isox 3.0.0

Download Linux ISOs from the fastest mirror and verify them against published checksums

ISOx

A command-line tool that downloads Linux distribution ISOs, races mirrors to find the fastest available source, and cryptographically verifies file integrity against the checksum published by the distribution itself, so you never have to manually hunt down hashes or skip verification because it's tedious.

Select distro -> Compare mirror speeds -> Download .iso -> Verify checksum
                                          (fastest mirror)  (distro's own host)

The ISO comes from whichever mirror is fastest right now. The checksum comes from the distribution's own server, so the mirror that hands you the bytes is not also the one vouching for them. See What verification does and doesn't cover.

Why

I distro-hop a lot across laptops, tablets, Pis, and spare hardware. Manually visiting each project's download page, picking a mirror, and copy-pasting checksums to verify against every time got tedious enough that I started skipping the verification step entirely. This poses an integrity risk (modified ISOs, corruption, etc.), so I built a tool that automates the whole pipeline and makes verification the default, and not an extra step.

Furthermore, I simply love Linux. It's been my daily driver ever since I discovered it, and I want to see it continue to grow. I hope this tool makes getting started with Linux a little faster, easier, and safer for anyone who wants to use it.

Usage

List every supported distro:

python isox.py --list

Save somewhere other than ./ISOx_Downloads:

python isox.py arch --output-dir /mnt/usb

Download and verify a distro:

python isox.py arch
python isox.py debian
python isox.py kali
python isox.py alpine
python isox.py mint
python isox.py fedora
python isox.py opensuse
python isox.py gentoo
python isox.py void
python isox.py garuda
python isox.py ubuntu
python isox.py rocky

Downloaded ISOs are saved to the created folder ISOx_Downloads/. Output looks like:

A Normal Run

https://fastly.mirror.pkgbuild.com/iso/latest/archlinux-x86_64.iso sampled at 3.33 MB/s
https://geo.mirror.pkgbuild.com/iso/latest/archlinux-x86_64.iso sampled at 1.98 MB/s
https://mirror.rackspace.com/archlinux/iso/latest/archlinux-x86_64.iso sampled at 0.08 MB/s
Downloading archlinux-x86_64.iso from https://fastly.mirror.pkgbuild.com/iso/latest ...
[##############################] 100.0%    3.41 MB/s
Checksum matches, file is good.

Resumed Output

https://fastly.mirror.pkgbuild.com/iso/latest/archlinux-x86_64.iso sampled at 3.28 MB/s
https://geo.mirror.pkgbuild.com/iso/latest/archlinux-x86_64.iso sampled at 2.04 MB/s
https://mirror.rackspace.com/archlinux/iso/latest/archlinux-x86_64.iso is unreachable
Downloading archlinux-x86_64.iso from https://fastly.mirror.pkgbuild.com/iso/latest ...
Resuming from 743.6 MB ...
[##############################] 100.0%    3.26 MB/s
Checksum matches, file is good.

