Linux

DNS, DDNS, and DHCP on a Linux router – Part 2

In a previous post I described how to set up a simple and efficient router and perimeter firewall on just about any computer.

What I kind of glossed over was DNS and DHCP. The barebones solution I described will work for automatically connecting new devices to the network and allowing them to reach Internet resources as you would expect from any home router. But suppose you present network resources from devices on your own network – a NAS or server of some kind, for example: Wouldn’t it be nice to be able to reach those using their actual names rather than their IP address?
And wouldn’t it be nice if anything you connected to your network got its device name automatically registered in DNS and pointing at its current IP address rather than having to manually edit your zone file and manually set an IP address for anything you might potentially want to reach?

A properly configured combination of DNS and DHCP makes this possible: The relevant configuration to achieve is that the two services trust each other so that the DNS server registers the device name the DHCP server reports back when a device receives an IP address lease.

DNS

DNS – or Domain Name System – is how our computer knows which IP address corresponds to the domain name we just typed in the address bad in our browser. To install such a service, just install BIND, as we saw in the previous article:

sudo apt install bind9

You would expect that this should pretty much be it, but for some reason I had to fight the systemd-resolved subsystem to make my router resolve its own DNS queries: In other words other devices on the network worked fine, but the router itself kept using systemd rather than Bind for its DNS queries. The short of it is that I needed to override the systemd resolver. I edited /etc/systemd/resolved.conf adding the following lines:

[Resolve]
DNS=10.199.200.1
Domains=mydomain.com

I also ensured /etc/resolve.conf pointed at the correct file instead of the default stub:

sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

This effectively forced the router to perform real DNS lookups against the proper DNS service.

DHCP

Next we need to allow devices on our network to receive addresses from our router. First let’s install ISC’s DHCP server – again exactly what I showed in the previous article:

apt install isc-dhcp-server

Now we need a configuration file. Edit /etc/dhcp/dhcpd.conf. It’s partially pre-filled, but make sure it contains sections similar to the following:

option domain-name "mydomain.com";
option domain-name-servers 10.199.200.1;

default-lease-time 600;
max-lease-time 7200;

ddns-update-style none;

authoritative;

subnet 10.199.200.0 netmask 255.255.255.0 {
    range 10.199.200.100 10.199.200.254;
    option subnet-mask 255.255.255.0;
    option routers 10.199.200.1;
}

Restart the DHCP server to make the new rules take:

sudo systemctl restart isc-dhcp-server.service

OK, so to reiterate: we now have a service that performs domain name lookups for us. We also have a service that will lease IP addresses to devices that request one, and that will tell them how to reach other networks (effectively the Internet), and where to find the DNS service. Let’s take it to the next level!

The DNS Zone File

The DNS server effectively needs to carry a database of sorts, containing key parts of its configuration. This database is called a zone file. It may look a bit daunting at first, but persevere through this short introduction, and you’ll have something workable and a basic understanding of it in a short while.

We’re used to having the system configuration in the /etc directory tree, and as expected we find a directory in /etc/bind/ with a bunch of Bind-related stuff. Remember what I said about the zone file being a database, though: We want Bind to be able to update the database in runtime, and so the correct place to put our zone file is actually in /var/lib/bind/. Let’s build a skeleton file to start with:

$ORIGIN .
$TTL 604800	; 1 week
mydomain.com		IN SOA	gateway.mydomain.com. (
				1000       ; serial
				14400      ; refresh (4 hours)
				3600       ; retry (1 hour)
				604800     ; expire (1 week)
				300        ; minimum (5 minutes)
				)
			NS	gateway.mydomain.com.
gateway                 A       10.199.200.1

This zone file is enough to start with, and it doesn’t matter if you don’t understand it all at this point. The key parts here are a Start Of Authority record, which tells clients that this DNS server is authoritative for the mydomain.com domain. The first value in this record is a serial number, which is relevant if we ever need to perform any manual changes to the DNS zone – something I will provide an example for further down in this article. Next we have a Name Server record: In a larger environment you’d expect to see at least two of these for high availability purposes. Finally you have an A – or server – record for this specific router, unimaginatively called gateway – this of course is the hostname of the router. It’s not unlikely that this is the only static record you’ll need, as most other addresses could just as well be dynamically assigned via DHCP, and if need be made semi-static using DHCP reservations.

