Summary: increasing security by verifying the identity of the machine that you connect to with ssh by eye and with SSHFP in DNS.
ssh allows you to connect to a remote machine with the data between the two machines being encrypted. You can then securely do: command line login, file copy, tunnel other protocols (eg: rsync, X11), ... You authenticate by either typing a password or key exchange.
A big vulnerability is a man in the middle attack where some malevolent snooping entity (Mallory) between you and the remote machine intercepts all traffic, decrypts it & reencrypts before sending it on. Mallory can do this fast enough so that you are not aware of his existance.
To protect you against this your ssh program checks the remote ssh server's fingerprint with the fingerprint saved from the last time it connected.
If the fingerprint has changed you will be warned and asked if you wish to continue.
In openssh (the ssh used on most Linux systems) this fingerprint is stored in
The fingerprint is a short version of the server's public key; it is easier for you to verify than the full key. It is very hard to spoof another public key with the same fingerprint.
When you connect to a machine for the first time you do not have the fingerprint in your
known_hosts, so ssh
has nothing to compare it to, so it asks you.
This page describes ways in which you can do more than blindly say
When you connect to a machine for the first time you will be told that the authenticity can't be established and presented with a key fingerprint to check. Something like this:
The authenticity of host 'mint.phcomp.co.uk (188.8.131.52)' can't be established. RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23. Are you sure you want to continue connecting (yes/no)? no
You might find that the fingerprint is shown in a different format — just keep reading.
To be able to check the fingerprint you need to know what it is.
You will probably find the
.pub files in
/etc/ssh/ that contain RSA & DSA keys.
This will generate the fingerprints that you can check:
cd /etc/ssh for file in *sa_key.pub do ssh-keygen -lf $file done
There is little point in doing this after you have logged in, a sufficiently ingenious Mallory could modify what the above generates on the fly so that you see what he wants you to see. This needs to be done before hand and you can then check it with what you see. You might print the fingerprints out and keep them in your briefcase or wallet.
You might find that the fingerprint is generated in a different format from what you have.
This section tells you how, when connecting, you get the
ssh client to show them in different
formats and, on the server, have
ssh-keygen generate different format references.
You may need to mix and match depending on what you have in front of you and where you are able to run commands.
This is the format shown above. You can force
ssh to display this thus:
$ ssh -o FingerprintHash=md5 host.example.org
This new format looks as below:
The authenticity of host 'mint.phcomp.co.uk (184.108.40.206)' can't be established. RSA key fingerprint is SHA256:jP0pfKJ9OAXt2F+LM7j3+BMalQ/2Koihl5eH/kli6A4. Are you sure you want to continue connecting (yes/no)?
You can request this with:
$ ssh -o FingerprintHash=sha256 host.example.org
You can get
ssh-keygen to display it:
ssh-keygen -l -E sha256 -f $file
This is used in OpenSSH 6.8 & later. The
FingerprintHash is not available in old versions.
This displays the host key in a box and is, hopefully, easier to recognise than a string of numbers. It can be used to display both MD5 and SHA256 keys. It looks like this:
The authenticity of host 'mint.phcomp.co.uk (220.127.116.11)' can't be established. RSA key fingerprint is SHA256:jP0pfKJ9OAXt2F+LM7j3+BMalQ/2Koihl5eH/kli6A4. +---[RSA 2048]----+ | | | | | . . | | +. . = | | . S= o + | | .oo+.. o o| | E.=***.= + | | .=*=BoXo+ | | .o+=o=+*o. | +----[SHA256]-----+ Are you sure you want to continue connecting (yes/no)?
