On Mon, 2021-08-30 at 10:08 +1000, Damien Miller wrote: > Hi, > > RSA/SHA1, a.k.a the "ssh-rsa" signature type is now disabled by default > in OpenSSH. > > While The SSH protocol confusingly uses overlapping names for key and > signature algorithms, this does not stop the use of RSA keys and there > is no need to regenerate "ssh-rsa" keys - most servers released in the > last five years will automatically negotiate the use of RSA/SHA-256/512 > signatures. > > This has been coming for a long time, but I do expect it will be > distruptive for some people as there are likely to be some devices > out there that cannot be upgraded to support the safer algorithms. > > In these cases, it is possible to selectively re-enable RSA/SHA1 > support by specifying PubkeyAcceptedAlgorithms=+ssh-rsa in the > ssh_config(5) or sshd_config(5) for the endpoint. > > Please report any problems here, to bugs@ or to openssh@ > > Thanks, > Damien > Just did an update to the latest snapshot and this breaks connection to one of the older hosts I still need to connect to from time to time.
Reverting this diff fixes the issue for me. According to -G it should work: $ ssh -G -oPubkeyAcceptedAlgorithms=ssh-rsa 10.255.3.242 | grep -i PubkeyAcceptedAlgorithms pubkeyacceptedalgorithms ssh-rsa But when trying it for real I get the following: martijn$ ssh -vvvv -oPubkeyAcceptedAlgorithms=ssh-rsa x.x.x.x OpenSSH_8.7, LibreSSL 3.4.0 debug1: Reading configuration data /etc/ssh/ssh_config debug2: resolve_canonicalize: hostname x.x.x.x is address debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' -> '/home/martijn/.ssh/known_hosts' debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' -> '/home/martijn/.ssh/known_hosts2' debug3: ssh_connect_direct: entering debug1: Connecting to x.x.x.x [x.x.x.x] port 22. debug3: set_sock_tos: set socket 3 IP_TOS 0x48 debug1: Connection established. debug1: identity file /home/martijn/.ssh/id_rsa type 0 debug1: identity file /home/martijn/.ssh/id_rsa-cert type -1 debug1: identity file /home/martijn/.ssh/id_dsa type -1 debug1: identity file /home/martijn/.ssh/id_dsa-cert type -1 debug1: identity file /home/martijn/.ssh/id_ecdsa type -1 debug1: identity file /home/martijn/.ssh/id_ecdsa-cert type -1 debug1: identity file /home/martijn/.ssh/id_ecdsa_sk type -1 debug1: identity file /home/martijn/.ssh/id_ecdsa_sk-cert type -1 debug1: identity file /home/martijn/.ssh/id_ed25519 type -1 debug1: identity file /home/martijn/.ssh/id_ed25519-cert type -1 debug1: identity file /home/martijn/.ssh/id_ed25519_sk type -1 debug1: identity file /home/martijn/.ssh/id_ed25519_sk-cert type -1 debug1: identity file /home/martijn/.ssh/id_xmss type -1 debug1: identity file /home/martijn/.ssh/id_xmss-cert type -1 debug1: Local version string SSH-2.0-OpenSSH_8.7 debug1: Remote protocol version 2.0, remote software version OpenSSH_5.9p1 Debian-5ubuntu1.9 debug1: compat_banner: match: OpenSSH_5.9p1 Debian-5ubuntu1.9 pat OpenSSH_5* compat 0x0c000002 debug2: fd 3 setting O_NONBLOCK debug1: Authenticating to x.x.x.x:22 as 'martijn' debug3: record_hostkey: found key type RSA in file /home/martijn/.ssh/known_hosts:4 debug3: load_hostkeys_file: loaded 1 keys from x.x.x.x debug1: load_hostkeys: fopen /home/martijn/.ssh/known_hosts2: No such file or directory debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory debug3: order_hostkeyalgs: prefer hostkeyalgs: rsa-sha2-512-cert-...@openssh.com,rsa-sha2-256-cert-...@openssh.com,rsa-sha2-512,rsa-sha2-256 debug3: send packet: type 20 debug1: SSH2_MSG_KEXINIT sent debug3: receive packet: type 20 debug1: SSH2_MSG_KEXINIT received debug2: local client KEXINIT proposal debug2: KEX algorithms: curve25519-sha256,curve25519-sha...@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16- sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,ext-info-c debug2: host key algorithms: rsa-sha2-512-cert-...@openssh.com,rsa-sha2-256-cert-...@openssh.com,rsa-sha2-512,rsa-sha2- 256,ssh-ed25519-cert-...@openssh.com,ecdsa-sha2-nistp256-cert-...@openssh.com,ecdsa-sha2-nistp384-cert-...@openssh.com,ecdsa-sha2-nistp521-cert-...@openssh.com,sk-ssh-ed25519-cert-...@openssh.com,s k-ecdsa-sha2-nistp256-cert-...@openssh.com,ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ssh-ed25...@openssh.com,sk-ecdsa-sha2-nistp...@openssh.com debug2: ciphers ctos: chacha20-poly1...@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-...@openssh.com,aes256-...@openssh.com debug2: ciphers stoc: chacha20-poly1...@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-...@openssh.com,aes256-...@openssh.com debug2: MACs ctos: umac-64-...@openssh.com,umac-128-...@openssh.com,hmac-sha2-256-...@openssh.com,hmac-sha2-512-...@openssh.com,hmac-sha1-...@openssh.com,umac...@openssh.com,umac-...@openssh.com,hmac- sha2-256,hmac-sha2-512,hmac-sha1 debug2: MACs stoc: umac-64-...@openssh.com,umac-128-...@openssh.com,hmac-sha2-256-...@openssh.com,hmac-sha2-512-...@openssh.com,hmac-sha1-...@openssh.com,umac...@openssh.com,umac-...@openssh.com,hmac- sha2-256,hmac-sha2-512,hmac-sha1 debug2: compression ctos: none,z...@openssh.com,zlib debug2: compression stoc: none,z...@openssh.com,zlib debug2: languages ctos: debug2: languages stoc: debug2: first_kex_follows 0 debug2: reserved 0 debug2: peer server KEXINIT proposal debug2: KEX algorithms: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman- group1-sha1 debug2: host key algorithms: ssh-rsa,ssh-dss debug2: ciphers ctos: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-...@lysator.liu.se debug2: ciphers stoc: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-...@lysator.liu.se debug2: MACs ctos: hmac-md5,hmac-sha1,umac...@openssh.com,hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,hmac-ripemd...@openssh.com,hmac-sha1-96,hmac-md5-96 debug2: MACs stoc: hmac-md5,hmac-sha1,umac...@openssh.com,hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,hmac-ripemd...@openssh.com,hmac-sha1-96,hmac-md5-96 debug2: compression ctos: none,z...@openssh.com debug2: compression stoc: none,z...@openssh.com debug2: languages ctos: debug2: languages stoc: debug2: first_kex_follows 0 debug2: reserved 0 debug1: kex: algorithm: ecdh-sha2-nistp256 debug1: kex: host key algorithm: (no match) Unable to negotiate with x.x.x.x port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss Same difference when using -oPubkeyAcceptedAlgorithms=+ssh-rsa, or placing it in the ssh_config(5).
