Here’s a puzzle I’m having trouble figuring out. This afternoon, ssh from my workstation or laptop stopped working to any of my servers (at OVH). The servers are all running wheezy, the local machines jessie. This happens on both my DSL and when tethered to my mobile phone. They had not applied any updates since the last time ssh worked. When looking at it with ssh -v, they were all hanging after:

debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr umac-64@openssh.com none
debug1: kex: client->server aes128-ctr umac-64@openssh.com none
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

Now, I noticed that a server on my LAN — running wheezy — could successfully connect. It was a little different:

debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

And indeed, if I run ssh -o MACs=hmac-md5, it works fine.

Now, I tried rebooting machines at multiple ends of this. No change. I tried connecting from multiple networks. No change. And then, as I was writing this blog post, all of a sudden it works normally again. Supremely weird! Any ideas what I can blame here?