I would like to disable strict host key checking in ssh for Ubuntu 11.04. How to do it?

  • 13
    Hi karthick87, I hope you understand the security implications of making that change ;)
    – Panther
    Commented Dec 13, 2011 at 16:18
  • 1
    It should be noted however, that you want to know if a host key has changed. That is a big red flag that someone may be spoofing the host. So UserKnownHostFile /dev/null is a really bad idea.
    – user274148
    Commented Apr 25, 2014 at 13:48
  • 11
    SSH is used not only for remote connections, you know. All hosts I'm connecting to are in heap on my table and share the same IP, so I always have the new host warning. Commented Sep 10, 2014 at 8:34
  • 1
    If you just want to remove the message for a particular host, delete the corresponding line ~/.ssh/known_hosts. Commented Feb 27, 2016 at 3:32
  • 5
    If you just need to do a one-time connect without errors: ssh -o UserKnownHostsFile=/dev/null Commented Aug 29, 2017 at 9:35

7 Answers 7


In your ~/.ssh/config (if this file doesn't exist, just create it):

Host *
    StrictHostKeyChecking no

This will turn it off for all hosts you connect to. You can replace the * with a hostname pattern if you only want it to apply to some hosts.

Make sure the permissions on the file restrict access to yourself only:

sudo chmod 400 ~/.ssh/config
  • 7
    Make one - the entire contents of the file are in my quote above. Note it's in the .ssh subdirectory of your homedir as well.
    – Caesium
    Commented Dec 14, 2011 at 14:44
  • Is the indentation required? My entries look like blocks divided by a empty line.
    – Andi Giga
    Commented Jun 20, 2017 at 10:04
  • 26
    This is unwise in many cases, often you just want to disable it once: ssh -o UserKnownHostsFile=/dev/null Commented Aug 29, 2017 at 9:34
  • 3
    mkdir -p ~/.ssh && echo "Host *" > ~/.ssh/config && echo " StrictHostKeyChecking no" >> ~/.ssh/config Commented Apr 25, 2018 at 12:24
  • 1
    I think chmod 600 is enough
    – Ivan
    Commented May 11, 2019 at 5:58

Rather than adding it to your ~/.ssh/config file for all Host *, it would be a safer to specify a particular host.

You can also pass a parameter on the command-line like this:

ssh -o StrictHostKeyChecking=no yourHardenedHost.com

This will automatically add the host key to your known_hosts file if it's not already there.

If there's a mismatch, it will display a big warning and not update known_hosts. It will also disable password-based authentication to prevent MITM attacks. Private key authentication will still automatically get through though, which you may not want.

  • 1
    Note that you generally only need to do this once per host since it says this the first time: Warning: Permanently added 'frxxx.blaps.net,' (RSA) to the list of known hosts.
    – MarkHu
    Commented Jul 24, 2013 at 0:49
  • 50
    That won't work. It should be ssh -o UserKnownHostsFile=/dev/null instead.
    – qwertzguy
    Commented Oct 29, 2014 at 2:51
  • 5
    @qwertzguy It does work. Your option will make it so that the host key is lost each time, which is useful and more secure, but not what the question asked for. Commented Nov 30, 2015 at 0:04
  • 3
    use both works for me, ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no
    – netawater
    Commented Nov 12, 2017 at 5:35
  • 3
    @MarkHu To avoid show this warning just add -o LogLevel=ERROR
    – MMJ
    Commented Oct 8, 2022 at 22:44

It's worth pointing out that setting in your ssh config:

StrictHostKeyChecking no

Will mean hostkeys are still added to .ssh/known_hosts - you just won't be prompted about whether you trust them, but should hosts change I'm willing to bet you'll get the big warning about it. You can work around this problem by adding another parameter:

UserKnownHostsFile /dev/null

This will add all these "newly discovered" hosts to the trash bin. If a host key changes, no troubles.

I would be remiss not to mention that circumventing these warnings on hostkeys has obvious security ramifications - you should be careful that you're doing it for the right reasons & that what you're connecting to actually is what you mean to connect to and not a malicious host, since at this point you've eroded a major part of the security in ssh as a solution.

For example if you were to try and set this with the commandline, the full command would be:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null user@host

That would be silly though - given that the working examples above for ssh config files is likely to make more sense in all cases.

