The DHCP server configuration excerpt above already includes the directives required for DNS zone updates: they are the ddns-update-style interim; and ddns-domain-name "internal.falcot.com"; lines in the block describing the subnet.
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 0.0.0.0:22 0.0.0.0:* LISTEN 2224/sshd
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 994/exim4
tcp 0 0 192.168.1.241:22 192.168.1.128:47372 ESTABLISHED 2944/sshd: roland [
tcp 0 0 192.168.1.241:22 192.168.1.128:32970 ESTABLISHED 2232/sshd: roland [
tcp6 0 0 :::22 :::* LISTEN 2224/sshd
tcp6 0 0 ::1:25 :::* LISTEN 994/exim4
udp 0 0 0.0.0.0:68 0.0.0.0:* 633/dhclient
udp 0 0 192.168.1.241:123 0.0.0.0:* 764/ntpd
udp 0 0 127.0.0.1:123 0.0.0.0:* 764/ntpd
udp 0 0 0.0.0.0:123 0.0.0.0:* 764/ntpd
udp6 0 0 fe80::a00:27ff:fe6c:123 :::* 764/ntpd
udp6 0 0 2002:52e0:87e4:0:a0:123 :::* 764/ntpd
udp6 0 0 ::1:123 :::* 764/ntpd
udp6 0 0 :::123 :::* 764/ntpd
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 scouzmir
Starting Nmap 5.00 ( http://nmap.org ) at 2010-10-12 18:52 CEST
Interesting ports on 192.168.1.101:
Not shown: 998 closed ports
PORT STATE SERVICE
22/tcp open ssh
111/tcp open rpcbind
MAC Address: 52:54:00:99:01:01 (QEMU Virtual NIC)
Nmap done: 1 IP address (1 host up) scanned in 2.11 seconds
# nmap -A localhost
Starting Nmap 5.00 ( http://nmap.org ) at 2010-10-12 18:59 CEST
Warning: Hostname localhost resolves to 2 IPs. Using 127.0.0.1.
Interesting ports on localhost (127.0.0.1):
Not shown: 997 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 5.5p1 Debian 4 (protocol 2.0)
| ssh-hostkey: 1024 af:07:60:17:16:64:6f:ee:c4:ca:b5:64:1e:4a:4c:22 (DSA)
|_ 2048 25:b0:aa:6b:11:5a:56:b6:8d:2d:ed:b3:16:17:96:33 (RSA)
25/tcp open smtp Exim smtpd 4.72
| smtp-commands: EHLO scouzmir.internal.placard.fr.eu.org Hello localhost [127.0.0.1], SIZE 52428800, PIPELINING, HELP
|_ HELP Commands supported: AUTH HELO EHLO MAIL RCPT DATA NOOP QUIT RSET HELP
111/tcp open rpcbind
| rpcinfo:
| 100000 2 111/udp rpcbind
| 100024 1 53273/udp status
| 100000 2 111/tcp rpcbind
|_ 100024 1 41127/tcp status
No exact OS matches for host (If you know what OS is running on it, see http://nmap.org/submit/ ).
TCP/IP fingerprint:
OS:SCAN(V=5.00%D=10/12%OT=22%CT=1%CU=34421%PV=N%DS=0%G=Y%TM=4CB4941A%P=i686