Ssh and sshfp dns records v04
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The document discusses the use of SSH and DNS SSHFP records to enhance security when connecting to hosts. It explains how to verify the ECDSA fingerprint of a host using a Python program, the necessity of a DNSSEC-aware resolver, and the requirements for building a compatible SSH client. Additionally, it covers the process of logging in securely and highlights potential issues with older SSH clients and configurations.
![python program that gets the md5,
sha1 and sha256 fingerprints
from hashlib import md5, sha1, sha256
from base64 import b16encode, b64decode
def main():
f = open("ssh_host_ecdsa_key.pub")
line = f.read()
parts = line.split()
decodedKey = b64decode(parts[1])
h_md5 = md5(decodedKey).digest()
h_sha1 = sha1(decodedKey).digest()
h_sha256 = sha256(decodedKey).digest()
b16_md5 = b16encode(h_md5).lower()
b16_sha1 = b16encode(h_sha1).lower()
b16_sha256 = b16encode(h_sha256).lower()
print "md5 hash=%s" % (b16_md5, )
print "%s IN SSHFP 3 1 ( %s )" % (parts[2].split('@')[1], b16_sha1, )
print "%s IN SSHFP 3 2 ( %s )" % (parts[2].split('@')[1], b16_sha256, )
if __name__ == "__main__":
main()
3/3/2014
4](https://image.slidesharecdn.com/sshandsshfpdnsrecordsv04-140304063551-phpapp01/85/Ssh-and-sshfp-dns-records-v04-4-320.jpg)
![login with debug enabled
$ ssh -v -o VerifyHostKeyDNS=yes bob.novas@shinkuro.com
debug1: SSH2_MSG_NEWKEYS sent
OpenSSH_6.1p1-snap20120802, OpenSSL 1.0.0e 6 Sep 2011
debug1: expecting SSH2_MSG_NEWKEYS
debug1: Reading configuration data /usr/local/etc/ssh_config
debug1: SSH2_MSG_NEWKEYS received
debug1: Connecting to shinkuro.com [66.92.164.104] port 22.
debug1: Roaming not allowed by server
debug1: Connection established.
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: identity file /home/bob/.ssh/id_rsa type 1
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: identity file /home/bob/.ssh/id_rsa-cert type -1
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: identity file /home/bob/.ssh/id_dsa type 2
debug1: Next authentication method: publickey
debug1: identity file /home/bob/.ssh/id_dsa-cert type -1
debug1: Offering RSA public key: /home/bob/.ssh/id_rsa
debug1: identity file /home/bob/.ssh/id_ecdsa type -1
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: identity file /home/bob/.ssh/id_ecdsa-cert type -1
debug1: Offering DSA public key: /home/bob/.ssh/id_dsa
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.8
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: match: OpenSSH_5.8 pat OpenSSH_5*
debug1: Trying private key: /home/bob/.ssh/id_ecdsa
debug1: Enabling compatibility mode for protocol 2.0
debug1: Next authentication method: keyboard-interactive
debug1: Local version string SSH-2.0-OpenSSH_6.1
Password:
debug1: SSH2_MSG_KEXINIT sent
debug1: Authentication succeeded (keyboard-interactive).
debug1: SSH2_MSG_KEXINIT received
Authenticated to shinkuro.com ([66.92.164.104]:22).
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: channel 0: new [client-session]
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: Requesting no-more-sessions@openssh.com
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: Entering interactive session.
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
Last login: Sat Aug 4 16:09:24 2012 from pool-108-28-91-61.washdc.fios.verizon.net
debug1: Server host key: ECDSA c5:3c:97:64:e4:99:ba:21:1c:bf:46:35:8c:d3:48:a1
Have a lot of fun...