The Reverse Zone File

DNS can not only help us with converting a human-readable hostname to an IP address that your computer can use. It can also be used for reverse lookups: If you know the IP address of a device, you can learn its hostname.

The reverse zone file is by convention named after your subnet range in reverse, and contains much of the same we see in the regular file:

$ORIGIN .
$TTL 3600	; 1 hour
200.199.10.in-addr.arpa	IN SOA	gateway.mydomain.com. (
				1000       ; serial
				14400      ; refresh (4 hours)
				3600       ; retry (1 hour)
				604800     ; expire (1 week)
				300        ; minimum (5 minutes)
				)
			NS	gateway.mydomain.com.
$ORIGIN 200.199.10.in-addr.arpa.
1			PTR	gateway.mydomain.com.

An astute observer will see that instead of starting a host line with the name of the host, it starts with the host address in the subnet and indicates that to be a pointer to the hostname.

The trust key

As mentioned earlier, an important part of dynamically updated DNS is the trust relationship between the DHCP and the DNS services. In our setup we simply use the rndc key that’s auto generated upon installation of Bind9. In a production environment I would prefer to generate a separate key for DHCP updates, using the rndc-confgen command.

We’ll tell Bind to import this key on startup by editing /etc/bind/named.conf.local and adding the following line to the bottom of the configuration:

include "/etc/bind/rndc.key";

DHCP Zone Updates

While still editing /etc/bind/named.conf.local we’ll configure the service to use the zone file we created earlier, and to accept updates to it if properly authenticated. Add the following zone blocks to the bottom of the file:

zone "mydomain.com" {
  type master;
  notify yes;
  file "/var/lib/bind/db.mydomain.com";
  allow-update { key rndc-key; };
};

zone "200.199.10.in-addr.arpa" IN {
  type master;
  notify yes;
  file "/var/lib/bind/db.200.199.10.in-addr.arpa.rev";
  allow-update { key rndc-key; };
};

Making the DHCP server update DNS

Bind is now configured to understand its part of our network environment, and we’ve told it to allow updates to its zones provided the update request is authenticated using a key. Let’s turn to the DHCP server and add the relevant configuration:

option domain-name "mydomain.com";
option domain-name-servers 10.199.200.1;

default-lease-time 600;
max-lease-time 7200;

ddns-update-style standard;
update-static-leases on;
authoritative;
key "rndc-key" {
	algorithm hmac-sha256;
	secret "<thesecret>";
};
allow unknown-clients;
use-host-decl-names on;

zone mydomain.com. {
    primary 10.199.200.20;
    secondary 10.199.200.1;
    key rndc-key;
}
zone 200.199.10.in-addr.arpa. {
    primary 10.199.200.20;
    secondary 10.199.200.1;
    key rndc-key;
}

subnet 10.199.200.0 netmask 255.255.255.0 {
    range 10.199.200.100 10.199.200.254;
    option subnet-mask 255.255.255.0;
    option routers 10.199.200.1;
    option domain-name "mydomain.com";
    ddns-domainname "mydomain.com.";
    ddns-rev-domainname "in-addr.arpa.";
}

Note an important difference to the previous version of the file: In addition to the rest of the changes, we’ve switched the value for ddns-update-style from none to standard.

We’ve also added the block key "rndc-key" that contains the actual contents of /etc/bind/rndc.key – remember I wrote that in a production environment I would generate a separate key for dhcp update authentication.

Once we restart the Bind9 and ISC-DHCP-Server services, by now we should have working forward and reverse DNS with dynamic DNS updates from the DHCP server.

Addendum 1: Updating DNS manually

If, for some reason, we need to add a host to a DNS zone manually, we’ll want the DNS server to temporarily stop dynamic updates.

sudo rndc freeze mydomain.com

Once we’ve changed the relevant zone file and increased the value for serial to indicate that the zone file has changed, we reload the zone and allow dynamic updates again:

sudo rndc thaw mydomain.com

Addendum 2: Adding static DHCP leases

Sometimes we’re not content with being able to reach a server by its hostname: For example when opening a pinhole through a firewall, we may want a server to have a predictable IP address. In this case we add a host clause to /etc/dhcp/dhcpd.conf like this:

host websrv1 {
    hardware ethernet 52:54:00:de:ad:ef;
    fixed-address 10.199.200.2;
}

The hardware ethernet field is of course the server’s MAC address.

