Date: Fri, 25 Feb 2011 10:29:29 +0300 From: c0re <nr1c0re@gmail.com> To: FreeBSD <freebsd-questions@freebsd.org>, freebsd-net@freebsd.org Subject: Strange behavior of MTU on loopback interfaces. Message-ID: <AANLkTi=HBE_D-RYNcdTYWXzr7w4z=%2BQ-JUiOBpQVYLhP@mail.gmail.com>
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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.
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