$ ssh-keygen -t ed25519 -f ./testkey -C testkey
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in ./testkey
Your public key has been saved in ./testkey.pub
The key fingerprint is:
SHA256:TjoOlsc/EIGNjnkfNcIcme188Dn/o19IJ5cLkXb4qCs testkey
The key's randomart image is:
+--[ED25519 256]--+
|     *.=         |
|    o O =     o  |
|   +   * + . = . |
|  o o o o = . = .|
|   . . oS. o oooo|
|     oo+    o..=.|
|    + =..  . ....|
|   . + o.E  . o. |
|      . .....o.. |
+----[SHA256]-----+
$ cat ./testkey.pub 
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIBugIbEgGArl/mdkOGuYIEXtdGqEl1OhUZnEuF3JPbhb testkey
$ rm ./testkey.pub 
$ ssh-keygen -e -f ./testkey | tee ./testkey.pub
---- BEGIN SSH2 PUBLIC KEY ----
Comment: "256-bit ED25519, converted by matoken@t430s from OpenSSH"
AAAAC3NzaC1lZDI1NTE5AAAAIBugIbEgGArl/mdkOGuYIEXtdGqEl1OhUZnEuF3JPbhb
---- END SSH2 PUBLIC KEY ----

tailscale経由で接続しようとすると失敗

$ ssh ${IP_ADDRESS} 
kex_exchange_identification: read: Connection reset by peer
Connection reset by ${IP_ADDRESS} port 22

登録していないipアドレスから接続しようとしたのでエラー．

変更が少ないならtailscaleで利用しているアドレスを /etc/hosts.allow に登録．

$ tailscale status | awk '{print $1}' | xargs -I{} echo 'sshd: {}' | sudo tee -a /etc/hosts.allow

台数が多かったり，増減がよくあるようならtailscaleの場合ここ( https://tailscale.com/kb/1015/100.x-addresses/ )にアドレス範囲があったので， /etc/hosts.allow に以下を追記．

sshd: 100.64.0.0/255.192.0.0

firewallや sshd_config(5) で Match Address 等を設定している場合はそちらも設定．

Match Address 100.64.0.0/10

こんな感じで怒られた．実際は62文字の長いホスト名

unix_listener: path "/home/matoken/.ssh/tmp/ssh-user@hostname:22.r3NYV60KUEAXUuRZ" too long for Unix domain socket

hostnameが長くて， ControlPath で設定したファイルの作成時にファイルシステムの制限で失敗しているよう．

とりあえず ssh のオプションで -o ControlMaster=no してみると繋がる

~/.ssh/config の対象hostで ControlMaster no とかしてみたけど効かない?

Host TARGETHOST
  ControlMaster no
  ControlPath no

man 5 ssh_config から %C で %l%h%p%r のハッシュになるようなので

     ControlPath
             Specify the path to the control socket used for connection sharing as described in the ControlMaster section above or the string none to disable connection sharing.  Arguments to ControlPath may use the tilde
             syntax to refer to a user's home directory, the tokens described in the TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section.  It is recommended that any ControlPath used
             for opportunistic connection sharing include at least %h, %p, and %r (or alternatively %C) and be placed in a directory that is not writable by other users.  This ensures that shared connections are uniquely
             identified.

TOKENS
     Arguments to some keywords can make use of tokens, which are expanded at runtime:

           %%    A literal ‘%’.
           %C    Hash of %l%h%p%r.
           %d    Local user's home directory.
           %f    The fingerprint of the server's host key.
           %H    The known_hosts hostname or address that is being searched for.
           %h    The remote hostname.
           %I    A string describing the reason for a KnownHostsCommand execution: either ADDRESS when looking up a host by address (only when CheckHostIP is enabled), HOSTNAME when searching by hostname, or ORDER when
                 preparing the host key algorithm preference list to use for the destination host.
           %i    The local user ID.
           %K    The base64 encoded host key.
           %k    The host key alias if specified, otherwise the original remote hostname given on the command line.
           %L    The local hostname.
           %l    The local hostname, including the domain name.
           %n    The original remote hostname, as given on the command line.
           %p    The remote port.
           %r    The remote username.
           %T    The local tun(4) or tap(4) network interface assigned if tunnel forwarding was requested, or "NONE" otherwise.
           %t    The type of the server host key, e.g.  ssh-ed25519.
           %u    The local username.

     CertificateFile, ControlPath, IdentityAgent, IdentityFile, KnownHostsCommand, LocalForward, Match exec, RemoteCommand, RemoteForward, and UserKnownHostsFile accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p,
     %r, and %u.

     KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.

     Hostname accepts the tokens %% and %h.

     LocalCommand accepts all tokens.

     ProxyCommand accepts the tokens %%, %h, %n, %p, and %r.

これを ~/.ssh/config に設定してみると( ControlPath ~/.ssh/tmp/%C )こんな感じの a1e724af86ad309c968737a2fb73a3f1c0298e05 ハッシュのファイル名になってとりあえず解決した．人間には優しくないけどそう見るものではないのでいいかな?

