Vigneswaran R <vignesh@xxxxxxxxxxx> writes:
> On 09/13/2013 10:40 AM, Nikolaus Rath wrote:
>> I've enabled packet forwarding and SNAT on the "ebox" computer as
>> follows:
>>
>> root@ebox:~# ip route
>> default via 23.92.25.1 dev eth0
>> 23.92.25.0/24 dev eth0 proto kernel scope link src 23.92.25.96
>> 192.168.12.0/24 dev rath proto kernel scope link src 192.168.12.1
>>
>> root@ebox:~# iptables -L -n -v
>> Chain INPUT (policy ACCEPT 1314 packets, 1736K bytes)
>> pkts bytes target prot opt in out source destination
>>
>> Chain FORWARD (policy DROP 0 packets, 0 bytes)
>> pkts bytes target prot opt in out source destination
>> 150K 62M ACCEPT all -- rath eth0 0.0.0.0/0 0.0.0.0/0
>> 86746 200M ACCEPT all -- eth0 rath 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
>> 319 22076 LOG all -- * * 0.0.0.0/0 0.0.0.0/0 limit: avg 1/min burst 30 LOG flags 0 level 4 prefix "Rejected forwarding: "
>> 393 26172 REJECT all -- * * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-net-prohibited
>>
>> Chain OUTPUT (policy ACCEPT 1142 packets, 2412K bytes)
>> pkts bytes target prot opt in out source destination
>> root@ebox:~# iptables -t nat -L -n -v
>> Chain PREROUTING (policy ACCEPT 36378 packets, 2383K bytes)
>>
>> Chain INPUT (policy ACCEPT 19982 packets, 1334K bytes)
>> pkts bytes target prot opt in out source destination
>>
>> Chain OUTPUT (policy ACCEPT 61430 packets, 4601K bytes)
>> pkts bytes target prot opt in out source destination
>>
>> Chain POSTROUTING (policy ACCEPT 8333 packets, 564K bytes)
>> pkts bytes target prot opt in out source destination
>> 69488 5081K SNAT all -- * eth0 0.0.0.0/0 0.0.0.0/0 to:23.92.25.96
>>
>> >From a second computer "vostro", I can now use ebox as a
>> gateway:
>>
>> root@vostro:~# ip route add 190.93.249.164 via 192.168.12.1
>>
>> This works fine, now connections to whatismyip.com (190.93.249.164) go
>> through ebox.
>>
>> However, when I try to be a bit more selective on vostro and use a
>> special routing table, things don't work anymore:
>>
>> root@vostro:~# iptables -t mangle -L -n
>> Chain PREROUTING (policy ACCEPT)
>> target prot opt source destination
>>
>> Chain INPUT (policy ACCEPT)
>> target prot opt source destination
>>
>> Chain FORWARD (policy ACCEPT)
>> target prot opt source destination
>>
>> Chain OUTPUT (policy ACCEPT)
>> target prot opt source destination
>> MARK tcp -- 0.0.0.0/0 190.93.249.164 tcp dpt:80 MARK set 0x1
>> LOG tcp -- 0.0.0.0/0 190.93.249.164 tcp dpt:80 LOG flags 0 level 4 prefix "marked: "
>>
>> Chain POSTROUTING (policy ACCEPT)
>> target prot opt source destination
>>
>> root@vostro:~# ip route del 190.93.249.164 via 192.168.12.1
>> root@vostro:~# ip route add default via 192.168.12.1 table tovpn
>> root@vostro:~# ip rule add fwmark 0x1 table tovpn
>>
>> Now connections from vostro to 190.93.249.164 still make it to ebox, and
>> from ebox to 190.93.249.164, but the answers get stuck on ebox:
>>
>> Sep 13 04:47:53 ebox kernel: Rejected forwarding: IN=eth0 OUT=eth0 MAC=f2:3c:91:69:db:07:84:78:ac:0d:79:c1:08:00 SRC=190.93.249.164 DST=192.168.17.47 LEN=60 TOS=0x00 PREC=0x00 TTL=58 ID=0 DF PROTO=TCP SPT=80 DPT=39024 WINDOW=14480 RES=0x00 ACK SYN URGP=0
>>
>> It seems that ebox tries to send the packet destined to go trough the
>> rath to eth0 instead, and consequency rejects them because forwarding is
>> only enabled from eth0 to rath.
>>
>> However, this only happens when vostro has the gateway route set in a
>> special routing table rather than the default table -- but how does ebox
>> even know about that?
>>
>> Can someone explain to me what is happening here and why?
>
> I have a doubt. It seems, rath of ebox is assigned with IP address in
> the range 192.168.12.0/24.
> However, IP address of vostro seems to be
> 192.168.17.47 (assuming /24). Ebox doesn't have any route to this
> range. So it try to use default route via eth0.
>
> What I assume is, 'vostro' has IP addresses in (atleast) two ranges
> (192.168.12.0/24, 192.168.17.0/24).
That's correct.
nikratio@vostro:~$ ip addr
5: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP
link/ether c8:60:00:bf:a2:7f brd ff:ff:ff:ff:ff:ff
inet 192.168.17.47/24 brd 192.168.17.255 scope global br0
valid_lft forever preferred_lft forever
inet6 fe80::ca60:ff:febf:a27f/64 scope link
valid_lft forever preferred_lft forever
6: rath: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 500
link/none
inet 192.168.12.4/24 scope global rath
valid_lft forever preferred_lft forever
> In the default routing table, the src IP is set to 192.168.12.x (for
> the packets originating from vostro). However, the 'tovpn' table
> didn't specify the src IP. So, when the 'tovpn' table is being used,
> the packets may have got the src IP as 192.168.17.x.
Hmm. This would make sense, but looking at the default table, the source
address for the route via 192.168.12.x is actually also not set:
nikratio@vostro:~$ ip route
default via 192.168.17.1 dev br0
190.93.249.164 via 192.168.12.1 dev rath
192.168.12.0/24 dev rath proto kernel scope link src 192.168.12.4
192.168.17.0/24 dev br0 proto kernel scope link src 192.168.17.47
This works just fine, despite the entry having no source address. So why
is it working in the default table, but not in the tovpn table?
> I think, you can avoid this by explicitly specifying the src IP when
> adding the route to 'tovpn' table,
>
> ip route add default via 192.168.12.1 src 192.168.12.x table tovpn
I'll of course try this nevertheless, thanks!
Best,
Nikolaus
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