You can request
ssh to display it thus:
$ ssh -o VisualHostKey=yes host.example.org
You can request MD5 or SHA256 keys, eg:
$ ssh -o VisualHostKey=yes -o FingerprintHash=sha256 host.example.org
On the server
ssh-keygen will display when given the
Combine this with
-E md5 or
-E sha256 for the fingerprint hash algorithm:
ssh-keygen -lv -E sha256 -f $file
You can put the server keys fingerprint in DNS (Domain Name System) and get ssh to tell you if what it the two fingerprints match. This is not a guarantee but it makes Mallory's job harder since he needs to spoof DNS as well as ssh, which can be done as few domains yet implement DNSSEC.
Generate the SSHFP fingerprint information to go into DNS:
cd /etc/ssh for file in *sa_key.pub do ssh-keygen -r freshmint.phcomp.co.uk -f $file -g done freshmint.phcomp.co.uk IN TYPE44 \# 22 02 01 e8fe15b374207a2e6ee99bbbadc87ecd068c17f8 freshmint.phcomp.co.uk IN TYPE44 \# 22 01 01 e5091496bda76d015e89db0bf925ffb1b9d8facb
The above output should be used for bind versions earlier than
9.5.0a1, after that the
SSHFP RR type is understood:
freshmint.phcomp.co.uk IN SSHFP 02 01 e8fe15b374207a2e6ee99bbbadc87ecd068c17f8 freshmint.phcomp.co.uk IN SSHFP 01 01 e5091496bda76d015e89db0bf925ffb1b9d8facb
Enter the above 2 RR (resource records) into the DNS for the machine. The complete record for the machine looks like:
freshmint IN A 18.104.22.168 IN AAAA 2001:4d48:ad51:2f00::2:2 IN MX 10 freshmint IN SSHFP 1 1 b4b8f2f051a16f57f69590c7c06aeaad039a3882 IN SSHFP 2 1 ea35c2064a5fc2ec9f51da2e7c790966f9844e59 mint IN CNAME freshmint
Check that this DNS update is correct with
host -a your.machine.name. You should see the
SSHFP lines as above, although
with old versions of
host it may not display properly but like:
freshmint.phcomp.co.uk. 259200 IN TYPE44 \# 22 0101b4b8f2f051a16f57f69590c7c06aeaad039a3882 freshmint.phcomp.co.uk. 259200 IN TYPE44 \# 22 0201ea35c2064a5fc2ec9f51da2e7c790966f9844e59
Configure your local ssh client, do this for just you be editing
$HOME/.ssh/config or system wide
/etc/ssh/ssh_config (on Minix:
You can also specify this on the command line when connecting. Note how you are told how the DNS fingerprint matches:
ssh -o VerifyHostKeyDNS=ask freshmint.phcomp.co.uk The authenticity of host 'freshmint.phcomp.co.uk (2001:4d48:ad51:2f00::2:2)' can't be established. RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23. Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?
If the fingerprint in the DNS does not match you will see the following — you may have seen similar before, but this is on a first connection:
ssh -o VerifyHostKeyDNS=ask freshmint.phcomp.co.uk @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! It is also possible that the RSA host key has just been changed. The fingerprint for the RSA key sent by the remote host is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23. Please contact your system administrator. Update the SSHFP RR in DNS with the new host key to get rid of this message. The authenticity of host 'freshmint.phcomp.co.uk (2001:4d48:ad51:2f00::2:2)' can't be established. RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23. No matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?
You don't really need to understand this bit to use the above; however if you are a technical nerd you will want to know.
Key types, these are the first number in the SSHFP RR:
Where the key types are used:
The second number in the SSHFP RR is the fingerprint type:
Thanks to Katie Foster <Katie.Foster@auspost.com.au> for reporting an error in a previous version of this page.
All description & sample files copyright (c) 2012, 2016 Parliament Hill Computers. Author: Alain D D Williams.
You may used these files as the basis your own (or organisation's/company's) project(s) (under whatever licence that you see fit). You may not claim ownership or copyright of any substantially unmodified files. Acknowledgement would be appreciated, but is not necessary.
These demonstrations are made available in the hope that they are useful. There may be errors: there is no warranty at all, use at your own risk.