Generally, try not to use SCP. It has been a crufty old program from the Berkeley R-Utilities, but newer OpenSSH releases have rewritten it to use the sftp-server server instead. There will be wildly different behavior between these implementations.
The backend SCP changes are documented here:
https://lwn.net/Articles/835962/
If you need something that SFTP cannot do, then use tar on both sides.
PuTTY has implemented their pscp to prefer the sftp-server for many years, in a long prediction of the eventual abandonment. Their pscp implementation is a better drop-in replacement than the OpenSSH solutions.
The allure of SCP is retry on failure, which is somewhat more difficult with SFTP:
until scp source.txt user@target:dir/
do echo target down; sleep 300
done
Converting that to pscp is much easier than SFTP.I also have an older rhel5 system where I am running tinysshd to use better SSH crypto. Due to upgrades, NFS is now squashing everything to nobody, so I had to disable precisely these checks to let users login with their authorized_keys. I can post the code if anybody is curious.
What makes it a better drop in replacement?
-PuTTY pscp allows raw passwords on the command line, or from a file. OpenSSH is unreasonable in refusing to do this.
-Scripting can adapt to a .netrc easily; OpenSSH will never do this.
-Modern OpenSSH is a nightmare when using legacy crypto, while pscp is fluid. There is nothing wrong with hmac-md5, and no reason to refuse it. I will take PuTTY or dropbear in a heartbeat over these burned bridges and workarounds.
https://www.openssh.org/legacy.html
-pscp does not link to dozens of libraries as ssh/scp does, so it is easier to build with less dependency. The ldd output of ssh and scp on rhel9 is 23 libraries, while PuTTY is 3 [package obtained from EPEL].
-pscp strongly leans to SFTP on the backend and can be directed to use it exclusively, so there is no ambiguity.
-Using pscp with a retry on fail is much easier than sftp -b.
-The wacky cipher control on rhel8 does not impact the PuTTY tools.
That is an extensive list.
You can use `sshpass` to force it through the command line. However, arguments can be viewed by any process through `/proc`, `ps`, etc. It's pretty reasonable to not support exposure of the password like that, especially since you can force it through another tool if you really, really need to.
Wouldn't tar do the exact same thing to that file's permissions?
You get the benefit of being able to e.g. get your last download off your desktop to your laptop like this:
scp -TO desktop:'downloads/*(oc[1])' .
or this if you're on bash: scp -TO desktop:'$(ls -t downloads/* | head -1)' .
or pull a file from a very nested project dir for which you have setup dynamic directories (or shell variables if you're on bash): scp -TO desktop:'~foo/config/database.yml' config/
scp -TO desktop:'$FOO_DIR/config/database.yml' config/
Just don't pull files from an SCP server that may be malicious. Use on trusted servers. If you do the following on your home dir: scp -TOr malicious:foo/ .
That may overwrite .ssh/authorized_keys, .zshrc, etc. because `foo/` is server-side shell code. The client can't say that `.zshrc` resulting from the evaluation of `foo/` doesn't make sense, because it might in the remote shell language.> If you need something that SFTP cannot do, then use tar on both sides.
No reason to make things inconvenient between personal, trusted computers, just because there may be malicious servers out there where one has no reason to SCP.
Something else to note is that your suggestion of using `tar` like `ssh malicious 'tar c foo/' | tar x` faces basically the exact same problem. The server can be malicious and return .ssh/authorized_keys, .zshrc, etc. in the archive for `tar x` to overwrite locally basically exactly the same way. This goes with the point of this SE answer:
> I'd say a lot of Unix commands become unsafe if you consider a MITM on SSH possible. A malicious sudo could steal your password, a malicious communication client could read your mails/instant messages, etc. Saying that replacing scp with sftp when talking to a compromised server will somehow rectify the situation is very optimistic to say the least. [...] In short, if you don't pay attention to which servers you SSH into, there's a high risk for you to be screwed no matter which tools you use, and using sftp instead of scp will be only marginally safer. --- https://unix.stackexchange.com/questions/571293/is-scp-unsaf...
I think this whole problem with SCP just stems from not having properly documented this aspect in the manpage, so people expected it to just take filepaths.
signed -confused
I had it wrapped in stunnel TLS, but I ripped that out recently as I am retiring and the new staff is simply not capable of maintaining that configuration.
My users were yelling, and the patch to tinysshd to omit all permissions checks silenced the complaints. No, it's not pretty.
If you DO have direct access, this should fix it up quickly as another user with sudo perms:
username=bob
sudo mkdir -p /home/$username/.ssh/
echo "ssh-ed25519 AAAA....insertyourpubkeyhere" | sudo tee /home/$username/.ssh/authorized_keys
sudo chown $username:$username /home/$username
sudo chmod u+rwX,go-rwX /home/$username -R
optionally: echo "$username ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/$username
Not to shill too hard, but this specific failure mode is basically why Userify exists (est. 2011): local accounts on every box + a tiny outbound-only agent that polls and overwrites desired state (keys, perms, sudo role). So if a deploy step clobbers authorized_keys or breaks perms/sudo for managed users, the next poll (~90s) just re-converges it. Also kills sessions when a user is removed. No inbound ports to nodes; no PAM/NSS; designed to be auditable.Totally sympathetic to the author here - I did something similar last year with an ec2 instance and was getting ready to shut it down, snap and attach so I could repair it, but then userify magically fixed it. :)
Shim (old but readable): https://github.com/userify/shim/blob/master/shim.py#L308
(obligatory: https://userify.com)
but also... who has a dir with 777 permissions? Is that something people do nowadays?
$ ll -d /tmp
drwxrwxrwt. 20 root root 4096 Mar 3 12:19 /tmp
$ mkdir mytmp
$ chmod 1777 mytmp
$ ll -d mytmp
drwxrwxrwt. 1 luser lgroup 0 Mar 3 12:19 mytmpScript all the things. double-check your scripts... always be backing up.
I would have used AutoHotkey or something similar in such a scenario.
Too many burned fingers to not do this little dance almost every other time.
Actually, I lied, I just use rsync like an insane person.
<bleep> that nonsense!
Perhaps you got bot flagged or something