SSH Piper


SSh Piper works as a proxy-like ware, and route connections by username, src ip , etc.

+---------+                      +------------------+          +-----------------+
|         |                      |                  |          |                 |
|   Bob   +----ssh -l bob----+   |   SSH Piper   +------------->   Bob' machine  |
|         |                  |   |               |  |          |                 |
+---------+                  |   |               |  |          +-----------------+
                             +---> pipe-by-name--+  |                             
+---------+                  |   |               |  |          +-----------------+
|         |                  |   |               |  |          |                 |
|  Alice  +----ssh -l alice--+   |               +------------->  Alice' machine |
|         |                      |                  |          |                 |
+---------+                      +------------------+          +-----------------+


 Downstream                         SSH Piper                       Upstream                     

Install

go get github.com/tg123/sshpiper/sshpiperd
go install github.com/tg123/sshpiper/sshpiperd

with pam module support

go get -tags pam github.com/tg123/sshpiper/sshpiperd
go install -tags pam github.com/tg123/sshpiper/sshpiperd

Docker image

Pull

docker pull farmer1992/sshpiperd

Run ❓ what is WORKING_DIR

docker run -d -p 2222:2222 \
  -v /etc/ssh/ssh_host_rsa_key:/etc/ssh/ssh_host_rsa_key \
  -v /YOUR_WORKING_DIR:/var/sshpiper \
  farmer1992/sshpiperd

Run with Additional Challenge

use env CHALLENGER to specify which challenger to use

docker run -d -p 2222:2222 \
  -e CHALLENGER=pam \
  -v /YOUR_PAM_CONFIG:/etc/pam.d/sshpiperd \
  -v /etc/ssh/ssh_host_rsa_key:/etc/ssh/ssh_host_rsa_key \
  -v /YOUR_WORKING_DIR:/var/sshpiper \
  farmer1992/sshpiperd

Quick start

Just run showme.sh in sshpiperd exmaple directory

$GOPATH/src/github.com/tg123/sshpiper/sshpiperd/example/showme.sh

the example script will setup a sshpiper server using

ssh 127.0.0.1 -p 2222 -l github # connect to github.com:22
ssh 127.0.0.1 -p 2222 -l linode # connect to lish-atlanta.linode.com:22

connect to linode

$ ssh 127.0.0.1 -p 2222 -l linode.com:22
linode@127.0.0.1's password:

connect to github.com:22

$ ssh 127.0.0.1 -p 2222 -l github
Permission denied (publickey).

Configuration

$ sshpiperd -h
  -c="": Additional challenger name, e.g. pam, emtpy for no additional challenge
  -h=false: Print help and exit
  -i="/etc/ssh/ssh_host_rsa_key": Key file for SSH Piper
  -l="0.0.0.0": Listening Address
  -p=2222: Listening Port
  -w="/var/sshpiper": Working Dir

Files inside Working Dir

Working Dir is a /home-like directory. SSHPiperd read files from workingdir/[username]/ to know upstream's configuration.

e.g.

workingdir tree

.
├── github
│   └── sshpiper_upstream
└── linode
    └── sshpiper_upstream

when ssh sshpiper_host -l github, sshpiper reads workingdir/github/sshpiper_upstream and the connect to the upstream.

User files

These file MUST be in mode 400

• sshpiper_upstream

  one line file upstream_host:port e.g. github.com:22

• authorized_keys

  OpenSSH format authorized_keys (see ~/.ssh/authorized_keys). Used for publickey sign again(see below).

• id_rsa

  RSA key for publickey sign again(see below).

Publickey sign again

During SSH publickey auth, RFC 4252 Section 7, ssh client sign session_id and some other data using private key into a signature sig. This is for server to verify that the connection is from the client not the man in the middle.

However, sshpiper actually holds two ssh connection, and it is doing what the man in the middle does. the two ssh connections' session_id will never be the same, because they are hash of the shared secret. RFC 4253 Section 7.2.

To support publickey auth, sshpiper will modify the sig using a private key (id_rsa) in the workingdir/[username]/.

How this work

+------------+        +------------------------+                       
|            |        |                        |                       
|   client   |        |   SSH Piper            |                       
|   PK_X     +-------->      |                 |                       
|            |        |      v                 |                       
|            |        |   Check PK_X           |                       
+------------+        |   in authorized_keys   |                       
                      |      |                 |                       
                      |      |                 |     +----------------+
                      |      v                 |     |                |
                      |   sign agian           |     |   server       |
                      |   using PK_Y  +-------------->   check PK_Y   |
                      |                        |     |                |
                      |                        |     |                |
                      +------------------------+     +----------------+

e.g.

on client

ssh-copy-id -i PK_X test@sshpiper

on ssh piper server

ln -s ~test/.ssh/authorized_keys workingdir/test/authorized_keys
ssh-keygen -N '' -f workingdir/test/id_rsa  # this is PK_Y
ssh-copy-id -i workingdir/test/id_rsa test@server

now ssh test@sshpiper -i -i PK_X, sshpiper will send PK_Y to server instead of PK_X.

Additional Challenge

ssh piper allows you run your own challenge before dialing to the upstream. if a client failed in this challenge, connection will be closed. however, the client has to pass the upstream server's auth in order to establish the whole connection. Additional Challenge is required, but not enough.

This is useful when you want use publickey and something like google-authenticator together. OpenSSH do not support use publickey and other auth together.

Available Challengers

• pam

  Linux-PAM challenger

  this module use the pam service called sshpiperd

  you can configure the rule at /etc/pam.d/sshpiperd

API

sshpiper use a modified version of golang.org/x/crypto/ssh. sshpiperd now is the front-end of the modified ssh.

TODO List

• deb package
• live upgrade
• unit test
• API doc
• man page
• hostbased auth support
• ssh-copy-id support or tools