Table of Contents

SSH Config

No matching key exchange method

Problem connecting from a Debian 11 Bullseye to an old OpenWRT Chaos Calmer: 

ssh root@192.168.31.45
Unable to negotiate with 192.168.31.45 port 22:
    no matching key exchange method found.
    Their offer: diffie-hellman-group14-sha1,diffie-hellman-group1-sha1,kexguess2@matt.ucc.asn.au

You can edit your $HOME/.ssh/config file and put ah Host stanza like this: 

Host hostname hostname.domain.tld 192.168.31.45
    KexAlgorithms +diffie-hellman-group1-sha1

This will force the use of diffie-hellman-group1-sha1 key exchange method, which is considered less secure nowday.

"no matching host key type" and "no mutual signature algorithm"

Problem connecting from an Android environment with Termux version 0.118.0 (February 2022) to an old Debian 6 Squeeze: 

Unable to negotiate with 192.168.0.250 port 22:
  no matching host key type found. Their offer: ssh-rsa,ssh-dss

So the first option to add is HostKeyAlgorithms, but there is another and more subtle problem, because no error message is printer unless you add the -v option (for verbose), 

debug1: send_pubkey_test: no mutual signature algorithm
debug1: No more authentication methods to try.
root@192.168.0.250: Permission denied (publickey).

So also the PubkeyAcceptedKeyTypes is required: 

ssh -o HostKeyAlgorithms=+ssh-rsa -o PubkeyAcceptedKeyTypes=+ssh-rsa root@192.168.0.250