Otdfbt

Question

Here's my situation: I'm setting up a test harness that will, from a central client, launch a number of virtual machine instances and then execute commands on them via ssh. The virtual machines will have previously unused hostnames and IP addresses, so they won't be in the ~/.ssh/known_hosts file on the central client.

The problem I'm having is that the first ssh command run against a new virtual instance always comes up with an interactive prompt:

The authenticity of host '[hostname] ([IP address])' can't be established.
RSA key fingerprint is [key fingerprint].
Are you sure you want to continue connecting (yes/no)?

Is there a way that I can bypass this and get the new host to be already known to the client machine, maybe by using a public key that's already baked into the virtual machine image ? I'd really like to avoid having to use Expect or whatever to answer the interactive prompt if I can.

For a test environment which is self-contained and physically secure, automated key acceptance may work just fine. But automatically accepting public keys in a production environment or across an untrusted network (such as the Internet) completely bypasses any protection against man-in-the-middle attacks that SSH would otherwise afford. The only valid way to make sure you're secure against MITM attacks is to verify the host's public key through some out-of-band trusted channel. There is no secure way to automate it without setting up a moderately complicated key signing infrastructure. — Feb 15 '18 at 20:04

Jason 1356 · Accepted Answer · 2014-10-03 09:53:21Z

140

Set the StrictHostKeyChecking option to no, either in the config file or via -o :

ssh -o StrictHostKeyChecking=no username@hostname.com

edited Oct 3 '14 at 9:53

Jason

1356

answered Apr 16 '10 at 4:34

Ignacio Vazquez-Abrams

39.8k46377

60

This leaves you open to man in the middle attacks, probably not a good idea.

– JasperWallace
Sep 23 '13 at 7:23

9

@JasperWallace, while this is usually good advice, the specific use case (deploying test VMs and sending commands to them) should be safe enough.

– Massimo
Oct 21 '14 at 17:33

8

This gives a Warning: Permanently added 'hostname,1.2.3.4' (RSA) to the list of known hosts. To avoid the warning, and to avoid the entry being added to any known_hosts file, I do: ssh -o StrictHostKeyChecking=no -o LogLevel=ERROR -o UserKnownHostsFile=/dev/null username@hostname.com

– Peter V. Mørch
May 21 '15 at 9:19

9

Downvoting as this does not answer the question and opens to serious security vulnerabilities.

– marcv81
Jan 20 '16 at 4:48

11

@Mnebuerquo: If you were worried about security then you wouldn't have anything at all to do with this question. You'd have the correct host key in front of you, gathered from the console of the system you wanted to connect to, and you would manually verify it upon first connecting. You certainly wouldn't do anything "automatically".

– Ignacio Vazquez-Abrams
Jun 15 '16 at 17:31

|
show 6 more comments

score 218 · Accepted Answer · 2011-09-27 22:14:06Z

218

IMO, the best way to do this is the following:

ssh-keygen -R [hostname]
ssh-keygen -R [ip_address]
ssh-keygen -R [hostname],[ip_address]
ssh-keyscan -H [hostname],[ip_address] >> ~/.ssh/known_hosts
ssh-keyscan -H [ip_address] >> ~/.ssh/known_hosts
ssh-keyscan -H [hostname] >> ~/.ssh/known_hosts

That will make sure there are no duplicate entries, that you are covered for both the hostname and IP address, and will also hash the output, an extra security measure.

edited Sep 27 '11 at 22:14

answered Sep 27 '11 at 20:51

yardena

2,3441105

4

Why do you need all 3 ssh-keyscan's? Can't you get by with just the first one since it works for both hostname and ip?

– Robert
May 24 '13 at 22:00

6

Can you be sure that the machine replying to the ssh-keyscan request is really the one you want to talk to? If not you've opened yourself to a man in the middle attack.

– JasperWallace
Sep 23 '13 at 7:24

2

@JasperWallace Yes, for that you need at least the fingerprint or even better the public key, in which case you can add it directly to known_hosts, turning this question moot. If you only have the fingerprint, you will have to write an extra step which verifies the downloaded public key with your fingerprint...

– user68729
Apr 28 '14 at 21:57

1

Calls to ssh-keyscan were failing for me because my target host doesn't support the default version 1 key type. Adding -t rsa,dsa to the command fixed this.

– phasetwenty
Aug 6 '14 at 18:11

4

This is probably a bad idea. You are opening yourself to a man-in-the-middle attack by updating these keys. To avoid duplicate entries, check the return status of ssh-keygen -F [address] instead. medium.com/@wblankenship/…

– retrohacker
Sep 29 '15 at 3:04

|
show 4 more comments

score 89 · Accepted Answer · 2019-05-15 12:10:42Z

89

For the lazy ones:

ssh-keyscan -H <host> >> ~/.ssh/known_hosts

-H hashes the hostname / IP address

edited May 15 at 12:10

answered Sep 25 '14 at 10:03

fivef

99963

2

"ssh-keyscan -H <host> >> ~/.ssh/known_hosts" produces an entry more like what ssh does with user interaction. (The -H hashes the name of the remote host.)

– Sarah Messer
Sep 4 '15 at 20:19

3

Vulnerable to MITM attacks. You're not checking the key fingerprint.

– Mnebuerquo
Jun 15 '16 at 17:20

7

@Mnebuerquo You say what to do but not how, which would be helpful.

– Jim
Mar 24 '17 at 21:20

3

@jameshfisher Yes its vulnerable to MITM attacks, but have you ever compared the RSA fingerprint, which was shown to you with the actual one of the server, when you were doing this manually? No? So this answer is the way to do it for you. If yes, you shouldn't use this answer and do it manually or implement other security measures...

– fivef
Nov 30 '17 at 9:07

1

@Mnebuerquo I would be really glad if you also let us know a better way to handle this, when we need to clone a repo using batch scripts un-attended and we want to by-pass this prompt. Please shed some light on a real solution if you think this is not the right one!

