[Shorewall-users] Way OT: MSS/MTU question.

Joshua Banks l0f33t at yahoo.com
Sat Dec 13 23:36:09 PST 2003


Hello,

I've been trying to track down the answer to this and am coming up
empty handed. I'm hoping someone might shed some light on the
following regarding mtu/mss values.

When I do: "netstat -rn"  or "route -nee"
*************************
root at deadmeat jbanks # netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window 
irtt Iface
63.187.254.9    0.0.0.0         255.255.255.255 UH       40 0         
0 ppp0
192.168.1.0     0.0.0.0         255.255.255.0   U        40 0         
0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U        40 0         
0 eth1
127.0.0.0       127.0.0.1       255.0.0.0       UG       40 0         
0 lo
0.0.0.0         63.187.254.9    0.0.0.0         UG       40 0         
0 ppp0


***********************

Why is the "mss" value "40" ? Everything works just fine. (e.g...
downloading email attatchments and doing file tranfers so I'm
thinking that this is a bug of some sort.)

When I do "ifconfig", it shows the appropriate mtu value but this isn't
reflected in the routing table?

eth0      Link encap:Ethernet  HWaddr 00:10:4B:72:F8:7E
          inet addr:192.168.1.1  Bcast:192.168.1.255 
Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3962 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12601 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:482119 (470.8 Kb)  TX bytes:11268641 (10.7 Mb)
          Interrupt:9 Base address:0xa000

eth1      Link encap:Ethernet  HWaddr 00:A0:CC:D3:DE:3A
          inet addr:192.168.1.2  Bcast:192.168.1.255 
Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:11720 errors:1 dropped:0 overruns:0 frame:0
          TX packets:40 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:1579402 (1.5 Mb)  TX bytes:1680 (1.6 Kb)
          Interrupt:5 Base address:0xa400

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:216 errors:0 dropped:0 overruns:0 frame:0
          TX packets:216 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:17141 (16.7 Kb)  TX bytes:17141 (16.7 Kb)

ppp0      Link encap:Point-to-Point Protocol
          inet addr:63.187.232.34  P-t-P:63.187.254.9 
Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1524  Metric:1
          RX packets:7208 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7826 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:5965919 (5.6 Mb)  TX bytes:940048 (918.0 Kb)

I know that -IP header (20 bytes) and -TCP heard (20 bytes) would give
you MTU -40 bytes wich would give you a MSS value or 1460 bytes for
ethernet. Does anyone have any ideas why or links that point to an
explanation of this low MSS value.

I'm running a 2.4.20r7 kernel.

Thanks,
JBanks




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