Features

• Config-driven distro support - supported distros are defined in distros.json, not hardcoded, meaning adding a new distro is a JSON entry, not a code change.
• Three ISO-discovery strategies - covers distros that publish their ISOs in very different ways.
• Checksums from the distro, ISO from the fastest mirror - a mirror serving a modified ISO could serve a matching hash just as easily, so the hash is fetched from the distribution's own host rather than from whichever mirror won the speed race.
• Version-folder auto-discovery - for distros with no stable "latest" alias, the current version-numbered directory is discovered automatically by scanning a parent directory and numerically sorting version-like folder names, so outdated ISOs aren't retrieved.
• Pre-release filtering - distros publish release candidates into the same directory as final releases, and _rc2 sorts above the release it precedes. Those are filtered out so isox alpine means the release, not the candidate.
• No silent guessing - if a distro's config matches more than one ISO in a checksum file, the run stops and names the candidates instead of picking one. A wrong image would still verify cleanly against its own published hash, so guessing is the one thing worse than failing.
• Mirror speed checks - samples ~2MB from each candidate mirror via a ranged request to measure real throughput, then downloads from the fastest.
• Resumable downloads - interrupted transfers are written to a .part file and continued via an HTTP Range request on the next run, so a drop at 90% doesn't cost you the 90% you already downloaded, even if a different mirror wins the race next time.
• Stale-partial detection - a .part left over from a previous release of a rolling distro is detected and discarded rather than merged into the new one.
• Live progress bar - shows percentage and real-time throughput, and degrades to a plain byte counter if the server won't report a total size.
• Streamed downloads - files are downloaded in large chunks (requests with stream=True) rather than loaded into memory all at once, so multi-GB ISOs don't hog RAM.
• Checksum verification across three real-world formats - the standard <hash> <filename> format, a single-hash-per-file format, and a BSD-style format are all normalized into the same lookup and compared with hashlib.
• Multi-algorithm support - uses hashlib.new(algo) rather than hardcoding a specific hash function, so the same code path supports SHA256, SHA512, or anything else hashlib supports.
• Failure quarantine - an ISO that fails verification is renamed rather than left in place, so it can't be mistaken for a verified file.
• Single-point error handling - every failure path exits with a one-line explanation and a non-zero status code, not a traceback.
• Path-traversal protection - filenames discovered from remote HTML listings are validated before ever being used in a URL or local file path.

How it works

Config format (distros.json)

Every distro entry needs mirrors, checksum_filename, and hash_algo at minimum. Everything else is optional and only needed if that distro deviates from the simplest cases such as Arch.

Fedora is shown as a more complex example on purpose. It demonstrates the additional options available when a distro needs version discovery, mirror scanning, or custom checksum handling. Most distributions only require the basic fields plus one or two optional ones.

If the included mirrors are not ideal for your location, you can easily update them. Just find a suitable mirror from the distro's official mirror list and replace the URL in distros.json. The tool will then handle the rest. Mirror, checksum and version-discovery URLs must be HTTPS. ISOx refuses a config with a plain-HTTP URL, but most distros have moved to HTTPS-only mirrors already, so this shouldn't narrow your options much.

{
    "arch": {
        "mirrors": ["https://fastly.mirror.pkgbuild.com/iso/latest/"],
        "checksum_base": "https://geo.mirror.pkgbuild.com/iso/latest/",
        "checksum_filename": "sha256sums.txt",
        "hash_algo": "sha256",
        "iso_filename": "archlinux-x86_64.iso"
    },
    "fedora": {
        "mirrors": [
            "https://dl.fedoraproject.org/pub/fedora/linux/releases/{version}/Workstation/x86_64/iso/",
            "https://mirror.cs.princeton.edu/pub/mirrors/fedora/linux/releases/{version}/Workstation/x86_64/iso/",
            "https://mirror.arizona.edu/fedora/linux/releases/{version}/Workstation/x86_64/iso/"
        ],
        "version_directory" : true,
        "version_discovery_url" : [
            "https://dl.fedoraproject.org/pub/fedora/linux/releases/",
            "https://mirror.arizona.edu/fedora/linux/releases/"
        ],
        "checksum_base": "https://dl.fedoraproject.org/pub/fedora/linux/releases/{version}/Workstation/x86_64/iso/",
        "checksum_filename" : "CHECKSUM",
        "checksum_discovery_method" : "html_scan",
        "checksum_format" : "bsd",
        "discovery_method": "html_scan",
        "hash_algo": "sha256",
        "iso_filename_contains": ["Workstation", "x86_64"]
    }
}

