Questions about the OpenSSH implementation of the ssh protocol.

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51
votes
5answers
43k views

What is the point of sshd “UseDNS” option?

I know what it does, but i don't know why. What attack(s) does it prevent ? Is it relevant for all kind of authentication methods ? (hostbased, password, publickey, keyboard-interactive ...)
32
votes
3answers
45k views

Restricting an SSH/SCP/SFTP user to a directory

Is there a simple way to restrict an SCP/SFTP user to a directory? All methods that I've come across require me to set a chroot jail up by copying binaries, but I don't think that should be necessary. ...
29
votes
2answers
4k views

What's the purpose of the randomart image for user (not host) SSH keys?

The ssh-keygen generates the following output: The key fingerprint is: dd:e7:25:b3:e2:5b:d9:f0:25:28:9d:50:a2:c9:44:97 user@machine The key's randomart image is: +--[ RSA 2048]----+ | .o o.. ...
26
votes
1answer
17k views

OpenSSH: How to end a match block

I'm using a Match block in OpenSSH's /etc/ssh/sshd_config (on debian) to restrict some users to SFTP: # my stuff Match group sftponly X11Forwarding no AllowTcpForwarding no ForceCommand ...
23
votes
6answers
61k views

Where are my sshd logs?

I can't find my sshd logs in the standard places. What I've tried: Not in /var/log/auth.log Not in /var/log/secure Did a system search for 'auth.log' and found nothing I've set /etc/ssh/sshd_config ...
20
votes
2answers
69k views

how to connect ssh 'with' specified port?

I know how to connect 'to' a certain port when ssh'ing. ssh user@remotehostip -p XXX but is there a way to establish a ssh 'with' a certain port? I mean, is there a way to specify the port which ...
19
votes
3answers
29k views

What difference between openssh key and putty key?

I've found that ssh-keygen ("ssh" package) produces different keys from puttygen ("putty" package). If I create public and private keys with ssh-keygen some ssh-servers will not accept my keys. If I ...
19
votes
1answer
25k views

Single command to login to SSH and run program?

Is there a way to structure a single command to login via SSH to a remote server and run a program on the remote login shell? In the OpenSSH manual, it reads "If command is specified, it is executed ...
17
votes
3answers
39k views

How do I convert a ssh-keygen public key into a format that openssl PEM_read_bio_RSA_PUBKEY() function will consume?

I'm having an issue generating a public key that the openssl PEM_read_bio_RSA_PUBKEY() function can consume. I keep getting errors. Obviously I cannot simply use the ASCII string in the ssh-keygen ...
15
votes
1answer
15k views

Convert OpenSSH private key into SSH2 private key

Is there a way to convert existing pair of OpenSSH keys to the SSH2 (ssh.com format) pair of keys? UPD: since there are some answers about ssh-keygen suddenly appeared, I'll explain where I came ...
15
votes
2answers
7k views

Limits of SSH multiplexing

I have the following entry in my .ssh/config file Host AAA User BBB HostName CCC ControlMaster auto ControlPath ~/.ssh/%r@%h:%p The above allows me to multiplex multiple ssh ...
13
votes
5answers
1k views

what's the purpose of ssh-agent?

I've read the official definition: ssh-agent is a program to hold private keys used for public key authentication (RSA, DSA, ECDSA). The idea is that ssh-agent is started in the beginning of an ...
13
votes
2answers
58k views

SSH key-based authentication: known_hosts vs authorized_keys

I read about setting up ssh keys in Linux and have some questions. Correct me if I'm wrong… Let's say host tr-lgto wants to connect to host tr-mdm using ssh. If we want to be sure that it's the real ...
12
votes
4answers
7k views

Given keys in ~/.ssh/authorized_keys format, can you determine key strength easily?

~/.ssh/authorized_keys[2] contains the list of public keys. Unfortunately, each public key does not specify the key strength ( number of bits ). Is there a utility that can process this file line by ...
10
votes
2answers
13k views

Multiple 'Host *' in ssh_config?

My understanding of the way ~/.ssh/config works is that each 'Host ' line takes effect for any host matching after that point in the config file. I have a number of personal servers and work servers ...
9
votes
1answer
24k views

Installing fail2ban on CentOS 7

I am using @GarethTheRed 's answer to this question to install fail2ban on a remote CentOS 7 server. I am able to complete all the steps up until tail -f /var/log/fail2ban.log, at which point I get ...
9
votes
5answers
3k views

Use two different ip addresses per host in SSH

I have a server, named gamma, constantly up and running at work. Sometimes I connect to it from at home, in which case I use the public IP address 55.22.33.99. Sometimes, I connect to it when I'm at ...
9
votes
2answers
19k views

What do the channel numbers in ssh error message refer to?

