croc – another file transfer method

I have lost count of the number of times I have had to send a large file to someone at work, usually in a hurry. I’ve used Dropbox, ownCloud, Firefox Send (no longer available) etc. Transferring large files became a bit easier when e-mail service providers increased the size limit for attachments, but that is still not a solution for very large files. The xkcd cartoon FILE TRANSFER sums up the situation nicely.

I recently discovered the command line utility croc, which the author claims is a way to ‘easily and securely transfer stuff from one computer to another.’ I thought I’d give it a try, if only to have another tool to fall back on in an emergency. It does rely on both ends having croc installed, but hopefully that should not be a show-stopper as croc is available for Linux, Windows, macOS and BSD. To quote the author:

croc differs from a utility like scp because it doesn’t require any two computers to have enabled port-forwarding. Instead, croc will uses a relay – a temporary server setup locally (if both computers are on lan) or publicly (default is at Any two computers can connect to the relay, and after securing their channel with PAKE [password authenticated key exchange], they can transfer encrypted metadata and data through the relay. The relay works by first having the computers communicate the PAKE protocol via websockets, and then exchanging encrypted metadata, and then stapling the TCP connections directly so that they can transfer directly.

So, to use croc you will be dependent on the public relay provided by the author unless you set up your own relay (instructions are provided in the author’s original 2018 blog post introducing croc – see link above – and in various third-party articles about croc, such as ‘Securely Transfer Files and Folders Between Computers Using Croc‘ and ‘Transfer Files And Folders Between Computers With Croc‘).

Anyway, I installed croc in Lubuntu and Gentoo Linux from the author’s GitHub repository and indeed it is easy to use and works fine. The binary releases for the various OSs and Linux distributions can be found on the Releases page of the GitHub repository or via the OS package manager.

Lubuntu 20.10:

user $ wget
user $ sudo dpkg -i croc_9.1.6_Linux-64bit.deb

Gentoo Linux:

root # emerge net-misc/croc

(Note that croc ebuilds are not currently marked as Stable in the Gentoo Linux Portage tree, so you’ll have to unmask them by keyword if you are using the Stable branch.)


I even installed croc in Termux on my Samsung Galaxy Note 20 Ultra 5G, and it works in Android too:

$ pkg install croc

Other OSs and other Linux distributions:

See the instructions in the README file online.

Using croc

Using croc is as simple as entering a command on one computer, informing (via e-mail, telephone, SMS, Signal or other social media) the person using the other computer of the command to use, and entering that command on the other computer. For example:


user $ croc send Documents/flight-times.ods
Sending 'flight-times.ods' (16.6 kB)
Code is: 8878-salary-courage-roger
On the other computer run

croc 8878-salary-courage-roger


user $ croc 8878-salary-courage-roger
Accept 'flight-times.ods' (16.6 kB)? (Y/n) 

If the receiving user then enters ‘Y’, the sending user sees something similar to this:

user $ croc send Documents/flight-times.ods
Sending 'flight-times.ods' (16.6 kB)
Code is: 8878-salary-courage-roger
On the other computer run

croc 8878-salary-courage-roger

Sending (->
 100% |████████████████████| (17/17 kB, 10.918 MB/s)
user $ 

and the receiving user sees something similar to this:

user $ croc 8878-salary-courage-roger
Accept 'flight-times.ods' (16.6 kB)? (Y/n) Y

Receiving (<-[::1]:39442)
 100% |████████████████████| (17/17 kB, 3.989 MB/s)
user $ 

The observant reader will notice that the above example shows a file being transferred on the same computer. When transferred between different computers the IP addresses of each computer will be displayed instead. I have used croc to transfer files between different computers on my home network (I would normally just use my NAS for this, though), between remote computers on the Internet, and between my computers and my phone via mobile broadband, and croc works in all cases.

I have not mentioned all croc’s features. I’ll leave you to read up on croc in more detail in the links I’ve given above. It looks like it might be a useful tool to have installed.


About Fitzcarraldo
A Linux user with an interest in all things technical.

One Response to croc – another file transfer method

  1. Not Given says:

    Try Google Drive. Then just go to File->Share. If the other party doesn’t have a Gmail account then I normally choose to the option to “share with anyone who has the link.”

    By the way, the same user name and password that work for Gmail work for Google Drive.

    Microsoft OneDrive has worked well for me in the past as well.

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