/etc/services

[j773303]

Any command should be run after modified /etc/services?

[Michael Tully · Accepted solution]

Simple answer is no. The /etc/services file is used as a reference for the port numbers and their aliases.

# man services

[j773303]

But if I disable
#TCP 25 SMTP

Does this disable port 25 immediately?

[Michael Steele_2]

As long as there is no conflict with other applications and its not own by root, (* < 1024 *) then you shouldn't have a problem. Order your /etc/services file with sort a check for duplicates:

cat /etc/services | sort -n | more

Don't confuse this with starting the network utility like oracle's listener or one of the inetd utilities. When you make a port you reserve buffer space and allow the system to create a system file for parsing. That's why ports are analyzed with lsof, the list open files command.

lsof -i tcp:23
lsof -i udp:123, etc.

[Michael Steele_2]

Regarding: "...But if I disable #TCP 25 SMTP Does this disable port 25 immediately? ..."

Probably not. You've got to stop the application. Run:

lsof -i smtp:25

Is there a sendmail daemon perhaps?

killsm

[Bill Hassell]

Think of /etc/services as a documentation file. A port may be used even though it is not defined in /etc/services. The file /etc/inetd.conf is used to associate a service with a daemon and route appropriate packets to the listed daemons. However, sendmail is not managed by inetd. To stop sendmail (the typical program that handles port 25), use the command:

/sbin/init.d/sendmail stop

Sendmail will no longer run and port 25 will be unused and will not respond to probes. To keep sendmail from running upon a reboot, edit the config file:

/etc/rc.config.d/mailservs

and change this line to read:

export SENDMAIL_SERVER=0

That will keep sendmail from running.

[Michael Steele_2]

Now j773303, don't you think that 'killsm' is a lot easier to use than '/sbin/init.d/sendmail stop?'

[Michael Tully]

The man pages (killsm) also states

"HP recommends that system administrators use ``/sbin/init.d/sendmail start'' and ``/sbin/init.d/sendmail stop'' to start and stop sendmail;"

[Steven E. Protter]

I think of /etc/services as a documentation or reservation file.

If you comment out the port 25 reference you have no functional effect.

You can control stmp traffic by making changes to the sendmail.mc file, generating a new sendmail.cf file and block incoming mail traffic on any port or IP address.

If you want to block traffic further, you need to have some kind of firewall between your box and your users. HP has a free one, or you can have an external pc based firewall.

I refer to /etc/services as a reservation file because when you install oracle on a system it takes up several ports. Oracle doesn't bother adding entries to the /etc/services file and everything works great.

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