请教：LVS/NAT模式的问题

我配置了一个LVS/NAT 的应用。
所有的机器都是fedora linux ，kernel ：2.6.16
配件：
1. 三个realserver，配置分别为：192.168.3.2 , 192.168.3.3 , 192.168.3.4。他们的网关均为192.168.3.1 ，子网掩码均为255.255.255.0 广播地址均为:192.168.3.255
2. 一个LD，二个网卡，eth2，eth3 。 配置分别为：192.168.1.172 ，192.168.3.1
3. 1个clinet做测试。
结构：
client（192.168.1.212）---------（192.168.1.172）LD（192.168.3.1）----switch-----（192.168.3.x）realserver(共3个)

配置：
在LD上：
echo 1 >/proc/sys/net/ipv4/ip_forward
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
echo 0 > /proc/sys/net/ipv4/conf/default/send_redirects
echo 0 > /proc/sys/net/ipv4/conf/eth2/send_redirects

ipvsadm -C
ipvsadm -A -t 192.168.1.172:80 -s rr
ipvsadm -a -t 192.168.1.172:80 -r 192.168.3.2:80 -m
ipvsadm -a -t 192.168.1.172:80 -r 192.168.3.3:80 -m
ipvsadm -a -t 192.168.1.172:80 -r 192.168.3.4:80 -m

确认内核已经支持ipvs。内核编译配置如下：
<*> IP virtual server support (EXPERIMENTAL)
[*] IP virtual server debugging
(12) IPVS connection table size (the Nth power of 2)
--- IPVS transport protocol load balancing support
[*] TCP load balancing support
[*] UDP load balancing support
[*] ESP load balancing support
[*] AH load balancing support
--- IPVS scheduler
<*> round-robin scheduling
<*> weighted round-robin scheduling
<*> least-connection scheduling
<*> weighted least-connection scheduling
<*> locality-based least-connection scheduling
<*> locality-based least-connection with replication scheduling
<*> destination hashing scheduling
<*> source hashing scheduling
<*> shortest expected delay scheduling
<*> never queue scheduling
--- IPVS application helper
<*> FTP protocol helper

现在的问题是：
从client发送过去的syn请求，realserver能够收到，但是不响应，3次syn包过后，断开连接。
LD上的ipvsadm查看如下：
# ipvsadm -l
IP Virtual Server version 1.2.1 (size=4096)
Prot LocalAddress:Port Scheduler Flags
-> RemoteAddress:Port Forward Weight ActiveConn InActConn
TCP 192.168.1.172:www rr
-> 192.168.3.4:www Masq 1 0 0
-> 192.168.3.3:www Masq 1 0 0
-> 192.168.3.2:www Masq 1 0 0

在LD上的内网 网口（192.168.3.1）上抓包如下：

15:32:00.723152 00:91:27:74:f2:2a > 00:91:27:76:00:64, ethertype IPv4 (0x0800), length 62: IP (tos 0x0, ttl 128, id 59533, offset 0, flags [DF], proto 6, length: 48) 192.168.1.212.50601 > 192.168.3.4.80: S [tcp sum ok] 3522911210:3522911210(0) win 65535

15:32:03.728032 00:91:27:74:f2:2a > 00:91:27:76:00:64, ethertype IPv4 (0x0800), length 62: IP (tos 0x0, ttl 128, id 59543, offset 0, flags [DF], proto 6, length: 48) 192.168.1.212.50601 > 192.168.3.4.80: S [tcp sum ok] 3522911210:3522911210(0) win 65535

15:32:09.747632 00:91:27:74:f2:2a > 00:91:27:76:00:64, ethertype IPv4 (0x0800), length 62: IP (tos 0x0, ttl 128, id 59555, offset 0, flags [DF], proto 6, length: 48) 192.168.1.212.50601 > 192.168.3.4.80: S [tcp sum ok] 3522911210:3522911210(0) win 65535

在realserver上抓包如下：
15:27:50.123656 00:91:27:74:f2:2a > 00:91:27:76:00:64, ethertype IPv4 (0x0800), length 62: IP (tos 0x0, ttl 128, id 59533, offset 0, flags [DF], proto 6, length: 48) 192.168.1.212.50601 > 192.168.3.4.80: S [tcp sum ok] 3522911210:3522911210(0) win 65535

15:27:53.128441 00:91:27:74:f2:2a > 00:91:27:76:00:64, ethertype IPv4 (0x0800), length 62: IP (tos 0x0, ttl 128, id 59543, offset 0, flags [DF], proto 6, length: 48) 192.168.1.212.50601 > 192.168.3.4.80: S [tcp sum ok] 3522911210:3522911210(0) win 65535

15:27:59.148699 00:91:27:74:f2:2a > 00:91:27:76:00:64, ethertype IPv4 (0x0800), length 62: IP (tos 0x0, ttl 128, id 59555, offset 0, flags [DF], proto 6, length: 48) 192.168.1.212.50601 > 192.168.3.4.80: S [tcp sum ok] 3522911210:3522911210(0) win 65535

也就是说，realserver是可以收到请求的，但是为什么不响应呢？而且可以肯定的是，realserver上跑的webserver是好的。内部相互访问web都是可以的。
是不是哪里没配置好？请不吝赐教。谢谢！