Since we’re changing the daemon’s configuration here, we need to restart it to make the change stick, with sudo systemctl restart isc-dhcp-server.

Reordering systemd services

Use case

As I still only have one public IP address I run my private mail server behind an HAProxy instance. At the same time I use Postfix on my servers to provide me with system information (anything from information on system updates to hardware failures). Naturally the mail service listeners in HAProxy collide with those of the local Postfix installation on the reverse proxy server. Every now and then this caused issues when Postfix managed to start before HAProxy, and stole the network ports from under its feet.

Solution

In systemd based distributions one “right way” to get around this issue is to override the service defaults for the Postfix service, so it doesn’t attempt to start until after HAProxy has started. We don’t want to mess with the package maintainer’s service files as they can change over time by necessity. Instead we should override the service defaults.

sudo systemctl edit postfix.service

The above command does the magic involved in creating an override (creates a file /etc/systemd/system/servicename.service.d/override.conf, and then runs systemctl daemon-reload once you’re done editing so the changes can take hold on the next service start).

Inside the override configuration file we just add a Unit block and add an After clause:

[Unit]
After=haproxy.service

That’s all, really. Save the file and on the next system reboot the services should start in the correct order.

(As I write this we’re approaching the tenth anniversary of World IPv6 Launch Day and most ISPs in Sweden still don’t hand out native IPv6 subnets to their clients but increasingly move them to IPv4 CGNAT despite the obvious issues this creates when attempting to present anything to the Internet, from “serious” web services to game servers!)

Build your own router with nftables – Part 1

Introduction

A few years ago, Jim Salter wrote a number of articles for Ars Technica related to his “homebrew routers“. Much of what he wrote then still stands, but time marches on, and now that I rebuilt my home router, I figured the lessons should be translated to a modern Ubuntu installation and the more approachable nftables syntax.

The hardware

Any old thing with a couple of network interfaces will do fine. In my case I already had a nice machine for the purpose; a solid state 4-NIC mini PC from Qotom.

The goal

What I wanted to achieve was to replicate my current pfSense functionality with tools completely under my control. This includes being able to access the Internet (router), convert human-readable names into IP addresses and vice versa (DNS), and automatically assign IP addresses to devices on my networks (DHCP) – all of these of course are standard functionality you get with any home router. Since I run some web services from home, I also need to allow select incoming traffic to hit the correct server in my house.

Base installation

I chose the latest LTS release of Ubuntu server for my operating system. Other systems are available, but this is an environment in which I’m comfortable. The installation is mostly a matter of pressing Next a lot, with a couple of exceptions:

First of all, there’s a network configuration screen that fulfills an important purpose: Connect your network cable to a port in the computer and take note of which logical network interface reacts in the user interface. In my case the NIC marked 1 (which I intended to use for my Internet connection or WAN) is called enp1s0, and Interface 4 (which I intended to use for my local network or LAN) is called enp2s0. This will become important further down.

Second we want to make sure to enable the Secure Shell service already here in the installer, to allow remote access after the router goes headless.

After installation has finished, it’s good practice to patch the computer by running sudo apt update && sudo apt upgrade and then rebooting it.

Basic network configuration

The first thing to do after logging in, is to configure the network. The WAN port usually gets its address information automatically from your ISP, so for that interface we want to enable DHCP. The LAN port on the other hand will need a static configuration. All this is configured using Netplan in Ubuntu. The installer leaves a default configuration file in /etc/netplan, so let’s just edit that one:

network:
  ethernets:
    enp1s0:
      dhcp4: true
    enp2s0:
      dhcp4: false
      addresses: [10.199.200.1/24]
      nameservers:
        search: [mydomain.com]
        addresses: [10.199.200.1]
    enp3s0:
      dhcp4: false
    enp5s0:
      dhcp4: false
  version: 2

At this point it’s worth noting that if you already have something on the IP address 10.199.200.1 the two devices will fight it out and there’s no telling who will win – that’s why I chose an uncommon address in this howto.

To perform an initial test of the configuration, run sudo netplan try. To confirm the configuration, run sudo netplan apply.