Key	Purpose
mirrors	Required. Where the ISO may be downloaded from. Raced on every run.
checksum_filename	Required. Literal name, or a {iso_filename}.sha256 template.
hash_algo	Required. Anything hashlib supports. Validated before any download starts.
checksum_base	Host to fetch the checksum from, instead of the winning mirror. Should be the distro's own server. Also decides the ISO filename, so name and hash always agree.
iso_filename	For distros whose filename never changes.
iso_filename_contains	Substrings every candidate filename must contain.
iso_filename_excludes	Substrings that disqualify a filename, on top of the built-in pre-release filter. Use this when a distro publishes variants your substrings can't tell apart, like Debian's -edu- and -mac- images.
discovery_method	checksum_scan (default) or html_scan.
checksum_discovery_method	html_scan when the checksum filename itself has to be scraped.
checksum_format	multi (default), bsd, or single.
version_directory	true when the current version has to be discovered first.
version_discovery_url	One URL or a list of them, tried in order.
version_scheme	ubuntu_lts to select only LTS releases.

Where distros.json is loaded from

Searched in this order, first hit wins:

1. $ISOX_DISTROS, if set - this short-circuits the rest, so a typo is reported rather than silently falling back
2. ~/.config/isox/distros.json (%APPDATA%\isox\distros.json on Windows)
3. Beside isox.py - the git clone case
4. share/isox/distros.json under the install scheme's data directory, the user base, or sys.prefix

Four locations for the install case rather than one because a wheel's data files land wherever the install scheme puts them, and that varies by how pip was invoked. venv and pipx have sys.prefix and the data path coincide; pip install --user puts it under the user base; Debian's patched system Python defaults to /usr/local while sys.prefix stays /usr. Duplicates collapse, so a normal install shows one path, not three.

If you customise mirrors on a pip install, put your copy at (2). pip install -U isox replaces what it installed, so edits made directly to (4) revert on upgrade without warning. A copy in your config directory survives.

mkdir -p ~/.config/isox
isox --list                      # prints the config path currently in use
cp "$(isox --list | sed -n 's/^config: //p')" ~/.config/isox/distros.json

isox --list prints which file won, which is the quickest way to answer "why is it using the wrong mirrors".

ISO filename discovery, three strategies

Not every distro publishes ISOs the same way, so the tool picks a strategy per distro based on which config fields are present. No per-distro code exists anywhere in the script.

• "iso_filename" - for distros with one fixed, unchanging filename.
• "iso_filename_contains" + default discovery - scans a shared checksum file for a filename matching all the given substrings, since versioned filenames would be inefficient to hardcode.
• "iso_filename_contains" + "discovery_method": "html_scan" - for distros with no single shared checksum file to scan. Scrapes the actual directory listing HTML with BeautifulSoup and filters <a href> links ending in .iso that match all the necessary substrings.

Both scanning strategies filter out pre-release artifacts by default. This matters more than it sounds: Alpine publishes alpine-extended-3.24.2_rc1-x86_64.iso into the same directory as the final release, and because _ sorts above -, a plain "newest wins" comparison hands you the release candidate. An RC has a perfectly valid published checksum, so it would download and verify without complaint.

The two strategies also differ in how they break ties, deliberately. A directory listing legitimately holds several versions, so the newest wins. A checksum file describes one release, so several matches means the config is ambiguous rather than that there is a choice to make - the run stops and names the candidates:

Error: 'debian' matched 3 ISOs in the checksum file at
https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/SHA256SUMS:
debian-13.6.0-amd64-netinst.iso, debian-edu-13.6.0-amd64-netinst.iso,
debian-mac-13.6.0-amd64-netinst.iso. Narrow iso_filename_contains or add
iso_filename_excludes in distros.json.

Only .iso files are considered. Checksum files cover every artifact of a release, and Kali's SHA256SUMS pairs each ISO with a .iso.torrent that matches exactly the same substrings - without the extension filter, a 100KB torrent could be downloaded, verified against its own hash, and reported as a good ISO.