– Waqas Shah
Jan 4 '18 at 4:22

|
show 6 more comments

kasperd 26.8k1252104 · Accepted Answer · 2015-07-27 19:55:20Z

41

As mentioned, using key-scan would be the right & unobtrusive way to do it.

ssh-keyscan -t rsa,dsa HOST 2>&1 | sort -u - ~/.ssh/known_hosts > ~/.ssh/tmp_hosts
mv ~/.ssh/tmp_hosts ~/.ssh/known_hosts

The above will do the trick to add a host, ONLY if it has not yet been added. It is also not concurrency safe; you must not execute the snippet on the same origin machine more than once at the same time, as the tmp_hosts file can get clobbered, ultimately leading to the known_hosts file becoming bloated...

edited Jul 27 '15 at 19:55

kasperd

26.8k1252104

answered Mar 6 '12 at 9:00

ysawej

51944

Is there a way to check whether the key is in known_hosts before ssh-keyscan? The reason is that it requires some time and additional network connection.

– utapyngo
May 23 '14 at 7:49

1

The original poster's version of this file had cat ~/.ssh/tmp_hosts > ~/.ssh/known_hosts, but a subsequent edit changed it to >>. Using >> is an error. It defeats the purpose of the uniqueness in the first line, and causes it to dump new entries into known_hosts every time it runs. (Just posted an edit to change it back.)

– paulmelnikow
Jul 27 '15 at 17:46

1

This is subject to the same MITM attacks as the others.

– Mnebuerquo
Jun 15 '16 at 17:18

@utapyngo ssh-keygen -F will give you the current fingerprint. If it comes back blank with return code of 1, then you don't have it. If it prints something and return code is 0, then it's already present.

– Rich L
Aug 17 '17 at 13:58

1

If you care that much about MITM, deploy DNSSEC and SSHFP records or use some other secure means of distributing the keys and this kludge solution will be irrelevant.

– Zart
Nov 28 '17 at 17:09

|
show 1 more comment

kenorb 3,2933042 · Accepted Answer · 2015-09-21 21:08:54Z

18

You could use ssh-keyscan command to grab the public key and append that to your known_hosts file.

edited Sep 21 '15 at 21:08

kenorb

3,2933042

answered Apr 16 '10 at 5:09

Alex

5,94311731

3

Make sure you check the fingerprint to ensure it is the correct key. Otherwise you open yourself up to MITM attacks.

– Mnebuerquo
Jun 15 '16 at 17:20

3

@Mnebuerquo Fair point in the general context, but why would someone be trying to programmatically gather keys if they already knew what the correct key was?

– Brian Cline
Aug 25 '17 at 19:27

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:39

add a comment |

ZartZart 29436 · Accepted Answer · 2016-02-03 03:12:46Z

7

This is how you can incorporate ssh-keyscan into your play:

---
# ansible playbook that adds ssh fingerprints to known_hosts
- hosts: all
 connection: local
 gather_facts: no
 tasks:
 - command: /usr/bin/ssh-keyscan -T 10 ansible_host 
 register: keyscan
 - lineinfile: name=~/.ssh/known_hosts create=yes line= item 
 with_items: ' keyscan.stdout_lines '

answered Feb 3 '16 at 3:12

Zart

29436

1

Are you uploading a known valid known_hosts file, or are you doing ssh-keyscan and dumping the output into known_hosts without verifying fingerprints?

– Mnebuerquo
Jun 15 '16 at 17:22

1

This is simply dumps output of a keyscan, yes. So in effect it's the same as StrictHostKeyChecking=no, just with silent known_hosts updating without fiddling with ssh options. This solution also doesn't work well due to ssh-keyscan returning multiple lines which causes this task always be flagged as 'changed'

– Zart
Jun 16 '16 at 21:42

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:39

2

@jameshfisher I would be really glad if you also let us know a better way to handle this, when we need to clone a repo using batch scripts un-attended and we want to by-pass this prompt. Please shed some light on a real solution if you think this is not the right one! Please let us know "how" to do it, if you think this is not the right way to do it!

– Waqas Shah
Jan 4 '18 at 4:25

It is a perfectly valid method of adding values to known_hosts, but yes it is susceptible to MITM. However, for internal use it is fine.

– Cameron Lowell Palmer
Nov 23 '18 at 14:15

|
show 1 more comment

score 6 · Accepted Answer · 2016-11-23 15:06:46Z

this would be a complete solution, accepting host key for the first time only

#!/usr/bin/env ansible-playbook
---
- name: accept ssh fingerprint automatically for the first time
 hosts: all
 connection: local
 gather_facts: False

 tasks:
 - name: "check if known_hosts contains server's fingerprint"
 command: ssh-keygen -F inventory_hostname 
 register: keygen
 failed_when: keygen.stderr != ''
 changed_when: False

 - name: fetch remote ssh key
 command: ssh-keyscan -T5 inventory_hostname 
 register: keyscan
 failed_when: keyscan.rc != 0 or keyscan.stdout == ''
 changed_when: False
 when: keygen.rc == 1

 - name: add ssh-key to local known_hosts
 lineinfile:
 name: ~/.ssh/known_hosts
 create: yes
 line: " item "
 when: keygen.rc == 1
 with_items: 'default([]) '

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:40 — Nov 28 '17 at 16:40

VenomFangsVenomFangs 234249 · Accepted Answer · 2014-10-21 17:27:56Z

I had a similar issue and found that some of the provided answers only got me part way to an automated solution. The following is what I ended up using, hope it helps:

ssh -o "StrictHostKeyChecking no" -o PasswordAuthentication=no 10.x.x.x

It adds the key to known_hosts and doesn't prompt for the password.

Vulnerable to MITM attacks. You're not checking the fingerprint. — Jun 15 '16 at 17:23

BradChesney79BradChesney79 21738 · Accepted Answer · 2016-10-05 21:18:00Z

So, I was searching for a mundane way to bypass the unkown host manual interaction of cloning a git repo as shown below:

brad@computer:~$ git clone git@bitbucket.org:viperks/viperks-api.git
Cloning into 'viperks-api'...
The authenticity of host 'bitbucket.org (104.192.143.3)' can't be established.
RSA key fingerprint is 97:8c:1b:f2:6f:14:6b:5c:3b:ec:aa:46:46:74:7c:40.
Are you sure you want to continue connecting (yes/no)?

Note the RSA key fingerprint...

