IPMI (Intelligent Platform Management Interface) provides out-of-band management and monitoring of servers (Supermicro X9SCL/X9SCM in my case). This is similar to Dell's DRAC or HP's iLO, but cheaper.
SOL (Serial over LAN) is a mechanism that enables the input and output of the serial port of a managed system to be redirected over IP. SOL can be accessed with Java (using the IPMI web interface) or with the linux "ipmitool" binary. Since Java is a nightmare to make it work, I've opted for the second method.
I was an operator of that server, not the administrator, so the IPMI and SOL configuration was unknown. I had only the IPMI IP, Username and Password (and root access on the operating system). I needed to guess how the console redirection was made (I think that the server is shipped SOL-enabled, but I was not able to find this information for my server model).
By default, you only get access to BIOS, the boot loader and/or the booted system requires additional settings.
Get OS redirection:
# server was configured to redirect console to COM2 => ttyS2
# and it used 115200 as a bit-rate
$ echo 'T0:23:respawn:/sbin/getty -L ttyS2 115200 vt100' >> /etc/inittab
$ reboot
jokeru@local:~$ ipmitool -C 3 -I lanplus -H 101.102.103.104 -U IPMI_USER -P IPMI_PASS -L operator sol activate
[SOL Session operational. Use ~? for help]
Debian GNU/Linux 7 node-05 ttyS2
node-05 login: