[Bridge] Why TTL is changing when sending a ping?

Stephen Hemminger shemminger at linux-foundation.org
Sun Apr 1 12:05:19 PDT 2007


On Fri, 30 Mar 2007 22:30:36 +0200 (CEST)
"ArcosCom Linux User" <linux at arcoscom.com> wrote:

> The situation is this:
> 
> INTERNET -- ROUTER -- ETHERNET+WIFI -- PC's
> 
> The conection between INTERNET and ROUTER is done with 2 LINKs with static
> IP's.
> 
> The conection between ROUTER and PC's is done via ETHERNET lan with many
> bridges and ACCESSPOINTS.
> 
> The PC's are in a IP subnet, the BRIDGES in another IP subnet, the AP's in
> another IP subnet. The ROUTER has 1 bridge interface (2 real ethernets in
> the bridge) connected to the LAN.
> 
> In the router exists then br0, br0:1, br0:2, br0:3 (PCs, APs, BRIDGEs IP
> subnets) to allow IP connection over the ETHERNET+WIFI between ROUTER and
> clients.
> 
> The principal purpose of the ROUTER is to allow internet acces to PC's.
> The BRIDGES and AP's have got implemented STP protocol and appears to be
> working fine (ap's and bridges are embedded linux boxes).
> 
> In router I have enabled rp_filter in all interfaces, default and each one.
> The ip routing is enabled too (obviously).
> 
> I detected that a normal ping from ROUTER to one PC usually has a TTL=64,
> but many times that TTL changes to 128.
> 
> What could be the problem? The "routing" enabled in bridge devices?
> Some TCP/IP parameter I don't configured fine?
> Any idea?
> 

Are you using some form of connection tracking filtering on the bridge?
If the packet has to be regenerated as part of filtering it might
restart the TTL hop count.



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