Version-folder auto-discovery ("version_directory": true) is a separate, earlier step for distros with no stable "latest" URL alias at all. Before any ISO discovery happens, the parent directory is scraped, version-numbered folder names are parsed and sorted numerically, and the newest one is spliced into every {version} placeholder across the mirror URLs.

version_discovery_url takes a list as well as a single URL, and they are tried in order. This step decides which directory everything else is fetched from, so a single unreachable host here would otherwise fail the whole run even when every configured mirror is healthy.

For Ubuntu, version_scheme is optional and only applies alongside version_directory. Left out, the newest version-numbered folder is selected. Set to "ubuntu_lts", only even-year .04 folders match, so python isox.py ubuntu resolves to the latest LTS. Interim releases are skipped intentionally, since LTS is the better default for a daily driver.

Checksum parsing

• "multi" (default) - the standard <hash> <filename> format used by sha256sum's own output.
• "single" - the whole file content is treated as the hash, with the filename supplied from context rather than parsed. Available for distros that publish a genuinely bare hash.
• "bsd" - parses lines shaped like SHA256 (filename) = hash, used by some distros. Only lines starting with the configured hash_algo are read, so a file listing multiple algorithms for the same filename can't have the wrong one picked.

Mirror selection

Each candidate mirror is sampled with a ranged GET request, pulling the first ~2MB of the actual ISO via an HTTP Range header, and the real transfer speed (bytes/second) is measured over that sample. The mirror with the highest sampled throughput is selected for the ISO download. The checksum is fetched separately, from checksum_base.

Mirrors that time out or return an error status are caught (requests.exceptions.RequestException) and skipped rather than crashing the whole run.

All Mirrors Down

https://fastly.mirror.pkgbuild.com/iso/latest/archlinux-x86_64.iso is unreachable
https://geo.mirror.pkgbuild.com/iso/latest/archlinux-x86_64.iso is unreachable
https://mirror.rackspace.com/archlinux/iso/latest/archlinux-x86_64.iso is unreachable
Error: none of the mirrors for this distro are reachable right now. Try again soon, or swap the mirrors in distros.json.

Resumable downloads

Downloads are written to <filename>.part and only renamed to the final name once the transfer completes, so a partial file can never be mistaken for a finished one. On the next run, if a .part exists, its size becomes the offset in a Range: bytes=N- request and the transfer continues from there.

Four things can go wrong with a resume, and each is handled:

• The .part is larger than the file on the server. The server answers 416 Range Not Satisfiable, which means the partial is stale: it's deleted and the download restarts.
• The .part belongs to an older release. Rolling distros like Arch reuse the same filename (archlinux-x86_64.iso) for a new image every month, so a partial from June would happily append onto July's file and produce a corrupt iso. ISOx stores the mirror's ETag/Last-Modified alongside the .part and discards the partial if it no longer matches. If the server publishes neither header, the partial is discarded after 24 hours instead.
• A different mirror wins the race this time. ETags are per-server - nginx derives them from mtime and size, which differ between mirrors holding identical bytes - so a fingerprint from one mirror says nothing about another. The URL is recorded alongside the fingerprint and they're only compared when they came from the same place, otherwise the 24-hour age check applies. Without this, changing mirrors between runs looks exactly like the file changing, and a 90%-complete download gets thrown away.
• The server ignores the Range header and sends the whole file with a 200. The offset is reset and the file is rewritten from scratch rather than appended to.

A transfer that ends short of the advertised Content-Length keeps its .part and exits non-zero rather than being promoted. A short read is the exact case resuming exists for, so turning it into a .FAILED ISO would throw away work that was still good.

Stale Partial Downloads

Downloading archlinux-x86_64.iso from https://fastly.mirror.pkgbuild.com/iso/latest ...
Partial download doesn't match file on server. Starting fresh.
[##############################] 100.0%    3.39 MB/s
Checksum matches, file is good.

Checksum verification remains the final stop: a bad merge that somehow slipped through would fail verification and be quarantined.

