General Question IPTABLES with DHCP

laurens · 08-04-2009, 06:01 PM

On my gateway 192.168.0.1 I have the following configuration for my network interfaces

Code:

debian1:/home# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:0c:29:3a:64:49
          inet addr:192.168.126.128  Bcast:192.168.126.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:29ff:fe3a:6449/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:48240 errors:0 dropped:0 overruns:0 frame:0
          TX packets:21633 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:60015843 (57.2 MiB)  TX bytes:1623577 (1.5 MiB)
          Interrupt:19 Base address:0x2000

eth1      Link encap:Ethernet  HWaddr 00:0c:29:3a:64:53
          inet addr:192.168.0.1  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:29ff:fe3a:6453/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:24433 errors:0 dropped:0 overruns:0 frame:0
          TX packets:44292 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2138382 (2.0 MiB)  TX bytes:53612898 (51.1 MiB)
          Interrupt:19 Base address:0x2080

eth2      Link encap:Ethernet  HWaddr 00:0c:29:3a:64:5d
          inet addr:192.168.1.1  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:29ff:fe3a:645d/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:5017 errors:0 dropped:0 overruns:0 frame:0
          TX packets:62 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:582408 (568.7 KiB)  TX bytes:3036 (2.9 KiB)
          Interrupt:16 Base address:0x2400

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:67 errors:0 dropped:0 overruns:0 frame:0
          TX packets:67 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:19932 (19.4 KiB)  TX bytes:19932 (19.4 KiB)

Now, I never really understood if IPTABLES works as good with DHCP on one of the adapters.

E.g. First, situation1 where eth1 (connected to internet) has a static IP address, 172.23.81.248

Code:

# Natting
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 172.23.81.248

Situation2: when you don't have a static IP but a DHCP IP, can you replace the IP address -simply- with the interface (here eth1) ?!

Code:

# Natting
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to eth1

If so, is this the best method?

Thanks a advance for clarifying !

laurens · 08-05-2009, 01:45 AM

No one ?

fotoguy · 08-05-2009, 04:35 AM

If your internet interface is using DHCP then you use masquerading instead of snat.

Quote:

/sbin/modprobe ipt_MASQUERADE
echo "1" > /proc/sys/net/ipv4/ip_dynaddr
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

laurens · 08-05-2009, 05:43 AM

Quote:

Originally Posted by fotoguy

If your internet interface is using DHCP then you use masquerading instead of snat.

Seems logical, thanks! But, anyway, it is possible use IPTABLES with their interface id like "from eth0 -to- eth1" (hypothetical)