NsMon (Network Security MONintor) is a program which can check your servers which are connected to the internet/intranet via TCP/IP. The check are the following: ping and port check. Ping check is a simple 5 ICMP echo with a timout. Port check is a simple connect() via TCP socket. If one of the checks was unsuccessfull NSMON sends a mail to the admin of that host which was wrong. You have to define an smtp mailer, NSMON will connet into it directly and send an RFC compatible message.
NsMon is a free, GNU/GPL software. Click here to read the terms and conditions for copying, ditribution and modification.
My name is Tibor Koleszar, i live in Hungary and workng for an ISP corporation called IntegraNET. You can reach me via email( t.koleszar@somogy.hu) or on ircNet (on #linux, or hungarian #linux.hu or irc.debian.org's #debian). If you have a problem with nsmon, please send me an email, and i will correct the problem.
By the way... If you like nsmon, and want to show it to others add a link to this page with the following image.

Check http://www.debian.org for the binary or for the source.

So, if you've got the source untar and unzip it with the following command : tar xzvf nsmon-X.X.tar.gz. After type cd nsmon-X.X and run make. Now run make install it will copy nsmon binary into /usr/sbin. After you have to make the magical nsmon config file.
Here is some help to do it:

KEYWORD	DESCRIPTION	TYPE
logfile	the path and name of the nsmon logfile. (logfile=/nsmon/nsmon.log)	global required
lock	the path of the nsmon lockfiles. (lock=/nsmon/lock)	global required
servername	It will be the sender of the mails. It have to be a real address because some intelligent mailer checks is. (servername=pluto.somogy.hu)	global required
username	It will be the sender of the mails. (username=root)	global required
gateway	Before every port or ping check nsmon checks the gateway ping. It has to be a 123.123.123.123 ip address because if there is no DNS there can be some problem. (gateway=193.91.74.3)	global required
checkhost	Checkhost is a beginer word of a section. Begins with checkhost { and ends with a }	global required
name	Name of the host what you want to check. NsMon will get its inet address at first time, so there will be no DNS problem during the tests. (name=mail.somogy.hu)	member of checkhost required
email	The "To:" field of the mail header (email=123123@sms.pgsm.hu)	member of checkhost required
smtpserver	The smtp mail server. (smtpserver=localhost)	member of checkhost required
sendalarmtime	If your checkhost is down nsmon wont send you email in every check time, just the first time and in every sendalarmtime min. (sendalarmtime=60)	member of checkhost required
pingtimeout	Timeout of the ping test in sec. (pingtimeout=15)	member of checkhost depends on pintest
pingtest	Test ping in every pingtest min. (pingtest=5)	member of checkhost depends on pingtimeout
portchecktime	Check ports in every portchecktime min. (portchecktime=3)	member of checkhost depends on portcheck
portcheck	Portcheck is a section to declare those ports which you want to check in this checkhost. Example: portcheck = ( 80 = http 25 = smtp ) It checks port 80 called http in the logfile and port 25 called smtp.	member of checkhost depends on portchecktime

Now let's see an example:
nsmon.conf:

    logfile = /var/log/nsmon.log
    lock = /var/lock/nsmon
    servername = www.foo.org
    username = root
    gateway = 123.123.123.123

    checkhost {
               name = mail.foo.org
               email = jsmith@foo2.org
               smtpserver = mail.foo.org
               sendalarmtime = 60
               pingtimeout = 20
               pingtest = 2
               portchecktime = 5
               portcheck = (
                            80 = http
                            3128 = squid
                            25 = smtp
                           ) 
     }