Network Configuration Management on Amazon Linux 2023

This guide details how to override the default gateway assigned by DHCP.

Opening and Editing Network Configuration

Open the Network Configuration File Use nano (or any text editor of your choice) to open the network configuration file:
```
sudo nano /usr/lib/systemd/network/80-ec2.network
```
to add the Gateway=172.31.96.100 line.
View the Network Configuration File Content After editing, you can view the contents to confirm the changes:
```
cat /usr/lib/systemd/network/80-ec2.network
```

Network Configuration File Content

Below is an example of the content inside the 80-ec2.network file:

[Match]
Driver=ena ixgbevf vif

[Link]
MTUBytes=9001

[Network]
DHCP=yes
IPv6DuplicateAddressDetection=0
LLMNR=no
DNSDefaultRoute=yes
Gateway=172.31.96.100

[DHCPv4]
UseHostname=no
UseDNS=yes
UseNTP=yes
UseDomains=yes

[DHCPv6]
UseHostname=no
UseDNS=yes
UseNTP=yes
WithoutRA=solicit

Viewing Current Route Table

Check the Current Route Table Before restarting the network service, check the current route table:

route -n

Expected Output:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.31.96.1     0.0.0.0         UG    512    0        0 enX0
172.31.0.2      172.31.96.1     255.255.255.255 UGH   512    0        0 enX0
172.31.96.0     0.0.0.0         255.255.240.0   U     512    0        0 enX0
172.31.96.1     0.0.0.0         255.255.255.255 UH    512    0        0 enX0

Restarting Network Service

Restart the Network Service Apply any changes made by restarting the systemd-networkd service:
```
sudo systemctl restart systemd-networkd
```

Verifying Changes

Check the Updated Route Table After restarting, verify that the changes are effective:

route -n

Updated Output:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.31.96.100   0.0.0.0         UG    0      0        0 enX0
0.0.0.0         172.31.96.1     0.0.0.0         UG    512    0        0 enX0
172.31.0.2      172.31.96.1     255.255.255.255 UGH   512    0        0 enX0
172.31.96.0     0.0.0.0         255.255.240.0   U     512    0        0 enX0
172.31.96.1     0.0.0.0         255.255.255.255 UH    512    0        0 enX0

Testing Connectivity

Ping Test Finally, perform a ping test to confirm network connectivity:

ping -c3 www.google.com

Output:

PING www.google.com (172.253.62.99) 56(84) bytes of data.
64 bytes from bc-in-f99.1e100.net (172.253.62.99): icmp_seq=1 ttl=104 time=2.75 ms
64 bytes from bc-in-f99.1e100.net (172.253.62.99): icmp_seq=2 ttl=104 time=2.30 ms
64 bytes from bc-in-f99.1e100.net (172.253.62.99): icmp_seq=3 ttl=104 time=2.48 ms

--- www.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 2.298/2.506/2.747/0.184 ms

jdmedeiros/override_dhcp.md