debug1: found 4 secure fingerprints in DNS
bob.novas@linux:~>
debug1: matching host key fingerprint found in DNS
debug1: ssh_ecdsa_verify: signature correct
3/3/2014
ssh verifies fingerprint from
host against fingerprint in
signed DNS SSHFP record
10](https://image.slidesharecdn.com/sshandsshfpdnsrecordsv04-140304063551-phpapp01/85/Ssh-and-sshfp-dns-records-v04-10-320.jpg)
Ssh and sshfp dns records v04
- 1. Secure Shell (ssh) and SSHFP DNS Records (and Secure Copy - scp)
- 2. “old” ssh For security, manually compare this fingerprint to shinkuro.com’s known ECDSA fingerprint... $ ssh bob.novas@shinkuro.com The authenticity of host 'shinkuro.com (66.92.164.104)' can't be established. ECDSA key fingerprint is c5:3c:97:64:e4:99:ba:21:1c:bf:46:35:8c:d3:48:a1. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added 'shinkuro.com,66.92.164.104' (ECDSA) to the list of known hosts. Password: Last login: Mon Jul 30 14:14:20 2012 from pool-108-28-91-61.washdc.fios.verizon.net bob.novas@linux:~> Next time you login, the fingerprint from shinkuro.com is compared to the fingerprint saved in .ssh/known_hosts (no user question unless the fingerprint doesn’t match). The $64,000 question: How do you get the “known ECDSA fingerprint” from shinkuro.com? 3/3/2014 2
- 3. ecdsa fingerprint • shinkuro.com:/etc/ssh/ssh_host_ecdsa_key.pub: ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHA yNTYAAABBBKnI9kF9QPkqqsLh7Ak7JhOtBoHjCDa928 OAApjnMerQkvrTulachl3Es30sqUGilyHkhmXVdRVQ4h Ls8qxBqNk= root@linux You essentially want to decode this base64 string and get the md5, sha1 and sha256 hash of that big number 3/3/2014 3
- 4. python program that gets the md5, sha1 and sha256 fingerprints from hashlib import md5, sha1, sha256 from base64 import b16encode, b64decode def main(): f = open("ssh_host_ecdsa_key.pub") line = f.read() parts = line.split() decodedKey = b64decode(parts[1]) h_md5 = md5(decodedKey).digest() h_sha1 = sha1(decodedKey).digest() h_sha256 = sha256(decodedKey).digest() b16_md5 = b16encode(h_md5).lower() b16_sha1 = b16encode(h_sha1).lower() b16_sha256 = b16encode(h_sha256).lower() print "md5 hash=%s" % (b16_md5, ) print "%s IN SSHFP 3 1 ( %s )" % (parts[2].split('@')[1], b16_sha1, ) print "%s IN SSHFP 3 2 ( %s )" % (parts[2].split('@')[1], b16_sha256, ) if __name__ == "__main__": main() 3/3/2014 4
- 5. run the python program: And, here’s the number that matches shinkuro.com’s ECDSA key fingerprint and proves you’re talking to the right host. > main.py md5 hash=c53c9764e499ba211cbf46358cd348a1 linux IN SSHFP 3 1 ( 3406cfb84d4be7ec8e593382c92db207bc982ee2 ) linux IN SSHFP 3 2 ( cc71ddcdb0d212d6fb88fc3df349cdc64f9d36fbb12336fd0173e01429fc14ac ) But wait... you really had to get this number “out of band” from some authoritative source who vouches that they actually ran the python program on the “right” host. 3/3/2014 5
- 6. The “new” ssh with SSHFP • now you need... – the host you want to login to must have signed SSHFP records with the host’s ecdsa sha1 and sha256 fingerprint (if it’s been recently updated to have ecdsa keys - if not, old ssh clients will work with rsa/dsa keys) – a DNSSEC aware resolver – a copy of a ssh client built with the –with-ldns option using a recent snapshot with the ecdsa code 3/3/2014 6
- 7. host: signed DNS SSHFP records >dig +dnssec @127.0.0.1 shinkuro.com SSHFP ; <<>> DiG 9.8.1 <<>> +dnssec @127.0.0.1 shinkuro.com SSHFP ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6803 ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 5, AUTHORITY: 4, ADDITIONAL: 4 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags: do; udp: 4096 ;; QUESTION SECTION: ;shinkuro.com. IN SSHFP ;; ANSWER SECTION: shinkuro.com. 14390 IN SSHFP 2 1 B6B102F02D6C791AAC26C7CCCD9F6C7290DC7B3F shinkuro.com. 14390 IN SSHFP 3 1 3406CFB84D4BE7EC8E593382C92DB207BC982EE2 shinkuro.com. 14390 IN SSHFP 3 2 CC71DDCDB0D212D6FB88FC3DF349CDC64F9D36FBB12336FD0173E014 29FC14AC shinkuro.com. 14390 IN SSHFP 1 1 295D4D75EF9BCC25BA55C98378313643853E06E0 shinkuro.com. 14390 IN RRSIG SSHFP 5 2 14400 20120911172329 20120802172329 51936 shinkuro.com. jSMtsJ 8Ff2bzHv524H8JlxFagdM+jiELbpLGjC4/YeBGd+J3gYF44Kiv ZvB93Fqlh0StQ6Z1MivXJAA2bweB4vkdC4Obktsk017XZKjebxBqkeZE 5tT50lSd/iCH 05qgcJ2hshzNeu0JRcDzHm2m4MVaKEKNZqlQVgrKixMJ pLk= 3/3/2014 7