A router will also need to be able to forward network packets from one interface to another. This is enabled by telling the kernel that we allow this functionality. By editing /etc/sysctl.conf we make the change permanent, and by reloading it using sysctl -p we make the changes take effect immediately.

(Bonus knowledge: The effect of the sed commandline below is to inline replace (-i) the effects of substituting (s) the commented-out string (starting with #) with the active one. We could edit the file instead – and if we don’t know exactly what we’re looking for that’s probably a faster way to get it right – but since I had just done it I knew the change I wanted to perform.)

sudo sed -i 's/#net.ipv4.ip_forward=1/net.ipv4.ip_forward=1/' /etc/sysctl.conf
sudo sysctl -p

Great, so our computer can get an IP address from our ISP, it has an IP address on our local network, and it can technically forward packets but we haven’t told it how yet. Now what?

Router

As mentioned, routing functionality in this case will be provided by nftables:

sudo apt install nftables

This is where things get interesting. This is my current /etc/nftables.conf file. This version is thoroughly commented to show how the various instructions fit together

#!/usr/sbin/nft -f

# Clear out any existing rules
flush ruleset

# Our future selves will thank us for noting what cable goes where and labeling the relevant network interfaces if it isn't already done out-of-the-box.
define WANLINK = enp1s0 # NIC1
define LANLINK = enp2s0 # NIC4

# I will be presenting the following services to the Internet. You perhaps won't, in which case the following line should be commented out with a # sign similar to this line.
define PORTFORWARDS = { http, https }

# We never expect to see the following address ranges on the Internet
define BOGONS4 = { 0.0.0.0/8, 10.0.0.0/8, 10.64.0.0/10, 127.0.0.0/8, 127.0.53.53, 169.254.0.0/16, 172.16.0.0/12, 192.0.0.0/24, 192.0.2.0/24, 192.168.0.0/16, 198.18.0.0/15, 198.51.100.0/24, 203.0.113.0/24, 224.0.0.0/4, 240.0.0.0/4, 255.255.255.255/32 }

# The actual firewall starts here
table inet filter {
    # Additional rules for traffic from the Internet
	chain inbound_world {
                # Drop obviously spoofed inbound traffic
                ip saddr { $BOGONS4 } drop
	}
    # Additional rules for traffic from our private network
	chain inbound_private {
                # We want to allow remote access over ssh, incoming DNS traffic, and incoming DHCP traffic
		ip protocol . th dport vmap { tcp . 22 : accept, udp . 53 : accept, tcp . 53 : accept, udp . 67 : accept }
	}
        # Our funnel for inbound traffic from any network
	chain inbound {
                # Default Deny
                type filter hook input priority 0; policy drop;
                # Allow established and related connections: Allows Internet servers to respond to requests from our Internal network
                ct state vmap { established : accept, related : accept, invalid : drop} counter

                # ICMP is - mostly - our friend. Limit incoming pings somewhat but allow necessary information.
		icmp type echo-request counter limit rate 5/second accept
		ip protocol icmp icmp type { destination-unreachable, echo-reply, echo-request, source-quench, time-exceeded } accept
                # Drop obviously spoofed loopback traffic
		iifname "lo" ip daddr != 127.0.0.0/8 drop

                # Separate rules for traffic from Internet and from the internal network
                iifname vmap { lo: accept, $WANLINK : jump inbound_world, $LANLINK : jump inbound_private }
	}
        # Rules for sending traffic from one network interface to another
	chain forward {
                # Default deny, again
		type filter hook forward priority 0; policy drop;
                # Accept established and related traffic
		ct state vmap { established : accept, related : accept, invalid : drop }
                # Let traffic from this router and from the Internal network get out onto the Internet
		iifname { lo, $LANLINK } accept
                # Only allow specific inbound traffic from the Internet (only relevant if we present services to the Internet).
		tcp dport { $PORTFORWARDS } counter
	}
}

# Network address translation: What allows us to glue together a private network with the Internet even though we only have one routable address, as per IPv4 limitations
table ip nat {
        chain  prerouting {
		type nat hook prerouting priority -100;
                # Send specific inbound traffic to our internal web server (only relevant if we present services to the Internet).
		iifname $WANLINK tcp dport { $PORTFORWARDS } dnat to 10.199.200.10
        }
	chain postrouting {
		type nat hook postrouting priority 100; policy accept;
                # Pretend that outbound traffic originates in this router so that Internet servers know where to send responses
		oif $WANLINK masquerade
	}
}

To enable the firewall, we’ll enable the nftables service, and load our configuration file:

sudo systemctl enable nftables.service && sudo systemctl start nftables.service
sudo /etc/nftables.conf

To look at our active ruleset, we can run sudo nft list ruleset.

