forwarding with ttl=1

Lee Dilkie lee at
Thu Feb 5 05:47:25 PST 2004

> Hi All
> I am newbie with configuring networks under FreeBSD . I have
> small network with gateway running on FreeBSD 5.2 Release .My
> ISP offers me pppoe service for connecting to Internet . I
> didn't have problems with configuring ppp with pppoe . I used
> nat option wchich works fine for masquerading the local
> network from the world . The problem is that the ISP's
> gateway returns every time packets with ttl=1 which makes
> further forwarding impossible . My gateway returns icmp error
> mesage "time exceeded" and discards packets . I want to know
> if I made some mistake with configuring nat service or if not
> what is the solution of the problem ? Is there any service
> that can increment ttl and process the packet ? I tried to
> avoid the checking of ttl in the ip_forward() function in
> ip_input.c and skipping the decrement of ttl and everything
> works fine but i think that this is very ugly kernel hack .
> Probably there is an easy and elegant solution . Any ideas ?

TTL (Time To Live) is a counter value in an IP packet that gets decremented
by every "hop" (router). When it reaches a value of zero, the packet is

Using a TTL of 1 isn't going to be very useful as your packets will all be
discarded at the first router. This is working as intended.

Normally, TTL is set to 128-ish.

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