The timezone in Linux is defined in the file /etc/localtime. To change this setting you should modify the contents of this file to reflect the desired zone information. By default, the operating system will keep the zone information files for all timezones under /usr/share/zoneinfo. To set a timezone, you can either copy the file as /etc/localtime or link /etc/locatime to the target file. The preferred method is to use soft links.
I will show you how I changed the timezone in my server from PDT to IST.
Current date and timezone:
[root@www ~]# date
Sat Aug 9 09:32:11 PDT 2008
[root@www ~]# ls -l /etc/localtime
lrwxrwxrwx 1 root root 39 Aug 2 07:15 /etc/localtime -> /usr/share/zoneinfo/America/Los_Angeles
As you can see, the timezone of the system is PDT (Pacific Daylight Time) . Now I want to change this to Indian Standard Time (IST). So I am going to change the link to /etc/localtime. My timezone file will be at the location /usr/share/zoneinfo/Asia/Calcutta.
[root@www ~]# ln -sf /usr/share/zoneinfo/Asia/Calcutta /etc/localtime
[root@www ~]# date
Sat Aug 9 22:10:23 IST 2008
As you can see, the timezone is IST now.
Saturday, August 9, 2008
VPS hosting @ Linode.com
It is one week since I signed up for the Linode VPS package. So far so good is my impression about their service. It is a very short time to evaluate a hosting service. Especially as I am running only a couple of low traffic websites on top of apache. But still I should say something about my initial experience.
The signup process was very quick. After I filled up my personal details, I got a mail with a link to continue signup process. I filled up remaining information - my choice was the basic package, Linode 360 for 19.95$ PM. Immediately afterwards I got an invoice mail followed by a payment receipt. Five more minutes and I got the account activation mail. All these took only ten minutes. I logged into the control panel (Linode manager), set my root password, partitioned the hard disk and installed a CentOS image. Within twenty minutes my server was up and running. Now that is cool, right? Your own server in thirty minutes. I was really impressed.
I will post some features of linode manager soon.
Saturday, May 3, 2008
Bash Date Math
There were many occasions when I had to find out something like "one week after current date" or "one month and two days earlier", something like that. All this lead me to searching for 'date mathematics' in bash. And here is the result I found. (Warning: this may not work in older systems)
This is achieved using the -d option of date which has a syntax:
date -d STRING
where string defines, the format of date. Here is were you can implement date math. As you can see below:
safeer@enjoyfast-lx:~$ date
Wed Dec 3 00:11:06 IST 2008
Now find tomorrows date - just use "+1 day" as format string
safeer@enjoyfast-lx:~$ date -d "+1 day"
Thu Dec 4 00:11:07 IST 2008
So the general format of the string is "+/-N UNIT" where UNIT is any one of "second|minute|hour|day|week|month|year" and N is the number of units. + or - sign determines whether to add or subtract a date. A few examples:
15 hours after current time:
safeer@enjoyfast-lx:~$ date;date -d "+15 hour"
Wed Dec 3 00:14:32 IST 2008
Wed Dec 3 15:14:32 IST 2008
3 months earlier:
safeer@enjoyfast-lx:~$ date;date -d "-3 months"
Wed Dec 3 00:18:39 IST 2008
Wed Sep 3 00:18:39 IST 2008
You can also combine different units as needed:
safeer@enjoyfast-lx:~$ date;date -d "+15 week -2 hours"
Wed Dec 3 00:21:21 IST 2008
Tue Mar 17 22:21:21 IST 2009
This way we can get whatever date we need. Combining this with the output formating option for date will give you good formatted dates to use in your scripts and commands. For eg:
safeer@enjoyfast-lx:~$ date -d "-2 year +42 hours"
Mon Dec 4 18:23:22 IST 2006
safeer@enjoyfast-lx:~$ date -d "-2 year +42 hours" +%F
2006-12-04
This is achieved using the -d option of date which has a syntax:
date -d STRING
where string defines, the format of date. Here is were you can implement date math. As you can see below:
safeer@enjoyfast-lx:~$ date
Wed Dec 3 00:11:06 IST 2008
Now find tomorrows date - just use "+1 day" as format string
safeer@enjoyfast-lx:~$ date -d "+1 day"
Thu Dec 4 00:11:07 IST 2008
So the general format of the string is "+/-N UNIT" where UNIT is any one of "second|minute|hour|day|week|month|year" and N is the number of units. + or - sign determines whether to add or subtract a date. A few examples:
15 hours after current time:
safeer@enjoyfast-lx:~$ date;date -d "+15 hour"
Wed Dec 3 00:14:32 IST 2008
Wed Dec 3 15:14:32 IST 2008
3 months earlier:
safeer@enjoyfast-lx:~$ date;date -d "-3 months"
Wed Dec 3 00:18:39 IST 2008
Wed Sep 3 00:18:39 IST 2008
You can also combine different units as needed:
safeer@enjoyfast-lx:~$ date;date -d "+15 week -2 hours"