In the example below what do the channel numbers correspond to? Which are on the server? Which are on the client? $ ssh -L1570:127.0.0.1:8899 root@thehost Password: Last login: Fri Aug 9 ...
9
votes
3answers
44k views

How to ssh to remote server using a private key?

I have two servers. Both servers are in CentOS 5.6. I want to SSH from Server 1 to Server 2 using a private key I have (OpenSSH SSH-2 Private Key). I don't know how to do it over unix. But what I did ...
9
votes
2answers
3k views

Why not simply use %h in OpenSSH ssh's ControlPath option?

Why do the "ssh_config(5)" manpages recommend that the ControlPath option should contain at least the %h, %p and %r placeholders in order to uniquely identify each shared connection? I thought that ...
9
votes
3answers
27k views

ssh, start a specific shell, and run a command on the remote machine?

I am in a situation where several users are sharing the same user account on a remote machine. I have a "personal" directory where I wrote my own .zshrc file, and I would like to have a way to: ...
9
votes
4answers
6k views

Local, timestamped logging of all ssh commands?

How can I keep a local, timestamped record of all remote commands I use in ssh (command-line openssh client started through bash)? Requirements: Essential: 100% client-side without relying on ...
7
votes
2answers
2k views

Should I be backing up my SSH host keys?

In my automated backups, I'm not sure if I should be doing this. If the hard drive fails or if I wipe the hard disk and start over, what will be the effect of starting with a new SSH host key to my ...
7
votes
2answers
4k views

Can't login to a remote machine with key

I used to login to a remote machine( I have to root of this machine) using key. Both my local machine and remote machine is in f23. From last few days, I can't login to this machine using key. It is ...
7
votes
2answers
825 views

Determine dynamically allocated port for OpenSSH RemoteForward

Question (TL;DR) When assigning ports dynamically for remote forwarding (a.k.a. -R option), how can a script on the remote machine (for instance sourced from .bashrc) determine which ports were ...
7
votes
3answers
621 views

OpenSSH - map aliases in ~/.ssh/config to IP addresses in command line

Is it possible to query ~/.ssh/config in the command line? Let's say I would like to retrieve what IP address does the particular alias point to in a separate program, is it possible?
7
votes
1answer
4k views

how to set the sftp -s subsystem option in a configuration file

The sftp command supports a subsystem option (-s) which allows the remote user to select the remote sftp-server executable, and optionally upgrade to sudo in the process like so; sftp -s ...
6
votes
4answers
961 views

SSH “sleeping” for a long time before connect? [duplicate]

Possible Duplicate: How to speed my too-slow ssh login? I have a very strange issue with my SSH server. Whenever I attempt to connect, it literally waits about 30 seconds to a minute before ...
6
votes
5answers
2k views

ssh command unexpectedly continues on other system after ssh terminates

I'm running the below command, and monitoring the output file on the other system: ssh $ip_address 'for n in 1 2 3 4 5; do sleep 10; echo $n >>/tmp/count; done' If I kill the ssh command ...
6
votes
2answers
8k views

making ssh hosts global to all the users on the computer

So SSH has these files that configure settings for a specific user. ~/.ssh/authorized_keys ~/.ssh/config ~/.ssh/id_rsa ~/.ssh/id_rsa.pub ~/.ssh/known_hosts I'd like to globalise some of these ...
6
votes
2answers
3k views

Can ssh_config's ProxyCommand run a local command before connecting to a remote machine?

Before I can to connect to a particular remote machine I have to run a certain local command. So instead of ssh me@remote.machine I have to do local_command ssh me@remote.machine I would like to ...
6
votes
1answer
6k views

sFTP server fails to start

I'm having problems getting sFTP working while there are no problems with ssh. I'm basically building zlib, openssl, and openssh for an ARM processor using an existing embedded Linux filesystem. After ...
6
votes
1answer
2k views

All about ssh ProxyCommand

I am looking for an in-depth explanation of the following ProxyCommand, down to the nuts and bolts of its operation, please. Can you kindly completely dissect it for me and improve on it if you can? ...
6
votes
3answers
3k views

How to prevent “Last Login:” message from showing up when using sftp?

