====== Configure Networking And Hostname Resolution Statically Or Dynamically ======

**General Information**

Networking has changed from RHEL 6 to 7. You can still edit network scripts in /etc/sysconfig/network-scripts/, but the "preferred" method is using front end tools that tie into the NetworkManager service.

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===== Troubleshooting Tools =====

Show IP Address of Devices
<code bash>
ip addr show
</code>
  * ifconfig is deprecated in RHEL 7.

\\
Show all Established Connections
<code bash>
ss
</code>

\\
Show Established and Listening TCP Connections
<code bash>
ss -ant
</code>

flags:
  * a => show both established and listening
  * t => show tcp connections
  * n => do not resolve service names (ports)

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Other tools to use:
  * ping -c 5 (send 5 pings)
  * traceroute
  * tracepath
  * netstat - netstat has been deprecated (use ss instead), but can still be installed as part of the net-tools package

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===== Network Manager =====

NetworkManager is the default network service in RHEL 7. 
  * Main Service = NetworkManager.service
  * Tools to interface with NetworkManager
    * nm-connection-editor (GUI)
    * nmtui (ncurses based)
    * nmcli (cli)
  * Fallback service = network.service
    * This uses the traditional network scripts in /etc/sysconfig/network-scripts/ as a fallback if NetworkManager is not in control of an interface.

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Open GUI for network connections (GUI Only)
<code bash>
nm-connection-editor
</code>

\\
Open text based wizard
<code bash>
nmtui
</code>

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==== CLI Tool: nmcli ====

nmcli makes configuration changes to files in: /etc/sysconfig/network-scripts/

These files can alternatively be modified manually.

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Network Device Status
<code bash>
nmcli dev status
</code>

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Show all connection configurations
<code bash>
nmcli con show
</code>
  * Configurations location: /etc/sysconfig/network-scripts/

\\
Example: Create new connection (dhcp)
<code bash>
nmcli con add con-name "mycon" autoconnect yes type ethernet ifname eth1
</code>

\\
Example: Create new connection (static ip)
<code bash>
nmcli con add con-name "mycon-static" autoconnect yes type ethernet ifname eth1 ipv4.method manual ip4 10.0.0.5/24 gw4 10.0.0.254
</code>
  * The above produces the following config:
    * /etc/sysconfig/network-scripts/ifcfg-mycon-static<code bash>TYPE=Ethernet    # from: 'type ethernet'
BOOTPROTO=none    # from: 'ipv4.method manual'
IPADDR=10.0.0.5    # from: 'ip4 10.0.0.5/24'
PREFIX=24    # from: 'ipv 10.0.0.5/24'
GATEWAY=10.0.0.254    # from: 'gw4 10.0.0.254'
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_PEERDNS=yes
IPV6_PEERROUTES=yes
IPV6_FAILURE_FATAL=no
NAME=mycon-static    # from: 'con-name "mycon-static"'
UUID=f7e0c9af-715d-43da-9576-e6ce218d0c28
DEVICE=eth1    # from: 'ifname eth1'
ONBOOT=yes    # from: 'autoconnect yes'</code>

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Show only active connections
<code bash>
nmcli con show --active
</code>

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Bring connections up/down
<code bash>
nmcli con down "mycon-static"
nmcli con up "mycon"
</code>

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Change an existing connection's IP and Gateway
<code bash>
nmcli con mod eth0 ipv4.addresses 192.168.1.50/24
nmcli con mod eth0 ipv4.gateway 192.168.1.254
nmcli con mod eth0 ipv4.method manual
nmcli con up eth0
</code>
  * Changes do not take place until "nmcli con up eth0"

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===== Hostname Configuration =====

View current hostname
<code bash>
hostname
</code>

\\
Set **temporary**(transient) hostname
<code bash>
hostname myserver.domain.com
exec bash
</code>

\\
View detailed hostname information
<code bash>
hostnamectl
</code>
  * static hostname (stored in: /etc/hostname) - used to initialize kernel at boot (persistent)
  * transient hostname - assigned temporarily due to network configuration, may revert back to static if network connectivity is lost
  * pretty hostname (stored in: /etc/machine-info) - little restrictions on characters used

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Set **persistent**(static) hostname
<code bash>
hostnamectl set-hostname myserver.com
</code>

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Order of name resolution (hosts entry)
<code bash>
/etc/nsswitch.conf

#....other entries above

# hosts: search local files and then DNS
hosts:      files dns

#....other entries below
</code>

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System wide dns name server entries (static or network manager generated)
<code bash>
/etc/resolv.conf

# Generated by NetworkManager
search us-west-2.compute.internal
nameserver 172.31.0.2
</code>

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Local system hostname resolution and example entry
<code bash>
/etc/hosts

# IP      Hostname                Aliases
10.1.1.2  myserver01.example.com  website.domain.com
</code>

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**Add DNS** (not replace) for eth0 with nmcli or with config file
  * nmcli method<code bash>
nmcli con show
(observe NAME field)
nmcli con mod "System eth0" +ipv4.dns 8.8.8.8</code>
  * Edit files directly method<code bash>
vim /etc/sysconfig/network-scripts/ifcfg-eth0

DNS1=8.8.8.8</code>
  * Apply changes: Issue nmcli command or restart NetworkManager
    * nmcli method<code bash>nmcli con up eth0</code>
    * Service restart<code bash>systemctl restart NetworkManager</code>

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