Centos, Acquiring an IP address from the DHCP server failed.

[feriance]

Hi, really thanks for this great software.
I try to config VPNServer as a gateway. At this time, I failed to to create a successful L2TP/IPSEC connection.

Server Host is Centos 5.9,
I created a tap named "tap_soft", bridged Virtual HUB that user belongs to tap "soft". assigned 192.168.7.1 ip to tap_soft.
installed dnsmasq and configed it like this:
/etc/dnsmasq.conf:
"interface=tap_soft
dhcp-range=tap_soft,192.168.7.80,192.168.7.250,12h
dhcp-option=tap_soft,3,192.168.7.1
port=0
dhcp-option=option:dns-server,8.8.8.8"

I've added rule to iptables to allow and forward connections from 192.168.7.0/24 to VpsIPAddress.
"iptables -t filter -A FORWARD -s 192.168.7.0/24 -j ACCEPT
iptables -t nat -A POSTROUTING -o eth0 -s 192.168.7.0/24 -j MASQUERADE"

Note that I sit on a fully-restricted network with deep packet inspection. and I'm not expert in linux and networking, but i know some basics.
After Enabling "VPN Over ICMP Server Function", client authenticated successfully and just right after creating new session "SID-TEST0-[L2TP]-7", "Acquiring an IP address from the DHCP server" fails and connections goes down.
I tried with "Softether Client Manager" and immediately after connection, requesting IP Address fails and connection terminates.
Enabling Virtual DHCP (DhcpEnable) didn't help.
I'm so confused and not sure what i missed?!

[feriance]

The problem exists, any help would be appreciated.