So, this is a SSH thing, this will work for git over SSH and just SSH related things in general...

brad@computer:~$ nmap bitbucket.org --script ssh-hostkey

Starting Nmap 7.01 ( https://nmap.org ) at 2016-10-05 10:21 EDT
Nmap scan report for bitbucket.org (104.192.143.3)
Host is up (0.032s latency).
Other addresses for bitbucket.org (not scanned): 104.192.143.2 104.192.143.1 2401:1d80:1010::150
Not shown: 997 filtered ports
PORT STATE SERVICE
22/tcp open ssh
| ssh-hostkey:
| 1024 35:ee:d7:b8:ef:d7:79:e2:c6:43:9e:ab:40:6f:50:74 (DSA)
|_ 2048 97:8c:1b:f2:6f:14:6b:5c:3b:ec:aa:46:46:74:7c:40 (RSA)
80/tcp open http
443/tcp open https

Nmap done: 1 IP address (1 host up) scanned in 42.42 seconds

First, install nmap on your daily driver. nmap is highly helpful for certain things, like detecting open ports and this-- manually verifying SSH fingerprints. But, back to what we are doing.

Good. I'm either compromised at the multiple places and machines I've checked it-- or the more plausible explanation of everything being hunky dory is what is happening.

That 'fingerprint' is just a string shortened with a one way algorithm for our human convenience at the risk of more than one string resolving into the same fingerprint. It happens, they are called collisions.

Regardless, back to the original string which we can see in context below.

brad@computer:~$ ssh-keyscan bitbucket.org
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-128
no hostkey alg
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-129
bitbucket.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw==
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-123
no hostkey alg

So, ahead of time, we have a way of asking for a form of identification from the original host.

At this point we manually are as vulnerable as automatically-- the strings match, we have the base data that creates the fingerprint, and we could ask for that base data (preventing collisions) in the future.

Now to use that string in a way that prevents asking about a hosts authenticity...

The known_hosts file in this case does not use plaintext entries. You'll know hashed entries when you see them, they look like hashes with random characters instead of xyz.com or 123.45.67.89.

brad@computer:~$ ssh-keyscan -t rsa -H bitbucket.org
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-128
|1|yr6p7i8doyLhDtrrnWDk7m9QVXk=|LuKNg9gypeDhfRo/AvLTAlxnyQw= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw==

The first comment line infuriatingly shows up-- but you can get rid of it with a simple redirect via the ">" or ">>" convention.

As I've done my best to obtain untainted data to be used to identify a "host" and trust, I will add this identification to my known_hosts file in my ~/.ssh directory. Since it will now be identified as a known host, I will not get the prompt mentioned above when you were a youngster.

Thanks for sticking with me, here you go. I'm adding the bitbucket RSA key so that I can interact with my git repositories there in a non-interactive way as part of a CI workflow, but whatever you do what you want.

#!/bin/bash
cp ~/.ssh/known_hosts ~/.ssh/known_hosts.old && echo "|1|yr6p7i8doyLhDtrrnWDk7m9QVXk=|LuKNg9gypeDhfRo/AvLTAlxnyQw= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw==" >> ~/.ssh/known_hosts

So, that's how you stay a virgin for today. You can do the same with github by following similar directions on your own time.

I saw so many stack overflow posts telling you to programmatically add the key blindly without any kind of checking. The more you check the key from different machines on different networks, the more trust you can have that the host is the one it says it is-- and that is the best you can hope from this layer of security.

WRONG
~~ssh -oStrictHostKeyChecking=no hostname [command]~~

WRONG
~~ssh-keyscan -t rsa -H hostname >> ~/.ssh/known_hosts~~

Don't do either of the above things, please. You're given the opportunity to increase your chances of avoiding someone eavesdropping on your data transfers via a man in the middle attack-- take that opportunity. The difference is literally verifying that the RSA key you have is the one of the bona fide server and now you know how to get that information to compare them so you can trust the connection. Just remember more comparisons from different computers & networks will usually increase your ability to trust the connection.

I think this is the best solution to this problem. However, be very careful while using Nmap on something like Amazon EC2, I got a warning about the port scanning that Nmap does! Fill in their form before doing portscanning! — Jan 5 '18 at 4:37
...well, yeah. I don't know why you would do the port scanning from EC2. If you are logged in to your account, you can just get the keys from the actual machines. This is more for machines you don't have control over. I would assume you would have a local machine not subject to AWS port scanning restrictions to use. But, if you are in that edge case situation where you must run nmap with AWS, I suppose this warning would be helpful. — Jan 15 '18 at 19:51
Using nmap to read the SSH host key from your workstation and then trusting that value is no different than connecting via SSH with StructHostKeyChecking turned off. Its just as vulnerable to a man-in-the-middle attack. — Feb 7 '18 at 17:40
...@MicahRLedbetter which is why I suggested that "more comparisons from different computers & networks will usually increase your ability to trust the connection". But, that is my point. If you only ever check your target host from one set of environment conditions then how would you ever know of any discrepancies? Did you have any better suggestions? — Feb 7 '18 at 19:06
This is security theater. Doing something complicated to create the appearance of greater security. It doesn't matter how many different methods you use to ask the host for its key. Like asking the same person multiple times if you can trust them (maybe you call, email, text, and snail mail) . They'll always say yes, but if you're asking the wrong person, it doesn't matter. — Jun 26 '18 at 17:28

Felipe AlcacibarFelipe Alcacibar 15616 · Accepted Answer · 2017-04-18 21:01:35Z

I do a one-liner script, a bit long but useful to make this task for hosts with multiples IPs, using dig and bash

(host=github.com; ssh-keyscan -H $host; for ip in $(dig @8.8.8.8 github.com +short); do ssh-keyscan -H $host,$ip; ssh-keyscan -H $ip; done) 2> /dev/null >> .ssh/known_hosts

Amadu BahAmadu Bah 1813 · Accepted Answer · 2017-07-02 11:41:47Z

4

The following avoid duplicate entries in ~/.ssh/known_hosts:

if ! grep "$(ssh-keyscan github.com 2>/dev/null)" ~/.ssh/known_hosts > /dev/null; then
 ssh-keyscan github.com >> ~/.ssh/known_hosts
fi

answered Jul 2 '17 at 11:41

Amadu Bah

1813

1

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:42

I like this answer best. For scripting initial setup of a random VPS of no importance to anyone but me, MITM risk is vanishingly small. Infinitesimal quibble ... first line needs to be mkdir -p ~/.ssh/known_hosts;

– Martin Bramwell
Mar 5 at 23:00

add a comment |

Curt J. SampsonCurt J. Sampson 8411821 · Accepted Answer · 2016-12-12 02:27:10Z

To do this properly, what you really want to do is collect the host public keys of the VMs as you create them and drop them into a file in known_hosts format. You can then use the -o GlobalKnownHostsFile=..., pointing to that file, to ensure that you're connecting to the host you believe you should be connecting to. How you do this depends on how you're setting up the virtual machines, however, but reading it off the virtual filesystem, if possible, or even getting the host to print the contents of /etc/ssh/ssh_host_rsa_key.pub during configuration may do the trick.

