Monthly Archives: July 2018

Web

Rewrite HTTP host request with Varnish

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I had a stubborn WordPress plugin that somehow was remembering the URL of the site it was installed on. It became a problem when I changed the site URL. Despite changing the URL everywhere I could think of, this particular plugin was calling CSS files for the URL of the original site. I did a search and replace in the site database and searched all files for any reference to that site but couldn’t find anything. I never did find the culprit. My workaround was to use varnish to rewrite the request before it hit the browser.

Thanks to this answer by Jorge Nerin on Stack Overflow, I found my answer on how to do this.

backend www {
  .host = "www.example.com";
  .port = "http";
}

sub vcl_recv {
  if (req.http.host ~ "(?i)^(www.)?example.com$") {
    set req.backend_hint = www;
  }
}

In my case I had a default backend (no other backends configured) so my varnish config was simply adding these line in sub vcs_recv (varnish 4 syntax)

if (req.http.host ~ "(?i)^(www.)?old.host.name$") {
     set req.backend_hint = default;
}

That did the trick!

CLI, OS

Mount encfs folder on startup with systemd

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A quick note on how to encrypt a folder with encfs and then mount it on boot via a systemd startup script. In my case the folder is located on a network drive and I wanted it to happen whether I was logged in or not.

Create encfs folder:

encfs <path to encrypted folder> <path to mount decrypted folder>

Follow the prompts to create the folder and set a password.

Next create a file which will contain your decryption password

echo "YOUR_PASSWORD" > /home/user/super_secret_password_location
chmod 700 /home/user/super_secret_password_location

Create a simple script to be called by systemd on startup using cat to pass your password over to encfs

#!/bin/bash
cat super_secret_password_location | encfs -S path_to_encrypted_folder path_to_mount_decrypted_folder

Finally create a systemd unit to run your script on startup:

vim /etc/systemd/system/mount-encrypted.service
[Unit] 
Description=Mount encrypted folder 
After=network.target 

[Service] 
User=<YOUR USER> 
Type=oneshot 
ExecStartPre=/bin/sleep 20 
ExecStart=PATH_TO_SCRIPT
TimeoutStopSec=30 
KillMode=process 

[Install] 
WantedBy=multi-user.target

Then enable the unit:

sudo systemctl daemon-reload
sudo systemctl enable mount-encrypted.service
CLI, Virtualization

CPU Pinning in Proxmox

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Proxmox uses qemu which doesn’t implement CPU pinning by itself. If you want to limit a guest VM’s operations to specific CPU cores on the host you need to use taskset. It was a bit confusing to figure out but fortunately I found this gist by ayufan which handles it beautifully.

Save the following into taskset.sh and edit VMID to the ID of the VM you wish to pin CPUs to. Make sure you have the “expect” package installed.

#!/bin/bash

set -eo pipefail

VMID=200

cpu_tasks() {
	expect <<EOF | sed -n 's/^.* CPU .*thread_id=\(.*\)$/\1/p' | tr -d '\r' || true
spawn qm monitor $VMID
expect ">"
send "info cpus\r"
expect ">"
EOF
}

VCPUS=($(cpu_tasks))
VCPU_COUNT="${#VCPUS[@]}"

if [[ $VCPU_COUNT -eq 0 ]]; then
	echo "* No VCPUS for VM$VMID"
	exit 1
fi

echo "* Detected ${#VCPUS[@]} assigned to VM$VMID..."
echo "* Resetting cpu shield..."

for CPU_INDEX in "${!VCPUS[@]}"
do
	CPU_TASK="${VCPUS[$CPU_INDEX]}"
	echo "* Assigning $CPU_INDEX to $CPU_TASK..."
	taskset -pc "$CPU_INDEX" "$CPU_TASK"
done

Update 9/29/18: Fixed missing done at the end. Also if you want to offset which cores this script uses, you can do so by modifying  the $CPU_INDEX variable to do a bit of math, like so:

        taskset -pc "$[CPU_INDEX+16]"

The above adds 16 to each process ID, so instead of staring on thread 0 it starts on thread 16.

Gaming, Virtualization

Synergy hyperactive mouse in FPS games fix

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I’ve been using Synergy to share my keyboard & mouse between my machine and my Windows gaming VM. A very annoying thing I’ve encountered is that in some games, first person shooters mainly, the mouse goes haywire. Barely touching the mouse causes the shooter to look / spin in circles hundreds of times.

I finally found a fix to this, thanks to this post

The solution is to modify the Synergy server configuration and check the box “Use Relative mouse moves” under Advanced server settings. Then, when in the game, hit scroll lock to lock your mouse to that screen. This fixed the hyperactive mouse issue for me!