Wed Dec 3 00:21:21 IST 2008
Tue Mar 17 22:21:21 IST 2009
This way we can get whatever date we need. Combining this with the output formating option for date will give you good formatted dates to use in your scripts and commands. For eg:
safeer@enjoyfast-lx:~$ date -d "-2 year +42 hours"
Mon Dec 4 18:23:22 IST 2006
safeer@enjoyfast-lx:~$ date -d "-2 year +42 hours" +%F
2006-12-04
Wednesday, March 12, 2008
TCP Wrappers
TCP wrappers is used to implement host based access restrictions to network services running on a Linux server. In order for a network service to work with TCP wrappers, it should be compiled with libwrap.a library. To check whether a network service has TCP wrapping enabled use:
strings -f /path/to/service/binary|grep hosts_access, for example to check whether ssh is compiled with TCP wrappers support,
[root@LinuxBox1 ~]#strings -f /usr/sbin/sshd|grep hosts_access
/usr/sbin/sshd: hosts_access
Host access is allowed or denied based on the entries in the files /etc/hosts.allow & hosts.deny ( together known as host access files) . Each of these files contain a list of services and corresponding list of hosts along with certain additional options.
As the name indicates, hosts.allow lists the hosts allowed to access listed services. When a host tries to access a network service on the server, hosts.allow is parsed from top to bottom to see a match for the service-host pair. The parsing will stop with the first match found and the access will be allowed. If no match is found in this file, hosts.deny is parsed. If a match is found access is denied for the host. If neither file contain a match or the files does not exist, access is allowed for the host.
Access files contain a number of rows, each with following format
daemon-list:host-list:[option1]:[option2]:..........
daemon list : Comma separated list of service binary names. Dont take this for the service name, for example, the service name for telnet is 'telnet' but the binary is 'in.telnetd' (/usr/sbin/in.telnetd).
host list : Comma separated list of hosts to be allowed/denied access to the daemon list. Hosts can be specified in any of the following format.
host names : machine1.mydomain.com represents a single machine
host names starting with a period: .mydomain.com represents all host names under this name. Example: machine2.mydomain.com, switch2.sub1.mydomain.com ......
Hostnames containing ? and/or * : Within a hostname or IP you can use this wild cards, '*' can substitute any number of characters while '?' can replace only a single character.
Eg:
*.mydomain.com : mac1.mydomain.com, mac2.sub1.mydomain.com ....
mac?.mydomain.com : mac1.mydomain.com,mac2.mydomain.com,...mac9.mydomain.com,maca.mydomain.com,macb.mydomain.com etc...
IP address ending with period : 192.168. represents all IPs in the network 192.168.0.0/16
Network & Netmasks : 192.168.1.0/255.255.255.128, 10.10.0.0/255.255.224.0 etc...
File path beginning with leading '/' : The specified file contains a list of hosts. Each line of the file will contain a number of host/ip patterns separated by white spaces.
Explicit wild cards: This wild cards has special purposes, even though there are many the popular one is 'ALL' which matches anything ( applicable to daemons as well as hosts)
The 'EXCEPT' operator: This can be used with both hosts and daemons as follows:
a) In the daemon list, a rule that is applicable to all services except vsftpd
ALL EXCEPT vsftpd
b) In the clients list all machine in the network 192.168.0.0 except 192.168.0.15
192.168.0. EXCEPT 192.168.0.15
options : This fields can be used to alter the log behaviour, launch shell commands etc when a rule is matched, but this feature is specific toRedhat version of tcp wrappers.
There are two approaches for any firewall/access control design, mostly closed and mostly open. In mostly closed all accesses arerestricted and only minimum required access is granted. For TCP wrappers mostly closed means a single line ALL:ALL in hosts.deny and necessary services in hosts.allow. In mostly open, all client access is allowed and restrictions applied as and when necessary. InTCP wrappers: ALL:ALL in hosts.allow and necessary restrictions in hosts.deny.
Some examples rules:
Allow telnet from local networks (192.168.0.0./24 & 192.168.1.0/24)
hosts.allow
in.telnetd:192.168.0.,192.168.1.
Suppose, in addition to this, you want to deny telnet access from all other hosts.
hosts.deny
in.telnetd:ALL
Since hosts.allow is parsed first the access rule ther will match request from local networks and allow them. If the request is from an external host, the rules in hosts.allow will not match and will be passed to hosts.deny where the rule is to deny all telnet communication. This rule will be matched and access denied for that host.
