Fix for SSH Keys Not Working on Newer Versions of Debian / Ubuntu

If your old SSH keys are not working on newer versions of Ubuntu / Debian, and you're being prompted to login (~/.ssh/config configuration being ignored), the fix is to add the following line to the bottom of the /etc/ssh/ssh_config file:

    PubkeyAcceptedKeyTypes +ssh-rsa

That's it.  It will work again.  You may need to restart the ssh service

sudo service ssh restart

https://askubuntu.com/questions/1404049/ssh-without-password-does-not-work-after-upgrading-from-18-04-to-22-04

OpenVPN Expired CRL – VPN Won't Connect

Recently, I ran into an issue where OpenVPN was no longer working for existing clients.  After looking at the OpenVPN log in /var/log/openvpn.log, I found the following:

VERIFY ERROR: depth=0, error=CRL has expired:

If you see an OpenVPN error about an expired certificate revocation list (CRL), here's how to generate a new CRL:

cd /etc/openvpn/easy-rsa
EASYRSA_CRL_DAYS=3650 ./easyrsa gen-crl
cp /etc/openvpn/easy-rsa/pki/crl.pem /etc/openvpn/crl.pem
chown nobody:nogroup /etc/openvpn/crl.pem
service openvpn restart

Problem solved!

cURL and wget Issues on Ubuntu 16.04

When using wget or curl to make HTTP requests from a no longer supported installation of Ubuntu 16.04 Xenial, if you get any of the following errors:

curl gnutls_handshake() failed: Error in protocol version
curl: (35) error:1407742E:SSL routines:SSL23_GET_SERVER_HELLO:tlsv1 alert protocol version  /home/mohan/mesg
[SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:727) 

The solution is to add SavOS Rob Savoury PPAs to get updated curl and wget packages:

sudo add-apt-repository ppa:savoury1/build-tools
sudo add-apt-repository ppa:savoury1/backports
sudo add-apt-repository ppa:savoury1/python
sudo add-apt-repository ppa:savoury1/encryption
sudo add-apt-repository ppa:savoury1/curl34
sudo apt-get update && sudo apt-get upgrade
sudo apt-get install wget curl python2.7

Ubuntu: Allow Automatic Updates for Specific Packages Only

If you want to allow Google products and packages to update automatically, follow this guide.

You can also add additional sources that should update automatically following the same process.

This is helpful when using Selenium, WebDriver for Chrome, and Python.  Doing this allows you to always use the most up-to-date version of all of these dependent packages.

Tested in Ubuntu 20.04

Setup Remote Logging on an Ubuntu rsyslog Server for DD-WRT to Use

Enable remote logging on an Ubuntu server by configuring rsyslog to allow remote connections from port 514 (adjust as needed):

sudo nano /etc/rsyslog.conf

Uncomment the imudp and imtcp load module statements like so (adjusting as needed):

# provides UDP syslog reception
module(load="imudp")
input(type="imudp" port="514")

# provides TCP syslog reception
module(load="imtcp")
input(type="imtcp" port="514")

Create a logging template and apply it only to remote hosts that start with "c-" (comcast connection remote host prefix [followed by the IP address of the device which can change])

# Comcast remote logging
$template remote-incoming-logs, "/var/log/remote_logs/%HOSTNAME%/%PROGRAMNAME%.$
if $fromhost startswith "c-" then -?remote-incoming-logs

Save and quit.

Restart the rsyslog daemon:

sudo service rsyslog restart

Remote logs will be stored in /var/log/remote_logs

Configure logrotate to process and rotate these logs automatically (so you don't lose them and have a history on them):

sudo nano /etc/logrotate.d/ddwrt

Paste these contents into the file:

/var/log/remote_logs/*.log /var/log/remote_logs/*/*.log {
    daily
    missingok
    compress
    delaycompress
    su syslog adm
}

Save and quit.

Everything has been configured, and remote logging should work from your DD-WRT router once you set the remote URL to your server's IPAddress:port combo and apply the changed settings.

Installing and Configuring CentOS to Host KVM Virtual Machines

GUI

When configuring a fresh install of CentOS for a KVM host machine (the main server that hosts all of the virtual machines), I like to run a GUI to make managing some of the virtual machines easier.  Thus, during install, choose the options for CentOS with Minimal GUI:

RAID 10 LVM Partitions

When configuring the hard drive partitions, set it up to use RAID 10 LVM SOFTWARE RAID:

Create volume group called "vms" without the quotes that is setup as RAID 10 (set volume group space to be as large as possible).

Set the "/" partition to 100GB XFS LVM (RAID10).

Set the "swap" partition to 32GB.

