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Host Your Own Home VOIP System using Asterisk


Some cable ISPs like comcast offer home VOIP systems for a hefty monthly fee. In addition to the extra cost, they often require you to rent a gateway from them. By renting a gateway from them, you not only incur an additional monthly fee for this, but more importantly you are giving them a backdoor into your cable modem and allowing them to use it as a wifi hotspot for strangers, often without your knowledge. To regain full control over your home network and VOIP system, you need to buy your own modem and host the VOIP system yourself using free (as in freedom) software.

A VOIP system could just be used for communicating between rooms in your house much like the phones in hotel rooms can call each other. But most likely you will want to be able to receive calls from outside of your house from the PSTN (Public Switched Telephone Network). To do this you will need to purchase a DID (Direct Inward Dialing) number from a SIP provider, which typically involves a small recurring charge.


On Debian 9 GNU/Linux, the asterisk package needs to be installed.

$ sudo apt-get install -y asterisk

The following information about your SIP provider must be known:

  • SIP_USER – SIP username
  • SIP_SECRET – SIP password
  • SIP_HOST – SIP server hostname (DID Point of Presence)
  • SIP_IP – SIP server IP
  • SIP_PORT – SIP port
  • DID_PHONE_NUMBER – phone number from VoIP provider (10 digits with no delimiters)

The following information about your IP Phone must be known:

  • EXT_NUMBER – a 4 digit extension number
  • EXT_PASSWORD – the extension password
  • VM_NUMBER – a 4 digit voicemail number

The following information must be known about your network:

  • EXTERNHOST – hostname that resolves to your public WAN IP

Decide on the following information:

  • VOICEMAIL_PIN – numeric password for voicemail
  • EMAIL_ADDRESS – email address of voicemail account
Configure sip.conf

There are a ton of possible settings that can go in this file. The file is divided into several sections whose names are enclosed in square brackets.

  • context specifies the name of the context to which each extension is associated and is defined later in extensions.conf
  • register specifies the SIP registration parameters including the username, secret, host-name, and port
  • allowguest=no disables unauthenticated calls
  • allowoverlap=no disables overlap dialing support
  • srvlookup=no disables looking up SRV DNS records on outbound calls, which is unnecessary because we explicitly specified both the SIP IP address and port number in the register line
  • disallow and allow restricts the audio codecs that a device will accept or offer
  • alwaysauthreject=yes enables a security feature where the reject for a wrong password and wrong username appear the same way, which prevents a cracker from gaining knowledge about the system they are attempting to attack
  • canreinvite=no disables SIP re-invites, forcing asterisk to stay in the middle of the media
  • nat=force_rport,comedia modifies the behavior related to a server behind NAT (Network Address Translation); force_rport disables symmetric RTP support and comedia enables symmetric RTP support if the remote side requests it
  • qualify=yes enables sending a SIP OPTIONS command every 2 seconds to check that the device is still online
  • session-timers=refuse disables session timers
  • externhost=EXTERNHOST  specifies the FQDN to be resolved to acquire the public IP address of the asterisk server
  • externrefresh=15 specifies that externhost should be resolved every 15 seconds
  • localnet= excludes addresses in the subnet from any NATing efforts by asterisk, allowing the source addresses of SIP requests/response to include the internal address
  • host=HOST specifies the hostname of the sip peer
  • host=dynamic means that the sip peer must register
  • secret=SECRET allows a SIP peer to securely prove its identity
  • type=friend specifies that an entity is both a sip peer (receives calls) and user (places calls)
  • defaultuser=SIP_USER specifies the username for authentication in SIP INVITE requests from remote clients
  • fromuser=SIP_USER overrides the username when calling to this peer from asterisk
  • trustrpid=yes trust Remote-Party-ID SIP header
  • sendrpid=yes send Remote-Party-ID SIP header
  • insecure=invite disables authentication requirement for incoming SIP INVITE requests
  • deny and permit restrict the addresses of hosts that can attempt registration


Create a Dialplan

The dialplan is stored in extensions.conf.