That said, this may not be worthwhile, depending on what sort of environment you're working in and who your anticipated adversaries are. Doing a simple "store on first connect" (via a scan or simply during the first "real" connection) as described in several other answers above may be considerably easier and still provide some modicum of security. However, if you do this I strongly suggest you change the user known hosts file (-o UserKnownHostsFile=...) to a file specific
for this particular test installation; this will avoid polluting your personal known hosts file with test information and make it easy to clean up the now useless public keys when you delete your VMs.

score 3 · Accepted Answer · 2017-08-25 15:39:54Z

This whole

ssh-key-scan

ssh-copy-id

ECSDA key warning

business kept annoying me so I opted for

One script to rule them all

This is a variant of the script at https://askubuntu.com/a/949731/129227 with Amadu Bah's answer https://serverfault.com/a/858957/162693 in a loop.

example call

./sshcheck somedomain site1 site2 site3

The script will loop over the names sites and modify the .ssh/config and .ssh/known_hosts file and do ssh-copy-id on request - for the last feature just the let the ssh test calls fail e.g. by hitting enter 3 times on the password request.

sshcheck script

#!/bin/bash
# WF 2017-08-25
# check ssh access to bitplan servers

#ansi colors
#http://www.csc.uvic.ca/~sae/seng265/fall04/tips/s265s047-tips/bash-using-colors.html
blue='33[0;34m' 
red='33[0;31m' 
green='33[0;32m' # 'e[1;32m' is too bright for white bg.
endColor='33[0m'

#
# a colored message 
# params:
# 1: l_color - the color of the message
# 2: l_msg - the message to display
#
color_msg() 
 local l_color="$1"
 local l_msg="$2"
 echo -e "$l_color$l_msg$endColor"


#
# error
#
# show an error message and exit
#
# params:
# 1: l_msg - the message to display
error() 
 local l_msg="$1"
 # use ansi red for error
 color_msg $red "Error: $l_msg" 1>&2
 exit 1


#
# show the usage
#
usage() 
 echo "usage: $0 domain sites"
 exit 1 


#
# check known_hosts entry for server
#
checkknown() 
 local l_server="$1"
 #echo $l_server
 local l_sid="$(ssh-keyscan $l_server 2>/dev/null)" 
 #echo $l_sid
 if (! grep "$l_sid" $sknown) > /dev/null 
 then
 color_msg $blue "adding $l_server to $sknown"
 ssh-keyscan $l_server >> $sknown 2>&1
 fi


#
# check the given server
#
checkserver() yes) ssh-copy-id $l_server
 esac
 fi


#
# check all servers
#
checkservers() 
me=$(hostname -f)
for server in $(echo $* 

#
# check configuration
#
checkconfig() 
#https://askubuntu.com/questions/87449/how-to-disable-strict-host-key-checking-in-ssh
 if [ -f $sconfig ]
 then
 color_msg $green "$sconfig exists"
 ls -l $sconfig
 fi


sconfig=~/.ssh/config
sknown=~/.ssh/known_hosts

case $# in
 0) usage ;;
 1) usage ;;
 *) 
 domain=$1 
 shift 
 color_msg $blue "checking ssh configuration for domain $domain sites $*"
 checkconfig
 checkservers $* 
 #for server in $(echo $* | sort)
 ##do
 # checkknown $server 
 #done
 ;;
esac

score 3 · Accepted Answer · 2017-08-28 12:41:36Z

How are you building these machines? can you run a dns update script? can you join an IPA Domain?

FreeIPA does this automatically, but essentially all you need is SSHFP dns records and DNSSEC on your zone (freeipa provides as configurable options (dnssec disabled by default)).

You can get the existing SSHFP records from your host by running.

ssh-keygen -r jersey.jacobdevans.com

jersey.jacobdevans.com IN SSHFP 1 1 4d8589de6b1a48e148d8fc9fbb967f1b29f53ebc jersey.jacobdevans.com IN SSHFP 1 2 6503272a11ba6d7fec2518c02dfed88f3d455ac7786ee5dbd72df63307209d55
jersey.jacobdevans.com IN SSHFP 3 1 5a7a1e8ab8f25b86b63c377b303659289b895736 > jersey.jacobdevans.com IN SSHFP 3 2 1f50f790117dfedd329dbcf622a7d47551e12ff5913902c66a7da28e47de4f4b

then once published, you'd add VerifyHostKeyDNS yes to your ssh_config or ~/.ssh/config

If/When google decides to flip on DNSSEC, you could ssh in without a hostkey prompt.

ssh jersey.jacobdevans.com

BUT my domain is not signed yet, so for now you'd see....

debug1: Server host key: ecdsa-sha2-nistp256 SHA256:H1D3kBF9/t0ynbz2IqfUdVHhL/WROQLGan2ijkfeT0s

debug1: found 4 insecure fingerprints in DNS

debug1: matching host key fingerprint

found in DNS The authenticity of host 'jersey.jacobdevans.com (2605:6400:10:434::10)' can't be established. ECDSA key fingerprint is
SHA256:H1D3kBF9/t0ynbz2IqfUdVHhL/WROQLGan2ijkfeT0s. Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)? no

Vackar AfzalVackar Afzal 111 · Accepted Answer · 2018-01-23 10:51:06Z

Here is how to do a collection of hosts

define a collection of hosts

ssh_hosts:
 - server1.domain.com
 - server2.domain.com
 - server3.domain.com
 - server4.domain.com
 - server5.domain.com
 - server6.domain.com
 - server7.domain.com
 - server8.domain.com
 - server9.domain.com

Then define two tasks to add the keys to known hosts:

- command: "ssh-keyscan item"
 register: known_host_keys
 with_items: "ssh_hosts"
 tags:
 - "ssh"

 - name: Add ssh keys to know hosts
 known_hosts:
 name: "item.item"
 key: "item.stdout"
 path: ~/.ssh/known_hosts
 with_items: "known_host_keys.results"

Jason 1356 · Accepted Answer · 2014-10-03 09:53:21Z

140

Set the StrictHostKeyChecking option to no, either in the config file or via -o :

ssh -o StrictHostKeyChecking=no username@hostname.com

edited Oct 3 '14 at 9:53

Jason

1356

answered Apr 16 '10 at 4:34

Ignacio Vazquez-Abrams

39.8k46377

60

This leaves you open to man in the middle attacks, probably not a good idea.