At this point we have a working router and perimeter firewall for our network. What’s missing is DHCP, so that other devices on the network can get an IP address and access the network, and DNS, so that they can look up human-readable names like duckduckgo.com and convert them to IP addresses like 52.142.124.215. The basic functionality is extremely simple and I’ll detail it in the next few paragraphs, but doing it well is worth its own article, which will follow.

DNS

The simplest way to achieve DNS functionality is simply to install what the Internet runs on:

sudo apt install bind9

DHCP

We’ll run one of the most common DHCP servers here too:

sudo apt install isc-dhcp-server

DHCP not only tells clients their IP address, but it also tells them which gateway to use to access other networks and it informs them of services like DNS. To set up a basic configuration let’s edit /etc/dhcp/dhcpd.conf:

subnet 10.199.200.0 netmask 255.255.255.0 {
    range 10.199.200.100 10.199.200.254;
    option subnet-mask 255.255.255.0;
    option routers 10.199.200.1;
    option domain-name-servers 10.199.200.1;
}

Load the new settings by restarting the DHCP server:

systemctl restart isc-dhcp-server

And that’s it, really. Check back in for the next article which will describe how to make DNS and DHCP cooperate to enhance your local network quality of life.

Reflections on Proxmox VE

I’ve now been using Proxmox VE as a hypervisor in my home lab for a couple of years, and as I’ve reverted to plain Ubuntu Server + KVM, I figured I would try to summarize my thoughts on the product.

Proxmox VE can be described as a low-cost and open-source alternative to VMware vSphere with aspects of vSAN and NSX. The prospect is excellent, and the system scales beautifully all the way from a single (home) lab server with a single traditionally formatted hard drive up to entire clusters with distributed object storage via Ceph; all in a pretty much turnkey solution. If I was involved in setting up an on-prem IT environment for a small- to medium-sized business today, Proxmox VE would definitely be on my shortlist.

So if it’s so good, what made me go back to a regular server distribution?

Proxmox VE, like all complete solutions, works best when you understand the developers’ design paradigm and follow it – at least roughly. It is theoretically based on a Debian core, but the additional layers of abstraction want to take over certain functionality and it’s simply best to let them. Trying to apply a configuration that somehow competes with Proxmox VE will introduce some occasional papercuts to your life: containers that fail to come back up after a restart now and then, ZFS pools that occasionally don’t mount properly, etc. Note that I’m sure I caused these problems on my own by various customizations, so I’m not throwing any shade on the product per se, but the fact remains that I wanted to manage my specific physical hosts in ways that differed from how Proxmox VE would like me to manage them, and that combination made the environment less than optimal.

As these servers are only used and managed by me and I do perfectly fine in a command line interface or using scripts and playbooks, I’ve come to the conclusion that I prefer a minimalist approach and so I’m back to running simple Ubuntu servers with ZFS storage pools for virtual machines and backups, and plain KVM for my hypervisor layer. After the initial setup – a weekend project I will write up for another post – I have the best kind of server environment at home: One I more or less never have to touch unless I want to.

Email address tags in Postfix and Dovecot

What if you could tag the mail address you provide when registering for various services to simplify the management of the inevitable stream of unsolicited mail that follows? If you could register myname+theservicename@mydomain.tld it would make it very easy to recognize mail from that service – and it would make it easy to pinpoint common leaks, whether they’d got their customer database cracked or just sold it to the highest bidder.

The most famous provider of such a service might be Google’s Gmail. But if you run a Postfix server, this functionality is included and may actually already be turned on out-of-the-box. In your main.cf it looks like this:

recipient_delimiter = +

The delimiter can basically be any character that’s valid in the local part of an email address, but obviously you want to avoid using characters that actually are in use in your environment (dots (.) and dashes (-) come to mind).