exten => s,1,Answer()
exten => s,2,Dial(SIP/EXT_NUMBER,10)
exten => s,3,Playback(vm-nobodyavail)
exten => s,4,VoiceMail(EXT_NUMBER@main)
exten => s,5,Hangup()
exten => EXT_NUMBER,1,Answer()
exten => EXT_NUMBER,2,Dial(SIP/EXT_NUMBER,10)
exten => EXT_NUMBER,3,Playback(vm-nobodyavail)
exten => EXT_NUMBER,4,VoiceMail(EXT_NUMBER@main)
exten => EXT_NUMBER,5,Hangup()

exten => _*97,1,VoiceMailMain(${CALLERID(num)}@main)
exten => _*97,2,Hangup()

include => voipms-inbound
include => voipms-outbound

exten => _1NXXNXXXXXX,1,Dial(SIP/${EXTEN}@voipms)
exten => _1NXXNXXXXXX,n,Hangup()
exten => _NXXNXXXXXX,1,Dial(SIP/1${EXTEN}@voipms)
exten => _NXXNXXXXXX,n,Hangup()
exten => _011.,1,Dial(SIP/${EXTEN}@voipms)
exten => _011.,n,Hangup()
exten => _00.,1,Dial(SIP/${EXTEN}@voipms)
exten => _00.,n,Hangup()

exten => DID_PHONE_NUMBER,1,Answer()
Configure voicemail.conf
Reload Asterisk
$ sudo asterisk -rx reload

Monitor Your GNU/Linux Server


One of the challenging of hosting services on your own GNU/Linux server is ensuring that the services remain running properly. There are free software monitoring packages that can be used to automatically detect various problems and notify you via email. There are some limitations to running the monitoring software on the same host that you want to monitor, because if the host becomes unreachable, you won’t be notified. However, it can still be useful for verifying that ports are open, processes are running, and performance is normal. If your server uses RAID, it is also critical to get notifications when a drive fails to avoid outages and most importantly prevent data loss. To accomplish these goals, my preference is to use icinga2.


On Debian 9 GNU/Linux, install the icinga2 package.

sudo apt-get install -y icinga2
Setup IcingaWeb2

Install the icingaweb2 package.

sudo apt-get install -y icingaweb2

Enable the icingaweb2 apache configuration and reload apache.

sudo a2enconf icingaweb2
sudo systemctl reload apache2

Finally open a browser and navigate to /icingaweb2 and follow the installation wizard.

Modify http check to check for https

Add the following bolded line to /etc/icinga2/conf.d/hosts.conf:

object Host NodeName {
 vars.http_vhosts["http"] = {
 http_uri = "/"
 http_ssl = "true"


Host Your Own WordPress Blog


On a Debian 9 GNU/Linux, install the wordpress package and adjust file ownership and permissions for www-data user.

sudo apt-get install -y wordpress
sudo chown -R www-data:www-data /usr/share/wordpress/
sudo chmod -R g+w /usr/share/wordpress/
Configure Apache

Copy the following into /etc/apache2/conf-available/wordpress.conf.

Alias /blog /usr/share/wordpress
<Directory /usr/share/wordpress>
 Options FollowSymLinks
 AllowOverride Limit Options FileInfo
 DirectoryIndex index.php
 Require all granted

Enable the configuration and reload apache.

sudo a2enconf wordpress
sudo systemctl reload apache2


Create a Free Software Baby Monitor with Zoneminder


When trying to figure out what baby monitor to buy, I found that there are three types of baby monitors: “cloud”-based monitors, IP cameras, and short-range RF monitors without network connectivity. The first two are capable of being accessed remotely, but the last one can only be accessed from within a few hundred feet. The “cloud”-based monitors are convenient but are a huge invasion of privacy because they give a third party access to your baby monitor. If you want to be able to view your baby monitor remotely without sacrificing your privacy, I have found the best choice is using an IP camera and using a free software package called zoneminder to access it.


On Debian 9 GNU/Linux, install the zoneminder package.

sudo apt-get install -y zoneminder
Enable Web Interface

To enable the web interface, simply enable the zoneminder configuration in apache2 and tell it to reload.

sudo a2enconf zoneminder
sudo systemctl reload apache2
Hardening Zoneminder

Click the Options link in the upper right hand corner, navigate to the Users tab, and click the “Add New User” button. Give the user full permissions and delete the default user. Then navigate to the System tab and enable OPT_USE_AUTH to enable authentication.

Adding a Monitor

Note that the specific settings you want to choose may vary depending on the model of your IP camera. To add a monitor, click the “Add New Monitor” button, choose a unique name for the monitor, and select the Ffmpeg source type. Navigate to the source path and set the source path to “rtsp://user:password@hostname”. Navigate back to the general tab, enable the monitor, and click save.

Host Your Own Music Streaming Service with Subsonic


Many people use music streaming services for the convenience and portability they can offer. However, nearly all of the ones I could find require the use of non-free software, use Digital Restrictions Management (DRM), and prevent you from sharing music with friends and family. All of these problems can be avoided by hosting your own music streaming service on your own server.


