Every now and then I have to work on servers running CentOS or Fedora Linux. Very typical problem that I observe on many servers which I have to inherit is the previous administrator did not know about the existence of NTP (Network Time Protocol) or forgot to install the ntpd server. As a consequence the many installed server services did not have a correct clock and at some specific cases this caused issues for web applications running on the server or any CMS installed etc.
The NTP Daemon is existing in GNU / linux since the early days of Linux and it served quite well so far. The NTP protocol has been used since the early days of the internet and for centuries is a standard protocol for BSD UNIX.
ntp is available in I believe all Linux distributions directly as a precompiled binary and can be installed on Fedora, CentOS with:
[root@centos ~]# yum install ntp
ntpd synchronizes the server clock with one of the /etc/ntp.conf defined RedHat NTP list
server 0.rhel.pool.ntp.org
server 1.rhel.pool.ntp.org
server 2.rhel.pool.ntp.org
To Synchronize manually the server system clock the ntp CentOS rpm package contains a tool called ntpdate :
Hence its a good practice to use ntpdate to synchronize the local server time with a internet server, the way I prefer to do this is via a government owned ntp server time.nist.gov, e.g.
[root@centos ~]# ntpdate time.nist.gov
8 Feb 14:21:03 ntpdate[9855]: adjust time server 192.43.244.18 offset -0.003770 sec
Alternatively if you prefer to use one of the redhat servers use:
[root@centos ~]# ntpdate 0.rhel.pool.ntp.org
8 Feb 14:20:41 ntpdate[9841]: adjust time server 72.26.198.240 offset 0.005671 sec
Now as the system time is set to a correct time via the ntp server, the ntp server is to be launched:
[root@centos ~]# /etc/init.d/ntpd start
...
To permanently enable the ntpd service to start up in boot time issue also:
[root@centos ~]# chkconfig ntpd on
Using chkconfig and /etc/init.d/ntpd cmds, makes the ntp server to run permanently via the ntpd daemon:
[root@centos ~]# ps ax |grep -i ntp
29861 ? SLs 0:00 ntpd -u ntp:ntp -p /var/run/ntpd.pid -g
If you prefer to synchronize periodically the system clock instead of running permanently a network server listening (for increased security), you should omit the above chkconfig ntpd on and /etc/init.d/ntpd start commands and instead set in root crontab the time to get synchronize lets say every 30 minutes, like so:
[root@centos ~]# echo '30 * * * * root /sbin/ntpd -q -u ntp:ntp' > /etc/cron.d/ntpd
The time synchronization via crontab can be also done using the ntpdate cmd. For example if you want to synchronize the server system clock with a network server every 5 minutes:
[root@centos ~]# crontab -u root -e
And paste inside:
*/5 * * * * /sbin/ntpdate time.nist.gov 2>1 > /dev/null
ntp package is equipped with ntpq – Standard NTP Query Program. To get very basic stats for the running ntpd daemon use:
[root@centos ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
======================================================
B1-66ER.matrix. 192.43.244.18 2 u 47 64 17 149.280 41.455 11.297
*ponderosa.piney 209.51.161.238 2 u 27 64 37 126.933 32.149 8.382
www2.bitvector. 132.163.4.103 2 u 1 64 37 202.433 12.994 13.999
LOCAL(0) .LOCL. 10 l 24 64 37 0.000 0.000 0.001
The remote field shows the servers to which currently the ntpd service is connected. This IPs are the servers which ntp uses to synchronize the local system server clock. when field shows when last the system was synchronized by the remote time server and the rest is statistical info about connection quality etc.
