iptables 新手问一个本地发出流量修改 mss 的问题

查一下 iptables-extensions 的手册页，我记得里面好像有关于这方面的资料的，进去之后搜索 mss

iptables -A $MY_CHAIN -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1400

确定要动 mss 不是 mtu?

ip route add 0.0.0.0/0 dev eth0 advmss 1200

@lcdtyph 请问这个$MY_CHAIN 应该填成什么？ OUTPUT? 试了一下好像不行

@tomychen 我改成 ip route change 0.0.0.0/dev eth0 advmss 1200，机器直接失去连接，重启才行。

@tomychen ip route change 0.0.0.0/ via xxx.xxx.xxx.xxx advmss 1200 无用，
wget 测试后，web 服务器端收到的 SYN 包 mss 还是 1460

-I OUTPUT -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1200

@wsycqyz #4 debian 9 亲测有效，请确认 OUTPUT 链中这条规则之前没有规则被成功匹配。

@wsycqyz

看看这样行不行：

iptables -t mangle -I POSTROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1200

放在 mangle 表中的 OUTPUT 链也是可以的，但 POSTROUTING 是包最后经过的一个链，所以会覆盖 OUTPUT 里的设置。
另外 OUTPUT 里的设置只针对本机发出的包，而 POSTROUTING 对本机发出的包和经过本机的包都有效。

建议看看这个图：
https://upload.wikimedia.org/wikipedia/commons/3/37/Netfilter-packet-flow.svg

man iptables:

TCPMSS

This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing interface's MTU minus 40). Of course, it can only be used in conjunction with -p tcp. It is only valid in the mangle table.
This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets:
1)
Web browsers connect, then hang with no data received.

2)

Small mail works fine, but large emails hang.

3)

ssh works fine, but scp hangs after initial handshaking.

Workaround: activate this option and add a rule to your firewall configuration like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu
--set-mss value
Explicitly set MSS option to specified value.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40).
These options are mutually exclusive.
TOS

This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.
--set-tos tos
You can use a numeric TOS values, or use
iptables -j TOS -h
to see the list of valid TOS names.

@wsycqyz 抱歉，没测试，刚测一下确实不行

不服气又试了一下...
是可以的
root@ubuntu-virtual-machine:~# netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
192.168.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
root@ubuntu-virtual-machine:~# ip route change 0.0.0.0/0 via 192.168.1.1 dev eth0 advmss 1140
root@ubuntu-virtual-machine:~# netstat -nr
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 192.168.1.1 0.0.0.0 UG 1180 0 0 eth0
192.168.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0

只是在设置为 1140 的时候会 MSS 的值为 1180，但是服务器 tcpdump 得到的 mss 是 1140

@tomychen 我的 ubuntu16.04 确实不行，不确定问题在哪里

@pheyx 谢谢