古い機器で短い鍵しか使えないなどといったときにどうにか出来ないかなって感じのときに?

※この方法はOpenSSH 7.2p2 では行けるらしいが，OpenSSH_7.6p1, OpenSSH 8.1p1ではNGだった． changelogを確認すると7.1/7.1p1からNGぽいけど7.2p1で動く? https://www.openssh.com/txt/release-7.1 * Refusing all RSA keys smaller than 1024 bits (the current minimum is 768 bits).

サポートされていない短い鍵を作ろうとすると作れない

$ ssh-keygen -t rsa -b 768
Invalid RSA key length: minimum is 1024 bits

opensslで作れる

$ openssl genrsa -out key.pem 768
Generating RSA private key, 768 bit long modulus (2 primes)
........+++++++
................................+++++++
e is 65537 (0x010001)

パスフレーズありの場合

$ openssl genrsa -out id_rsa768 -aes256 768 
Generating RSA private key, 768 bit long modulus (2 primes)
..+++++++
..........+++++++
e is 65537 (0x010001)
Enter pass phrase for id_rsa768:
Verifying - Enter pass phrase for id_rsa768:

秘密鍵の確認

$ openssl rsa -text < key.pem

対応する公開鍵を作る
作れなかった．7.2p2では行けるらしいが今回試しているのは8.1p1

$ ssh-keygen -y -f key.pem > key.pub
Load key "key.pem": Invalid key length

公開鍵もopensslで作る

$ openssl rsa -pubout < key.pem > key.pub
writing RSA key

公開鍵の確認

$ openssl rsa -text -pubin < key.pub

しかし実際使おうとすると使えない．

$ ssh -v -i ~/.ssh/key.pem localhost
   :
debug1: Trying private key: /home/matoken/.ssh/key.pem
Load key "/home/matoken/.ssh/key.pem": Invalid key length

• Settable minimum RSA key sizes on the client end for legacy devices\.
  • Settable minimum RSA key sizes on the client end for legacy devices\.

検証中

$ openssl ecparam -genkey -name prime256v1
-----BEGIN EC PARAMETERS-----
BggqhkjOPQMBBw==
-----END EC PARAMETERS-----
-----BEGIN EC PRIVATE KEY-----
MHcCAQEEII/p1OH2ZQ6B6k1CRk1WHr4iPxOuv+N/DzK0znQ6DWpAoAoGCCqGSM49
AwEHoUQDQgAEhO6Eb8d+0awb9XXFBGcBH3C7QPh+LfZDOtpzIeAoiektPlJOVDff
CeV/6xhBDuYuVkdZVen1JIrcP4mjhCpZzA==
-----END EC PRIVATE KEY-----
$ openssl ecparam -genkey -name secp521r1
-----BEGIN EC PARAMETERS-----
BgUrgQQAIw==
-----END EC PARAMETERS-----
-----BEGIN EC PRIVATE KEY-----
MIHcAgEBBEIAt6966Pl0MVeg629y+0SKPtXB1Y0jXVT1SRNrKPIrs22tbkmxGsTZ
H6l3BxbdB9A6cvQggW+hNLO2FhsptOb7/2agBwYFK4EEACOhgYkDgYYABAEm6obx
je5KY9CgO60nvX/2AsTqMyJNcqm5yOXS7Y7CWldu2fvxTz87n5E17hzNb/Eob+VD
5trMIGbUht+gjRwW9AC4seiIdZp9/pVEKVXIgL25IsqvPg7DRKRJXgYloDtpjKPV
mMH96l9GloIV295s/l7VK9MweghstzyxhOiLfNGjtA==
-----END EC PRIVATE KEY-----

検証中

$ openssl genpkey -algorithm ed25519

up14-sha1

古いマシンにつなごうとしてアルゴリズムが対応していない．

snippet.bash

$ ssh 192.168.1.182                                                                      
Unable to negotiate with 192.168.1.182 port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1,diffie-hellman-gro
up14-sha1                                                 

オプションでアルゴリズムを指定して繋ぐ

snippet.bash

$ ssh -o KexAlgorithms=diffie-hellman-group1-sha1,diffie-hellman-group14-sha1 root@192.168.1.182

繋いでみると Angstrom の OpenSSH 6.0p1 だった

snippet.bash

# ssh -V
OpenSSH_6.0p1, OpenSSL 1.0.1e 11 Feb 2013
# lsb_release -a
Distributor ID: Angstrom
Description:    Angstrom GNU/Linux v2012.12 (Core edition)
Release:        v2012.12
Codename:       Core edition

$ ssh localhost 
kex_exchange_identification: read: Connection reset by peer
Connection reset by ::1 port 22

::1 ってことは ipv6 でつなぎに行こうとして失敗していそう -4 付けたら繋がった．

hosts.allow の 127.0.0.1 を LOCALHOST にして sshd_config の Match Address ::1 を追記して繋がるように

https://nitter.matoken.org/matoken1/status/1495779062060498945#mb