<div class="news_item">#457<div class="news_title">SSH hopping with the ProxyCommand
Posted by dandriff on Wednesday August 29, 2012@04:18PM
I started off with a basic ProxyCommand configuration, like this:
Host 10.0.0.15 HostName 10.0.0.15 ProxyCommand ssh 10.0.0.2 nc %h %p IdentityFile ~/.ssh/id_rsa
$ ssh 10.0.0.15 Permission denied (publickey). $ ssh -v 10.0.0.15 OpenSSH_5.6p1, OpenSSL 0.9.8r 8 Feb 2011 debug1: Reading configuration data /Users/wi-fi/.ssh/config debug1: Applying options for * debug1: Applying options for api1b debug1: Reading configuration data /etc/ssh_config debug1: Applying options for * debug1: Executing proxy command: exec ssh -C 10.0.0.2 nc 10.0.0.15 22 debug1: permanently_drop_suid: 501 debug1: identity file /Users/wi-fi/.ssh/id_rsa type 1 debug1: identity file /Users/wi-fi/.ssh/id_rsa.pub type -1 debug1: Remote protocol version 2.0, remote software version OpenSSH_5.9p1 Debian-5ubuntu1 debug1: match: OpenSSH_5.9p1 Debian-5ubuntu1 pat OpenSSH* debug1: Enabling compatibility mode for protocol 2.0 debug1: Local version string SSH-2.0-OpenSSH_5.6 debug1: SSH2_MSG_KEXINIT sent debug1: SSH2_MSG_KEXINIT received debug1: kex: server->client aes128-ctr hmac-md5 email@example.com debug1: kex: client->server aes128-ctr hmac-md5 firstname.lastname@example.org debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent 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 '10.0.0.15' is known and matches the RSA host key. debug1: Found key in /Users/wi-fi/.ssh/known_hosts:89637 debug1: ssh_rsa_verify: signature correct debug1: SSH2_MSG_NEWKEYS sent debug1: expecting SSH2_MSG_NEWKEYS debug1: SSH2_MSG_NEWKEYS received debug1: Roaming not allowed by server debug1: SSH2_MSG_SERVICE_REQUEST sent debug1: SSH2_MSG_SERVICE_ACCEPT received debug1: Authentications that can continue: publickey debug1: Next authentication method: publickey debug1: Offering RSA public key: /Users/wi-fi/.ssh/id_rsa debug1: Authentications that can continue: publickey debug1: No more authentication methods to try. Permission denied (publickey).
But what each of these publications fails to mention as a possible problem is that if your username doesn't match on both the intermediate and destination hosts, the ProxyCommand won't work until you specify the usernames in your ~/.ssh/config like this:
Host 10.0.0.15 HostName 10.0.0.15 User wi-fizzle ProxyCommand ssh 10.0.0.2 nc %h %p IdentityFile ~/.ssh/id_rsa
The transparent multi-hop connections can be very useful but you may find that it takes a second or two to establish each connection. This delay can become annoying if it happens a lot (e.g.: every time you save a file from the text editor).
The good news is that if you can configure SSH to reuse an existing connection. This means that for example if you have an SSH shell session running then a new connection for SCP can skip the connection setup phase. Two steps are required:
First, you must create a directory (or 'folder') which SSH will use to keep track of established connections:
ControlMaster auto ControlPath /home/YOUR-NAME/.ssh/tmp/%h_%p_%r