By default, though, such mail won’t actually get delivered if you use Dovecot with a relatively default configuration for storing mail. The reason is that the + character needs to be explicitly allowed. To fix this, find the auth_username_chars setting and add the + character to it (remembering to uncomment the line):

auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@+

That’s it: A single step to enable some additional useful functionality on your mail server.

IPv6 guests in KVM

I’ve been experimenting with IPv6 at home, and spent some time trying to get it working in my virtual machines.

The first symptom I got was that VMs got a “Network unreachable” error when trying to ping6 anything but their own address. The cause was a complete brainfart on my side: We need a loopback interface network definition for IPv6 in /etc/network/interfaces:

auto lo
iface lo inet loopback
iface lo inet6 loopback

The second problem took a bit more digging to understand: I would get an IPv6 address, and I could ping stuff both on my own network and on the Internet from the VM, but no other computers could reach the virtual machine over IPv6.

According to this discussion, QEMU/KVM has support for multicast (required for proper IPv6 functioning), but it’s turned off by default. Remedy this by running virsh edit [vm-name] and adding trustGuestRxFilters='yes' to the appropriate network interface definition:

    
      
      
      
      

As usual, when you understand the problem the solution is simple.

Frustrations in Ubuntu 18.04

My first frustration with Ubuntu 18.04 came yesterday. I created a template VM with my basic toolkit that any machine in my network should have. I then deployed the VM and asked vSphere to set the hostname to the value of the VM name. Strangely, this didn’t happen: The new machine booted up alright, but its name remained that of the template.

Remember the old way to manually change the name of a machine in Linux? It went something like this:

  1. Add the new hostname to your /etc/hosts so sudo doesn’t get confused.
  2. Replace the old hostname in /etc/hostname with the new one.
  3. Reboot the computer or restart all affected services.

The new way goes like this:

  1. Add the new hostname to your /etc/hosts so sudo doesn’t get confused.
  2. Replace the old hostname in /etc/hostname with the new one.
  3. Reboot the computer.
  4. Notice that the hostname is the same as it was before you attempted to change it.
  5. Web search “change hostname ubuntu 18.04”.
  6. Discover that there’s a new utility, hostnamectl, which has a command, change-hostname, that takes the new hostname as an argument.
  7. Run hostnamectl change-hostname [newname]
  8. Run hostnamectl without any arguments to confirm that “Static hostname” has the correct value.
  9. Log off and back on again and be happy that everything seems to be working.
  10. Reboot the computer after doing some changes.
  11. Notice that the hostname is back to what it was.
  12. Run hostnamectl change-hostname [newname] again, and check in /etc/hostname just to see that it actually did change the file to contain the new hostname.
  13. Check in /etc/hosts and see that the new name appears there too.
  14. Scour the web some more for additional information.
  15. Find some mention of cloud-init.
  16. Read up on it and see the point of it – but also that it doesn’t apply to my current environment.
  17. Run sudo apt remove cloud-init
  18. Reboot the server and see that it works as expected again.
  19. (In the future: Learn more about cloud-init and re-evaluate whether it should be implemented in my environment as a complement to Ansible).

Transport security with Postfix

I had a “Face: Meet Palm” moment today, and as usual when that happens, I learned something new:

What happened was that I noticed that mail from a Postfix server I use for sending mail from a couple of domains was marked with the red “no encryption” label rather than the expected grey “standard encryption” icon when I looked at the message details in Gmail. I was sure that I had set the server to use what they call “opportunistic TLS”; that is: Attempt to use TLS but fall back to no encryption if that’s unavailable.

Reading the Postfix documentation, however, I saw the problem: there are two sets of TLS rules in the main.cf configuration file: those starting with “smtpd_“, which deal with how the server responds to its clients, and those who start with “smtp_“, which deal with how Postfix acts when working in client mode towards other servers.

So now I have the following two lines in my /etc/postfix/main.cf:

smtp_tls_security_level = may
smtpd_tls_security_level = may

Resizing the system volume on a Linux VM

Background

With LVM, the preferred way of adding storage space to a computer running a Linux-based operating system seems to be to add disks, judging by my search results. Naturally, this is a great way of minimizing disruption in a physical machine, but what if you’re running your machines virtually? Adding virtual disks tends to get messy after a while, and hypervisors allow you to simply grow the vdisk, so why not do that?

Problem is, the old way I used to do it (using partprobe after growing the partition) required a system reboot to see the entire available new space if I attempted it on the system volume. Documented below is a better way.

