Re-read The Partition Table Without Rebooting Linux System

Posted on in Categories File system, Linux, RedHat/Fedora Linux, Sys admin, Tips, Troubleshooting, Ubuntu Linux last updated May 8, 2006

If you are using hot swappable hard disk and created new partition using fdisk then you need to reboot Linux based system to get partition recognized. Without reboot you will NOT able to create filesystem on your newly created or modified partitions with the mke2fs command.

However with partprobe command you should able to create a new file system without rebooting the box. It is a program that informs the operating system kernel of partition table changes, by requesting that the operating system re-read the partition table.

No Route to Host error and solution

Posted on in Categories Debian Linux, Gentoo Linux, Linux, Networking, RedHat/Fedora Linux, Tips, Troubleshooting, Ubuntu Linux last updated May 5, 2006

I am getting error that read as No Route to Host. I am trying to ping my ISP gateway as well as DNS server but I am getting this error. How do I solve this problem?

This problem indicate networking conflicts or some sort of networking configuration problem.

Here are things to check:

Can you ping to your local router interface (such as 192.168.1.254)?

Make sure your card (eth0) is properly configured with correct IP address and router address. Use ifconfig command to configure IP address and route command to setup correct router address. If you prefer to use GUI tools:

  • redhat-config-network – Works on Red Hat and Fedora Linux/Cent OS.
  • network-admin – Debian and Other Linux distribution use this GUI too

Use above two GUI tools to setup correct IP address, DNS address and router address.

b) Make sure firewall is not blocking your access

iptables is default firewall on Linux. Run following command to see what iptables rules are setup:
# /sbin/iptables -L -n

You can temporary clear all iptables rules so that you can troubleshoot problem. If you are using Red Hat or Fedora Linux type command:
# /etc/init.d/iptables save
# /etc/init.d/iptables stop

If you are using other Linux distribution type following commands:
# iptables -F
# iptables -X
# iptables -t nat -F
# iptables -t nat -X
# iptables -t mangle -F
# iptables -t mangle -X

c) Finally make sure you are using a router and not a proxy server. Proxy servers are good for Internet browsing but not for other work such as ftp, sending ICMP request and so on.

See also:

Configure Static Routes In Debian or Red Hat Enterprise Linux

Posted on in Categories CentOS, Debian Linux, Linux, Networking, RedHat/Fedora Linux, Suse Linux, Ubuntu Linux last updated April 13, 2006

Static routes improves overall performance of your network (especially bandwidth saving). They are also useful in stub networks (i.e. there is only one link to the network). For example, each LAN (located at different offices) is connected to HQ IDC (Internet data center) using single T1/LL/Wan links.

For example under Red Hat/Fedora Linux you can add static router for eth0 network interface by editing /etc/sysconfig/network-scripts/route-eth0 file. Under Debian Linux add static route by editing /etc/network/interface file.

RHEL: Linux Bond / Team Multiple Network Interfaces (NIC) Into a Single Interface

Posted on in Categories fedora linux, Howto, Networking, RedHat/Fedora Linux, Tips last updated April 3, 2006

Finally, today I had implemented NIC bounding (bind both NIC so that it works as a single device). Bonding is nothing but Linux kernel feature that allows to aggregate multiple like interfaces (such as eth0, eth1) into a single virtual link such as bond0. The idea is pretty simple get higher data rates and as well as link failover. The following instructions were tested on:

  1. RHEL v4 / 5 / 6 amd64
  2. CentOS v5 / 6 amd64
  3. Fedora Linux 13 amd64 and up.
  4. 2 x PCI-e Gigabit Ethernet NICs with Jumbo Frames (MTU 9000)
  5. Hardware RAID-10 w/ SAS 15k enterprise grade hard disks.
  6. Gigabit switch with Jumbo Frame

HowTo: Recovering Linux Grub Boot Loader Password

Posted on in Categories CentOS, Debian Linux, Howto, Linux, RedHat/Fedora Linux, Suse Linux, Sys admin, Tips, Troubleshooting, Ubuntu Linux last updated February 20, 2006

If you have, a password protected grub boot loader and you forgot both root and grub password, then you can recover grub-boot loader password using the following method/procedure:

* Use Knoppix cd
* Remove the password from Grub configuration file
* Reboot the system
* Change the root password
* Setup new Grub password if required (optional)

Logging to a centralized loghost from Router or other hosts

Posted on in Categories Backup, CentOS, Debian Linux, Howto, Linux, UNIX, Windows server last updated February 16, 2006

It is really a good idea to have one central logging host for security and performance reason. For example monitoring log files will help you to detect:
* Security risks (you can see failed login attempt, port scan etc) analysis
* Troubleshoot user login problem
* Save disk space
* If hard disk crashed on other hosts old logs will be available from centralized loghost

Linux (and other UNIX like systems) use sysklogd (or syslogd) utility. It is system logging facility. It support of both internet and unix domain sockets enables this utility package to support both local and remote logging from DSL/ADSL router or other hosts in your network.

