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10.8. Network Diagnosis Tools

When a network application does not run as expected, it is important to be able to look under the hood. Even when everything seems to run smoothly, running a network diagnosis can help ensure everything is working as it should. Several diagnosis tools exists for this purpose; each one operates on a different level.

10.8.1. Local Diagnosis: netstat

Let's first mention the netstat command (in the net-tools package); it displays an instant summary of a machine's network activity. When invoked with no argument, this command lists all open connections; this list can be very verbose since it includes many Unix-domain sockets (widely used by daemons) which do not involve the network at all (for example, dbus communication, X11 traffic, and communications between virtual filesystems and the desktop).
Common invocations therefore use options that alter netstat's behavior. The most frequently used options include:
  • -t, which filters the results to only include TCP connections;
  • -u, which works similarly for UDP connections; these options are not mutually exclusive, and one of them is enough to stop displaying Unix-domain connections;
  • -a, to also list listening sockets (waiting for incoming connections);
  • -n, to display the results numerically: IP addresses (no DNS resolution), port numbers (no aliases as defined in /etc/services) and user ids (no login names);
  • -p, to list the processes involved; this option is only useful when netstat is run as root, since normal users will only see their own processes;
  • -c, to continuously refresh the list of connections.
Other options, documented in the netstat(8) manual page, provide an even finer control over the displayed results. In practice, the first five options are so often used together that systems and network administrators practically acquired netstat -tupan as a reflex. Typical results, on a lightly loaded machine, may look like the following:
# netstat -tupan
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0   *               LISTEN      397/rpcbind     
tcp        0      0    *               LISTEN      431/sshd        
tcp        0      0 *               LISTEN      407/rpc.statd   
tcp        0      0  *               LISTEN      762/exim4       
tcp        0    272     ESTABLISHED 1172/sshd: roland [
tcp6       0      0 :::111                  :::*                    LISTEN      397/rpcbind     
tcp6       0      0 :::22                   :::*                    LISTEN      431/sshd        
tcp6       0      0 ::1:25                  :::*                    LISTEN      762/exim4       
tcp6       0      0 :::35210                :::*                    LISTEN      407/rpc.statd   
udp        0      0 *                           916/dhclient    
udp        0      0   *                           397/rpcbind     
udp        0      0*                           407/rpc.statd   
udp        0      0    *                           916/dhclient    
udp        0      0 *                           451/avahi-daemon: r
udp        0      0   *                           397/rpcbind     
udp        0      0*                           539/ntpd        
udp        0      0 *                           539/ntpd        
udp        0      0   *                           539/ntpd        
udp        0      0  *                           451/avahi-daemon: r
udp        0      0 *                           407/rpc.statd   
udp6       0      0 :::996                  :::*                                397/rpcbind     
udp6       0      0 :::34277                :::*                                407/rpc.statd   
udp6       0      0 :::54852                :::*                                916/dhclient    
udp6       0      0 :::111                  :::*                                397/rpcbind     
udp6       0      0 :::38007                :::*                                451/avahi-daemon: r
udp6       0      0 fe80::5054:ff:fe99::123 :::*                                539/ntpd        
udp6       0      0 2001:bc8:3a7e:210:a:123 :::*                                539/ntpd        
udp6       0      0 2001:bc8:3a7e:210:5:123 :::*                                539/ntpd        
udp6       0      0 ::1:123                 :::*                                539/ntpd        
udp6       0      0 :::123                  :::*                                539/ntpd        
udp6       0      0 :::5353                 :::*                                451/avahi-daemon: r
As expected, this lists established connections, two SSH connections in this case, and applications waiting for incoming connections (listed as LISTEN), notably the Exim4 email server listening on port 25.

10.8.2. Remote Diagnosis: nmap

nmap (in the similarly-named package) is, in a way, the remote equivalent for netstat. It can scan a set of “well-known” ports for one or several remote servers, and list the ports where an application is found to answer to incoming connections. Furthermore, nmap is able to identify some of these applications, sometimes even their version number. The counterpart of this tool is that, since it runs remotely, it cannot provide information on processes or users; however, it can operate on several targets at once.
A typical nmap invocation only uses the -A option (so that nmap attempts to identify the versions of the server software it finds) followed by one or more IP addresses or DNS names of machines to scan. Again, many more options exist to finely control the behavior of nmap; please refer to the documentation in the nmap(1) manual page.
# nmap mirtuel

Starting Nmap 6.47 ( ) at 2015-03-09 16:46 CET
Nmap scan report for mirtuel (
Host is up (0.000013s latency).
rDNS record for
Not shown: 998 closed ports
22/tcp  open  ssh
111/tcp open  rpcbind

Nmap done: 1 IP address (1 host up) scanned in 2.41 seconds
# nmap -A localhost

Starting Nmap 6.47 ( ) at 2015-03-09 16:46 CET
Nmap scan report for localhost (
Host is up (0.000013s latency).
Other addresses for localhost (not scanned):
Not shown: 997 closed ports
22/tcp  open  ssh     OpenSSH 6.7p1 Debian 3 (protocol 2.0)
|_ssh-hostkey: ERROR: Script execution failed (use -d to debug)
25/tcp  open  smtp    Exim smtpd 4.84
| smtp-commands: mirtuel Hello localhost [], SIZE 52428800, 8BITMIME, PIPELINING, HELP, 
111/tcp open  rpcbind 2-4 (RPC #100000)
| rpcinfo: 
|   program version   port/proto  service
|   100000  2,3,4        111/tcp  rpcbind
|   100000  2,3,4        111/udp  rpcbind
|   100024  1          36568/tcp  status
|_  100024  1          39172/udp  status
Device type: general purpose
Running: Linux 3.X
OS CPE: cpe:/o:linux:linux_kernel:3
OS details: Linux 3.7 - 3.15
Network Distance: 0 hops
Service Info: Host: mirtuel; OS: Linux; CPE: cpe:/o:linux:linux_kernel

OS and Service detection performed. Please report any incorrect results at .
Nmap done: 1 IP address (1 host up) scanned in 11.54 seconds
As expected, the SSH and Exim4 applications are listed. Note that not all applications listen on all IP addresses; since Exim4 is only accessible on the lo loopback interface, it only appears during an analysis of localhost and not when scanning mirtuel (which maps to the eth0 interface on the same machine).

10.8.3. Sniffers: tcpdump and wireshark

Sometimes, one needs to look at what actually goes on the wire, packet by packet. These cases call for a “frame analyzer”, more widely known as a sniffer. Such a tool observes all the packets that reach a given network interface, and displays them in a user-friendly way.
The venerable tool in this domain is tcpdump, available as a standard tool on a wide range of platforms. It allows many kinds of network traffic capture, but the representation of this traffic stays rather obscure. We will therefore not describe it in further detail.
A more recent (and more modern) tool, wireshark (in the wireshark package), has become the new reference in network traffic analysis due to its many decoding modules that allow for a simplified analysis of the captured packets. The packets are displayed graphically with an organization based on the protocol layers. This allows a user to visualize all protocols involved in a packet. For example, given a packet containing an HTTP request, wireshark displays, separately, the information concerning the physical layer, the Ethernet layer, the IP packet information, the TCP connection parameters, and finally the HTTP request itself.
The wireshark network traffic analyzer

Figure 10.1. The wireshark network traffic analyzer

In our example, the packets traveling over SSH are filtered out (with the !tcp.port == 22 filter). The packet currently displayed was developed at the HTTP layer.