SSH

Server configuration paramenters

• file to edit

/etc/ssh/sshd_config

• Allowed users

AllowUsers username username@ipaddress user@hostname

• Client keep alive

ClientAliveInterval 6000

• subsystems

Subsystem       sftp    /usr/lib64/misc/sftp-server

• X11 options

X11Forwarding no
X11DisplayOffset 10
X11UseLocalhost yes

• Authentication
  • disable tunneled clear text passwords

PasswordAuthentication no

PermitEmptyPasswords no
RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile     .ssh/authorized_keys

• • use host keys in /etc/ssh/ssh_known_hosts

RhostsRSAAuthentication no

• • similar for protocol version 2

HostbasedAuthentication no

Client configuration paramenters

System wide

don't just put all of them in your config file, the list is just informational

/etc/ssh/ssh_config

• Timeout (keep alive)

ServerAliveInterval 50

1. Host *
2. ForwardAgent no
3. ForwardX11 no
4. RhostsRSAAuthentication no
5. RSAAuthentication yes
6. PasswordAuthentication yes
7. HostbasedAuthentication no
8. GSSAPIAuthentication no
9. GSSAPIDelegateCredentials no
10. BatchMode no
11. CheckHostIP yes
12. AddressFamily any
13. ConnectTimeout 0
14. StrictHostKeyChecking ask
15. IdentityFile ~/.ssh/identity
16. IdentityFile ~/.ssh/id_rsa
17. IdentityFile ~/.ssh/id_dsa
18. Port 22
19. Protocol 2,1
20. Cipher 3des
21. Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
22. MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160
23. EscapeChar ~
24. Tunnel no
25. TunnelDevice any:any
26. PermitLocalCommand no
27. VisualHostKey no
28. ProxyCommand ssh -q -W %h:%p gateway.example.com

User Side

• config file ~/.ssh/config.

• matches wildcard domain

Host *.porcelinux.com

• match any host in the 192.168.0.[0-9] network

Host 192.168.0.?

• matches any 10.0.0.0/8

host 10.*

Host foo
HostName 192.168.0.1
Port 2200
User username

• specify different key

Host 192.168.1.*
IdentityFile KeyFile

• fix the

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

if you know what you're doing, the following command will remove the old known_host keys from the .ssh/known_hosts file

ssh-keygen -f "/home/username/.ssh/known_hosts" -R "computername"

Tunnel

• Col comando qui sotto è possibile creare un tunnel dal server (dietro il firewall) al client attraverso cui far passare in senso contrario la connessione al servizio che ci occorre, in questo caso 'telnet'.

Sul server lanciare il comando per creare il tunnel fino al nostro client:

$ ssh -f -N -R 2333:localhost:23 guest@80.100.100.100

• 2333 porta usata sul client
• 23 porta del servizio sul server (telnet)
• guest è un utente presente sul client
• 80.100.100.100 indirizzo IP (su internet) del client

• Il server si collega con il client usando ssh e gli dice di redirigere le richieste alla porta 2333 sul client alla porta 23 del server.
  • Se va tutto bene verrà chiesta la password per accedere sul client come utente guest (a meno che ci sia autenticazione con chiave criptata).

• Ora sul client è possibile collegarsi via telnet al server:

$telnet localhost 2333

(ssh -f -N -R 2333:localhost:22 user@dyndns-ip)

ssh -f -N -R 2333:localhost:22 -p22251 user@dyndns-ip

X11 forwarding

• dependencies

dnf install xorg-x11-xauth
...
apt install xorg-x11-xauth

• forwarding X11 from client to local machine

ssh -X user@host

• insecure

ssh -X -Y ...

putty

• ssh client for Windows

http://www.putty.org

putty -D 8080 -P 443 -ssh homeIP

Troubleshooting

send_pubkey_test: no mutual signature algorithm

• must re-enable ssh-rsa support to connect to server

add to client's config:

PubkeyAcceptedKeyTypes +ssh-rsa