{/* banner prompt: Of course! Here are a few creative and engaging prompts for a banner image, designed to capture the essence of the blog post.

### Main Prompt (Cinematic & Evocative)

This prompt focuses on the "resurrection" theme of bringing old hardware back to life with modern, efficient software.

**Prompt:**
> A cinematic, dramatic shot of a dusty, forgotten 90s-era beige PC tower sitting on a workbench in a dimly lit workshop. A brilliant, glowing blue USB cable connects a modern Android smartphone to the PC's front port, acting as a lifeline. Ethereal, glowing data streams, shaped subtly like the Alpine Linux mountain logo, emanate from the PC's vents, illuminating the scene and casting long shadows. The old machine hums with new purpose, transformed into a powerful network gateway. Moody, high-contrast lighting, tech noir aesthetic, hyper-realistic, 8k. --ar 16:9

---

### Alternative Prompts

Here are two other options with different artistic styles.

#### Option 2 (Technical & Stylized)

This prompt emphasizes the technical, blueprint-like nature of the project.

**Prompt:**
> A stylized, futuristic blueprint schematic on a dark grid background, illustrating the post's network topology. At the top, a sleek icon of an Android phone connects via a glowing USB data line to a central, semi-transparent block representing an old computer. The iconic Alpine Linux mountain logo glows brightly from within this central block. From the block, a solid ethernet data stream flows out to a home router icon. The entire image has a clean, technical, and cyberpunk feel, with digital annotations and connection labels. Widescreen, vector illustration style, highly detailed. --ar 16:9

#### Option 3 (Minimalist & Symbolic)

This prompt focuses on the core components in a clean, modern, and symbolic way.

**Prompt:**
> A minimalist composition on a clean, dark grey background. In the center, a simple, iconic representation of an old PC case is given new life. A single, vibrant cyan USB cable plugs into it from one side, originating from an abstract smartphone shape. From the other side, a stable, glowing ethernet cable extends outwards. The Alpine Linux mountain symbol is subtly but clearly etched onto the side of the PC case, glowing with a soft white light. The image conveys elegance, efficiency, and the concept of "powerful simplicity." Professional product shot style, soft studio lighting. --ar 16:9 */}

# Building a 4G/5G Backup Mobile Gateway with Alpine Linux on Old Hardware

*How to repurpose an outdated PC into a reliable backup internet gateway using Android USB tethering*

---

## The Problem

Internet goes down. It always does at the worst possible time. However, even though that only happens a couple of times a year, an always-on 4G/5G backup isn't strictly necessary. It would be nice to have a plug-and-play solution—like using your phone (which probably has plenty of data) as a rescue for those few hours of downtime.

If you have a spare Android phone and an old PC gathering dust, you can build a proper backup gateway that kicks in when your main connection fails — no expensive hardware required.

This guide walks through the entire process, including the real-world gotchas encountered along the way.


## Hardware Requirements

- An old/low-powered PC (sub 1GHz, even under 512MB RAM is workable)
- One or two ethernet NICs (two is ideal, one works fine)
- A USB port for the Android phone
- An Android phone with a mobile data plan
- A USB stick (2GB+ is plenty) to install from

For this project, I used a rugged industrial PC made by **Advantech Australia**. It's equipped with an **Intel Atom D510 CPU** and **2GB of memory**. Its industrial build and dual NIC cards make it a perfect "warhorse" for a dedicated gateway role. 

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/2026/04/repurpose-your-old-pcs-how-to-install-alpine-linux-on-them/2026-04-20-19-42-33.png)

The machine used in this guide had two onboard NICs, an 8GB internal flash module (DOM — Disk on Module), and a larger hard drive set aside for backups. Very modest specs, perfectly adequate for a router role. Please execute me for the dust collected on this machine.


## Why Alpine Linux?

For low-powered hardware doing a single job, Alpine Linux is the right tool:

- Boots in under 5 seconds
- Idles at ~30–50MB RAM
- Tiny base install (~130MB on disk)
- Uses OpenRC (no systemd overhead)
- The `rndis_host` kernel module handles Android USB tethering natively — no extra drivers needed
- Actively maintained, security-focused

Other options like Debian minimal or OpenWrt on x86 are also viable, but Alpine wins on resource footprint for truly constrained hardware.