– JasperWallace
Sep 23 '13 at 7:23

9

@JasperWallace, while this is usually good advice, the specific use case (deploying test VMs and sending commands to them) should be safe enough.

– Massimo
Oct 21 '14 at 17:33

8

This gives a Warning: Permanently added 'hostname,1.2.3.4' (RSA) to the list of known hosts. To avoid the warning, and to avoid the entry being added to any known_hosts file, I do: ssh -o StrictHostKeyChecking=no -o LogLevel=ERROR -o UserKnownHostsFile=/dev/null username@hostname.com

– Peter V. Mørch
May 21 '15 at 9:19

9

Downvoting as this does not answer the question and opens to serious security vulnerabilities.

– marcv81
Jan 20 '16 at 4:48

11

@Mnebuerquo: If you were worried about security then you wouldn't have anything at all to do with this question. You'd have the correct host key in front of you, gathered from the console of the system you wanted to connect to, and you would manually verify it upon first connecting. You certainly wouldn't do anything "automatically".

– Ignacio Vazquez-Abrams
Jun 15 '16 at 17:31

|
show 6 more comments

score 218 · Accepted Answer · 2011-09-27 22:14:06Z

218

IMO, the best way to do this is the following:

ssh-keygen -R [hostname]
ssh-keygen -R [ip_address]
ssh-keygen -R [hostname],[ip_address]
ssh-keyscan -H [hostname],[ip_address] >> ~/.ssh/known_hosts
ssh-keyscan -H [ip_address] >> ~/.ssh/known_hosts
ssh-keyscan -H [hostname] >> ~/.ssh/known_hosts

That will make sure there are no duplicate entries, that you are covered for both the hostname and IP address, and will also hash the output, an extra security measure.

edited Sep 27 '11 at 22:14

answered Sep 27 '11 at 20:51

yardena

2,3441105

4

Why do you need all 3 ssh-keyscan's? Can't you get by with just the first one since it works for both hostname and ip?

– Robert
May 24 '13 at 22:00

6

Can you be sure that the machine replying to the ssh-keyscan request is really the one you want to talk to? If not you've opened yourself to a man in the middle attack.

– JasperWallace
Sep 23 '13 at 7:24

2

@JasperWallace Yes, for that you need at least the fingerprint or even better the public key, in which case you can add it directly to known_hosts, turning this question moot. If you only have the fingerprint, you will have to write an extra step which verifies the downloaded public key with your fingerprint...

– user68729
Apr 28 '14 at 21:57

1

Calls to ssh-keyscan were failing for me because my target host doesn't support the default version 1 key type. Adding -t rsa,dsa to the command fixed this.

– phasetwenty
Aug 6 '14 at 18:11

4

This is probably a bad idea. You are opening yourself to a man-in-the-middle attack by updating these keys. To avoid duplicate entries, check the return status of ssh-keygen -F [address] instead. medium.com/@wblankenship/…

– retrohacker
Sep 29 '15 at 3:04

|
show 4 more comments

score 89 · Accepted Answer · 2019-05-15 12:10:42Z

89

For the lazy ones:

ssh-keyscan -H <host> >> ~/.ssh/known_hosts

-H hashes the hostname / IP address

edited May 15 at 12:10

answered Sep 25 '14 at 10:03

fivef

99963

2

"ssh-keyscan -H <host> >> ~/.ssh/known_hosts" produces an entry more like what ssh does with user interaction. (The -H hashes the name of the remote host.)

– Sarah Messer
Sep 4 '15 at 20:19

3

Vulnerable to MITM attacks. You're not checking the key fingerprint.

– Mnebuerquo
Jun 15 '16 at 17:20

7

@Mnebuerquo You say what to do but not how, which would be helpful.

– Jim
Mar 24 '17 at 21:20

3

@jameshfisher Yes its vulnerable to MITM attacks, but have you ever compared the RSA fingerprint, which was shown to you with the actual one of the server, when you were doing this manually? No? So this answer is the way to do it for you. If yes, you shouldn't use this answer and do it manually or implement other security measures...

– fivef
Nov 30 '17 at 9:07

1

@Mnebuerquo I would be really glad if you also let us know a better way to handle this, when we need to clone a repo using batch scripts un-attended and we want to by-pass this prompt. Please shed some light on a real solution if you think this is not the right one!

– Waqas Shah
Jan 4 '18 at 4:22

|
show 6 more comments

kasperd 26.8k1252104 · Accepted Answer · 2015-07-27 19:55:20Z

41

As mentioned, using key-scan would be the right & unobtrusive way to do it.

ssh-keyscan -t rsa,dsa HOST 2>&1 | sort -u - ~/.ssh/known_hosts > ~/.ssh/tmp_hosts
mv ~/.ssh/tmp_hosts ~/.ssh/known_hosts

The above will do the trick to add a host, ONLY if it has not yet been added. It is also not concurrency safe; you must not execute the snippet on the same origin machine more than once at the same time, as the tmp_hosts file can get clobbered, ultimately leading to the known_hosts file becoming bloated...

edited Jul 27 '15 at 19:55

kasperd

26.8k1252104

answered Mar 6 '12 at 9:00

ysawej

51944

Is there a way to check whether the key is in known_hosts before ssh-keyscan? The reason is that it requires some time and additional network connection.

– utapyngo
May 23 '14 at 7:49

1

The original poster's version of this file had cat ~/.ssh/tmp_hosts > ~/.ssh/known_hosts, but a subsequent edit changed it to >>. Using >> is an error. It defeats the purpose of the uniqueness in the first line, and causes it to dump new entries into known_hosts every time it runs. (Just posted an edit to change it back.)

– paulmelnikow
Jul 27 '15 at 17:46

1

This is subject to the same MITM attacks as the others.

– Mnebuerquo
Jun 15 '16 at 17:18

@utapyngo ssh-keygen -F will give you the current fingerprint. If it comes back blank with return code of 1, then you don't have it. If it prints something and return code is 0, then it's already present.

