How to send a file from your phone to your computer

What everyone actually does

Email it to yourself. Message it to yourself. Put it in a cloud drive and wait for it to sync down.

These work. They also mean a photograph of your passport travels to a data centre and back in order to get from your hand to a machine one metre away, gets recompressed on the way, and stays in a sent-items folder for as long as the account exists.

For a holiday snap, who cares. For a screenshot of a bank statement, an ID photo, a contract, or anything for work, it is worth having a way that does not leave the room.

The technique

Both devices are on the same Wi-Fi. So:

1. The computer starts a small web server and prints its address — something like http://192.168.1.24:53317.
2. You open that address on the phone, or scan the QR code the computer prints.
3. The phone shows a plain upload form, or a download link. You pick the file.
4. The file goes directly from the phone to the computer over the local network.

No app on the phone — the browser it already has is the client. No account. Nothing leaves the network. And it is quick, because local Wi-Fi is a good deal faster than your upload speed to the internet.

The same thing works in reverse: the computer serves a file, and the phone downloads it.

What to check if it does not connect

Nearly every failure is one of four things.

• The two devices are on different networks. A phone on mobile data, or on a 5 GHz guest network the computer cannot see. Check the phone is on Wi-Fi and on the same network name.
• Client isolation. Most guest networks, and most hotel and café Wi-Fi, deliberately stop devices talking to each other. Nothing you do on either device fixes this; you need a network that allows it, or a phone hotspot that both join.
• The firewall. Windows in particular will ask, once, whether to allow the program on private networks, and if that dialog was dismissed the answer is stored as no. Check the firewall rules.
• The wrong address. A machine with several network interfaces — Ethernet plus Wi-Fi, or a VPN — has several addresses, and only one of them is reachable from the phone.

What to look for in a tool

It should bind to the local network only, by default. Serving files to the whole internet should be something you have to ask for explicitly, not something that happens because you left a default alone.

It should have a code or a PIN. Anyone else on the same Wi-Fi can reach the address. A short code that has to match keeps the coffee shop out, and comparing it in constant time keeps that check from being guessable.

It should stop when you are done. A transfer tool left running is a file share you have forgotten about.

The programs for this

• SnapBeam is the one built for the common case: a screenshot from the phone onto the desk in a couple of seconds. It speaks the LocalSend protocol, so a phone already running LocalSend finds it by itself; and it serves its own browser page with a QR code for a phone running nothing at all. The PIN is compared in constant time.
• PhoneBridge is the simplest possible version of the same idea, both ways: send serves a file or a folder for the phone to download, receive serves an upload form for the phone to upload to. --once shuts it down after a single transfer.
• DeviceDock is for the bigger job of emptying a camera roll: it reads each photo’s EXIF capture date and files it by the date the shutter fired rather than the date you copied it, which is the difference between a library sorted by when things happened and one sorted by when you got round to it.
• MovePhone moves a whole phone’s files onto a new handset and keeps a ledger proving every byte arrived.

Free while we are in preview, one file each, Windows and Mac.

The cable is still fine

If you are moving fifty gigabytes of video, plug it in. USB beats Wi-Fi by a wide margin for bulk, and none of this is trying to replace that. It replaces the twenty-times-a-week case of one screenshot, where getting the cable out is the slowest part.