ssh-keyfetch

ssh-keyfetch — Host key tool for the Secure Shell client

Synopsis

ssh-keyfetch [options...]
[host]

Description

ssh-keyfetch is a tool that downloads server host keys and optionally sets them as known host keys for the Secure Shell client. It is typically used by the system administrator during the initial setup phase.

By default the host key is fetched from the server and saved in file key_host_port.suffix in the current directory.

Options

The following options are available:

-a, --set-trusted

Instead of writing the public key to a file, add the public key as a known host key to the user-specific directory: $HOME/.ssh2/hostkeys. This option cannot be combined with -C or -K.

Caution

	Caution
When ssh-keyfetch is run with the `-a` option, it accepts the received host keys automatically without prompting the user. You should verify the validity of keys by verifying the key fingerprints after receiving them or you risk being subject to a man-in-the-middle attack. To validate the host key, obtain the host key fingerprint from a trusted source (for example by calling the server administrator) and verify it against the output from command: ssh-keygen-g3 --fingerprint <hostname>

When ssh-keyfetch is run with the -a option, it accepts the received host keys automatically without prompting the user. You should verify the validity of keys by verifying the key fingerprints after receiving them or you risk being subject to a man-in-the-middle attack.

To validate the host key, obtain the host key fingerprint from a trusted source (for example by calling the server administrator) and verify it against the output from command:

ssh-keygen-g3 --fingerprint <hostname>

-A, --fetch-any

Probe for and fetch either server public key or certificate.

-C, --fetch-certificate

Probe for and fetch the server certificate only.

-d, --debug debug-level

Enable debugging.

-D, --debug-default

Enable debugging with default level.

-f, --filename-format nameformat

Filename format for known host keys. Accepted values are plain and hashed. The default is plain.

-F, --fingerprint-type [ =babble | babble-upper | pgp-2 | pgp-5 | hex | hex-upper ]

Public key fingerprint type for fingerprints displayed in messages and log. Most popular types are babble (the SSH babble format) and hex. The default is babble. See also the option --rfc4716.

-H, --hash [ =md5 | sha1 ]

Specifies the digest algorithm for fingerprint generation. Valid options are md5 and sha1.

-K, --kex-key-formats typelist

Explicitly specify the host-key types accepted in protocol key exchange. For experts only. See RFC 4253 for details.

-l, --log

Report successfully received keys in log format. The log format consists of one line per key, six fields per line. The fields are:

accept|save
replace|append
hostname
ip-port
user-id
key-file-path
fingerprint

-o, --output-file output-file

Write result to output-file. A minus sign ("-") denotes standard output.

-O, --output-directory output-dir

Write result to output-dir. The default is the current directory.

-p, --port port

Server port (default: 22).

-P, --fetch-public-key

Probe for and fetch the server public key only. This is the default behaviour.

-q, --quiet

Quiet mode, report only errors.

-R, --rfc4716

Displays the public key fingerprints in the format specified in RFC 4716. The digest algorithm (hash) is md5, and the output format is the 16-bytes output in lowercase HEX separated with colons (:).

-S, --proxy-url socks-url

Specifies the SOCKS server to use.

-t, --timeout timeout

Connection timeout in seconds (default: 10 seconds).

--append [ =yes | no ]

Instead of appending a new host key, overwrite the existing known host keys for this host. Optional values are yes and no. The default is to append.

-V, --version

Displays version string and exits.

Environment Variables

In order to run ssh-keyfetch the following environment variables must be set:

_BPXK_AUTOCVT=ON: If this variable is not set correctly ssh-keyfetch fails to start.

SSH_SOCKS_SERVER: The address of the SOCKS server used by ssh-keyfetch.

Examples

Connect to the server through a SOCKS proxy:

$ ssh-keyfetch -S socks://fw.example.com:1080/10.0.0.0/8 server.outside.example
Public key from server.outside.example:22 saved.
 File: server.outside.example.pub
 Fingerprint: xucar-bened-liryt-lumup-minad-tozuc-pesyp-vafah-mugyd-susic-guxix

Accept the server key as a known key for Tectia Client and report in the more rigid log format:

$ ssh-keyfetch -a -l  newhost
Accepted newhost 22 testuser /home/testuser/.ssh2/hostkeys/key_22_newhost.pub 
xigad-hozuf-kykek-vogid-dumid-bydop-mulym-zegar-nybuv-muled-syxyx

Accept the server key as a known key for Tectia client tools for z/OS and store the key to global configuration hostkeys directory:

$ ssh-keyfetch -a --output-directory /etc/ssh2/hostkeys 
Accepted newhost 22 testuser /etc/ssh2/hostkeys/key_22_anotherhost.pub 
bydop-mulym-zegar-nybuv-muled-syxyx-xigad-hozuf-kykek-vogid-dumid

Accept the server key as a known key for Tectia Client and use an uninformative hash as the filename for the stored known key:

$ ssh-keyfetch -f hashed -a  newhost
Public key from newhost:22 accepted as trusted hostkey.
 File:
 /home/testuser/.ssh2/hostkeys/keys_420b23ca959ab165e52e117a90baa89d92ffc535
 Fingerprint:
 xigad-hozuf-kykek-vogid-dumid-bydop-mulym-zegar-nybuv-muled-syxyx

Fetch the X.509 certificate of the server running in port 222 and display the content with ssh-certview:

$ ssh-keyfetch -C -p 222 -o - newhost | ssh-certview -
Certificate = 
  SubjectName = <C=FI, O=SSH, OU=DEV, CN=newhost.ssh.com>
  IssuerName = <C=FI, O=SSH, CN=Sickle CA>
  SerialNumber= 24593438
  Validity = 
    NotBefore = 2007 Sep 13th, 15:10:00 GMT
    NotAfter  = 2008 Sep 12th, 15:10:00 GMT
  PublicKeyInfo = 
    PublicKey =
      Algorithm = RSA
      Modulus n  (1024 bits) :
...
  Fingerprints = 
    MD5 = 3c:71:17:9b:c2:12:26:cf:96:27:fb:d7:a8:19:37:89
    SHA-1 =
    14:72:f3:0f:20:5e:75:ed:d2:c3:86:4b:69:45:00:47:ae:fe:31:64

This explicit key exchange type list is equivalent to specifying option -A:

$ ssh-keyfetch -K ssh-rsa,ssh-dss,x509v3-sign-rsa,x509v3-sign-dss newhost 
Public key from newhost:22 saved.
 File: key_newhost_22.pub
 Fingerprint:
 xigad-hozuf-kykek-vogid-dumid-bydop-mulym-zegar-nybuv-muled-syxyx