The process

Start by confirming the current disk size so we know our baseline.

# fdisk -l

Disk /dev/sda: 26.8 GB, 26843545600 bytes, 52428800 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x000ba3e8

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048     2099199     1048576   83  Linux
/dev/sda2         2099200    52428799    25164800   8e  Linux LVM

Disk /dev/mapper/ol-root: 18.2 GB, 18249416704 bytes, 35643392 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

OK, so we have slightly less than 27 GB of disk space. Let’s grow the disk image in the hypervisor, and then re-scan the device.

# ls /sys/class/scsi_device/
1:0:0:0 2:0:0:0
# echo 1 > /sys/class/scsi_device/1\:0\:0\:0/device/rescan
# fdisk -l

Disk /dev/sda: 80.5 GB, 80530636800 bytes, 157286400 sectors
(...)

Now we have the disk space available, let’s perform the steps to grow our file system.

# fdisk /dev/sda


Command (m for help): p

Disk /dev/sda: 80.5 GB, 80530636800 bytes, 157286400 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x000ba3e8

Device Boot Start End Blocks Id System
/dev/sda1 * 2048 2099199 1048576 83 Linux
/dev/sda2 2099200 52428799 25164800 8e Linux LVM

Command (m for help): d
Partition number (1,2, default 2): 
Partition 2 is deleted

Command (m for help): n
Partition type:
 p primary (1 primary, 0 extended, 3 free)
 e extended
Select (default p): 
Using default response p
Partition number (2-4, default 2): 
First sector (2099200-157286399, default 2099200): 
Using default value 2099200
Last sector, +sectors or +size{K,M,G} (2099200-157286399, default 157286399): 
Using default value 157286399
Partition 2 of type Linux and of size 74 GiB is set

Command (m for help): t
Partition number (1,2, default 2): 
Hex code (type L to list all codes): 8e
Changed type of partition 'Linux' to 'Linux LVM'

Command (m for help): w
The partition table has been altered!

The above statement is followed by what used to be a problem:

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.

Partprobe won’t help us here, and kpartx for some reason doesn’t consistently catch the entire new disk size. The correct way, then, is the following:

# partx -u /dev/sda2

The result?

# partx -s /dev/sda
NR START END SECTORS SIZE NAME UUID
 1 2048 2099199 2097152 1G 
 2 2099200 157286399 155187200 74G

Now let’s finish extending everything up to the actual file system:

# pvresize /dev/sda2
 Physical volume "/dev/sda2" changed
 1 physical volume(s) resized / 0 physical volume(s) not resized
# lvextend -l 100%VG /dev/mapper/ol-root
 Size of logical volume ol/root changed from <17.00 GiB (4351 extents) to <72.00 GiB (18431 extents).
 Logical volume ol/root successfully resized.
# xfs_growfs /dev/mapper/ol-root

And finally let’s check that everything worked out as we expected:

# df -h
Filesystem Size Used Avail Use% Mounted on
(...)
/dev/mapper/ol-root 72G 17G 56G 24% /
(...)

Conclusion

The Windows family of operating systems has had the ability to grow any volume on the fly since Server 2008. I couldn’t imagine that Linux would lack this ability, but I didn’t know how to do it the right way. Now I do.

Test whether a git pull is needed from within a batch script

Just a quick hack I did to avoid having to sync a couple of scripts unnecessarily when deploying my load balancers. Underlying idea stolen from a post by Neil Mayhew on Stackoverflow.

Shell session script:

#!/bin/bash
UPSTREAM=${1:-'@{u}'}
LOCAL=$(git rev-parse @)
REMOTE=$(git rev-parse "$UPSTREAM")
BASE=$(git merge-base @ "$UPSTREAM")
if [ $LOCAL = $REMOTE ]; then
    GIT_STATUS=nochange
elif [ $LOCAL = $BASE ]; then
    GIT_STATUS=changed
    git pull
fi

Ansible playbook:

---
vars:
-   version_status: "{{ lookup ('env', 'GIT_STATUS') }}"
-   tasks:
    -   name: Update HAProxy scripts
        copy:
            src: "{{ config_root }}/etc/haproxy/scripts"
            dest: "/etc/haproxy"
        when: version_status=="changed"