Re: Can't connect with some clients

[Jakub Jelen <jjelen@xxxxxxxxxx>]

On 1/12/21 4:09 PM, Emiliano Gonzalez wrote:
> I have a ssh server.
>
> When I connect with the client on Fedora 32 ssh there are no problems.
>
> With PuTTY: Release 0.74 I can't connect with error: Couldn't agree a 
> host key algorithm (available: rsa-sha2-512, rsa-sha2-256)

That sounds like an issue of PuTTY, that it can not use the RSA SHA2 
hostkey algorithms from RFC8332.

As a workaround, you can configure your server with different hostkeys 
(ed25519) so incompatible clients can connect too.

>  From Centos 7 ssh server crash. In packet.c (line 1862):
>
> |/* For smaller blocks use limit of 1 GB as recommended in RFC4253 */ 
> cipher->max_blocks = ((uint64_t)1 << 30) / cipher->blocksize; |
>
> cipher->blocksize is 0;

What cipher is used for this connection? There is no cipher that would 
have blocksize == 0.

Can you get a debug log?

Regards,
--
Jakub Jelen
Senior Software Engineer
Crypto Team, Security Engineering
Red Hat, Inc.