Archive for the ‘Tips and Tricks’ Category

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vnstat

Thursday, November 18th, 2010

vnStat is a wonderful tool that will allow you to grab traffic information from you network interfaces.

From its homepage…

vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network traffic for the selected interface(s). It uses the network interface statistics provided by the kernel as information source. This means that vnStat won’t actually be sniffing any traffic and also ensures light use of system resources. However, in Linux at least a 2.2 series kernel is required.

(more…)

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Posted in General, Linux Debian, Tips and Tricks | No Comments »

Description of the file system hierarchy

Wednesday, August 19th, 2009

man hier

;)

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Posted in General, Tips and Tricks, Unix facts | No Comments »

Bash script: effectively check if a filesystem is mounted

Tuesday, August 18th, 2009

Whenever you want to check if a file system is mounted and perform some action in you script based on this, the simplest and most accurate way to do it in Linux is to check /proc/mounts:

if grep -qs ‘/mnt/sda2′ /proc/mounts; then
echo “fs mounted”
else
echo “fs not mounted”
fi

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How to disable IPv6 in Debian

Thursday, April 23rd, 2009

I’ve recently installed a new LDAP server on Debian Lenny and I wanted to disable IPv6 as it is unnecessary for me in this moment. With netstat I checked the listening processes:

netstat -tunlp

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:389 0.0.0.0:* LISTEN 2226/slapd
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 2215/sshd
tcp6 0 0 :::389 :::* LISTEN 2226/slapd
tcp6 0 0 :::22 :::* LISTEN 2215/sshd

and lsmod showed something like this:

Module Size Used by
ipv6 235364 12
...

So, to disable IPv6 I changed /etc/modprobe.d/aliases:
...
# alias net-pf-10 ipv6
# Disable ipv6
alias net-pf-10 off
alias ipv6 off
...

I also disabled these lines in /etc/hosts to avoid confusions:

...
## The following lines are desirable for IPv6 capable hosts
#::1 localhost ip6-localhost ip6-loopback
#fe00::0 ip6-localnet
#ff00::0 ip6-mcastprefix
#ff02::1 ip6-allnodes
#ff02::2 ip6-allrouters
#ff02::3 ip6-allhosts

Finally I restarted the server.

shutdown -r now

Now the situation is like this:

netstat -tunlp

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:389 0.0.0.0:* LISTEN 2233/slapd
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 2216/sshd

lsmod|grep ipv6

...

No IPv6.

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dig: basic usage

Wednesday, April 15th, 2009

Today we are going to take a quick look at dig. According to the man pages:

dig (domain information groper) is a flexible tool for interrogating DNS name servers. It performs DNS lookups and displays the answers that are returned from the name server(s) that were queried. Most DNS administrators use dig to troubleshoot DNS problems because of its flexibility, ease of use and clarity of output.

Let’s see how to lookup the DNS servers for the domain karkomaonline.com:

dig karkomaonline.com -t ns

The -t option specifies the query type (a, any, mx, ns, txt… ), being a the default. The -t ns option will look-up the Name Servers for the domain karkomaonline.com. You should get something like this:

; <<>> DiG 9.4.2-P2 <<>> karkomaonline.com -t ns
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4825
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1


;; QUESTION SECTION:
;karkomaonline.com.        IN    NS


;; ANSWER SECTION:
karkomaonline.com.    172800    IN    NS    dns010.d.register.com.
karkomaonline.com.    172800    IN    NS    dns024.c.register.com.
karkomaonline.com.    172800    IN    NS    dns071.a.register.com.
karkomaonline.com.    172800    IN    NS    dns150.b.register.com.


;; ADDITIONAL SECTION:
dns010.d.register.com.    48    IN    A    216.21.236.10


;; Query time: 172 msec
;; SERVER: 192.168.1.9#53(192.168.1.9)
;; WHEN: Wed Apr 15 22:46:35 2009
;; MSG SIZE  rcvd: 152

The interesting part is the ANSWER SECTION, that lists the name servers for the mentioned domain. You can get a shorter output of the same command:

dig karkomaonline.com -t ns +short

dns010.d.register.com.
dns071.a.register.com.
dns024.c.register.com.
dns150.b.register.com.

Now look up the mail servers for the same domain:

dig karkomaonline.com -t mx

...
;; ANSWER SECTION:
karkomaonline.com.    86400    IN    MX    0 mailhost.karkomaonline.com.
...

From the output of the first example you can see that by default dig queried my internal DNS server (configured in /etc/resolv.conf):

...
;; SERVER: 192.168.1.9#53(192.168.1.9)
...

You can change this behaviour by instructing dig to query a specific name server:

dig @dns010.d.register.com karkomaonline.com -t mx

...
;; ANSWER SECTION:
karkomaonline.com.    86400    IN    MX    0 mailhost.karkomaonline.com.


;; ADDITIONAL SECTION:
mailhost.karkomaonline.com. 86400 IN    A    94.75.208.171


;; Query time: 181 msec
;; SERVER: 216.21.236.10#53(216.21.236.10)
...

Note that the queried server now is 216.21.236.10.

More info::

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ISO image as a filesystem

Saturday, January 3rd, 2009

If you want to mount an ISO image as a filesystem, simply proceed as follows:

mkdir /mnt/iso0
mount -o loop -t iso9660 /tmp/debian-40r6-amd64-netinst.iso  /mnt/iso0

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System stats with saidar

Sunday, April 6th, 2008

saidar is a top like tool. It provides a curses based interface for viewing system statistics such as network I/O, disk I/O, CPU, memory and more. The core of the functionality is provided by the libstatgrab library:

libstatgrab is a library that provides cross platform access to statistics about the system on which it's run. It's written in C and presents a selection of useful interfaces which can be used to access key system statistics. The current list of statistics includes CPU usage, memory utilisation, disk usage, process counts, network traffic, disk I/O, and more.

On a Debian system install the package as usual:

apt-get install saidar

and then run:

saidar

Saidar in action

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Serial terminal support in GRUB

Saturday, February 9th, 2008

In some situations it is very useful to have the possibility to access the GRUB boot menu from a serial line. For example you could remotely reboot your machine with the new kernel you just compiled.

In my case I have a couple of remote machines linked together by a serial cable. I can log into machine A via ssh and get the Grub menu of machine B through minicom. Thus I can boot whatever OS/kernel I want as if I were in front of the machine. Kind of KVM IP for the rest of us.

Simply add this to Grub’s menu.lst:

serial –unit=0 –speed=9600 –word=8 –parity=no –stop=1
terminal serial

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MySQL: quick replacement

Thursday, December 6th, 2007

Say that you have a table called myTable with several rows and a column called URL that contains the IP address of a log server. Now imagine that you want to change the value of URL so IP address is substituted by the server name. One way to do that is as follows:

use dbname

update myTbl set URL=replace(URL,’172.16.1.10′,’logserver’);

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OpenSSH: simplifying logins

Thursday, December 6th, 2007

OpenSSH provides a per-user configuration file usually located in ~/.ssh/config. This file can help ease your life as Sysadmin. Let’s say that you frequently connect to a server in this way:

ssh admin@boring-servername.boring-domain.com

With OpenSSH there is an easy way to do the job. Edit or create a config file:

vi ~/.ssh/config

and then add the following:

Host servername
     User admin
     HostName boring-servername.boring-domain.com

Where Host is the alias for the remote server you want to connect to, HostName is the full name of the remote server and User is the login name.

References:

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