On 2/25/2011 9:29 AM, c0re wrote:
Hello all!
I'm testing setting lower MTU on loopback interfaces to avoid some MTU
problems with IPSEC in a path of traffic.
ifconfig lo1 create
ifconfig lo1 mtu 1300
ifconfig lo1 5.5.5.5/32
# ifconfig lo1
lo1: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 1300
inet 5.5.5.5 netmask 0xffffffff
#ifconfig em0
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=9b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>
ether 12:ac:29:7c:fa:39
inet 10.0.0.1 netmask 0xffffff00 broadcast 10.0.0.255
media: Ethernet autoselect (1000baseTX<full-duplex>)
status: active
And I set only one "Listen 5.5.5.5:80" in http.conf in apache 2.2
# sockstat -4 | grep 80
www httpd 96843 3 tcp4 5.5.5.5:80 *:*
www httpd 96838 3 tcp4 5.5.5.5:80 *:*
www httpd 96837 3 tcp4 5.5.5.5:80 *:*
www httpd 96836 3 tcp4 5.5.5.5:80 *:*
www httpd 96835 3 tcp4 5.5.5.5:80 *:*
www httpd 96834 3 tcp4 5.5.5.5:80 *:*
root httpd 96833 3 tcp4 5.5.5.5:80 *:*
I run tcpdump -ni em0 port 80. And made telnet 5.5.5.5 80 from other
host and saw something wrong.
10:26:01.640866 IP 10.0.0.2.57553> 5.5.5.5.80: S
1049284626:1049284626(0) win 65535<mss 1460,sackOK,eol>
10:26:01.640902 IP 5.5.5.5.80> 10.0.0.2.57553: S
2144222949:2144222949(0) ack 1049284627 win 65535<mss
1460,sackOK,eol>
10:26:01.642632 IP 10.0.0.2.57553> 5.5.5.5.80: . ack 1 win 65535
5.5.5.5:80 said that it has got tcp mss 1460. Why? I was waiting for
something like 1260.
It uses the MTU of the outgoing path, which is 1500.
You change the MTU for specific paths, using route and the mtu modifier.
Like this:
lab# ifconfig em0
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=9b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>
ether 08:00:27:17:c3:de
inet 192.168.73.193 netmask 0xffffff00 broadcast 192.168.73.255
media: Ethernet autoselect (1000baseT <full-duplex>)
status: active
lab# route change 192.168.73.0 -mtu 1100
change net 192.168.73.0
lab# route -n get 192.168.73.0
route to: 192.168.73.0
destination: 192.168.73.0
mask: 255.255.255.0
interface: em0
flags: <UP,DONE,STATIC>
recvpipe sendpipe ssthresh rtt,msec mtu weight expire
0 0 0 0 1100 1 0
lab#
All packets going to 192.168.73.0/24 will use IP packet sizes up to
1100. IMHO it's better to leave the physical interface's MTU unchanged
and use the routing subsystem to define the maximum IP packet size per
path.
HTH, Nikos
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