In Ubuntu GNU/Linux 12.04, I have a user johndoe that is part of an sftponly group, set up to sftp to a chroot jail using Subsystem sftp internal-sftp Match Group sftponly ChrootDirectory %h ...
6
votes
2answers
3k views

Force ssh to always use SSH_ASKPASS

I want to force ssh to always use the program specified by SSH_ASKPASS. In my case the program is just a bash script returning the passphrase. The problem is, as the ssh documentation says: If ...
5
votes
2answers
844 views

Why doesn't the ssh command follow RFC on URI?

The RFC: Uniform Resource Identifier (URI) Scheme for Secure File Transfer Protocol (SFTP) and Secure Shell (SSH) presents the SSH URI as: ssh://[<user>[;fingerprint=<host-key ...
5
votes
1answer
2k views

How to setup port redirection after a ssh connection has been opened?

I'm using openssh on both client and server. I know I can run ssh -L or ssh -R to setup some port redirections over ssh. However, since they are command-line parameters, such redirections must be ...
5
votes
3answers
437 views

Why are rules not combining in an ssh config file?

It seems like the following would work as expected, i.e., that the second rule, having a hostname that matches the first rule, would apply it. Host *.hostname.com User myuser IdentityFile ...
5
votes
1answer
128 views

Can an OpenSSH server forward inbound traffic to another server?

In my current work setup, I have: Windows 7 laptop Ubuntu virtual host / SSH Windows 2k8 guest I'd like to be able to RDP from my laptop into the Windows guest without having to open additional ...
5
votes
2answers
2k views

How do I list available host key algorithms for an SSH client?

So ssh has the option HostKeyAlgorithms. Sample usage: ssh -o "HostKeyAlgorithms ssh-rsa" user@hostname I'm trying to get the client to connect using the servers ecdsa key, but I can't find what ...
5
votes
3answers
4k views

SSH access from inside and outside a LAN using the same terminal command

I have a Raspberry Pi (RPi) and I am making remote connections to it using ssh. I have managed to set up ssh correctly such that I can access the RPi both from a local area network and from the ...
5
votes
1answer
1k views

Public Key Auth + Kerberos

I've setup public key authentication for ssh'ing into my university machine. However, it only logs me in on the local machine and doesn't give me Kerberos credentials which I need for accessing my ...
5
votes
1answer
202 views

Can I get SSH to use an askpass program even if it was run from a terminal?

man ssh says: SSH_ASKPASS If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh does not have a terminal associated with ...
5
votes
1answer
2k views

Generate own (stronger) RSA host key for OpenSSH?

I would like to replace the default 2048 bit host key generated when installing OpenSSH or starting it the first time with one of 4096 bit length. This leads to two questions: Does OpenSSH allow ...
5
votes
1answer
920 views

Avoiding duplicate entries in ~/.ssh/known_hosts

I have my DNS search path set to ".intranet", ie, /etc/resolv.conf contains: search intranet When I SSH to a host using a FQDN, like "mailserver.intranet", it adds an entry for ...
5
votes
2answers
68 views

options override for openssh client configuration

Since I want to protect my ssh connections, I set some global cipher suite options to restrict set of used algorithms. But recently I've encountered a server which doesn't support some of those ...
5
votes
1answer
3k views

How to exclude from a “Match Group” in SSHD?

There is a Match group in the SSHD config: cat /etc/ssh/sshd_config ... Match Group FOOGROUP ForceCommand /bin/customshell ... There are many users on the machine that are in the ...
5
votes
3answers
87 views

How to make SSH's `ControlPath` distinguish between IPv4 and IPv6?

I'm using ControlMaster auto ControlPath ~/.ssh/tmp/%l_%r@%h:%p in my ~/.ssh/config to open multiple sessions using the same connection when accessing a host via SSH. That works fine for ...
5
votes
2answers
5k views

sshd process inoperative

I have an AIX server which suddenly stopped servicing SSH connections. When I try to start the service through startsrc -s sshd it says: 0513-059 The sshd Subsystem has been started. Subsystem PID is ...
5
votes
2answers
3k views

ssh-add -D doesn't erase stored keys from the ssh-agent

How come this happens?: stan@tcpc:~/.ssh$ ssh-add -l 8192 e0:45:5e:cc:45:3e:17:2b:a6:54:6f:8d:53:1b:j2:e3 github (RSA) 2048 25:41:53:a6:45:5d:ac:eb:5c:45:f8:ce:42:a9:he:aa BITBUCKET (RSA) ...