Prepare syslogd to accept remote logging message

Open file /etc/init.d/sysklogd under Debian Linux to configure syslogd to accept remote message.
# vi /etc/init.d/sysklogd
Locate line SYSLOGD and edit it as follows:
SYSLOGD="-r"
The option (-r) will enable the facility to receive message from the network using an internet domain socket with the syslog service. The default is to not receive any messages from the network.

Save file and exit to shell prompt. Restart the sysklogd:
# /etc/init.d/sysklogd restart

A note about RHEL / CentOS / Fedora Linux User

If you are using Red Hat or Fedora Linux, edit file /etc/sysconfig/syslog:
# vi /etc/sysconfig/syslog
Make changes:
SYSLOGD="-r"
Restart syslogd:
# service syslog restart

Open UDP port 514

If you are, using iptables based firewall, insert following rule to your iptables script to accept connection from your network:

MYNET=192.168.1.0/24
SLSERVER=192.168.1.100

iptables -A INPUT -p udp -s $MYNET --sport 1024:65535 -d $SLSERVER --dport 514 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -p udp -s $SLSERVER --sport 514 -d $MYNET --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

192.168.1.100 is IP address of syslogd server. You need to restrict access to syslogd within your network (192.168.1.0/24) only.

Configure the Router to logging message to a centralized loghost

You can open web configuration interface and type IP address of centralized loghost (192.168.1.100) and port 514. Save configuration and reboot router.

Configure Linux or Unix host to logging message to a centralized loghost

You need to open syslog configuration file /etc/syslog.conf:
# vi /etc/syslog.conf
Setup syslogd to send all important message related to auth to loghost IP 192.168.1.100 (or use FQDN if configured)

*.*;auth,authpriv.none          @192.168.1.100

OR

*.*;auth,authpriv.none          @loghost.mydomain.com.

Restart sysklogd (Debian Linux):
# /etc/init.d/sysklogd restart
OR
Restart syslogd under Red Hat/Fedora / CentOS Linux
# service syslog restart
If required open outgoing UDP 514 port from other hosts:

# SYSLOG outgoing client request
iptables -A OUTPUT -p udp -s 192.168.1.100 --sport 1024:65535 -d 192.168.1.5 --dport 514 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -A INPUT -p udp -s 192.168.1.5 --sport 514 -d 192.168.1.100 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

Windows NT/2000/XP/Vista Desktop system

You can force your Windows NT/2000/XP desktop to log all messages to a centralized loghost. However, Windows do not have in build system to log message to remote Unix syslogd server. You can use NTsyslog program, which runs as a service under Windows NT based operating systems. It formats all System, Security, and Application events into a single line and sends them to a syslogd host.

Verify that message are logged in to your /var/log/messages# tail -f /var/log/messages
Output:

Feb 16 02:08:01 router  kernel: klogd started: BusyBox v1.00 (2005.09.22-19:11+0000)
Feb 16 02:08:01 router  kernel: Linux version 2.6.8.1 ([email protected]) (gcc version 3.4.2) #1 Thu Sep 22 15:07:47 EDT 2005
Feb 16 02:08:01 router  kernel: Total Flash size: 2048K with 39 sectors
Feb 16 02:08:01 router  kernel: 96338L-2M-8M prom init
Feb 16 02:08:01 router  kernel: CPU revision is: 00029010
Feb 16 02:08:01 router  kernel: Determined physical RAM map:
Feb 16 02:08:01 router  kernel:  memory: 007a0000 @ 0000000
..........
...
......
Feb 16 02:08:01 router  kernel: AdslCoreHwReset: AdslOemDataAddr = 0xA07E504C
Feb 16 02:08:01 router  kernel: ip_tables: (C) 2000-2002 Netfilter core team
Feb 16 02:08:01 router  kernel: ip_conntrack version 2.1 (61 buckets, 0 max) - 368 bytes
Feb 16 02:08:06 router  pppd[224]: pppd 2.4.1 started by admin, uid 0
Feb 16 02:08:07 router  pppd[224]: PPP: Start to connect ...
Feb 16 02:08:10 router  dnsprobe[272]: dnsprobe started!