Solution to Knockd won't work / open port in iptables

I had a struggle to get Portknocking with knockd to work on my Ubuntu 14.04 VPS. I've read and followed a lot of instructions, Ubuntus instruction among these. But nothing seemed to help me out here.

I did check my knockd log located to /var/log/knockd.log and the configuration for activating the knockd commands seemed to work. But I always ended up with "command returned non-zero status code (a number)"

So what I figured out that it had to do something with the start_command and stop_command that didn't do the job correctly. Everywhere I could read that you were "supposed to" control the IP tables by having e.g.

start_command = /sbin/iptables -A INPUT -s %IP% -p tcp --dport 22 -j ACCEPT

If it was say to open up the SSH-default port 22. But that didn't work for me.
The first I did was to check if /sbin/iptables even existed and it didn't. No wonder why nothing happen with my iptables configuration...

So one solution to this was for me to create two shell script to configure the iptables for me.
I created a knock-open.sh containing this.

#!/bin/sh 

iptables -I INPUT 1 -s $1 -p tcp -m tcp --dport 22 -j ACCEPT

Then I created a knock-close.sh:

#!/bin/sh

iptables -D INPUT -s $1 -p tcp -m tcp --dport 22 -j ACCEPT

And my knockd.conf-file (/etc/knockd.conf), I configured it like this:

[options] 

logfile = /var/log/knockd.log 

[SSH] 

sequence      = 1212:udp,3861:tcp,8721:udp 

seq_timeout   = 5 

tcpflags      = syn 

start_command = sh /var/scripts/knock-open.sh %IP% 

cmd_timeout   = 20 

stop_command  = sh /var/scripts/knock-close.sh %IP%

So this did the trick for me. After restarting the daemon (service knockd restart) and knocking the sequence ports, iptables was now configured correctly and working with knockd.

I hope this solution helps someone out there who's struggling with knockd and iptables.

Apt-get problems and Temporary failure resolving 'us.archive.ubuntu.com'

Experience problems when trying to install or running update with apt-get in Linux, Ubuntu or other distributions?

When running ie. apt-get update you may get error message like:

Temporary failure resolving 'us.archive.ubuntu.com'
Temporary failure resolving 'archive.ubuntu.com'

Err http://archive.ubuntu.com lucid Release.gpg Temporary failure resolving 'archive.ubuntu.com'

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/lucid/main/i18n/Translation-en_US.lzma Temporary failure resolving 'archive.ubuntu.com'

The problem is that apt-get can't reach the destination files for some reasons.

One possible answer to this problem is that this is an temporary problem because the servers are not available for the moment.
You may also want to check your source.list @ /etc/apt/sources.list

One possible solution for this problem is to check your iptables configuration so you aren't blocking outgoing connection that are needed to be reached.

I solved the "Temporary failure resolving", apt-get failure on my VPS by adding "-A OUTPUT -p udp -m udp --dport 53 -j ACCEPT" into my iptables configuration.

So making sure that this below are added to your iptables configuration can help and solve the Temporary failure resolving issue.

-A OUTPUT -p udp -m udp --dport 53 -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 80 -j ACCEPT