On Debian 9 GNU/Linux, install subsonic from the repository.

sudo apt-get install -y subsonic
Configure Subsonic

The configuration file for subsonic is located at /etc/default/subsonic. Modify the SUBSONIC_USER setting to be subsonic. This improves security because if the daemon is compromised, the intruder doesn’t get root access. Modify the SUBSONIC_ARGS to specify the context of /subsonic, the port as 8080 and the max-memory as 150.

SUBSONIC_ARGS="--context-path=/subsonic --port=8080 --max-memory=150"
Issues with the Default Configuration

By default subsonic runs on a non-standard port and does not use TLS encryption. Since you are entering credentials to subsonic, it a good idea to use TLS encryption to prevent eavesdroppers from discovering your password. While you can configure subsonic to run TLS encryption with self-signed certificate, I found it difficult to get it to use a trusted certificate. Running on a non-standard port may also require you to add an additional port-forwarding rule on your router for you to access subsonic remotely. If you already have apache setup with port-forwarding and a trusted TLS certificate, then this these problems can be solved by simply having apache act as a proxy to the subsonic service.

Configure Apache as a Web Proxy

First ensure that apache is installed and the proxy and proxy_http modules are enabled.

sudo apt-get install -y apache2
sudo a2enmod proxy proxy_http

Then create the text file /etc/apache2/conf-available/subsonic.conf and copy the following content into the file:

SSLProxyEngine On
ProxyPreserveHost Off
ProxyRequests Off
<Location /subsonic>
 ProxyPass http://localhost:8080/subsonic
 ProxyPassReverse http://localhost:8080/subsonic

Finally, enable the proxy and reload the apache2 service.

sudo a2enconf subsonic
sudo systemctl reload apache2

Host Your Own Samba File Server


A common problem with running a mixed-environment is sharing files between GNU/Linux and Windows computers. The best way to provide Windows computers access to your GNU/Linux file server is using Samba.


Install samba server and client packages.

$ sudo apt-get install -y samba smbclient
Configure Samba Server

I configure two shares: one read-only (called data) and one read-write (called data-rw). Append the following to the end of /etc/samba/smb.conf:

 comment = Data Share (read-only)
 path = /mnt/md0/
 available = yes
 valid users = myuser
 read only = yes
 browseable = yes
 public = yes
 writeable = no
 comment = Data Share
 path = /mnt/md0/
 available = yes
 valid users = myuser
 read only = no
 browseable = yes
 public = yes
 writeable = yes
Create Samba User

Next, create a new samba user using the following command and it will prompt you to specify a password for the user and then again to confirm the password.

sudo smbpasswd -a myuser
New SMB password:
Retype new SMB password:
Verify the Configuration with testparm

The testparm utility checks the /etc/samba/smb.conf configuration file for internal correctness.

$ testparm
Load smb config files from /etc/samba/smb.conf
rlimit_max: increasing rlimit_max (1024) to minimum Windows limit (16384)
WARNING: The "syslog" option is deprecated
Processing section "[homes]"
Processing section "[printers]"
Processing section "[print$]"
Processing section "[data]"
Processing section "[data-rw]"
Loaded services file OK.

Press enter to see a dump of your service definitions
Test the Samba Server with smbclient and smbget

smbclient can be used to verify that all the shares are available on the server.

$ smbclient -L euclid
Domain=[WORKGROUP] OS=[Windows 6.1] Server=[Samba 4.5.8-Debian]

 Sharename   Type    Comment
 ---------   ----    -------
 print$      Disk    Printer Drivers
 data        Disk    Data Share (read-only)
 data-rw     Disk    Data Share
 IPC$        IPC     IPC Service (Samba 4.5.8-Debian)
 myuser      Disk    Home Directories
Domain=[WORKGROUP] OS=[Windows 6.1] Server=[Samba 4.5.8-Debian]

 Server         Comment
 ---------      -------
 EUCLID         Samba 4.5.8-Debian

 Workgroup      Master
 ---------      -------

smbget is a wget-like utility for downloading files from a samba share

$ smbget smb://euclid/data/test.txt
Using workgroup WORKGROUP, user myuser
Downloaded 830b in 1 seconds

Customize Your Debian GNU/Linux Server


There are many GNU/Linux distributions available but my favorite is Debian. The latest version at the time of this post is Debian 9 “Strecth”. Debian does not come optimally prepared for my specific needs right out of the box. Other distributions come with some of these things pre-installed, but Debian does not want to force them on those who don’t want them. However, with a few quick configuration changes and additional packages, the system can be greatly improved. This post describes the initial changes I prefer to make after installing the stock images.

