The hotel in Thailand that I’m currently staying in is for some inexplicable reason blocking SSH, randomly. 6 hours ago it was working fine, but now when I need to do some work it’s decided to block me. At least that’s what I think is happening as all other requests work fine. I think it’s using some deep packet inspection too, as moving to another port also doesn’t work.
Anyway, the way round this is our friend, Tor. There are lots of ways to install Tor, but I like to have my Mac relatively clean of cruft, so I’ve opted for the TorBrowser bundle, which doesn’t come with any of the usual Tor utilities like proxychains, tsocks or torify.
Netcat is installed by default on a Mac though, and Netcat is the BOMB, and you can use it to proxy SSH over the Tor instance that comes with TorBrowser!
Provided TorBrowser is installed, running and connected, the following will let you proxy SSH over it:
ssh -o ProxyCommand=’nc -x 0:9150 %h %p’ example.com
Please note that the TorBrowser port has changed from 9050 to 9150, if you’re running a really old version of Tor you should probably update anyway.
And you thought netcat was just for reverse shells!
My home Ubuntu Jaunty installation often takes a good 40-60 seconds to connect to using SSH, none of the other servers I maintain have this same problem, they’re pretty much instantaneous, and today, on a Friday evening at 1am this irked me enough to fix it!
The first step is of course to make the SSH connection with debug output on.
Chill:~ idimmu$ ssh -v cordy
OpenSSH_5.2p1, OpenSSL 0.9.7l 28 Sep 2006
debug1: Reading configuration data /Users/idimmu/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: Connecting to cordy [192.168.0.10] port 22.
debug1: Connection established.
debug1: identity file /Users/idimmu/.ssh/identity type -1
debug1: identity file /Users/idimmu/.ssh/id_rsa type -1
debug1: identity file /Users/idimmu/.ssh/id_dsa type 2
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.1p1 Debian-5ubuntu1
debug1: match: OpenSSH_5.1p1 Debian-5ubuntu1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.2
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'cordy' is known and matches the RSA host key.
debug1: Found key in /Users/idimmu/.ssh/known_hosts:7
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
< 30 – 40 second pause occurs here >
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Trying private key: /Users/idimmu/.ssh/identity
debug1: Trying private key: /Users/idimmu/.ssh/id_rsa
debug1: Offering public key: /Users/idimmu/.ssh/id_dsa
debug1: Server accepts key: pkalg ssh-dss blen 433
debug1: read PEM private key done: type DSA
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Requesting firstname.lastname@example.org
debug1: Entering interactive session.
debug1: Requesting authentication agent forwarding.
Linux Cordy 2.6.28-15-generic #49-Ubuntu SMP Tue Aug 18 18:40:08 UTC 2009 i686
Last login: Sat Jan 23 01:22:08 2010 from chill.local
A quick Google for SSH2_MSG_SERVICE_ACCEPT and a read of man sshd_config gave me:
UseDNS Specifies whether sshd(8) should look up the remote host name and check that the resolved host name for the remote IP address maps back to the very same IP address. The default is ``yes''.
So I simply added
to the end of /etc/ssh/sshd_config and issued an sudo /etc/init.d/ssh reload and bamo, instant SSH access and I can sleep peacefully!