Use ssh with a specific network interface

Question

I'm using openconnect to connect to vpn. After entering my credentials, I get this:

POST https://domain.name/...
Got CONNECT response: HTTP/1.1 200 OK
CSTP connected. DPD 30, Keepalive 30
Connected tun0 as xxx.xxx.xxx.xxx, using SSL
Established DTLS connection

Running ifconfig shows I have a new network interface tun0 with a certain ip address.

Question: How do I make ssh use only the network interface tun0 so that I can access computers on that private network?

Edit:

My network configuration (route -n) seems to be this:

172.16.194.0    0.0.0.0         255.255.255.0   U     0      0        0 vmnet8
192.168.0.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
172.16.25.0     0.0.0.0         255.255.255.0   U     0      0        0 vmnet1
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
0.0.0.0         192.168.0.1     0.0.0.0         UG    100    0        0 eth0

Solution 1

It's not the ssh client that decides through which interface TCP packets should go, it's the kernel. In short, SSH asks the kernel to open a connection to a certain IP address, and the kernel decides which interface is to be used by consulting the routing tables.

(The following assumes you're on GNU/Linux; the general concept is the same for all Unices, but the specifics of the commands to run and the way the output is formatted may vary.)

You can display the kernel routing tables with the commands route -n and/or ip route show.

OpenConnect should have added a line for the tun0 interface; connections to any address matching that line will be routed through that interface. For example, running route -n on my laptop I get the following output:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.30.0.1       0.0.0.0         UG    0      0        0 eth0
10.30.0.0       0.0.0.0         255.255.255.0   U     1      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
192.168.122.0   0.0.0.0         255.255.255.0   U     0      0        0 virbr0

This means that connections to hosts in the 192.168.122.0/24 (i.e., addresses 192.168.122.0 to 192.168.122.255 according to CIDR notation) network will be routed through interface virbr0; those to 169.254.0.0/16 and 10.30.0.0/24 will go through eth0, and anything else (the 0.0.0.0 line) will be routed through eth0 to the gateway host 10.30.0.1.

Solution 2

I don't know when it was introduced but the OpenSSH client on RHEL7 has this in its manpage:

 -b bind_address
         Use bind_address on the local machine as the source address of the connection.  Only useful on systems with more than one address.

Not as good as being able to choose the interface, but close.

Solution 3

Just addition of an Answer. You can use -b flag and define your source IP at access time.

Format + Example

ssh -b interface-ip remote-ip
ssh -b 10.11.22.40 10.11.22.38 

Solution 4

If you are using Network Manager to manage your internet connections (as is the default manager on many systems), you may want to install both openconnect and network-manager-openconnect.

Once the OpenConnect plugin is installed for Network Manager, open Network Manager and click the + icon in the lower-left. You should be given a combo-box with the option VPN and then the ability to select OpenConnect Compatible VPN.

By using Network Manager to interface with OpenConnect, your routes will automagically appear and help you connect to the VPN. This is especially helpful for accessing servers over VPN, such as how FireHost does things.

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Comments

[Eli Heady]

Can you elaborate on your network configuration? With proper routing in place, any traffic destined for the network attached to tun0 will use that interface.

[axel22]

Thanks for clarifying this for me - it seems that openconnect did not add a line for the tun0 interface. I suppose I should do this manually.

[heyman]

@axel22 You might have a look here: bbs.archlinux.org/viewtopic.php?id=69064 for a script that uses openconnect and sets up the routes.

[Mathiewz]

@RiccardoMurri Would you like to answer my question

[Henrik]

Also the -B flag, which appears to allow for specifying the name of the network interface to use.

[John]

The option -b bind_address did not work for me, somehow. Changing routes temporarily should work. BTW: The -B option does not exist on the SSH version that comes with Ubuntu.

[iBug]

@John Even when bound to an interface, it still depends on the kernel routing table, except that it'll match iif (outgoing interface) routing rule entries (i.e. ip rule).