, you can use a simple command like:

# sudo route add 192.168.15.0/24 192.168.1.1

The above command make all request to 192.168.15.0/24 (i.e. 192.168.15.1 to 192.168.15.254) route thru the gateway 192.168.1.1

Answer:

To show the current system's routing table, you can try:

# sudo netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         102.147.87.1    0.0.0.0         UG        0 0          0 eth0
0.0.0.0         102.147.45.1    0.0.0.0         UG        0 0          0 eth0
102.147.45.0    0.0.0.0         255.255.255.0   U         0 0          0 eth0
102.147.87.0    0.0.0.0         255.255.255.0   U         0 0          0 eth0

Answer:

Wireshark is the De facto standard in network sniffing, but most of the time you might just want to analyse the content passing through your web browser or web server, then it is easy with tcpflow

For example, to show your communication with google.com to console, you can use

sudo tcpflow -c -i en0 src or dst host www.google.com

Answer:

The following codes illtsraeted how to get the hostname of the local machine using Java.

import java.net.InetAddress;

public class Test {

    public static void main(String[] args) {

        try {
            InetAddress addr = InetAddress.getLocalHost();

            // Get hostname
            String hostName = addr.getHostName();

            System.out.println(hostName);

        } catch (Exception e) {}
    }
}

Answer:

To show the current Postfix queue, you can try the command "postqueue"

# postqueue -p

-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
BD7E2BF973      972 Sat Aug 27 08:57:54  no-reply@example.com
(connect to alt4.gmail-smtp-in.l.google.com[209.85.143.27]:25: Connection timed out)
                                         johndoe@gmail.com

Answer:

You can simulate a HTTP form post, with a file being uploaded to a remote server using the curl command.

curl -F 'photo=@/upload/image.jpg' http://example.com/upload.php

Assume the name of the form element of your upload is called "photo"