## Network Topology

With two NICs the setup is clean — no double-NAT:

```
Android Phone (USB tethering)
        |
        | usb0
  [Alpine Linux PC]
   eth0  |  eth1
    |         |
Your Router  (spare / unused)
 (WAN port)
```

With a single NIC, the same approach works — `eth0` connects to your router's WAN port, `usb0` is the phone. Your router handles the LAN side as normal. The Alpine box just NATs between `usb0` and `eth0`.

> **Note on double-NAT:** With a single NIC and a typical home router, you'll have three layers of NAT (phone → Alpine → router). For general internet use this is completely fine. Port forwarding gets complicated, but for a backup scenario it's a non-issue.


## Step 1: Creating the Install USB

Download the Alpine Linux "standard" x86 ISO from [alpinelinux.org](https://alpinelinux.org/downloads/).

Write it to a USB stick using `dd`:

```bash
dd if=alpine-standard-x.x.x-x86.iso of=/dev/sdX bs=4M status=progress
```

Replace `/dev/sdX` with your USB stick's device. **Double-check this** — `dd` will happily wipe whatever you point it at.

> **Important:** Use `dd` directly, or Rufus in **DD mode** if on Windows. Do not let Rufus create a FAT32 partition layout — old BIOSes can choke on it. Alpine ISOs are hybrid MBR images and work correctly when written raw.


## Step 2: BIOS Boot Gotchas (American Megatrends)

Old AMI BIOS boards have a few quirks that can stop USB boot from working even when USB is set as first priority:

- **Plug the USB in before powering on.** USB drives often only appear in the boot device list if they're present at POST time.
- **Check for two separate boot priority menus.** AMI BIOS often has a top-level device-type priority (Removable → HDD → CD) *and* a separate submenu for ordering within each type. The USB stick needs to be first in *both* places.
- **Enable Legacy USB Support** — usually found under Advanced or Integrated Peripherals.
- **Use a rear USB 2.0 port.** Front panel ports and USB 3.0 ports are often ignored by old BIOSes during boot enumeration.
- **Use a USB 2.0 flash drive** if possible. USB 3.0 sticks can fail to enumerate on old BIOSes.
- **Try the one-time boot menu first** — on most AMI boards, hit **F8** or **F11** at POST. This bypasses the boot order entirely and lets you pick the USB stick directly, which is useful for confirming the stick itself is bootable before adjusting BIOS settings.

In my case, I was finally able to have it boot into the Alpine Linux installation interface after plugging the USB drive into the back (rear) of the machine.

---

## Step 3: Identifying Your Disks

Once booted into the Alpine live environment, identify your drives before installing:

```sh
cat /proc/partitions
```

Example output:

```
8    0  312571224  sda     ← ~150GB (Windows drive)
8   16    7880544  sdb     ← ~7.5GB (target install drive)
8   32    3910656  sdc     ← ~3.7GB (Alpine USB stick)
```

To confirm the nature of a drive — particularly whether it's a USB device or an internal flash module:

```sh
dmesg | grep sdb
```

If the output references `sd X:X:X:X` (SCSI enumeration) rather than `usb-storage`, the drive is connected internally — likely a DOM (Disk on Module), common on older appliance-grade hardware. These are perfectly fine for Alpine and handle the low write workload of a router OS well.

> **Tip:** A drive formatted as FAT32 with write cache disabled and a single partition spanning the whole disk is a typical signature of a factory-installed DOM or embedded flash device.

---

## Step 4: Installing Alpine

Run the setup script:

```sh
setup-alpine
```

Work through the prompts:

- **Keyboard layout:** your preference
- **Hostname:** something useful, e.g. `mobile-gateway`
- **Network interface:** configure `eth0` for DHCP during install; skip `eth1` for now
- **Root password:** set something strong
- **Timezone:** your local timezone
- **NTP client:** choose **chrony** (see note below)
- **Mirror:** pick a geographically close one or `r` for auto-detect
- **SSH:** `openssh` is fine
- **Disk mode:** type `sys` — this is a standard permanent install to disk
- **Which disk:** type the device name of your target drive (e.g. `sdb`)

The installer will wipe the selected disk and install Alpine. When finished, remove the USB stick and reboot.

### Why chrony for NTP?

For a machine that's offline frequently or has intermittent connectivity — exactly the backup router use case — chrony is the best choice. It handles time jumps gracefully when connectivity comes and goes, and compensates for clock drift between connections. `busybox-ntpd` is too basic for this, and `openntpd` handles irregular connectivity less gracefully than chrony.

