FAQ Updates: July/12/2011

Posted on in Categories FAQ, Sys admin last updated July 12, 2011

Our FAQ section is updated in last few days with new howtos:

Sendmail is only accepting connections from localhost ( IP)

Posted on in Categories FAQ, Troubleshooting last updated December 18, 2007

Q. My sendmail service is running under Linux and whenever I try to telnet it from other LAN IP, it gives connection refuse error. If I connect it from localhost it accept connection. I can only send mail from my server only. How do I force sendmail to accept mail from other hosts/LAN ips?

A. For security reason sendmail is by default configured to accept connection from local system ( This should avoid open mail relay problem.

To allow connections from ALL hosts/LAN IPs open sendmail.mc file (login as the root):

# vi /etc/mail/sendmail.mc

Look for line that read as follows:

DAEMON_OPTIONS(`Port=smtp,Addr=, Name=MTA’)dnl

Comment or remove above line and insert new line that read as follows:


Above line will force to accept connection from any host. Save the file. Regenerate sendmail configuration file using m4:

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Restart sendmail service :

# /etc/init.d/sendmail restart

Caution: You should configure firewall and other Sendmail Anti-Spam configuration control to avoid problems.

See also:

Howto Reboot or halt Linux system in emergency

Posted on in Categories CentOS, Debian Linux, Gentoo Linux, Howto, Linux, Linux distribution, Networking, RedHat/Fedora Linux, Security, Suse Linux, Sys admin, Tips, Troubleshooting, Ubuntu Linux last updated February 20, 2008

Linux kernel includes magic system request keys. It was originally developed for kernel hackers. However, you can use this hack to reboot, shutdown or halt computer safely (remember safe reboot/shutdown == flush filesystem buffers and unmount file system and then reboot so that data loss can be avoided).

This is quite useful when Linux based system is not available after boot or after a X server crashed ( svgalib program crashes) or no display on screen. Sysrq key combo forces the kernel to respond it regardless of whatever else it is doing, unless it is completely locked up (dead).

Using further extension to iptables called ipt_sysrq (new iptables target), which allows you to do the same as the magic sysrq key on a keyboard does, but over the network. So if your network server is not responding you can still reboot it. Please note that Magic SysRq support need to be compiled in your kernel. You need to say “yes” to ‘Magic SysRq key (CONFIG_MAGIC_SYSRQ)’ when configuring the kernel. I’m assuming that you have Magic SysRq key’ support is compiled in your kernel.

Enable sysrq keys

By default it is not enabled on many Linux distributions. Add or modify following line (as soon as new Linux system installed) /etc/sysctl.conf:
# vi /etc/sysctl.conf
Append following config directive:
Save and close the file. Reload settings:
# sysctl -p

Save and close the file and reboot system to take effect

How do I use the magic SysRq keys in emergency?

You need to use following key combination in order to reboot/halt/sync file system etc:

The ‘SysRq’ key is also known as the ‘Print Screen’ key. COMMAND-KEY can be any one of the following (all keys need to hit simultaneously) :

  • ‘b’ : Will immediately reboot the system without syncing or unmounting your disks.
  • ‘o’ : Will shutdown your system off (if configured and supported).
  • ‘s’: Will attempt to sync all mounted filesystems.
  • ‘u’ : Will attempt to remount all mounted filesystems read-only.
  • ‘e’ : Send a SIGTERM to all processes, except for init.
  • ‘h’: Show help, indeed this the one you need to remember.

So whey you need to tell your Linux computer to reboot or when your X server is crashed or you don’t see anything going across the screen then just press:

ALT+SysRQ+s : (Press and hold down ALT, then SysRQ (Print Screen) key and press ‘s’) -Will try to syn all mounted system

ALT+SysRQ+r : (Press and hold down ALT, then SysRQ (Print Screen) key and press ‘r’) -Will reboot the system.

If you wish to shutdown the system instead of reboot then press following key combination:

ipt_sysrq is a new iptables target that allows you to do the same as the magic sysrq key on a keyboard does, but over the network. Sometimes a remote server hangs and only responds to icmp echo request (ping). Every administrator of such machine is very unhappy because (s)he must go there and press the reset button. It takes a long time and it’s inconvenient. So use the Network Magic SysRq and you will be able to do more than just pressing a reset button. You can remotely sync disks, remount them read-only, then do a reboot. And everything comfortably and only in a few seconds. Please see Marek Zelem page to enableIP Tables network magic SysRq function.

For more information read official Documentation for sysrq.c version 1.15 stored in /usr/src/linux/Documentation/sysrq.txt and read man page of sysctl, sysctl.conf.