If the ntp server is to be run in daemon mode (ntpd to be running in the background). Its a good idea to allow ntp connections from the local network and filter incoming connections to port num 123 in /etc/sysconfig/iptables :
-A INPUT -s 192.168.1.0/24 -m state --state NEW -p udp --dport 123 -j ACCEPT
-A INPUT -s 127.0.0.1 -m state --state NEW -p udp --dport 123 -j ACCEPT
-A INPUT -s 0.0.0.0 -m state --state NEW -p udp --dport 123 -j DROP
Restrictions on which IPs can be connected to the ntp server can also be implied on a ntpd level through /etc/ntp.conf. For example if you would like to add the local network IPs range 192.168.0.1/24 to access ntpd, in ntpd.conf should be added policy:
# Hosts on local network are less restricted.
restrict 192.168.0.1 mask 255.255.255.0 nomodify notrap
To deny all access to any machine to the ntpd server add in /etc/ntp.conf:
restrict default ignore
After making any changes to ntp.conf , a server restart is required to load the new config settings, e.g.:
[root@centos ~]# /sbin/service ntpd restart
In most cases I think it is better to imply restrictions on a iptables (firewall) level instead of bothering change the default ntp.conf
Once ntpd is running as daemon, the server listens for UDP connections on udp port 123, to see it use:
[root@centos ~]# netstat -tulpn|grep -i ntp
udp 0 0 10.10.10.123:123 0.0.0.0:* 29861/ntpd
udp 0 0 80.95.28.179:123 0.0.0.0:* 29861/ntpd
udp 0 0 127.0.0.1:123 0.0.0.0:* 29861/ntpd
udp 0 0 0.0.0.0:123 0.0.0.0:* 29861/ntpd
How to install and configure Jabber Server (Ejabberd) on Debian Lenny GNU / Linux
Wednesday, December 28th, 2011I've recently installed a jabber server on one Debian Lenny server and hence decided to describe my installations steps hoping this would help ppl who would like to run their own jabber server on Debian . After some research of the jabber server softwares available, I decided to install Ejabberd
The reasons I choose Ejabberd is has rich documentation, good community around the project and the project in general looks like one of the best free software jabber servers available presently. Besides that ejabberd doesn't need Apache or MySQL and only depends on erlang programming language.
Here is the exact steps I followed to have installed and configured a running XMPP jabber server.
1. Install Ejabberd with apt
The installation of Ejabberd is standard, e.g.:
debian:~# apt-get --yes install ejabberd
Now as ejabberd is installed, some minor configuration is necessery before the server can be launched:
2. Edit /etc/ejabberd/ejabberd.cfg
Inside I changed the default settings for:
a) Uncomment%%override_acls.. Changed:
%%%% Remove the Access Control Lists before new ones are added.%%%%override_acls.
to
%%
%% Remove the Access Control Lists before new ones are added.
%%
override_acls.
b) Admin User from:
%% Admin user
{acl, admin, {user, "", "example.com"}}.
to
%% Admin user
{acl, admin, {user, "admin", "jabber.myserver-host.com"}}.
c) default %% Hostname of example.com to my real hostname:
%% Hostname
{hosts, ["jabber.myserver-host.com"]}.
The rest of the configurations in /etc/ejabberd/ejabberd.cfg can stay like it is, though it is interesting to read it carefully before continuing as, there are some config timings which might prevent the XMPP server from user brute force attacks as well as few other goodies like for example (ICQ, MSN , Yahoo etc.) protocol transports.
3. Add iptables ACCEPT traffic (allow) rules for ports which are used by Ejabberd
The minimum ACCEPT rules to add are:
/sbin/iptables -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT
/sbin/iptables -A INPUT -p tcp -m tcp --dport 5222 -j ACCEPT
/sbin/iptables -A INPUT -p udp -m udp --dport 5222 -j ACCEPT
/sbin/iptables -A INPUT -p tcp -m tcp --dport 5223 -j ACCEPT
/sbin/iptables -A INPUT -p udp -m udp --dport 5223 -j ACCEPT
/sbin/iptables -A INPUT -p tcp -m tcp --dport 5269 -j ACCEPT
/sbin/iptables -A INPUT -p udp -m udp --dport 5269 -j ACCEPT
/sbin/iptables -A INPUT -p tcp -m tcp --dport 5280 -j ACCEPT
/sbin/iptables -A INPUT -p udp -m udp --dport 5280 -j ACCEPT
/sbin/iptables -A INPUT -p tcp -m tcp --dport 4369 -j ACCEPT
/sbin/iptables -A INPUT -p udp -m udp --dport 4369 -j ACCEPT
/sbin/iptables -A INPUT -p tcp -m tcp --dport 53873 -j ACCEPT
Of course if there is some specific file which stores iptables rules or some custom firewall these rules has to be added / modified to fit appropriate place or chain.