Ensure the Packages are Updated

The first step is to ensure that all packages are up to date.

# apt-get update
# apt-get upgrade
Install etckeeper

The first thing I like to install is etckeeper, which keeps track of any changes made to etc configuration files with a version control system.  But before I install etckeeper, I first install and configure git. I prefer to do this first so that all my other configuration changes in /etc are tracked in git.

# apt-get install -y git
# git config --global "Jon Doe"
# git config --global ""
# apt-get install -y etckeeper
Install vim

Next I like to install vim and make sure it is selected as the default editor.

# apt-get install -y vim
# echo 'SELECTED_EDITOR="/usr/bin/vim.basic"' > ~/.selected_editor

I also run update-alternatives to set vim.basic as my editor of choice. You can optionally use readlink to confirm that the change worked.

# update-alternatives --set editor /usr/bin/vim.basic
# readlink /etc/alternatives/editor
Modify bashrc

Define PS1 in root’s .bashrc to add color to terminal prompt.

# sed -i \
 -e "s/^# export LS_OPTIONS='--color=auto'$/export LS_OPTIONS='--color=auto'/" \
 -e "s/^# eval \"\`dircolors\`\"$/eval \"\`dircolors\`\"/" \
 -e "s/^# alias ls='ls \$LS_OPTIONS'$/alias ls='ls \$LS_OPTIONS'/" \
 -e "s/^# alias ll='ls \$LS_OPTIONS -l'$/alias ll='ls \$LS_OPTIONS -l'/" \
 -e "s/^# alias l='ls \$LS_OPTIONS -lA'$/alias l='ls \$LS_OPTIONS -lA'/" \
 -e "s/^# alias rm='rm -i'$/alias rm='rm -i'/" \
 -e "s/^# alias cp='cp -i'$/alias cp='cp -i'/" \
 -e "s/^# alias mv='mv -i'$/alias mv='mv -i'/" \
# echo "PS1='${debian_chroot:+($debian_chroot)}\[\033[01;31m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]$ '" >> /root/.bashrc
Setup User Account

Next, I create a user account, add it to the sudo group, and update it’s .bashrc file.

# apt-get install -y sudo
# adduser danny
# adduser danny sudo
# sed -i \
 -e "s/^#force_color_prompt=yes$/force_color_prompt=yes/" \
 -e "s/^ #alias ls='ls --color=auto'$/ alias ls='ls --color=auto'/" \
 -e "s/^ #alias grep='grep --color=auto'$/ alias grep='grep --color=auto'/" \
 -e "s/^ #alias fgrep='fgrep --color=auto'$/ alias fgrep='fgrep --color=auto'/" \
 -e "s/^ #alias egrep='egrep --color=auto'$/ alias egrep='egrep --color=auto'/" \
 -e "s/^#alias ll='ls -l'/alias ll='ls -l'/" \
 -e "s/^#alias la='ls -A'/alias la='ls -A'/" \
 -e "s/^#alias l='ls -CF'/alias l='ls -CF'/" \
 -e "$a\\nexport PATH=$PATH:/usr/sbin:/sbin" \
Keep Clock Synchronized with NTP

To keep the clock synchronized with internet standard time servers, install the ntp daemon.

$ sudo apt-get install -y ntp
Prevent Catastrophic User Error

Next, I like to install several packages that help prevent catastrophic user error. Firstly, I like to install safe-rm, which is a wrapper around the rm command that prevents accidental deletions of files. Secondly, I like to install molly-guard, which guards against accidental shutdowns or reboots by prompting the user for the hostname before allowing the instructions to execute.

$ sudo apt-get install -y safe-rm molly-guard
Ensure Latest Version of Libraries Are In Use

needrestart checks which running daemons need to be restarted after library upgrades. This may help catch issues sooner and ensure that the latest version of the library is being used by all running daemons. It also informs you when a restart is required to use a newer version of the kernel.

$ sudo apt-get install -y needrestart
Make iptables Changes Persistent

By default changes to iptables will not be preserved on reboot. To fix this, I install iptables-persistent and have it save the current running configuration to /etc/iptables/rules.v4 and /etc/iptables/rules.v6 for IPv4 and IPv6, respsectively.

