service: Add scp support

[Xuanwo]

It's possible for OpenDAL to read/write data via scp over ssh. This issue seems cool for users who want to connect a ssh server and read/write data between them.

[Xuanwo]

Cc @NobodyXu and @silver-ymz for ideas. Can we use openssh for this task? Are there any obstacles preventing us from doing so?

[NobodyXu]

Can we use openssh for this task? Are there any obstacles preventing us from doing so?

I'm not familiar with scp but it should be possible.

Though it should be noted that it is already deprecated.

[Xuanwo]

Though it should be noted that it is already deprecated.

Do you mean scp is already deprecated? Or others?

[silver-ymz]

SCP is designed for single file transfers, which doesn't maintain a continuous interactive session like SFTP. As result, it will exist following problems.

• We must build SCP session for each transfer. Although openssh provides ControlMaster feature to reduce the cost of building ssh session, SCP performance will be much lower than SFTP
• SCP doesn't support transferring ranges of data within a file or metadata. We can only implement basic read/write operation.

I'd like to complete this feature. However, I am currently undergoing military training at school. Maybe, I can begin to complete it in next weekend.

[silver-ymz]

SCP is already deperated by openssh community due to many security issues, like CVE-2019-6111, CVE-2020-15778. Please see the detailed report here. https://lwn.net/Articles/835962/

Since OpenSSH 8.8, scp has use the SFTP protocol for transfers by default.

[Xuanwo]

Got it, seems we don't a separate scp service anymore

[Xuanwo]

Thank you all for the advice and research!