Solved

No Routing

Posted on 1997-09-08
3
289 Views
Last Modified: 2010-05-18
I can't ping the ppp0 port on my linux server.

I have a small LAN with the linux server acting as the S/W router to my ISP. All the hardware is setup and seems to be behaving itself. From within the LAN I can ping all the(legal) PC IP addresses. I can also ping all those addresses from the linux PC. I can also ping the ppp0 port and the rest of the world from the linux PC. What I can't do is ping beyond the ppp0 port from within the LAN from any other PC, other than the linux PC. For example, I can't ping the ISP's gateway from within the LAN.

When I try pinging the internet from within the LAN, there are no Tx or Rx lights flashing on the modem, as there is when pinging from the linux PC.

When I run the debug switch for routed, I get coninuous messages saying that "sendto: Network is unreachable" and "ADD Route: Broken Pipe"

I am running Slackware 2.0.0
0
Comment
Question by:vaughnm
3 Comments
 
LVL 3

Expert Comment

by:sauron
Comment Utility
Have you recomplied your kernel with IP forwarding turned on???
0
 

Expert Comment

by:DeeKs
Comment Utility
You need IP Forwarding switched on, but also unless you have been assigned more than 1 ip address for your link this wont work with out using masquarding etc...

0
 
LVL 1

Accepted Solution

by:
jantypas earned 50 total points
Comment Utility
There may be three things going on here:

- I'll assume your ISP has assigned you a series of IP
   addresses and you're not doing IP Masquerading

First, do the obvious and make sure your IP Forwarding option is enabled.  (I'm sure you already did, but it never hurts to check)

Next, obviously, be running routed or gateway to serve as a route broadcast... again, obvious, but let's get these out of the way.

Now some issues.  Some ISPs like to assign your ppp0 (your side) address out of the space you are alloted.  In effect, if you are assigned six addresses, your UNIX machine actually uses two of them.  This is a bad thing and makes Linux cry a lot.  Specifically, it requires that HOST ROUTING work correctly, it doesn't.  ISP's like this because they use less address space for you.  Linux hates it.

What happens in routing:

An interior PC sends a packet to its default gateway.
The linux box receives the packet and asks:

  Is this packet I've received destined for addresses
  within the netmask'ed address space I call my
  local ethernet or myself?

If your ISP assigned your ppp0 side of things an address
outside of "your IP space", the answer is no.  So your
kernel forwards the packet to the default route, down the
ppp0 link to the modem... easy enough.  Even if they assigned in "your space", the packet would still go out...

Here's the catch.  When an IP ACK is sent back in protocol, i9t must find its way BACK to the sending workstation,  It arrives at your PPP link and finds the UNIX box.  (No where else for it to go.)  Unix asks the same question:

   Is this packet destined for an address outside of my
   "owned" address space.  If so route it.

If your ISP assigned space "in your space" for ppp0,
the answer is NO.  Your kernel dutifully forwards the
link into or out of ppp0 again!  This is the host-routing bug.  Proper host routing requires the kernel recognize a point-to-point link and handle it special.

The solution is to ask your ISP to make sure your ppp0 link has addresses on each side that are NOT within your allotted space.  Then any packets into or out of your net will be handled correctly.  
0

Featured Post

Zoho SalesIQ

Hassle-free live chat software re-imagined for business growth. 2 users, always free.

Join & Write a Comment

I have seen several blogs and forum entries elsewhere state that because NTFS volumes do not support linux ownership or permissions, they cannot be used for anonymous ftp upload through the vsftpd program.   IT can be done and here's how to get i…
Note: for this to work properly you need to use a Cross-Over network cable. 1. Connect both servers S1 and S2 on the second network slots respectively. Note that you can use the 1st slots but usually these would be occupied by the Service Provide…
This video discusses moving either the default database or any database to a new volume.
This video explains how to create simple products associated to Magento configurable product and offers fast way of their generation with Store Manager for Magento tool.

771 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

9 Experts available now in Live!

Get 1:1 Help Now