$ sudo apt-get install -y iptables-persistent

Whenever changes are made to iptables, the files can be updated with the following commands for IPv4 and IPv6, respectively:

$ sudo iptables-save > /etc/iptables/rules.v4
$ sudo ip6tables-save > /etc/iptables/rules.v6
Email Notifications Listing Packages Pending an Upgrade

Some upgrades  have important bug or security patches that leave the system vulnerable if they remain unpatched. Email notifications may help keep the system administrator informed about important updates.

$ sudo apt-get install -y apticron
Install and Configure ufw

Uncomplicated firewall (ufw) is an easy to use front-end for netfilter.

$ sudo apt-get install ufw
$ sudo ufw allow ssh
$ sudo ufw enable
Install Optional Utilities

The tree utility lists contents of directories in a tree-like format. The file utility determines file type. The less utility is an alternative pager to the more utility that allows scrolling upwards.

$ sudo apt-get install -y tree file less

The dos2unix utility converts text files from DOS to Unix. The renameutils package provides utilities for quickly moving or copying files, editing the file name in a text editor. The bzip2 package provides utilities for bzip2 compression.

$ sudo apt-get install -y dos2unix renameutils bzip2

The sysstat package which provides iostat and mpstat. The dstat utility is an alternative to iostat and mpstat that is more pretty.

$ sudo apt-get install -y sysstat dstat

The htop utility is an alternative to top but allows you to scroll and looks pretty. The iotop utility is a top-like disk I/O monitor. The itop utility is a top-like interrupt load monitor.

$ sudo apt-get install -y htop iotop itop iftop

The dnsutils package provides dig and nslookup. The bind9-host provides the host utility.

$ sudo apt-get install -y dnsutils bind9-host

The mtr-tiny package provides the mtr utility which combines the ping and traceroute programs in a single diagnostic tool. Th telnet utility is useful for testing TCP connectivity. The nmap utility is a network exploration tool and port scanner. The tcpdump utility dumps traffic on a network.

$ sudo apt-get install -y mtr-tiny telnet nmap tcpdump

Create RAID6 Array with mdadm and xfs

I recently used mdadm to setup RAID6 on my GNU/Linux file servers. The following is a tutorial on the commands I ran on debian 9 stretch to accomplish this task.


You will need to install the parted, mdadm, and xfsprogs packages. GNU parted is used to create partition tables. mdadm is used to create and maintain multi devices. The xfsprogs package contains several xfs related utilities including mkfs.xfs and xfs_info.

$ sudo apt-get install parted mdadm xfsprogs
Create Partition Table

The first step in preparing your disk drives is to create the GPT partition table.

$ sudo parted /dev/sdb mklabel gpt
Create Partition

Next you need to create the partitions. While this step is optional since the RAID array can be built from devices, it is recommended that you use partitions so that down the road you have the option of replacing a drive with one of equal or greater capacity. For my purposes, I just created a single partition that consumes all of the available space.

The “-a optimal” option tells parted to use optimal alignment.

$ sudo parted -a optimal /dev/sdb mkpart primary 0% 100%
Enable the RAID Flag

Next you need to set the raid flag on the partition.

$ sudo parted /dev/sdb set 1 raid on
Verify the Configuration

You can verify that the Partition Table is type gpt, the primary partiton has been created with the appropriate size and that the raid flag is set on that partition.

$ sudo parted /dev/sdb print
Model: ATA HGST HDN724040AL (scsi)
Disk /dev/sdb: 4001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags:

Number Start  End    Size   File system Name    Flags
1      1049kB 4001GB 4001GB             primary raid
Create the Multi Device

After you have completed the previous steps on all disks, the next step is to create the multi device (md). In my case, I am creating a RAID6 multi device from 7 partitions.

$ sudo mdadm --create --verbose /dev/md0 --level=6 --raid-devices=6 /dev/sd{b,c,d,e,f,g,h}1

The multi device is created, but it will not be preserved on reboot. To preserve the configuration on reboot we need to append to the mdadm configuration file. You may also need to update initramfs and grub to ensure the multi device is automatically assembled on reboot.

$ sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
$ sudo update-initramfs -u
$ sudo update-grub
Create the XFS Filesystem

Now that we have a multi device, we can create the filesystem. I recommend using xfs if the effective capacity of your multi device may eventually grow to be more than 16TB of hard drive space.

The “-d” option allows us to specify one or more disk parameters.

The sunit disk parameter stands for Stripe Unit. It is optimal when assigned to the Stripe Size in bytes divided by 512. In my case the Stripe Size is 512KB, and thus the optimal Stripe Unit is 1024.

