Skip to main content

Reverse SSH tunnel: how can I send my port number to the server?




I have two machines, Client and Server.



Client (who is behind a corporate firewall) opens a reverse SSH tunnel to Server, which has a publicly-accessible IP address, using this command:




ssh -nNT -R0:localhost:2222 insecure@server.example.com




In OpenSSH 5.3+, the 0 occurring just after the -R means "pick an available port" rather than explicitly calling for one. The reason I'm doing this is because I don't want to pick a port that's already in use. In truth, there are actually many Clients out there that need to set up similar tunnels.




The problem at this point is that the server does not know which Client is which. If we want to connect back to one of these Clients (via localhost) then how do we know which port refers to which client?



I'm aware that ssh reports the port number to the command line when used in the above manner. However, I'd also like to use autossh to keep the sessions alive. autossh runs its child process via fork/exec, presumably, so that the output of the actual ssh command is lost in the ether.



Furthermore, I can't think of any other way to get the remote port from Client. Thus, I'm wondering if there is a way to determine this port on Server.



One idea I have is to somehow use /etc/sshrc, which is supposedly a script that runs for every connection. However, I don't know how one would get the pertinent information here (perhaps the PID of the particular sshd process handling that connection?) I'd love some pointers.



Thanks!



Answer



Wouldn't a VPN be more appropriate? OpenVPN is super simple to configure. Here is a sample config and some links to guide your through the certificate creation process:



apt-get install openvpn
mkdir /etc/openvpn/easy-rsa
mkdir -p /etc/openvpn/ccd/client_server
touch /etc/openvpn/ipp.txt
cp -a /usr/share/doc/openvpn/examples/easy-rsa/2.0/* /etc/openvpn/easy-rsa
cd /etc/openvpn/easy-rsa
source ./vars

./clean-all
./build-ca 
./build-dh
./build-key-server server
cd /etc/openvpn/easy-rsa/keys
openssl pkcs12 -export -out server.p12 -inkey server.key -in server.crt -certfile ca.crt


Then create a new file /etc/openvpn/client_server.conf and put the following in it, changing the SERVER_IP_ADDRESS as appropriate




local SERVER_IP_ADDRESS
port 8443
proto udp
dev tun
ca /etc/openvpn/easy-rsa/keys/ca.crt
pkcs12 /etc/openvpn/easy-rsa/keys/server.p12
dh /etc/openvpn/easy-rsa/keys/dh1024.pem
ifconfig-pool-persist /etc/openvpn/ipp.txt
server 192.168.100.0 255.255.255.0
client-config-dir /etc/openvpn/ccd/client_server

ccd-exclusive
keepalive 10 120
comp-lzo
persist-key
persist-tun
status /var/log/openvpn-status.log
verb 3
reneg-sec 0



Then build a key per user who is going to connect, and create the config file in the ccd dir



./build-key-pkcs12 user1@domain.com
echo "ifconfig-push 192.168.100.2 255.255.255.0" > /etc/openvpn/ccd/client_server/user1@domain.com


The IP address MUST be suitable for a /30 subnet (see http://www.subnet-calculator.com/cidr.php), as there is only 2 addresses available (server and client) per connection. So your next available client IP would be 192.168.100.6 and so on.



Then you now have static IPs per connecting user.




Then supply the user1@domain.com.p12 file to the end-user and use the following config file



client
dev tun
proto udp
remote SERVER_IP_ADDRESS 8443
pkcs12 user1@domain.com.p12
resolv-retry infinite
nobind
ns-cert-type server

comp-lzo
verb 3
reneg-sec 0

Comments

Post a Comment

Popular posts from this blog

linux - iDRAC6 Virtual Media native library cannot be loaded

When attempting to mount Virtual Media on a iDRAC6 IP KVM session I get the following error: I'm using Ubuntu 9.04 and: $ javaws -version Java(TM) Web Start 1.6.0_16 $ uname -a Linux aud22419-linux 2.6.28-15-generic #51-Ubuntu SMP Mon Aug 31 13:39:06 UTC 2009 x86_64 GNU/Linux $ firefox -version Mozilla Firefox 3.0.14, Copyright (c) 1998 - 2009 mozilla.org On Windows + IE it (unsurprisingly) works. I've just gotten off the phone with the Dell tech support and I was told it is known to work on Linux + Firefox, albeit Ubuntu is not supported (by Dell, that is). Has anyone out there managed to mount virtual media in the same scenario?
Post a Comment
Read more

ubuntu - Monitoring CPU, Mem, disk, on a single server

I've been looking for a simple starter solution for monitoring my [currently] single server hosted solution. Other than Nagios and similar, are there other good (simple) solutions people are using? Answer Everything depends on what you want. For example Munin is very simple, you can install and configure it in less then 10 minutes (on one server), it can sends alarms, make graphs from monitoring cpu, mem. apache connections, eaccellerator, disk io and many many more (it has many plugins). But if you are planning in future get some more machines, munin may not be enough. For example in munin you cant monitor state of individual processes, can't monitor changes in files (for security purpose). So if you wanna only see what is the utilization of basics parameters on your server and don't plan to buy some more servers Munin is what you are looking for, but if you wanna be alarmed when some of your service is down, take more control on what is happeninig on...
Post a Comment
Read more

hp proliant - Smart Array P822 with HBA Mode?

We get an HP DL360 G8 with an Smart Array P822 controller. On that controller will come a HP StorageWorks D2700 . Does anybody know, that it is possible to run the Smart Array P822 in HBA mode? I found only information about the P410i, who can run HBA. If this is not supported, what you think about the LSI 9207-8e controller? Will this fit good in that setup? The Hardware we get is used but all original from HP. The StorageWorks has 25 x 900 GB SAS 10K disks. Because the disks are not new I would like to use only 22 for raid6, and the rest for spare (I need to see if the disk count is optimal or not for zfs). It would be nice if I'm not stick to SAS in future. As OS I would like to install debian stretch with zfs 0.71 as file system and software raid. I have see that hp has an page for debian to. I would like to use hba mode because it is recommend, that zfs know at most as possible about the disk, and I'm independent from the raid controller. For us zfs have many benefits, ...
Post a Comment
Read more