---

## Step 5: Post-Install — Making It Boot

If the machine boots back into another OS after install, the BIOS is still preferring the wrong drive. Go back into BIOS and find the **Hard Disk Drive Priority** submenu — set your Alpine drive (the 8GB/DOM one) as first in that list.

Use the one-time boot menu (F8/F11) to test first — select your Alpine drive directly to confirm it boots before making the BIOS change permanent.

---

## Step 6: Configuring USB Tethering

Plug in the Android phone and enable **USB Tethering** from the phone's settings (usually under Settings → Network → Hotspot & Tethering).

The Alpine kernel will load `rndis_host` automatically and a new interface will appear — typically `usb0`:

```sh
ip link show
```

Bring it up and get an IP via DHCP from the phone:

```sh
udhcpc -i usb0
```

Test connectivity:

```sh
ping -c 3 1.1.1.1 -I usb0
```

---

## Step 7: NAT and IP Forwarding

Install iptables if not already present:

```sh
apk add iptables
```

Enable IP forwarding:

```sh
echo 1 > /proc/sys/net/ipv4/ip_forward
```

Set up NAT (assuming `usb0` = WAN, `eth0` = LAN out to router):

```sh
iptables -t nat -A POSTROUTING -o usb0 -j MASQUERADE
iptables -A FORWARD -i eth0 -o usb0 -j ACCEPT
iptables -A FORWARD -i usb0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
```

Make IP forwarding persistent — add to `/etc/sysctl.conf`:

```
net.ipv4.ip_forward = 1
```

Save and persist iptables rules across reboots:

```sh
rc-update add iptables
/etc/init.d/iptables save
```

---

## Step 8: Hotplug Script (Plug In and It Just Works)

Rather than running commands manually every time, Alpine's `mdev` can detect when the phone is plugged in via USB and automatically bring up the tethering interface and routing.

Create `/etc/mdev/usb-tether.sh`:

```sh
#!/bin/sh
# Called by mdev when a USB network interface appears

IFACE="usb0"

if [ "$ACTION" = "add" ] && ip link show "$IFACE" > /dev/null 2>&1; then
    udhcpc -i "$IFACE" -q
    iptables -t nat -A POSTROUTING -o "$IFACE" -j MASQUERADE
    iptables -A FORWARD -i eth0 -o "$IFACE" -j ACCEPT
    iptables -A FORWARD -i "$IFACE" -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
fi
```

Make it executable:

```sh
chmod +x /etc/mdev/usb-tether.sh
```

This gives you plug-in activation — connect the phone, USB tethering enables on the phone, and the gateway comes up automatically. Unplug to deactivate.

---

## Second NIC — What to Do With It

If your machine has two NICs, you have more flexibility:

- **eth0** — connect to your router's WAN port (LAN out)
- **eth1** — leave unconfigured unless you need a second downstream device connected directly

There's no need to bridge or assign an IP to `eth1` for a basic single-router setup. Leave it unconfigured and use it later if needed.

---

## Summary

{/* | Component | Choice | Reason |
|-----------|--------|--------|
| OS | Alpine Linux | Minimal footprint, fast boot, good hardware support |
| NTP | chrony | Handles intermittent connectivity gracefully |
| Tethering | Android USB (rndis_host) | Native kernel support, no extra drivers |
| Install mode | sys | Standard permanent disk install |
| Activation | mdev hotplug | Plug-in and it just works | */}

The end result is a machine that sits quietly on the shelf, draws minimal power, and becomes a functional mobile gateway the moment you plug a phone into it — no GUI, no fuss, no expensive hardware.

---

*Built and tested on an old low-powered PC with AMI BIOS, dual onboard NICs, and an internal DOM flash drive. Alpine Linux 3.x.*

c8c2ab02361c09d64f2eb621cac10b95098bd376448ee41d0eb5bbc43ad13478

A step-by-step guide to installing Alpine Linux on old, low-powered hardware and configuring it as a mobile backup gateway using Android USB tethering. Covers BIOS quirks, disk identification, NAT/iptables setup, and hotplug automation with mdev.

Turn an old, dusty PC into a reliable 4G/5G backup internet gateway using Alpine Linux and Android USB tethering — no expensive hardware required.

Tag: Home-lab

Tag: Home-lab

Repurpose Your Old PCs: How To Install Alpine Linux on Them