Category: Reliable Penguin

Reduce TCP WAIT Times

October 19, 2012

Interesting discussion here: http://stackoverflow.com/questions/410616/increasing-the-maximum-number-of-tcp-ip-connections-in-linux Especially, this section: Default Sysctl values on a typical linux box for tcp_tw_recycle & tcp_tw_reuse would be


net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_tw_reuse = 0

net.ipv4.tcp_tw_recycle = 0

net.ipv4.tcp_tw_reuse = 0

These do not allow a connection in wait state after use, and force them to last the complete time_wait cycle. I recommend setting them to:


net.ipv4.tcp_tw_recycle = 1
net.ipv4.tcp_tw_reuse = 1

net.ipv4.tcp_tw_recycle = 1

net.ipv4.tcp_tw_reuse = 1

This allows fast cycling of sockets in time_wait state and re-using them. But before you do this change make sure that this does not conflict with the protocols that you would use for the application that needs these sockets. Some other references: http://www.speedguide.net/articles/linux-tweaking-121 and http://wiki.docdroppers.org/index.php?title=Sysctl_Modifications

Zabbix Proxy Install

October 13, 2012

Zabbix is our favorite monitoring tool and the new 2.0 release contains a range of great new features. One of the most useful features of the Zabbix architecture is support for Zabbix proxies. A Zabbix proxy gathers monitoring data and forwards it to the central monitoring server. This is really useful if you have a group of servers inside a firewall that need to be monitored. Just setup a Zabbix proxy inside the firewall and all the monitoring data can be securely forwarded to the central external monitoring server. The documentation of proxy setup is here: http://www.zabbix.com/documentation/2.0/manual/distributed_monitoring/proxies For a CentOS 6 server here are the steps that I used to install and configure a Zabbix proxy. Download latest source distribution from http://www.zabbix.com/download.php. Unpack, compile and install according to docs at: http://www.zabbix.com/documentation/2.0/manual/installation/install Since we’re installing from source we need a selection of build tools which I added with:


yum install gcc make

yum install gcc make

The Zabbix

SED Trick – Add slashes to filenames if needed

August 30, 2011

This sed command will append / to the end of every line in filename.txt which doesn’t already have one.


sed -i '/\/$/!s/$/\//g' filename.txt

sed -i '/\/$/!s/$/\//g' filename.txt

Manage Apache Modules On Ubuntu

June 7, 2011

Configuration for enable modules is stored in: /etc/apache2/mods-enabled Available but inactive modules are stored in: /etc/apache2/mods-available A module can be enabled with:

a2enmod [modulename]

1	a2enmod [modulename]

Where [modulename] is the module to be enabled. For example:

a2enmod rewrite

1	a2enmod rewrite

will enable the rewrite module. A module can be disabled in a similar fashion with:

a2dismod [modulename]

1	a2dismod [modulename]

Don’t forget to restart apache after making changes with:

service apache2 restart

1	service apache2 restart

Syncing Content Between RackSpace Cloud Servers With Unison

June 2, 2011

We have two RackSpace Cloud Server running CentOS that need to have the web content kept in sync. Changes on either server need to be replicated to the other server. Easy way to get this accomplished is with the Unison File Syncronizer (http://www.cis.upenn.edu/~bcpierce/unison/). Here are the steps: 1. Install SSH keys for root on each server so SSH can run without passwords between the servers. 2. Add /etc/hosts file entries to map the private addresses of the servers to names.

192.168.10.1 web1-priv
192.168.10.2 web2-priv

192.168.10.1 web1-priv

192.168.10.2 web2-priv

3. Install Unison on each server:

yum install unison227

1	yum install unison227

4. Create a profile on each server at /root/.unison/sync_web.prf containing:


# Reasonable defaults
auto=true
confirmbigdeletes=true
contactquietly=true
group=true
maxthreads=20
numericids=true
owner=true
times=true

# Skip confirmation
batch=true

# Log all sync operations
log=true
logfile=/var/log/unison.log

# Backup deleted files
backup=Name *
backuplocation=central
backupdir=/var/www/vhosts/mydomain.com/backup/
maxbackups=5

# Local root
root=/var/www/vhosts/mydomain.com/httpdocs/

# Remote root (the double forward-slash between IP and remote path is correct)
root=ssh://<font style="color:red">web2-priv</font>///var/www/vhosts/mydomain.com/httpdocs/

# Resolve conflicts in favor of local root
prefer=newer

# Don't sync (can specify multiple ignore lines)
#ignore=Path */var/cache

# Reasonable defaults

auto=true

confirmbigdeletes=true

contactquietly=true

group=true

maxthreads=20

numericids=true

owner=true

times=true

# Skip confirmation

batch=true

# Log all sync operations

log=true

logfile=/var/log/unison.log

# Backup deleted files

backup=Name *

backuplocation=central

backupdir=/var/www/vhosts/mydomain.com/backup/

maxbackups=5

# Local root

root=/var/www/vhosts/mydomain.com/httpdocs/

# Remote root (the double forward-slash between IP and remote path is correct)

root=ssh://<font style="color:red">web2-priv</font>///var/www/vhosts/mydomain.com/httpdocs/

# Resolve conflicts in favor of local root

prefer=newer

# Don't sync (can specify multiple ignore lines)

#ignore=Path */var/cache

Notice the hostname “web2-priv” in red above. The file on web1 should reference web2-priv. The file on web2 should reference web1-priv. You’ll need to adjust the path listed in the file to match your environment. 5. Next create a script to run Unison with the profile created in the previous step.