Repair Synology DSM5.1 for use as a Linux backup repository.

After updating my Synology to DSM 5.1-5004, the following morning I was greeted by a rash of error messages from my Veeam B&R 7 backup jobs: "Error: Server does not support diffie-hellman-group1-sha1 for keyexchange"

I logged into the backup host and re-ran the repository resync process, to be greeted by the same error.

Synology DSM 5.1 error

The version of SSH on the Synology was OpenSSH 6.6p2:

As it turns out, this version of SSH doesn't enable the required key exchange protocol by default; luckily, that's an easy edit of the /etc/ssh/sshd_config file. And to play it safe, I added not only the needed Kex parameter, I also added the published defaults.

KexAlgorithms diffie-hellman-group1-sha1,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1

After restarting SSH in the DSM control panel, then re-scanning the repository, all was not quite fixed:

Back to the manfile for sshd_conf...

The list of supported ciphers is impressive, but rather than add all of them into the list, I thought it would be useful to get a log entry from the daemon itself as it negotiated the connection with the client. Unfortunately, it wasn't clear where it was logging, so it took some trial-and-error with the config settings before I found a useful set of parameters:

SyslogFacility USER
LogLevel DEBUG

At that point, performing a rescan resulted in an entry in /var/log/messages:

Armed with that entry, I could add the Ciphers entry in sshd_conf, using the options from the Veeam ssh client to the defaults available in this version of sshd:

Ciphers aes128-cbc,blowfish-cbc,3des-cbc,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com

One more rescan, and all was well, making it possible to retry the failed jobs.

Follow Up

There have been responses of both successes and failures from people using this post to get their repository back on line. I'm not sure what's going on, but I'll throw in these additional tips for editing sshd_config:

1. Each of these entries (KexAlgorithms and Ciphers) are single line entries. You must have the keyword—case sensitive— followed by a single space, followed by the entries without whitespace or breaks.
2. There's a spot in the default sshd_config that "looks" like the right place to put these entries; that's where I put them. It's a heading labelled "# Ciphers and keying." Just drop them into the space before the Logging section. In the screenshot below, you can see how there's no wrap, no whitespace, etc. This works for me.
   
3. Restart the SSH service. You can use the command line (I recommend using telnet during this operation, or you'll loose your SSH connection as the daemon cycles) or the GUI control panel. If using the latter, uncheck SSH, save, check SSH.