  • 1
    You're correct, you do get the big warning Commented Jan 20, 2014 at 17:35
  • 1
    I think this is the right answer. This works well for connecting to hosts on a private local network. Commented Jan 14, 2015 at 14:39
  • 10
    Could be convenient to have an alias to ssh -o StrictHostKeyChecking=no -o UserKnownHostFiles=/dev/null user@host. In my case I use issh to connect to hosts where I know the host key changes. Commented May 2, 2016 at 13:53
  • 2
    @ecerulm - just a small typo: it's UserKnownHostsFile not UserKnownHostFiles. Commented Oct 2, 2017 at 10:16
  • 3
    @LeeMeador There isn't a known_hosts file in your Docker containers because nobody put one there. Not because there can't be one or never is one. There can be one. You can copy one in at build time, or mount one at docker run time, or fetch one from inside the container using another secure mechanism. If this security issue is important enough to your organization, then you can create a discovery and delivery system in your CI/CD processes to have an updated known_hosts file in your Docker images.
    – ErikE
    Commented Jul 13, 2020 at 9:50

FYI. I prefer to disable host checking just when using cssh.

alias cssh='ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null'
  • 1
    cssh or ssh?
    – kenorb
    Commented Jan 16, 2014 at 14:29
  • 1
    Maybe he uses cssh.sourceforge.net
    – MarkHu
    Commented Jan 31, 2014 at 3:22
  • 1
    Am I wrong, or is the second -o unnecessary?
    – yckart
    Commented Nov 22, 2016 at 2:15
  • 2
    alias relay='ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null [email protected] -p 2222' work for me
    – arganzheng
    Commented Jul 25, 2018 at 8:12

If you want to disable on a one time basis use:

ssh -o UserKnownHostsFile=/dev/null

That will work also if the host key changes and will make sure not to save the key as trusted for added security.


From what it sounds like,

NoHostAuthenticationForLocalhost yes

may be good enough, for you. AND you'd still be able to maintain that semblance of security.


https://askubuntu.com/a/87452/129227 suggest to modify the config file which helps. But instead of opening things up for any host I wanted this to be done per host. The script below helps automating the process:

example call

./sshcheck somedomain site1 site2 site3

sshcheck script

# WF 2017-08-25
# check ssh access to bitplan servers

#ansi colors
green='\033[0;32m' # '\e[1;32m' is too bright for white bg.

# 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 the given server
checkserver() {
  local l_server="$1"
  grep $l_server $sconfig > /dev/null
  if [ $? -eq 1 ]
    color_msg $blue "adding $l_server to $sconfig"
    today=$(date "+%Y-%m-%d")
    echo "# added $today by $0"  >> $sconfig
    echo "Host $l_server" >> $sconfig
    echo "   StrictHostKeyChecking no" >> $sconfig
    echo "   userKnownHostsFile=/dev/null" >> $sconfig
    echo "" >> $sconfig
    color_msg $green "$l_server found in $sconfig"
  ssh -q $l_server id > /dev/null
  if [ $? -eq 0 ]
    color_msg $green "$l_server accessible via ssh"
    color_msg $red "ssh to $l_server failed" 
    color_msg $blue "shall I ssh-copy-id credentials to $l_server?"
    read answer
    case $answer in
      y|yes) ssh-copy-id $l_server

# check all servers
checkservers() {
me=$(hostname -f)
for server in $(echo $* | sort)
  case $os in
   # Mac OS X
     pingoption=" -t1";;
    *) ;;

  pingresult=$(ping $pingoption -i0.2 -c1 $server)
  echo $pingresult | grep 100 > /dev/null
  if [ $? -eq 1 ]
    checkserver $server
    checkserver $server.$domain
    color_msg $red "ping to $server failed"

# check configuration
checkconfig() {
  if [ -f $sconfig ]
    color_msg $green "$sconfig exists"
    ls -l $sconfig


case  $# in
  0) usage ;;
  1) usage ;;
    color_msg $blue "checking ssh configuration for domain $domain sites $*"
    checkservers $* 
  • 2
    grep $l_server $sconfig > /dev/null; if [ $? -eq 1 ]; then should be replaced with if ! grep -q "$l_server" "$sconfig"; then. 1) Don't check exit codes unless required, instead just use if directly. 2) grep has a -q option. Use it instead of > /dev/null. 3) All environment variable expansion should be double-quoted. Also, consider avoiding abbreviations and consider naming compound words like checkserver as check_server or checkServer so IDEs with spell check don't complain.
    – ErikE
    Commented Jul 13, 2020 at 9:58
  • @ErikE - please post your improved version - I'll happily upvote it. Commented Jul 13, 2020 at 12:20
  • It's okay, my comment is here and anyone who wants it will find it.
    – ErikE
    Commented Jul 15, 2020 at 18:30

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