Only setup those two partitions.  The remaining space in the RAID 10 volume group "vms" will be used for KVM containers (and the remaining space does NOT need to be assigned to any mount points).

That's all.

Increasing KVM Guest Hard Disk (Hard Drive) Space

Increasing the hard drive space in a KVM guest can be rather tricky.  The first step is to shutdown (completely turn off) the guest machine by running the below command from the guest system:

sudo shutdown -h now

Once the guest machine has been turned off (verify it is off by using sudo virt-manager on the host machine to see if it's no longer running), on the host machine, resize the LVM partition by running the following command (and adjust the size as necessary):

sudo lvextend -L+78G /dev/vg_vps/utils

If you need help identifying the name of the disk your guest has been assigned, run this command from the host:

sudo virsh domblklist {VIRSH_NAME_OF_VIRTUAL_MACHINE}

For my example, I would use this command:

sudo virsh domblklist utils

From the host machine, download the GParted live ISO image for your system's architecture (x86 or x64).  Start virt-manager:

sudo virt-manager

Assign a CD drive to the virtual machine you're expanding the hard drive space for, and assign / mount the GParted ISO to it.  Change the boot order so that the KVM guest boots from the CD first.  Save your settings and start the KVM guest virtual machine.  Boot into GParted Live.  GParted will run automatically.  Use GParted to expand the partitions so that they make use of the added storage based on your own preferences.  Apply the resize operation.  Exit GParted and shutdown the virtual machine so that it's off again. Remove the CD drive from the boot options from virt-manager, and then start the KVM guest again. 

If Guest Doesn't Use LVM Partitioning

If your KVM guest virtual machine hasn't been configured to use LVM, the added hard drive space should already be available to your system.  Verify it has been expanded by again running the df -h command.  You're done!

If Guest Uses LVM

Let the OS boot.  From the guest, the file system needs to be resized itself.  You can do this by running the following command to see the current space allocated to your system's partitions:

df -h

You'll see a bunch of output similar to:

Filesystem                  Size  Used Avail Use% Mounted on
udev                        2.9G     0  2.9G   0% /dev
tmpfs                       597M  8.3M  589M   2% /run
/dev/mapper/utils--vg-root  127G   24G   98G  20% /
tmpfs                       3.0G     0  3.0G   0% /dev/shm
tmpfs                       5.0M     0  5.0M   0% /run/lock
tmpfs                       3.0G     0  3.0G   0% /sys/fs/cgroup
/dev/vda1                   720M   60M  624M   9% /boot
tmpfs                       597M     0  597M   0% /run/user/1000

You'll notice that the added hard drive space doesn't show up on any of the partitions.  However, it is available to be assigned to these partitions.  To assign additional space, you will need to resize it using these commands (run from the guest virtual machine… the machine you're resizing):

lvextend /dev/mapper/utils--vg-root -L +78G
resize2fs /dev/mapper/utils--vg-root

Obviously, you need to substitute the name of the LVM partition with the one from your system shown in your output of the df -h command.

Resources

https://tldp.org/HOWTO/LVM-HOWTO/extendlv.html – Mirror if Offline

https://sandilands.info/sgordon/increasing-kvm-virtual-machine-disk-using-lvm-ext4 – Mirror if Offline

Adding SAS RAID Drivers to CentOS 8 and Red Hat Linux During Installation

CentOS 8 and Red Hat Linux 8 removed a lot of built in RAID controller and SAS drivers.  As such, you'll need to identify your SAS RAID controller card model number, and then during the installation of CentOS 8 or Red Hat, you will need to follow these instructions (modifying them for your hardware).

https://gainanov.pro/eng-blog/linux/rhel8-install-to-dell-raid/

If for some reason the link above is no longer available, I saved and archived a copy which can be read here.

Add El Repo Permanently

As updates are released to CentOS 8 / Rocky Linux / Red Hat 8, the kernel will often be upgraded.  To make sure the SAS drives are updated as well, you'll need to configure your system to pull updates from El Repo automatically by using the following commands:

sudo rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
sudo yum install https://www.elrepo.org/elrepo-release-8.el8.elrepo.noarch.rpm
sudo yum update -y

In case the above instructions no longer work, this guide should help.

Disable NetworkManager Wait Online Service

Prevent the boot from being halted on startup by network connection checks by running the below command:

sudo systemctl mask NetworkManager-wait-online.service

Do Not Use tcp_tw_recycle

I had a very strange connectivity issue recently that I was only able to reproduce intermittently on my own LAN network when connecting to a few of my servers hosting websites that process and receive tons of simultaneous connections at any point in time.  Basically, my connection to a specific set of websites that I host would timeout from my home internet connection.  However, I was never able to reproduce this issue when connecting to the same sites from other networks belonging to my family and friends. 