– Rich L
Aug 17 '17 at 13:58

1

If you care that much about MITM, deploy DNSSEC and SSHFP records or use some other secure means of distributing the keys and this kludge solution will be irrelevant.

– Zart
Nov 28 '17 at 17:09

|
show 1 more comment

kenorb 3,2933042 · Accepted Answer · 2015-09-21 21:08:54Z

18

You could use ssh-keyscan command to grab the public key and append that to your known_hosts file.

edited Sep 21 '15 at 21:08

kenorb

3,2933042

answered Apr 16 '10 at 5:09

Alex

5,94311731

3

Make sure you check the fingerprint to ensure it is the correct key. Otherwise you open yourself up to MITM attacks.

– Mnebuerquo
Jun 15 '16 at 17:20

3

@Mnebuerquo Fair point in the general context, but why would someone be trying to programmatically gather keys if they already knew what the correct key was?

– Brian Cline
Aug 25 '17 at 19:27

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:39

add a comment |

ZartZart 29436 · Accepted Answer · 2016-02-03 03:12:46Z

7

This is how you can incorporate ssh-keyscan into your play:

---
# ansible playbook that adds ssh fingerprints to known_hosts
- hosts: all
 connection: local
 gather_facts: no
 tasks:
 - command: /usr/bin/ssh-keyscan -T 10 ansible_host 
 register: keyscan
 - lineinfile: name=~/.ssh/known_hosts create=yes line= item 
 with_items: ' keyscan.stdout_lines '

answered Feb 3 '16 at 3:12

Zart

29436

1

Are you uploading a known valid known_hosts file, or are you doing ssh-keyscan and dumping the output into known_hosts without verifying fingerprints?

– Mnebuerquo
Jun 15 '16 at 17:22

1

This is simply dumps output of a keyscan, yes. So in effect it's the same as StrictHostKeyChecking=no, just with silent known_hosts updating without fiddling with ssh options. This solution also doesn't work well due to ssh-keyscan returning multiple lines which causes this task always be flagged as 'changed'

– Zart
Jun 16 '16 at 21:42

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:39

2

@jameshfisher I would be really glad if you also let us know a better way to handle this, when we need to clone a repo using batch scripts un-attended and we want to by-pass this prompt. Please shed some light on a real solution if you think this is not the right one! Please let us know "how" to do it, if you think this is not the right way to do it!

– Waqas Shah
Jan 4 '18 at 4:25

It is a perfectly valid method of adding values to known_hosts, but yes it is susceptible to MITM. However, for internal use it is fine.

– Cameron Lowell Palmer
Nov 23 '18 at 14:15

|
show 1 more comment

score 6 · Accepted Answer · 2016-11-23 15:06:46Z

this would be a complete solution, accepting host key for the first time only

#!/usr/bin/env ansible-playbook
---
- name: accept ssh fingerprint automatically for the first time
 hosts: all
 connection: local
 gather_facts: False

 tasks:
 - name: "check if known_hosts contains server's fingerprint"
 command: ssh-keygen -F inventory_hostname 
 register: keygen
 failed_when: keygen.stderr != ''
 changed_when: False

 - name: fetch remote ssh key
 command: ssh-keyscan -T5 inventory_hostname 
 register: keyscan
 failed_when: keyscan.rc != 0 or keyscan.stdout == ''
 changed_when: False
 when: keygen.rc == 1

 - name: add ssh-key to local known_hosts
 lineinfile:
 name: ~/.ssh/known_hosts
 create: yes
 line: " item "
 when: keygen.rc == 1
 with_items: 'default([]) '

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:40 — Nov 28 '17 at 16:40

VenomFangsVenomFangs 234249 · Accepted Answer · 2014-10-21 17:27:56Z

I had a similar issue and found that some of the provided answers only got me part way to an automated solution. The following is what I ended up using, hope it helps:

ssh -o "StrictHostKeyChecking no" -o PasswordAuthentication=no 10.x.x.x

It adds the key to known_hosts and doesn't prompt for the password.

Vulnerable to MITM attacks. You're not checking the fingerprint. — Jun 15 '16 at 17:23

BradChesney79BradChesney79 21738 · Accepted Answer · 2016-10-05 21:18:00Z

So, I was searching for a mundane way to bypass the unkown host manual interaction of cloning a git repo as shown below:

brad@computer:~$ git clone git@bitbucket.org:viperks/viperks-api.git
Cloning into 'viperks-api'...
The authenticity of host 'bitbucket.org (104.192.143.3)' can't be established.
RSA key fingerprint is 97:8c:1b:f2:6f:14:6b:5c:3b:ec:aa:46:46:74:7c:40.
Are you sure you want to continue connecting (yes/no)?

Note the RSA key fingerprint...

So, this is a SSH thing, this will work for git over SSH and just SSH related things in general...

brad@computer:~$ nmap bitbucket.org --script ssh-hostkey

Starting Nmap 7.01 ( https://nmap.org ) at 2016-10-05 10:21 EDT
Nmap scan report for bitbucket.org (104.192.143.3)
Host is up (0.032s latency).
Other addresses for bitbucket.org (not scanned): 104.192.143.2 104.192.143.1 2401:1d80:1010::150
Not shown: 997 filtered ports
PORT STATE SERVICE
22/tcp open ssh
| ssh-hostkey:
| 1024 35:ee:d7:b8:ef:d7:79:e2:c6:43:9e:ab:40:6f:50:74 (DSA)
|_ 2048 97:8c:1b:f2:6f:14:6b:5c:3b:ec:aa:46:46:74:7c:40 (RSA)
80/tcp open http
443/tcp open https

Nmap done: 1 IP address (1 host up) scanned in 42.42 seconds

First, install nmap on your daily driver. nmap is highly helpful for certain things, like detecting open ports and this-- manually verifying SSH fingerprints. But, back to what we are doing.

Good. I'm either compromised at the multiple places and machines I've checked it-- or the more plausible explanation of everything being hunky dory is what is happening.

That 'fingerprint' is just a string shortened with a one way algorithm for our human convenience at the risk of more than one string resolving into the same fingerprint. It happens, they are called collisions.

Regardless, back to the original string which we can see in context below.

brad@computer:~$ ssh-keyscan bitbucket.org
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-128
no hostkey alg
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-129
bitbucket.org ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw==
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-123
no hostkey alg

So, ahead of time, we have a way of asking for a form of identification from the original host.