4. Restart ejabberd via init.d script
debian:~# /etc/init.d/ejabberd restart
Restarting jabber server: ejabberd is not running. Starting ejabberd.
5. Create ejabberd necessery new user accounts
debian:~# /usr/sbin/ejabberdctl register admin jabber.myserver-host.com mypasswd1
etc.debian:~# /usr/sbin/ejabberdctl register hipo jabber.myserver-host.com mypasswd2
debian:~# /usr/sbin/ejabberdctl register newuser jabber.myserver-host.com mypasswd3
debian:~# /usr/sbin/ejabberdctl register newuser1 jabber.myserver-host.com mypasswd4
...
ejabberdctl ejabberd server client (frontend) has multiple other options and the manual is a good reading.
One helpful use of ejabberdctl is:
debian:~# /usr/sbin/ejabberdctl status
Node ejabberd@debian is started. Status: started
ejabberd is running
ejabberctl can be used also to delete some existent users, for example to delete the newuser1 just added above:
debian:~# /usr/sbin/ejabberdctl unregister newuser jabber.myserver-host.com
6. Post install web configurations
ejabberd server offers a web interface listening on port 5280, to access the web interface right after it is installed I used URL: http://jabber.myserver-host.com:5280/admin/
To login to http://jabber.myserver-host.com:5280/admin/ you will need to use the admin username previously added in this case:
admin@jabber.myserver-host.com mypasswd1
Anyways in the web interface there is not much of configuration options available for change.
7. Set dns SRV records
I'm using Godaddy 's DNS for my domain so here is a screenshot on the SRV records that needs to be configured on Godaddy:
In the screenshto Target is the Fually qualified domain hostname for the jabber server.
Setting the SRV records for the domain using Godaddy's DNS could take from 24 to 48 hours to propagate the changes among all the global DNS records so be patient.
If instead you use own custom BIND DNS server the records that needs to be added to the respective domain zone file are:
_xmpp-client._tcp 900 IN SRV 5 0 5222 jabber.myserver-host.com.
_xmpp-server._tcp 900 IN SRV 5 0 5269 jabber.myserver-host.com.
_jabber._tcp 900 IN SRV 5 0 5269 jabber.myserver-host.com.
8. Testing if the SRV dns records for domain are correct
debian:~$ nslookup
> set type=SRV
> jabber.myserver-host.com
...
> myserver-host.com
If all is fine above nslookup request should return the requested domain SRV records.
You might be wondering what is the purpose of setting DNS SRV records at all, well if your jabber server has to communicate with the other jabber servers on the internet using the DNS SRV record is the way your server will found the other ones and vice versa.
DNS records can also be checked with dig for example
$ dig SRV _xmpp-server._tcp.mydomain.net
[…]
;; QUESTION SECTION:
;_xmpp-server._tcp.mydomain.net. IN SRV
;; ANSWER SECTION:
_xmpp-server._tcp.mydomain.net. 259200 IN SRV 5 0 5269 jabber.mydomain.net.
;; ADDITIONAL SECTION:
jabber.mydomain.net. 259200 IN A 11.22.33.44
;; Query time: 109 msec
;; SERVER: 212.27.40.241#53(212.27.40.241)
;; WHEN: Sat Aug 14 14:14:22 2010
;; MSG SIZE rcvd: 111
9. Debugging issues with ejabberd
Ejabberd log files are located in /var/log/ejabberd , you will have to check the logs in case of any issues with the jabber XMPP server. Here is the three files which log messages from ejabberd:
debian:~$ ls -1 /var/log/ejabberd/
ejabberd.log
erl_crash.dump
sasl.log
I will not get into details on the logs as the best way to find out about them is to read them 😉
10. Testing ejabberd server with Pidgin
To test if my Jabber server works properly I used Pidgin universal chat client . However there are plenty of other multiplatform jabber clients out there e.g.: Psi , Spark , Gajim etc.
Here is a screenshot of my (Accounts -> Manage Accounts -> Add) XMPP protocol configuration
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