TTL returning 0 when /proc/sys/net/ipv4/ip_default_ttl is 64????

[Smith, Albert]

> We are a simulation site that uses UDP broadcasting to send packets to
> various sims.  We been using RHL 9.0 with success, but with the upgrade to
> RHEL-WS 3.3 we are now having problems with tcp TTL being set to 0.  Looking
> at /proc/sys/net/ipv4/ip_default_ttl it shows a value of 64. But yet via
> ethereal the time to live field shows 0 for all UDP packets.  This also
> appears on broadcast pings with TTL showing 0.  This does not occur on any
> of the RHL 9.0, only on the RHEL-WS systems.  
> To add, these systems have multiple interfaces (3), Intel and 3Com type
> cards and that these are UDP packets.  Is there something new under RHEL
> that is preventing UDP packets to have a TTL greater than 0?  Is there a way
> to set TTL for UDP??  Any help would be appreciated.  If  more info is
> needed, I be happy to give.
> 
> Thanks.
> 
> Steve Binyon
> System Administrator
> DMOC
> (505)853-0368 


Steve,

  Modify /etc/sysctl.conf

and hardcode the following line:

net.ipv4.ip_default_ttl = 64

save/exit

then do a sysctl -e -p

This will reset the values in /proc to their boot values plus add the change. This will hard set the TTL to 64. There is no default setting that I see for UDP only. I would check to see if you have your firewall running.

just type lokkit at the prompt and if it's enabled, disable it then do:

service iptables stop

this should shutdown your firewall altogther. If this doesn't work check the firewall on the receiving host and make sure it's allowed to send out pings and such. 

If that doesn't work check with your network admin to see if UDP is blocked on the local network.



Albert Smith
Sr. Unix Systems Administrator
Genex Services
440 E. Swedesford Rd.
Wayne, PA 19087
albert.smith at genexservices.com
(610) 964-5154