always remember

Nothing is foolproof to a sufficiently talented fool... Make something
idiot proof, and the world will simply make a bigger idiot.

Monitor Pending Connections – Zen/Zevenet Load Balancers

In my working environment, we use (rather extensively) ZenLB (or as they are now know, Zevenet) Load Balancers. In production systems, sometimes the back-ends of an infrastructure, or the “real servers” behind the load balancers, can become unresponsive for whatever reason. A typical one that I see quite often is when using clustered MS Exchange Client Access servers behind a load balanced pool. IIS may lock up on one or multiple CAS’s causing the connections coming in from clients to be stored at LB level as “pending”.

This is fine, but in my experience, once the Zevenet LB racks up 1500+ pending connections on one of its farms, it quickly exhausts it’s available memory.

The following check is called by the Nagios NRPE agent installed locally on the LB (It’s just Debian 8 afterall)

# ZenLB Pending/Established Connection Tracking v1.0 - Dave Byrne
hour=`date +%H`
pending=`cat /proc/net/nf_conntrack |grep SYN_SENT |grep dport='443|80' |wc -l`
established=`cat /proc/net/nf_conntrack |grep ESTABLISHED |grep dport='443|80' |wc -l`

if [ $pending -gt 5 ]
      printf "CRITICAL - Pending connections above threshold! Pending: $pending -- Established: $establishedn"
   exit 2
elif [ $established -eq 0 ] && [ $hour -ge 8 ] && [ $hour -le 23 ];
      printf "CRITICAL - No established connections! Pending: $pending -- Established: $establishedn"
   exit 2
      printf "OK - Pending connections at acceptable level. Pending: $pending -- Established: $establishedn"
   exit 0

The check will go CRITICAL if pending connections across ANY of the farms goes above 5. It will also go CRITICAL is the established connections drops to 0 (probably bad). But I have limited this to a certain time frame, as I appreciate that there may well be 0 established connections at 4am!!


dave / August 21, 2017 / Code, Nagios Monitoring

dave / July 27, 2016 / Code, Linux Bash, Nagios Monitoring

How To Upgrade PostgreSQL From 9.3 to 9.4 (In-Place)

To make use of the JSONB features implemented in 9.4, it is required that you upgrade your existing PgSQL 9.3 cluster to 9.4+. I cover the basics on how to perform an in-place upgrade.

1. Add the PostgreSQL repo to apt:

echo "deb utopic-pgdg main" > /etc/apt/sources.list.d/pgdg.list


2. Install the repo’s key:

wget -q -O - | sudo apt-key add

Read On… ->

dave / April 7, 2016 / Guide, PostgreSQL

How To Setup Binary Replication Between 2 PostgreSQL 9.4 Hosts (Hot-Standby)

Utilising a master/slave (hot-standby) setup to provide a resilience layer at database level can be easy. The following assumes you have 2 PgSQL hosts at and, both running Ubuntu 14.04 LTS and PostgreSQL 9.4 (9.4.5).

1. On the master, edit the following in postgresql.conf:

listen_addresses = '*'
wal_level = hot_standby
max_wal_senders = 3

listen_addresses can also be scoped down to single or multiple server bound IP addresses, for added security/best practice

Read On… ->

dave / April 7, 2016 / Guide, PostgreSQL

dave / April 6, 2016 / Code, Nagios Monitoring

Bash Application – Simpler Exim Queue Management

Ever get stuck trying to remember the exim/exiqgrep/xargs commands at that one critical moment when you need to stop 800,000 spam mail getting delivered to Hotmail accounts? Me too.. So I made this to help. My other monitoring & alert scripts should have told you by now that you have a spam outbreak, so you’ll know which sending host its coming from. Fire this application up on that host to quickly and effectively manage the items in the mail queue.

Read On… ->

dave / March 13, 2016 / Code, Linux Bash