Alternatively we can do both this in a single rule in hosts.deny
hosts.deny
in.telnetd: ALL EXCEPT 192.168.0.,192.168.1.
strings -f /path/to/service/binary|grep hosts_access, for example to check whether ssh is compiled with TCP wrappers support,
[root@LinuxBox1 ~]#strings -f /usr/sbin/sshd|grep hosts_access
/usr/sbin/sshd: hosts_access
Host access is allowed or denied based on the entries in the files /etc/hosts.allow & hosts.deny ( together known as host access files) . Each of these files contain a list of services and corresponding list of hosts along with certain additional options.
As the name indicates, hosts.allow lists the hosts allowed to access listed services. When a host tries to access a network service on the server, hosts.allow is parsed from top to bottom to see a match for the service-host pair. The parsing will stop with the first match found and the access will be allowed. If no match is found in this file, hosts.deny is parsed. If a match is found access is denied for the host. If neither file contain a match or the files does not exist, access is allowed for the host.
Access files contain a number of rows, each with following format
daemon-list:host-list:[option1]:[option2]:..........
daemon list : Comma separated list of service binary names. Dont take this for the service name, for example, the service name for telnet is 'telnet' but the binary is 'in.telnetd' (/usr/sbin/in.telnetd).
host list : Comma separated list of hosts to be allowed/denied access to the daemon list. Hosts can be specified in any of the following format.
host names : machine1.mydomain.com represents a single machine
host names starting with a period: .mydomain.com represents all host names under this name. Example: machine2.mydomain.com, switch2.sub1.mydomain.com ......
Hostnames containing ? and/or * : Within a hostname or IP you can use this wild cards, '*' can substitute any number of characters while '?' can replace only a single character.
Eg:
*.mydomain.com : mac1.mydomain.com, mac2.sub1.mydomain.com ....
mac?.mydomain.com : mac1.mydomain.com,mac2.mydomain.com,...mac9.mydomain.com,maca.mydomain.com,macb.mydomain.com etc...
IP address ending with period : 192.168. represents all IPs in the network 192.168.0.0/16
Network & Netmasks : 192.168.1.0/255.255.255.128, 10.10.0.0/255.255.224.0 etc...
File path beginning with leading '/' : The specified file contains a list of hosts. Each line of the file will contain a number of host/ip patterns separated by white spaces.
Explicit wild cards: This wild cards has special purposes, even though there are many the popular one is 'ALL' which matches anything ( applicable to daemons as well as hosts)
The 'EXCEPT' operator: This can be used with both hosts and daemons as follows:
a) In the daemon list, a rule that is applicable to all services except vsftpd
ALL EXCEPT vsftpd
b) In the clients list all machine in the network 192.168.0.0 except 192.168.0.15
192.168.0. EXCEPT 192.168.0.15
options : This fields can be used to alter the log behaviour, launch shell commands etc when a rule is matched, but this feature is specific toRedhat version of tcp wrappers.
There are two approaches for any firewall/access control design, mostly closed and mostly open. In mostly closed all accesses arerestricted and only minimum required access is granted. For TCP wrappers mostly closed means a single line ALL:ALL in hosts.deny and necessary services in hosts.allow. In mostly open, all client access is allowed and restrictions applied as and when necessary. InTCP wrappers: ALL:ALL in hosts.allow and necessary restrictions in hosts.deny.
Some examples rules:
Allow telnet from local networks (192.168.0.0./24 & 192.168.1.0/24)
hosts.allow
in.telnetd:192.168.0.,192.168.1.
Suppose, in addition to this, you want to deny telnet access from all other hosts.
hosts.deny
in.telnetd:ALL
Since hosts.allow is parsed first the access rule ther will match request from local networks and allow them. If the request is from an external host, the rules in hosts.allow will not match and will be passed to hosts.deny where the rule is to deny all telnet communication. This rule will be matched and access denied for that host.
Alternatively we can do both this in a single rule in hosts.deny
hosts.deny
in.telnetd: ALL EXCEPT 192.168.0.,192.168.1.
Sunday, March 9, 2008
Adding swap space to Linux
The virtual memory used by Linux is called swap space. It is the extension of physical memory in the sense that the total usable primary memory will be the sum of RAM and swap space. The kernel will write the memory blocks of unused programs to the swap space, thus freeing the RAM to be used by active programs. Hard disk access is much slower when compared to RAM access. So swap space can't be considered as a replacement for RAM, it is just a supporting techniques for RAM. Using of swap space imposes overhead on the system, but for systems with less physical memory it is a necessary evil.
When you install the operating system you will add a swap space, most probably up to double of the RAM. This will not be enough if you later add more RAM to your machine and you will have to increase the swap space. You can add swap space to your system in two ways, either by adding a swap partition or swap file.
