I found the answer to this, posting it for others' benefit. The configuration is as follows, but the parameters vary depending on the mode in which the GRE needs to work on UML/Ubuntu:

1. Boot-up UML with GRE enabled
./vmlinux ubd0=guest3,uml-ext2-img con0=null con=fd:0,fd:1 umid=guest3 mem=32m eth0=tapng,,tap1 eth1=gre,,10.0.0.1,10.0.0.2,0x0,0x0,0x9 cgroup_disable=memory init=/etc/preinit

2. Configure a 'Ethernet over GRE' tunnel on Ubuntu
#sudo ip link add gt0 type gretap remote <IP> local <IP> [csum] [seq]
#sudo ip addr add 10.20.30.70/24 dev gt0
#sudo ip link set gt0 up

With everything in between them working, the systems will now be able to talk over GRE.


From: madhusudan r <madhurjf@yahoo.com>
To: "user-mode-linux-user@lists.sourceforge.net" <user-mode-linux-user@lists.sourceforge.net>
Sent: Friday, 28 June 2013 12:12 PM
Subject: [uml-user] Configuring GRE tunnel between Ubuntu and UML

Hi,

I'm trying to setup a GRE tunnel between an Ubuntu system and a UML (running on a different system). This is how the systems were configured:

1. UML

Boot arguments: ./vmlinux ubd0=guest3,uml-ext2-img con0=null con=fd:0,fd:1 umid=guest3 mem=32m eth0=tapng,,tap1 eth1=gre,,10.0.0.1,10.0.0.2,9 cgroup_disable=memory init=/etc/preinit

root@OpenWrt:/# ifconfig
eth0      Link encap:Ethernet  HWaddr 5A:86:C4:0D:19:F5
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:43 errors:0 dropped:22 overruns:0 frame:0
          TX packets:777 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2321 (2.2 KiB)  TX bytes:312354 (305.0 KiB)
          Interrupt:5

eth1      Link encap:Ethernet  HWaddr 76:5A:7B:B7:D9:F6
          inet addr:192.168.252.1  Bcast:192.168.252.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:611 errors:0 dropped:611 overruns:0 frame:0
          TX packets:1 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:42518 (41.5 KiB)  TX bytes:402 (402.0 B)
          Interrupt:5

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

2. Ubuntu

This is the entry I added to /etc/network/interfaces:

auto gre1
iface gre1 inet static
        address 192.168.252.110
        netmask 255.255.255.252
        pre-up iptunnel add gre1 mode gre local 10.0.0.1 remote 10.0.0.2 ttl 255
        up ifconfig gre1 multicast
        pointtopoint 192.168.252.1
        post-down iptunnel del gre1

ubuntu# ifconfig

eth0      Link encap:Ethernet  HWaddr 78:e7:d1:c6:23:71
          inet addr:10.0.0.1  Bcast:10.78.49.255  Mask:255.255.255.0
          inet6 addr: fe80::7ae7:d1ff:fec6:2371/64 Scope:Link
          inet6 addr: 2001:220:abcd::20/64 Scope:Global
          inet6 addr: 2001:220:abcd:70::26/64 Scope:Global
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3331 errors:0 dropped:284 overruns:0 frame:0
          TX packets:543 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1200307 (1.2 MB)  TX bytes:114051 (114.0 KB)
          Interrupt:17

gre1      Link encap:UNSPEC  HWaddr 0A-4E-31-4B-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:192.168.252.110  P-t-P:192.168.252.110  Mask:255.255.255.252
          inet6 addr: fe80::5efe:a4e:314b/64 Scope:Link
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1476  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:168 (168.0 B)

The ubuntu box is able to ping the IP 192.168.252.1, but strangely I don't see packets going out to 10.0.0.2. Also, the UML is not able to ping 192.168.252.110.

Is this configuration correct or is there something missing?

Thanks!

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