Problem with routing decisions, and multihop

/dev/rob0 rob0 at gmx.co.uk
Mon Jul 4 20:06:08 CEST 2005


On Monday 04 July 2005 11:54, Lluís Batlle wrote:
> Thanks :) I answer between lines...

Thank you.

> On 7/4/05, /dev/rob0 <rob0 at gmx.co.uk> wrote:
>  > >>="masquerading.multi-eth" (misnamed: it does no masquerading)
>
> Ok. I tried with MASQUERADE, but by now I use SNAT.

Right. MASQUERADE will not work with multiple routing.

> > >>NE1=192.168.16.0/28
> > >>NE2=192.168.17.0/28
> >
> > Let's see, those are .0-.15 on the last quad.
> >
> > >>NLOCAL=192.168.0.0/20
> >
> > And this is 0.0 through 15.255 ... IOW, wrong, excluding both $NE1
> > and $NE2. Try 192.168.16.0/23. It would not hurt for you to brush
> > up on TCP/IP and subnetting basics.
>
> Oh. Is it wrong? I don't understand what's "IOW". Where should I try
> your proposed subnet? why?

IOW="in other words", a common Internet shorthand.

192.168.0.0/20, set as $NLOCAL in your iptables script, excludes your
IP addresses and networks. No packet hitting the rules which refer to 
that value will match, so the rules are ignored.

The rules to which I am referring:
$IPTABLES -t nat -A POSTROUTING -o eth1 -s $NLOCAL -j SNAT --to $IPE1
$IPTABLES -t nat -A POSTROUTING -o eth2 -s $NLOCAL -j SNAT --to $IPE2
Your SNAT rules.

Change "NLOCAL=192.168.0.0/20" to "NLOCAL=192.168.0.0/16", or as 
previously suggested, "NLOCAL=192.168.16.0/23". I suppose you could 
even omit the source specification altogether:
$IPTABLES -t nat -A POSTROUTING -o eth1 -j SNAT --to $IPE1
$IPTABLES -t nat -A POSTROUTING -o eth2 -j SNAT --to $IPE2

> > 1. IMO it's confusing to give chains the same name in different
> > tables.
>
> I agree... but by now does that matter?

Simply a point of style. You can give chains any names you wish, no 
matter how confusing they might be in context:

###  Kids, don't try this at home. Professional stunt driver on a
###  closed track.
iptables -N InputLogDrop
iptables -N ForwardAllow
iptables -A InputLogDrop -j ACCEPT
iptables -A FORWARD -j InputLogDrop
iptables -A ForwardAllow -j LOG
iptables -A ForwardAllow -p tcp -j REJECT
iptables -A ForwardAllow -j DROP
iptables -A INPUT -j ForwardAllow
###  For my next trick, I will campaign to be elected Prime Minister.
###  Thank you for your support in the polls.

> > 3. --state in -t nat? Is that possible? Does it work? Does it break
> > anything?
>
> It seems it's possible. I get no error from those commands. Anyway,

Perhaps it doesn't break anything, but I have read here that only 
packets of --state NEW hit the -t nat PREROUTING chain. I don't know 
about the relationship between connection tracking and NAT.

> I've thought that happens double application of that rule, through
> filter and nat tables. I've removed everything about 'keep_state' in
> the nat table. Everything is still working bad. Even from the

Likely because of NLOCAL in your script. If that's not the case it's 
beyond my limited understanding, and once again I'll suggest you take 
it to LARTC. Some people in LARTC know more about this than I do.

> > already-NAT'ed Linux machine. Why not do the DNAT in the external
> > routers? Also, those DNAT rules refer to other RFC 1918 netblocks.
>
> mmm I've never read RFC 1918. :) I'll take a look at it.

"RFC 1918 netblocks" is simply another form of shorthand to refer to 
IPv4 ranges which are reserved for private use, namely 10.0.0.0/8, 
172.16.0.0/12, and 192.168.0.0/16. I rarely read RFC's myself (but I 
must confess to a fondness for RFC 1149. :) )
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