# Use Elastic IP as Traffic Source

There are some cases where you may want to use Elastic IP as a source IP address - for example, if you run an SMTP server. It is possible to achieve that with manual Elastic IPs for applications, which allows you to configure their source IP. The traffic emitted in this setup is seen as coming from the Elastic IP address and not from the native IP address.

For instance, with Postfix, you can add the following line in `/etc/postfix/main.cf`:

```bash
smtp_bind_address = 203.0.113.202
```

Most software has a similar setting for outgoing connections. Search
for the `bind` keyword in the documentation.

> [!WARNING]
> Elastic IP addresses should not be shared across multiple traffic-emitting instances, as it will result in 
> asymmetric routing for returning traffic.

There might be cases when you need to use IPv6 addresses from the Elastic IPv6 prefix as a source for egress traffic. You will need to either:

Configure the Elastic IPv6 addresses as binding addresses on the application level
: For example with Postfix mail server you could use `smtp_bind_address6
  = 2001:240:587:0:250:56ff:fe89:1`

Configure the Elastic IPv6 address as the preferred source on the system's routing level
: There are several ways to implement a preferred source at the level of
  routing, depending on the operating system and personal preference. Here are
  two examples with `netplan` and `systemd-networkd`, two popular networking
  options in the Linux world.


## IPv6 modification with `netplan`
Modifying the default IPv6 source address with `netplan`.

Assuming you need to use `2a04:c44:c00:3a46:500:2::1337` as the source IPv6 address for all egress traffic:

```bash
sudo ip -6 address add 2a04:c44:c00:3a46:500:1:0:1337/128 dev lo
```

Then

```bash
cat <<EOF | sudo tee /etc/netplan/51-eipv6-source.yaml
---
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      routes:
        - to: default
          from: 2a04:c44:c00:3a46:500:2::1337
          via: $(ip -6 r show default | awk '{print $3}')
          metric: 99
EOF
```

and then invoke `sudo netplan apply`.

## IPv6 modification with `systemd-networkd`
Modifying the default IPv6 source address with `systemd-networkd`.

Assuming you need to use `2a04:c44:c00:3a46:500:2::1337` as the source IPv6 address for all egress traffic:

```bash
sudo ip -6 address add 2a04:c44:c00:3a46:500:1:0:1337/128 dev lo
```

Then

```bash
cat << EOF | sudo tee /etc/systemd/network/eth0.network
```

```bash
> [Match]
Name=eth0

[Network]
DHCP=ipv4
LinkLocalAddressing=ipv6
IPv6AcceptRA=yes

[Route]
Destination=::/0
Gateway=_ipv6ra
PreferredSource=2a04:c44:c00:3a46:500:1:0:1337
Metric=99
EOF
```

and then invoke `sudo systemctl restart systemd-networkd`.