ubuntu - 利用iptables做nat传输数据卡死?

Question

为了解决内外网访问问题，用一台ubuntu 14.04.1安装了OpenVPN，并利用ubuntu自带的iptables做NAT。但是做完后发现，简单的远程访问没有问题，但是当数据返回稍微一大，如查看一个文件，或者传输一个文件时，访问终端就会吊死。但是利用iftop查看网络流量确是正常的。
网络结构如下：

shoper@~ % ifconfig                                                                                                                      15-01-25 5:59
docker0   Link encap:Ethernet  HWaddr 56:84:7a:fe:97:99
          inet addr:172.17.42.1  Bcast:0.0.0.0  Mask:255.255.0.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

eth0      Link encap:Ethernet  HWaddr 00:50:56:b0:8a:0b
          inet addr:9.115.251.155  Bcast:9.115.251.255  Mask:255.255.255.0
          inet6 addr: fe80::250:56ff:feb0:8a0b/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:11790 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1070 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1010944 (1.0 MB)  TX bytes:149870 (149.8 KB)

eth1      Link encap:Ethernet  HWaddr 00:50:56:b0:1c:a7
          inet addr:10.10.10.5  Bcast:10.10.10.255  Mask:255.255.255.0
          inet6 addr: fe80::250:56ff:feb0:1ca7/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:492 errors:0 dropped:0 overruns:0 frame:0
          TX packets:104 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:60870 (60.8 KB)  TX bytes:11280 (11.2 KB)

需求是从eth0访问的用户能够访问到eth1网络内的机器。
iptables的规则如下：

shoper@~ % cat /etc/iptables-rules                                                                                                       15-01-25 5:59
# Generated by iptables-save v1.4.21 on Tue Jan  6 06:00:25 2015
*nat
:PREROUTING ACCEPT [1:78]
:INPUT ACCEPT [1:78]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:DOCKER - [0:0]
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE
-A POSTROUTING -o eth1 -j MASQUERADE
COMMIT
# Completed on Tue Jan  6 06:00:25 2015
# Generated by iptables-save v1.4.21 on Tue Jan  6 06:00:25 2015
*filter
:INPUT ACCEPT [80:10140]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [69:6479]
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
-A FORWARD -i eth1 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
# -A FORWARD -i eth1 -o eth0 -j ACCEPT
-A FORWARD -i eth0 -o eth1 -j ACCEPT
COMMIT
# Completed on Tue Jan  6 06:00:25 2015

换了几台终端都是同样的问题。但是在这台ubuntu上向内网访问，或者是外网终端直接访问ubuntu却都没有问题，所以估计是iptables的问题。

不知sf上有人能帮到我么？

Answer 1

After testing, it was found that it has nothing to do with iptables. I originally thought it was a problem with Ubuntu's MTU, but the test found it was invalid. Later, I captured packets and found that frames with a size of 1422 in the session were constantly being retransmitted. After debugging, I confirmed that it was an issue with the MTU size of OpenVPN.

After adding MTU configuration to both the OpenVPN server and client, the connection became normal.

tun-mtu 1500
fragment 1300
mssfix 1300

But what I am curious about is why it was OK without modifying the MTU on WINDOWS before?