The swidth disk parameter stands for Stripe Width. It is defined by the number of non-parity disks times Stripe Unit. In my case, I am using RAID6 with a total of 7 disks. Since RAID6 requires 2 parity disks, that leaves me with 5 non-parity disks. Thus the optimal Stripe Width is 5120 (5 non-parity disks times the 1024 Stripe Unit).

$ sudo mkfs.xfs -d sunit=1024,swidth=5120 /dev/md0

You can verify the sunit and swidth settings aftewards, but it can be tricky because the units are different. With mkfs.xfs they are defined as a multiple of 512-bytes. With xfs_info, they are defined as a multiples of the block size (bsize). In this case bsize is 4096. So the values appear as 1/8th of the value we specified in the mkfs.xfs command.

$ sudo xfs_info /dev/md0
meta-data=/dev/md0 isize=256 agcount=32, agsize=152612736 blks
 = sectsz=4096 attr=2, projid32bit=1
 = crc=0 finobt=0 spinodes=0 rmapbt=0
 = reflink=0
data = bsize=4096 blocks=4883607040, imaxpct=5
 = sunit=128 swidth=640 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=0
log =internal bsize=4096 blocks=521728, version=2
 = sectsz=4096 sunit=1 blks, lazy-count
Alternative: Create the ext4 Filesystem

If you are certain you will never exceed the 16TB limit, then ext4 is a reasonable alternative choice for a filesystem. Note that you should only run this command if you are using ext4 instead of xfs.

$ sudo mkfs.ext4 -F /dev/md0
Mount the Multi Device

A mount point is simply the directory to which the device is mounted. So we just need to make sure that directory exists and mount the multi device to the newly created mount point.

$ sudo mkdir -p /mnt/md0
$ sudo mount /dev/md0 /mnt/md0

Next we update fstab to preserve the changes on reboot.

$ echo '/dev/md0 /mnt/md0 xfs defaults,nofail 0 0' | sudo tee -a /etc/fstab

Finally df can be used to verify the disk is mounted and displays the usage.

$ df -h -t xfs
Filesystem Size Used Avail Use% Mounted on
/dev/md0   19T  0T   0T    0%   /mnt/md0
Verify the Setup

You can verify the setup with lsblk. It shows the disk device and capacity, the raid partition, and multi device and its capacity, filesystem type, raid level, and mount point.

$ lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT /dev/sd{b,c,d,e,f,g,h}
sdb 3.7T disk
└─sdb1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0
sdc 3.7T disk
└─sdc1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0
sdd 3.7T disk
└─sdd1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0
sde 3.7T disk
└─sde1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0
sdf 3.7T disk
└─sdf1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0
sdg 3.7T disk
└─sdg1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0
sdh 3.7T disk
└─sdh1 3.7T linux_raid_member part
 └─md0 18.2T xfs raid6 /mnt/md0

You can view the state of multi devices using the proc virtual filesystem.

$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]
md0 : active raid6 sdc1[4] sdg1[3] sde1[2] sdd1[7] sdf1[6] sdh1[0] sdb1[5]
 19534428160 blocks super 1.2 level 6, 512k chunk, algorithm 2 [7/7] [UUUUUUU]
 bitmap: 0/30 pages [0KB], 65536KB chunk

unused devices: <none>

Lastly, you can view detailed information about a multi device using mdadm.

$ sudo mdadm --detail /dev/md0
 Version : 1.2
 Creation Time : Mon May 22 21:28:46 2017
 Raid Level : raid6
 Array Size : 19534428160 (18629.48 GiB 20003.25 GB)
 Used Dev Size : 3906885632 (3725.90 GiB 4000.65 GB)
 Raid Devices : 7
 Total Devices : 7
 Persistence : Superblock is persistent

Intent Bitmap : Internal

Update Time : Fri Sep 8 20:07:07 2017
 State : clean
 Active Devices : 7
Working Devices : 7
 Failed Devices : 0
 Spare Devices : 0

Layout : left-symmetric
 Chunk Size : 512K

Name : euclid:0 (local to host euclid)
 UUID : a7faccce:58914d5e:b26cce7e:795b773b
 Events : 48719

Number Major Minor RaidDevice State
 0 8 113 0 active sync /dev/sdh1
 6 8 81 1 active sync /dev/sdf1
 7 8 49 2 active sync /dev/sdd1
 3 8 97 3 active sync /dev/sdg1
 4 8 1 4 active sync /dev/sdc1
 5 8 17 5 active sync /dev/sdb1
 2 8 65 6 active sync /dev/sde1