At this point we manually are as vulnerable as automatically-- the strings match, we have the base data that creates the fingerprint, and we could ask for that base data (preventing collisions) in the future.

Now to use that string in a way that prevents asking about a hosts authenticity...

The known_hosts file in this case does not use plaintext entries. You'll know hashed entries when you see them, they look like hashes with random characters instead of xyz.com or 123.45.67.89.

brad@computer:~$ ssh-keyscan -t rsa -H bitbucket.org
# bitbucket.org SSH-2.0-conker_1.0.257-ce87fba app-128
|1|yr6p7i8doyLhDtrrnWDk7m9QVXk=|LuKNg9gypeDhfRo/AvLTAlxnyQw= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw==

The first comment line infuriatingly shows up-- but you can get rid of it with a simple redirect via the ">" or ">>" convention.

As I've done my best to obtain untainted data to be used to identify a "host" and trust, I will add this identification to my known_hosts file in my ~/.ssh directory. Since it will now be identified as a known host, I will not get the prompt mentioned above when you were a youngster.

Thanks for sticking with me, here you go. I'm adding the bitbucket RSA key so that I can interact with my git repositories there in a non-interactive way as part of a CI workflow, but whatever you do what you want.

#!/bin/bash
cp ~/.ssh/known_hosts ~/.ssh/known_hosts.old && echo "|1|yr6p7i8doyLhDtrrnWDk7m9QVXk=|LuKNg9gypeDhfRo/AvLTAlxnyQw= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw==" >> ~/.ssh/known_hosts

So, that's how you stay a virgin for today. You can do the same with github by following similar directions on your own time.

I saw so many stack overflow posts telling you to programmatically add the key blindly without any kind of checking. The more you check the key from different machines on different networks, the more trust you can have that the host is the one it says it is-- and that is the best you can hope from this layer of security.

WRONG
~~ssh -oStrictHostKeyChecking=no hostname [command]~~

WRONG
~~ssh-keyscan -t rsa -H hostname >> ~/.ssh/known_hosts~~

Don't do either of the above things, please. You're given the opportunity to increase your chances of avoiding someone eavesdropping on your data transfers via a man in the middle attack-- take that opportunity. The difference is literally verifying that the RSA key you have is the one of the bona fide server and now you know how to get that information to compare them so you can trust the connection. Just remember more comparisons from different computers & networks will usually increase your ability to trust the connection.

I think this is the best solution to this problem. However, be very careful while using Nmap on something like Amazon EC2, I got a warning about the port scanning that Nmap does! Fill in their form before doing portscanning! — Jan 5 '18 at 4:37
...well, yeah. I don't know why you would do the port scanning from EC2. If you are logged in to your account, you can just get the keys from the actual machines. This is more for machines you don't have control over. I would assume you would have a local machine not subject to AWS port scanning restrictions to use. But, if you are in that edge case situation where you must run nmap with AWS, I suppose this warning would be helpful. — Jan 15 '18 at 19:51
Using nmap to read the SSH host key from your workstation and then trusting that value is no different than connecting via SSH with StructHostKeyChecking turned off. Its just as vulnerable to a man-in-the-middle attack. — Feb 7 '18 at 17:40
...@MicahRLedbetter which is why I suggested that "more comparisons from different computers & networks will usually increase your ability to trust the connection". But, that is my point. If you only ever check your target host from one set of environment conditions then how would you ever know of any discrepancies? Did you have any better suggestions? — Feb 7 '18 at 19:06
This is security theater. Doing something complicated to create the appearance of greater security. It doesn't matter how many different methods you use to ask the host for its key. Like asking the same person multiple times if you can trust them (maybe you call, email, text, and snail mail) . They'll always say yes, but if you're asking the wrong person, it doesn't matter. — Jun 26 '18 at 17:28

Felipe AlcacibarFelipe Alcacibar 15616 · Accepted Answer · 2017-04-18 21:01:35Z

I do a one-liner script, a bit long but useful to make this task for hosts with multiples IPs, using dig and bash

(host=github.com; ssh-keyscan -H $host; for ip in $(dig @8.8.8.8 github.com +short); do ssh-keyscan -H $host,$ip; ssh-keyscan -H $ip; done) 2> /dev/null >> .ssh/known_hosts

Amadu BahAmadu Bah 1813 · Accepted Answer · 2017-07-02 11:41:47Z

4

The following avoid duplicate entries in ~/.ssh/known_hosts:

if ! grep "$(ssh-keyscan github.com 2>/dev/null)" ~/.ssh/known_hosts > /dev/null; then
 ssh-keyscan github.com >> ~/.ssh/known_hosts
fi

answered Jul 2 '17 at 11:41

Amadu Bah

1813

1

This is not the way to do it. MITM.

– jameshfisher
Nov 28 '17 at 16:42

I like this answer best. For scripting initial setup of a random VPS of no importance to anyone but me, MITM risk is vanishingly small. Infinitesimal quibble ... first line needs to be mkdir -p ~/.ssh/known_hosts;

– Martin Bramwell
Mar 5 at 23:00

add a comment |

Curt J. SampsonCurt J. Sampson 8411821 · Accepted Answer · 2016-12-12 02:27:10Z

To do this properly, what you really want to do is collect the host public keys of the VMs as you create them and drop them into a file in known_hosts format. You can then use the -o GlobalKnownHostsFile=..., pointing to that file, to ensure that you're connecting to the host you believe you should be connecting to. How you do this depends on how you're setting up the virtual machines, however, but reading it off the virtual filesystem, if possible, or even getting the host to print the contents of /etc/ssh/ssh_host_rsa_key.pub during configuration may do the trick.