Checksum verification

The distro's checksum file is fetched new on every run, from checksum_base rather than from the mirror that served the ISO, and parsed into a {filename: hash} lookup dictionary. The downloaded file is then hashed via hashlib and compared against the expected hash with hmac.compare_digest, case-insensitively - hexdigest() is always lowercase and not every distro publishes it that way, and a case difference would otherwise be reported as tampering.

If the hashes don't match, the ISO is renamed to <filename>.FAILED. If no checksum entry could be found for the file at all, it's renamed to <filename>.UNVERIFIED. In both cases the process exits non-zero.

Verification Failure

[##############################] 100.0%    3.12 MB/s
WARNING: checksum mismatch, file may be corrupted or tampered with!
Renamed to ISOx_Downloads/archlinux-x86_64.iso.FAILED so it can't be mistaken for a verified ISO.

This was tested: I created a separate script to append garbage bytes to a previously-verified ISO, then ran the same verify_checksum() function used in the main program against it. It correctly returns False, confirming the verification logic detects tampering/corruption rather than always reporting success. The script I used to test corruption is below. Feel free to try for yourself.

from isox import verify_checksum
import requests

response = requests.get("https://fastly.mirror.pkgbuild.com/iso/latest/sha256sums.txt")
hash_lookup = {}
for line in response.text.splitlines():
    parts = line.split()
    if len(parts) == 2:
        hash_lookup[parts[1]] = parts[0]

result = verify_checksum("ISOx_Downloads/archlinux-x86_64.iso", "archlinux-x86_64.iso", hash_lookup, "sha256")
print("Verified:", result)  # should print False now, after corruption

What verification does and doesn't cover

Covered. Corruption in transit, truncated transfers, a bad disk, a botched resume, and a mirror serving a modified ISO. That last one is why the checksum is fetched from checksum_base - the distribution's own host - rather than from the mirror that served the bytes. A mirror that can hand you a tampered ISO can hand you a hash matching it just as easily, so verifying a mirror's file against that same mirror's hash is close to verifying nothing. Splitting the two means one rogue mirror can't supply both halves.

Not covered. ISOx does not perform GPG signature verification, so it cannot prove the checksum itself is authentic. If the distribution's own host is compromised, or someone holds a certificate for it, a matching ISO and hash could be served together and ISOx would report success.

GPG is not included as it would require maintaining trusted public keys (or fingerprints) for every supported distribution, along with key management and signature validation logic. That complexity conflicts with ISOx's goal of being a lightweight, easy to use, and config-driven Linux tool.

Worth knowing: several distros already publish signed checksums that ISOx fetches and parses while ignoring the signature - Gentoo's .sha256 is PGP-clearsigned, and Rocky ships a CHECKSUM.asc beside its CHECKSUM. If your threat model requires verifying the origin of a release, those signatures are right there, and the distribution's documentation covers its public signing keys and the verification steps.

Requirements

• Python 3.10+
• requests
• beautifulsoup4 - used for HTML directory-listing discovery

Everything else (hashlib, hmac, json, argparse, os, sys, time, re) is part of the Python standard library.

Run it straight from a clone:

pip install -r requirements.txt
python isox.py arch

Or install it and get an isox command on your PATH:

pip install .
isox arch

Development

pip install -r requirements.txt -r requirements-dev.txt
pytest
black --check .
ruff check .
bandit isox.py

The suite is hermetic and stubs the network. tests/test_live_mirrors.py is the one exception: it resolves every distro in distros.json against the real mirrors and checks the filename it lands on has a published checksum, without downloading any ISO. It's deselected by default and takes about 30 seconds:

pytest -m live       # all 12 distros
pytest -m live -s    # printing each resolved filename and hash
pytest -m live -k rocky

Contributing

Distro requests and additions are welcome - most new distros are a distros.json entry with no Python at all. See CONTRIBUTING.md.

License

MIT License: see LICENSE for details. Feel free to use, modify, or build on this.