From my home connection, I used TCPView and saw that SYN_SENT packets were supposedly sent to my servers to establish a connection.  Unfortunately, the server never replied to some of these requests.  As such, my connection would timeout at times, and work perfectly fine sometimes.  I looked at DD-WRT's connection table, and it also claimed that the packets had been sent, but that they were in an UNREPLIED state when I experienced issues.  Thus, packets were supposedly being sent, but the server was not responding at times.  After spending nearly a week trying to tackle this issue and buying new cable internet equipment (an officially supported Comcast modem), I tracked down the issue, and it ended up being a TCP configuration setting on my servers rather than my home LAN equipment.

Modem or Router's Fault?

Originally, I thought my issue was caused by the DD-WRT open source firmware I was running on my wireless router.  If I restored the router's settings to DD-WRT's factory defaults, I could always connect to the websites I was having intermittent connection timeout issues on.  I suspected it might be my router after trying an older router which didn't have any problems either.  I even upgraded the DD-WRT firmware to the latest version and rebuilt my complicated network configuration from scratch.  Unfortunately, the issue was still there.  Thus, despite mixed results with different routers, I started to wonder if the issue was on my server's end.

Finally Fixed

I started looking at sysctl TCP settings I could adjust on my router, and I ended up comparing some of these values to the ones used on my servers (that were hosting the problem websites).  Eventually, I came across configuration values I had changed myself several months ago which were supposed to help the server support multiple simultaneous connections.

After reading this StackOverflow thread (https://stackoverflow.com/questions/6426253/tcp-tw-reuse-vs-tcp-tw-recycle-which-to-use-or-both), I decided I would try disabling the tcp_tw_recycle setting.

/proc/sys/net/ipv4/tcp_tw_recycle was set to 1 (enabled) from tweaks I had run that I had found on the internet.  After I disabled it, /proc/sys/net/ipv4/tcp_tw_recycle was set to 0 (disabled).  By default, Linux keeps tcp_tw_recycle disabled.  Again, this is something I had changed for tuning reasons.  After disabling this setting and rebooting the server, I no longer have any issues connecting to the severs in question.  No more connection timeouts, and everything works properly again.

I have no idea why I wasn't able to reproduce this issue on other networks.  I thought it was my network equipment (modem and router), but it ended up being the server.

Lessons Learned

Be careful when applying settings you find online.  Sometimes, they may not work, or their usage may be buggy.  In fact, net.ipv4.tcp_tw_recycle has been removed from Linux in kernel versions newer than 4.12 by default.  I'm guessing this is because it doesn't work, as I experienced.  Do NOT use  net.ipv4.tcp_tw_recycle! I kept tcp_tw_reuse enabled, so you can enable this setting without running into problems.  Just don't for the love of anything use tcp_tw_recycle!  It doesn't work, and it will cause you headaches trying to track down strange intermittent issues!

Linux KVM:  Using Multiple NICs and Routing Traffic Properly Between Them

When setting up a KVM guest to use multiple network interface controllers (NICs), additional ip routes may be needed in order for the additional interfaces to work properly.  For example, if you configure a NIC with a public static IP address and a NIC with an internal private DHCP LAN IP address, you must create a route for any traffic that comes through the DHCP LAN IP address to respond via the interface from which the request originated.  Otherwise, forwarded NAT traffic from the main KVM host to the DHCP internal LAN IP will reach its destination, but no response will be sent back (because it will attempt to send the response via the configured static IP address interface which may NOT be the original destination of the senders request).

The Solution:

https://unix.stackexchange.com/questions/4420/reply-on-same-interface-as-incoming/23345#answer-23345

From the above link, the solution for me was to do the following in the KVM guest virtual machine:

Only needs to be done once:

sudo -i
echo 200 isp1 >> /etc/iproute2/rt_tables

Setting up the route (adjust variables as necessary):

sudo -i
ip rule add from <interface_IP> table isp1 priority 900
ip rule add from <interface_IP> dev <interface> table isp1
ip route add default via <gateway_IP> dev <interface> table isp

The command I used for my specific setup:

sudo -i
ip rule add from 192.168.122.10 table isp1 priority 900 
ip rule add from 192.168.122.10 dev ens9 table isp1 
ip route add default via 192.168.122.1 dev ens9 table isp1

Making it permanent (apply on system start up):

sudo -i
nano /etc/network/interfaces

I added the below post-up rules (adjust variables as necessary):

auto ens9
iface ens9 inet dhcp
        post-up ip rule add from <interface_IP> table isp1 priority 900
        post-up ip rule add from <interface_IP> dev <interface> table isp1
        post-up ip route add default via <gateway_IP> dev <interface> table isp1

The route is created whenever the dhcp interface is brought up.