I have a twofold question about OpenSSH rekeying. I use OpenSSH 6.4 on the server side and the connection between client and server is a SSH2 connection.
The questions:
How do I know when a rekey has happened? Should I check any logs, or any specific output? Running sshd in debug more (-d) does not show me anything related to rekeying.
It is printed only in debug mode, because it should not be a think you should care about. It should work out of the box as it is. But anyway, you can increase log level from your client using:
-vvv switch to your ssh command (DEBUG3)LogLevel DEBUG3 config option in your ~/.ssh/config~v escape sequence during your sessionHow can I trigger a rekey?
Rekey is handled by configuration option RekeyLimit in both client and server config (even though a bit broken until openssh-7.2). If you set up low value it should rekey more often, but there is no point in doing that.
By hand in session, you can trigger one, by escape sequence ~R. After that (in at least DEBUG log level or -v, you will see a lot of debug messages:
[root@f24 ~]# ~?
Supported escape sequences:
~. - terminate connection (and any multiplexed sessions)
~B - send a BREAK to the remote system
~C - open a command line
~R - request rekey
~V/v - decrease/increase verbosity (LogLevel)
~^Z - suspend ssh
~# - list forwarded connections
~& - background ssh (when waiting for connections to terminate)
~? - this message
~~ - send the escape character by typing it twice
(Note that escapes are only recognized immediately after newline.)
[root@f24 ~]# ~v [LogLevel VERBOSE]
~v [LogLevel DEBUG]
[root@f24 ~]# ~R
debug1: SSH2_MSG_KEXINIT sent
debug1: rekeying in progress
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: [email protected]
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: [email protected] MAC: <implicit> compression: none
debug1: kex: client->server cipher: [email protected] MAC: <implicit> compression: none
debug1: kex: [email protected] need=64 dh_need=64
debug1: kex: [email protected] need=64 dh_need=64
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: rekeying in progress
debug1: rekeying in progress
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:E9HuzpVQJ/5DavNIZhWzJrADNj5Ntw69RJ1obXJtlGo
debug1: set_newkeys: rekeying, input 5156 bytes 167 blocks, output 5864 bytes 0 blocks
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: set_newkeys: rekeying, input 5168 bytes 0 blocks, output 5864 bytes 0 blocks
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS received
According to this stackexchange Question Does RekeyLimit property in sshd_config enforce re-keying / key re-exchange? you need to make debug more talkative (-ddd). The debug mode (even with a single -d) is sufficient. All you need to do is trigger the rekeying process.
When using OpenSSH as client, simply enter ~R (capital R!) and rekeying will take place. On server side you will see
debug1: SSH2_MSG_KEXINIT received
debug1: SSH2_MSG_KEXINIT sent
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: expecting SSH2_MSG_KEX_ECDH_INIT
debug1: set_newkeys: rekeying
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: set_newkeys: rekeying
debug1: SSH2_MSG_NEWKEYS received
When using PuTTY as client you can force rekeying by 'Special Command' -> 'Repeat Key Exchange' which will result in a similar server log:
debug1: SSH2_MSG_KEXINIT received
debug1: SSH2_MSG_KEXINIT sent
debug1: kex: client->server aes256-ctr hmac-sha2-256 none
debug1: kex: server->client aes256-ctr hmac-sha2-256 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent
debug1: set_newkeys: rekeying
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: set_newkeys: rekeying
debug1: SSH2_MSG_NEWKEYS received