To use swap partition, you should have unallocated space left on your hard disk. Use fdisk program to create a new partition.
[root@LinuxBox1 ~]#fdisk /dev/hdb
and use "n" command to create a new partition, say /dev/hdb2
If you don't have unallocated space to create a new partition, but have free space on existing partitions, you can create swap files instead. IF you want to create 128 MB of swap space, create a file of that size.
[root@LinuxBox1 ~]#dd if=/dev/zero of=/vmem/swapfile01 bs=1024 count=131072
131072+0 records in
131072+0 records out
134217728 bytes (134 MB) copied, 3.0451 seconds, 44.1 MB/s
In the rest of this document, I will use this swap file as example, but the procedure is same for swap partition also.
To mark the created space usable as swap, a signature is needed to be written into the start of file/partition. This is done withmkswap command.
[root@LinuxBox1 ~]#mkswap /vmem/swapfile01
Setting up swapspace version 1, size = 134213 kB
Now tell the kernel to use this space for swap
[root@LinuxBox1 ~]#swapon /vmem/swapfile01
In order to keep this settings persistent over reboots, add a line to /etc/fstab.
/etc/fstab
/vmem/swapfile01 swap swap defaults 0 0
To list all the swap partitions/files in a system,
[root@LinuxBox1 ~]#swapon -s
Filename Type Size Used Priority
/dev/hda3 partition 1124540 1848 -1
/vmem/swapfile01 file 131064 0 -3
/vmem/swapfile02 file 65528 0 -4
To see the current primary memory status
[root@LinuxBox1 ~]#free -m
total used free shared buffers cached
Mem: 495 470 24 0 3 204
-/+ buffers/cache: 263 232
Swap: 1290 1 1288
You can see that Total Swap reported is the sum of 3 swap sizes as listed in the output of "swapon -s"
If you want to remove the added swap space, turn of swapping for the partition/file
[root@LinuxBox1 ~]#swapoff /vmem/swapfile01
If you want to save hard disk space, remove the file/partition.
Swap files are most useful when you need to run a memory intensive program for a short period of time. You can create a swap file of appropriate size and delete it when the application is finished. If you want to add swap for permanent use, better go for swap partitions- provided your hard disk has unallocated space left.
When you install the operating system you will add a swap space, most probably up to double of the RAM. This will not be enough if you later add more RAM to your machine and you will have to increase the swap space. You can add swap space to your system in two ways, either by adding a swap partition or swap file.
To use swap partition, you should have unallocated space left on your hard disk. Use fdisk program to create a new partition.
[root@LinuxBox1 ~]#fdisk /dev/hdb
and use "n" command to create a new partition, say /dev/hdb2
If you don't have unallocated space to create a new partition, but have free space on existing partitions, you can create swap files instead. IF you want to create 128 MB of swap space, create a file of that size.
[root@LinuxBox1 ~]#dd if=/dev/zero of=/vmem/swapfile01 bs=1024 count=131072
131072+0 records in
131072+0 records out
134217728 bytes (134 MB) copied, 3.0451 seconds, 44.1 MB/s
In the rest of this document, I will use this swap file as example, but the procedure is same for swap partition also.
To mark the created space usable as swap, a signature is needed to be written into the start of file/partition. This is done withmkswap command.
[root@LinuxBox1 ~]#mkswap /vmem/swapfile01
Setting up swapspace version 1, size = 134213 kB
Now tell the kernel to use this space for swap
[root@LinuxBox1 ~]#swapon /vmem/swapfile01
In order to keep this settings persistent over reboots, add a line to /etc/fstab.
/etc/fstab
/vmem/swapfile01 swap swap defaults 0 0
To list all the swap partitions/files in a system,
[root@LinuxBox1 ~]#swapon -s
Filename Type Size Used Priority
/dev/hda3 partition 1124540 1848 -1
/vmem/swapfile01 file 131064 0 -3
/vmem/swapfile02 file 65528 0 -4
To see the current primary memory status
[root@LinuxBox1 ~]#free -m
total used free shared buffers cached
Mem: 495 470 24 0 3 204
-/+ buffers/cache: 263 232
Swap: 1290 1 1288
You can see that Total Swap reported is the sum of 3 swap sizes as listed in the output of "swapon -s"
If you want to remove the added swap space, turn of swapping for the partition/file
[root@LinuxBox1 ~]#swapoff /vmem/swapfile01
If you want to save hard disk space, remove the file/partition.
Swap files are most useful when you need to run a memory intensive program for a short period of time. You can create a swap file of appropriate size and delete it when the application is finished. If you want to add swap for permanent use, better go for swap partitions- provided your hard disk has unallocated space left.
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