That said, this may not be worthwhile, depending on what sort of environment you're working in and who your anticipated adversaries are. Doing a simple "store on first connect" (via a scan or simply during the first "real" connection) as described in several other answers above may be considerably easier and still provide some modicum of security. However, if you do this I strongly suggest you change the user known hosts file (-o UserKnownHostsFile=...) to a file specific
for this particular test installation; this will avoid polluting your personal known hosts file with test information and make it easy to clean up the now useless public keys when you delete your VMs.

score 3 · Accepted Answer · 2017-08-25 15:39:54Z

This whole

ssh-key-scan

ssh-copy-id

ECSDA key warning

business kept annoying me so I opted for

One script to rule them all

This is a variant of the script at https://askubuntu.com/a/949731/129227 with Amadu Bah's answer https://serverfault.com/a/858957/162693 in a loop.

example call

./sshcheck somedomain site1 site2 site3

The script will loop over the names sites and modify the .ssh/config and .ssh/known_hosts file and do ssh-copy-id on request - for the last feature just the let the ssh test calls fail e.g. by hitting enter 3 times on the password request.

sshcheck script

#!/bin/bash
# WF 2017-08-25
# check ssh access to bitplan servers

#ansi colors
#http://www.csc.uvic.ca/~sae/seng265/fall04/tips/s265s047-tips/bash-using-colors.html
blue='33[0;34m' 
red='33[0;31m' 
green='33[0;32m' # 'e[1;32m' is too bright for white bg.
endColor='33[0m'

#
# a colored message 
# params:
# 1: l_color - the color of the message
# 2: l_msg - the message to display
#
color_msg() 
 local l_color="$1"
 local l_msg="$2"
 echo -e "$l_color$l_msg$endColor"


#
# error
#
# show an error message and exit
#
# params:
# 1: l_msg - the message to display
error() 
 local l_msg="$1"
 # use ansi red for error
 color_msg $red "Error: $l_msg" 1>&2
 exit 1


#
# show the usage
#
usage() 
 echo "usage: $0 domain sites"
 exit 1 


#
# check known_hosts entry for server
#
checkknown() 
 local l_server="$1"
 #echo $l_server
 local l_sid="$(ssh-keyscan $l_server 2>/dev/null)" 
 #echo $l_sid
 if (! grep "$l_sid" $sknown) > /dev/null 
 then
 color_msg $blue "adding $l_server to $sknown"
 ssh-keyscan $l_server >> $sknown 2>&1
 fi


#
# check the given server
#
checkserver() yes) ssh-copy-id $l_server
 esac
 fi


#
# check all servers
#
checkservers() 
me=$(hostname -f)
for server in $(echo $* 

#
# check configuration
#
checkconfig() 
#https://askubuntu.com/questions/87449/how-to-disable-strict-host-key-checking-in-ssh
 if [ -f $sconfig ]
 then
 color_msg $green "$sconfig exists"
 ls -l $sconfig
 fi


sconfig=~/.ssh/config
sknown=~/.ssh/known_hosts

case $# in
 0) usage ;;
 1) usage ;;
 *) 
 domain=$1 
 shift 
 color_msg $blue "checking ssh configuration for domain $domain sites $*"
 checkconfig
 checkservers $* 
 #for server in $(echo $* | sort)
 ##do
 # checkknown $server 
 #done
 ;;
esac

score 3 · Accepted Answer · 2017-08-28 12:41:36Z

How are you building these machines? can you run a dns update script? can you join an IPA Domain?

FreeIPA does this automatically, but essentially all you need is SSHFP dns records and DNSSEC on your zone (freeipa provides as configurable options (dnssec disabled by default)).

You can get the existing SSHFP records from your host by running.

ssh-keygen -r jersey.jacobdevans.com

jersey.jacobdevans.com IN SSHFP 1 1 4d8589de6b1a48e148d8fc9fbb967f1b29f53ebc jersey.jacobdevans.com IN SSHFP 1 2 6503272a11ba6d7fec2518c02dfed88f3d455ac7786ee5dbd72df63307209d55
jersey.jacobdevans.com IN SSHFP 3 1 5a7a1e8ab8f25b86b63c377b303659289b895736 > jersey.jacobdevans.com IN SSHFP 3 2 1f50f790117dfedd329dbcf622a7d47551e12ff5913902c66a7da28e47de4f4b

then once published, you'd add VerifyHostKeyDNS yes to your ssh_config or ~/.ssh/config

If/When google decides to flip on DNSSEC, you could ssh in without a hostkey prompt.

ssh jersey.jacobdevans.com

BUT my domain is not signed yet, so for now you'd see....

debug1: Server host key: ecdsa-sha2-nistp256 SHA256:H1D3kBF9/t0ynbz2IqfUdVHhL/WROQLGan2ijkfeT0s

debug1: found 4 insecure fingerprints in DNS

debug1: matching host key fingerprint

found in DNS The authenticity of host 'jersey.jacobdevans.com (2605:6400:10:434::10)' can't be established. ECDSA key fingerprint is
SHA256:H1D3kBF9/t0ynbz2IqfUdVHhL/WROQLGan2ijkfeT0s. Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)? no

Vackar AfzalVackar Afzal 111 · Accepted Answer · 2018-01-23 10:51:06Z

Here is how to do a collection of hosts

define a collection of hosts

ssh_hosts:
 - server1.domain.com
 - server2.domain.com
 - server3.domain.com
 - server4.domain.com
 - server5.domain.com
 - server6.domain.com
 - server7.domain.com
 - server8.domain.com
 - server9.domain.com

Then define two tasks to add the keys to known hosts:

- command: "ssh-keyscan item"
 register: known_host_keys
 with_items: "ssh_hosts"
 tags:
 - "ssh"

 - name: Add ssh keys to know hosts
 known_hosts:
 name: "item.item"
 key: "item.stdout"
 path: ~/.ssh/known_hosts
 with_items: "known_host_keys.results"

搜尋此網誌

Otdfbt

15 Answers
15

Your Answer

Post as a guest

15 Answers
15

15 Answers
15

Post as a guest

Popular posts from this blog

Vilaño, A Laracha Índice Patrimonio | Lugares e parroquias | Véxase tamén | Menú de navegación43°14′52″N 8°36′03″O / 43.24775, -8.60070

15 Answers 15

Your Answer

Sign up or log in

Post as a guest

Post as a guest

15 Answers 15

15 Answers 15

Sign up or log in

Post as a guest

Post as a guest

Sign up or log in

Post as a guest

Sign up or log in

Post as a guest

Sign up or log in

Post as a guest

Popular posts from this blog

Vilaño, A Laracha Índice Patrimonio | Lugares e parroquias | Véxase tamén | Menú de navegación43°14′52″N 8°36′03″O﻿ / ﻿43.24775, -8.60070

15 Answers
15

15 Answers
15

15 Answers
15

Vilaño, A Laracha Índice Patrimonio | Lugares e parroquias | Véxase tamén | Menú de navegación43°14′52″N 8°36′03″O / 43.24775, -8.60070