Strange problem about iptables DNAT.
 

Hi,

I have a Linux box with RedHat 9.0 installed, this box is a firewall&proxy. Now I want external user can access my internal web server via the firewall box.

According to RedHat 9.0's manual and the posts in internet, I used the following command.

#iptables -t nat -A PREROUTING -p TCP -d xxx.xxx.xxx.xxx --dport 80 -j DNAT --to 192.168.0.5:80

But I can't access the internal web server.

use tcpdump to get the following packets:

21:57:18.274817 192.168.0.85.1331 > 218.77.120.200.25460: udp 49
21:57:18.450579 218.77.120.200 > 192.168.0.85: icmp: 218.77.120.200 udp port 25460 unreachable [tos 0xc0]
21:57:18.968829 192.168.0.5.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:18.969963 0.00:30:48:23:04:33.455 > 0.ff:ff:ff:ff:ff:ff.455: ipx-netbios 50
21:57:19.057680 61.144.148.161.50660 > 192.168.0.5.http: S 5685372:5685372(0) win 8192 <mss 1452,nop,nop,sackOK> (DF)
21:57:19.718043 192.168.0.5.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:19.749255 0.00:30:48:23:04:33.455 > 0.ff:ff:ff:ff:ff:ff.455: ipx-netbios 50
21:57:20.468067 192.168.0.5.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:20.528584 0.00:30:48:23:04:33.455 > 0.ff:ff:ff:ff:ff:ff.455: ipx-netbios 50
21:57:22.020715 61.144.148.161.50660 > 192.168.0.5.http: S 5685372:5685372(0) win 8192 <mss 1460,nop,nop,sackOK> (DF)
21:57:23.444576 arp who-has 192.168.0.85 tell 192.168.0.1
21:57:23.444815 arp reply 192.168.0.85 is-at 0:e0:4c:ef:55:f8
21:57:23.533007 218.17.247.6.http > 192.168.0.85.1383: R 562882410:562882410(0) ack 2793007952 win 0
21:57:24.054574 arp who-has 192.168.0.5 tell 192.168.0.1
21:57:24.054674 arp reply 192.168.0.5 is-at 0:30:48:23:4:33
21:57:27.919595 0.00:30:48:23:04:33.4010 > 0.ff:ff:ff:ff:ff:ff.452:ipx-sap-resp[|ipx 64]
21:57:28.024632 61.144.148.161.50660 > 192.168.0.5.http: S 5685372:5685372(0) win 8192 <mss 1460,nop,nop,sackOK> (DF)
21:57:29.248044 192.168.0.100.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:29.248486 192.168.0.100.netbios-dgm > 192.168.0.255.netbios-dgm: NBT UDP PACKET(138)
21:57:33.839985 192.168.0.100.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:34.581878 192.168.0.100.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:35.332929 192.168.0.100.netbios-ns > 192.168.0.255.netbios-ns: NBT UDP PACKET(137): QUERY; REQUEST; BROADCAST
21:57:40.026871 61.144.148.161.50660 > 192.168.0.5.http: S 5685372:5685372(0) win 8192 <mss 1460,nop,nop,sackOK> (DF)
21:57:43.191397 arp who-has 192.168.0.5 tell 192.168.0.4
21:58:18.987637 arp who-has 192.168.0.5 tell 192.168.0.222

As I said, this box is a proxy too, so the above packets maybe contain unuseful message to analysis where the problem is.

Anyway, anybody can help me?

thanks,

The rule seems to be ok on condition that earlier rules don't drop the packets.
But you need also a rules for forwarding the packets.

For instance:
iptables -A FORWARD -m state --state NEW -i $INT_IF -p tcp --dport 80 -j ACCEPT
for forwarding the packets to internal box
and
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
for forwarding packets from internal LAN boxes