How to get gateway of network interface?

Question

Playing around with VPN connections on SoftEther, I created a virtual network interface for the VPN. Now in order to set iptables to route via this network interace, I need the gateway of the VPN server (local ip of the host). Is there an easy way to get it? I tried ip neigh but this does not list my virtual network interface.

Solution 1

When configuring the network interface using dhclientit is possible to set the verbose flag which will yield something like

~ $  dhclient -v vpn_se
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/vpn_se/00:ac:2b:d3:d9:e8
Sending on   LPF/vpn_se/00:ac:2b:d3:d9:e8
Sending on   Socket/fallback
DHCPDISCOVER on vpn_se to 255.255.255.255 port 67 interval 3 (xid=0xa0efb173)
DHCPREQUEST of 10.211.1.22 on vpn_se to 255.255.255.255 port 67 (xid=0x73b1efa0)
DHCPOFFER of 10.211.1.22 from 10.211.254.254
DHCPACK of 10.211.1.22 from 10.211.254.254
bound to 10.211.1.22 -- renewal in 1403 seconds.

where the lines starting with DHCPOFFER and DHCPACK contain the clients local IP and the VPN gateway.

Solution 2

• Try ip r. It will show info in the form

  default via aa.bb.cc.dd ...
  

• or netstat -r -

  Kernel IP routing table
  Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
  

• traceroute goo.gl or tracepath goo.gl will show all routers (VPN-gateway at the top)

Solution 3

To be honest , as you are on a private network, the easiest way is to ask your network administrator.

The next easiest way is to connect with a working connection, perhaps without VPN

you then run ifconfig and ip r, or similar, and you can get useful information about your network including netmask.

inet 10.0.0.8 netmask 255.255.255.0 broadcast 10.0.0.255

ip r default via 10.0.0.1 dev enp0s3 proto static metric 100 tracepath google.com

If you can not obtain your network architecture that way, it can be difficult to impossible.

Start with typical guesses such as x.x.x.1 or x.x.x.254

If that does not work it can be difficult to impossible, it is going to depend on the network architecture, the gateway can be firewalled or may only respond to certain features such as mac or ipaddress, or 802.1x networks. Some networks will not allow you to connect even if you know the gateway.

You would have to start with a packet sniffer, such as tcpdump or wireshark, capture network traffic, and make a guess at the gateway.

Or you can try to discover hosts using nmap

nmap -p 22 --open x.x.x.0/24

The nmap command assumes the gateway is running ssh on the standard port, 22

If that does not work, you have to be more liberal with nmap and guess the gateway

Note: Even if you discover or guess the gateway, you may not be able to connect.

Comments

[Suppenkasper]

The problem is that the gateway is not yet listed in iptables. For my VPN connection to work I want to add it as default but first I have to know the local ip address of the router that I am connected to via VPN

[N0rbert]

Normal PPTP and OpenVPN gateways are shown in the output of these commands. Does ifconfig -a list you virtual interface?

[N0rbert]

Try traceroute or tracepath.

[Suppenkasper]

That would require sending packages via VPN which is not possible because the routing is not done yet.

[Panther]

@N0rbert - your answer is correct, as long as one is already connected to the network / gateway. If you are not already connected, and do not know the gateway, it can be difficult to impossible to discover or access can be blocked even if you discover the correct ip, part of wireless security or otherwise.