- 8. rebuild ssh client • Build a tip version of openssh (not the latest release!) – openssh-SNAP-20120802 – openssl 1.0.1.c – ldns 1.6.13 – recent openssh snapshot with option: --with-ldns • Make sure /etc/resolv.conf points to a DNSSEC aware resolver (e.g., not FIOS cable modem!) • check ~/.ssh/known_hosts is empty 3/3/2014 8
- 9. login... $ ssh -o VerifyHostKeyDNS=yes bob.novas@shinkuro.com Password: Last login: Sat Aug 4 15:50:03 2012 from pool-108-28-91-61.washdc.fios.verizon.net Have a lot of fun... bob.novas@linux:~> or, set VerifyHostKeyDNS=yes in ~/.ssh/config: $ ssh bob.novas@shinkuro.com Password: Last login: Sat Aug 4 16:10:30 2012 from pool-108-28-91-61.washdc.fios.verizon.net Have a lot of fun... bob.novas@linux:~> 3/3/2014 9
- 10. login with debug enabled $ ssh -v -o VerifyHostKeyDNS=yes bob.novas@shinkuro.com debug1: SSH2_MSG_NEWKEYS sent OpenSSH_6.1p1-snap20120802, OpenSSL 1.0.0e 6 Sep 2011 debug1: expecting SSH2_MSG_NEWKEYS debug1: Reading configuration data /usr/local/etc/ssh_config debug1: SSH2_MSG_NEWKEYS received debug1: Connecting to shinkuro.com [66.92.164.104] port 22. debug1: Roaming not allowed by server debug1: Connection established. debug1: SSH2_MSG_SERVICE_REQUEST sent debug1: identity file /home/bob/.ssh/id_rsa type 1 debug1: SSH2_MSG_SERVICE_ACCEPT received debug1: identity file /home/bob/.ssh/id_rsa-cert type -1 debug1: Authentications that can continue: publickey,keyboard-interactive debug1: identity file /home/bob/.ssh/id_dsa type 2 debug1: Next authentication method: publickey debug1: identity file /home/bob/.ssh/id_dsa-cert type -1 debug1: Offering RSA public key: /home/bob/.ssh/id_rsa debug1: identity file /home/bob/.ssh/id_ecdsa type -1 debug1: Authentications that can continue: publickey,keyboard-interactive debug1: identity file /home/bob/.ssh/id_ecdsa-cert type -1 debug1: Offering DSA public key: /home/bob/.ssh/id_dsa debug1: Remote protocol version 2.0, remote software version OpenSSH_5.8 debug1: Authentications that can continue: publickey,keyboard-interactive debug1: match: OpenSSH_5.8 pat OpenSSH_5* debug1: Trying private key: /home/bob/.ssh/id_ecdsa debug1: Enabling compatibility mode for protocol 2.0 debug1: Next authentication method: keyboard-interactive debug1: Local version string SSH-2.0-OpenSSH_6.1 Password: debug1: SSH2_MSG_KEXINIT sent debug1: Authentication succeeded (keyboard-interactive). debug1: SSH2_MSG_KEXINIT received Authenticated to shinkuro.com ([66.92.164.104]:22). debug1: kex: server->client aes128-ctr hmac-md5 none debug1: channel 0: new [client-session] debug1: kex: client->server aes128-ctr hmac-md5 none debug1: Requesting no-more-sessions@openssh.com debug1: sending SSH2_MSG_KEX_ECDH_INIT debug1: Entering interactive session. debug1: expecting SSH2_MSG_KEX_ECDH_REPLY Last login: Sat Aug 4 16:09:24 2012 from pool-108-28-91-61.washdc.fios.verizon.net debug1: Server host key: ECDSA c5:3c:97:64:e4:99:ba:21:1c:bf:46:35:8c:d3:48:a1 Have a lot of fun... debug1: found 4 secure fingerprints in DNS bob.novas@linux:~> debug1: matching host key fingerprint found in DNS debug1: ssh_ecdsa_verify: signature correct 3/3/2014 ssh verifies fingerprint from host against fingerprint in signed DNS SSHFP record 10
- 11. DNSSEC SSHFP records • work “automatically” to do something that is otherwise difficult to do – how do you get that fingerprint? • not quite ready for prime time: – older ssh clients won’t work with hosts signed with ECDSA keys even in fallback mode – must have recent snapshot – ssh-keygen doesn’t yet generate ecdsa keys with sha1 or sha256 hash – python program to generate fingerprints – ssh isn’t built (by vendors) with option –with-ldns – but you can build your own – ssh doesn’t use VerifyHostKeyDNS option by default (but, you can set it in config file) • all of this holds true for scp (secure copy) as well 3/3/2014 11