Hosts with only OpenSSH: sftp://

sftp://[user@]host[:port]/path is a directory on a host where rcp is not installed but OpenSSH is: the endpoint opens ssh host -s sftp and speaks SFTP version 3 to the subsystem, one session per worker, with reads kept eight deep and writes sixteen deep. Everything above works across it: copy, sync, verify, scan, delta (the signing and matching run on this side, since the far side is only a filesystem). What the protocol cannot say is refused or approximated in the open: no inode numbers, so a hard link on an sftp source is a second copy of the file and one bound for an sftp destination is written as a file (hardlink@openssh.com is used where the server offers it); times to the second (verify accepts a whole-second destination against a source with nanoseconds); no holes, so runs of zeros are detected on read and skipped on write; no extended attributes, so every one is reported refused. The trust model is ssh’s, as for ssh://, and the same RCP_SSH hook reaches a stand-in for the suite.

Each read and write is the size the server says it takes. A server that offers limits@openssh.com (OpenSSH 8.7 and later) is asked, and OpenSSH answers 256 KiB less 1024. One that does not gets 32 KiB, which every version 3 server must accept and which OpenSSH’s own client uses in the same case. -loglevel DEBUG prints the sizes in use.

This used to be 256 KiB for every request, on the belief that it was the largest read or write OpenSSH accepts. OpenSSH’s 256 KiB limit is on the whole message, header included, so every file of 256 KiB or more failed against a real server. Measured against OpenSSH 10.3: 262,100 bytes crossed and 262,144 did not. The suite’s stand-in is paramiko, which accepts messages of up to 16 MB, so it could not see the problem. It now enforces OpenSSH’s limit and offers the extension as OpenSSH does, and it can also leave the extension out, as a server older than 8.7 does.

A session that ends mid-file is a failure the run reports. ERROR names the file, the run ends STATUS: FAILED, and the catalog keeps what completed for rcp resume. Before, a server that dropped the connection killed the run on its next write: exit 141, no message.