On Tue, 7 Apr 2009, Andrew Morton wrote:
Back in the days of 2.6.18 there was the SAME target which allowed, with the
option '--nodst' to SNAT internal hosts to the same address of a whole SNAT
range regardless of the destination address.
In cb76c6a597350534d211ba79d92da1f9771f8226 the SAME target was removed from
the kernel sources due to being obsolete, since the same functionality was now
in nf_nat. Shortly after that a discussion Patrick McHardy proposed a patch to
mimic the behaviour of SAME with --nodst in nf_nat by dropping the destination
IP from the jhash. The patch was dropped shortly after because it apparently
showed some uneven distribution.
The whole thread can be read at
http://thread.gmane.org/gmane.comp.security.firewalls.netfilter.devel/23275/focus=27670
.
This thread went dead, I tried to revive it but did not get an answer. We're
getting hit by this regression because we are currently NATing some thousand IP
addresses (student dorms) to an external /28. It works fine with our old
2.6.18+SAME setup, but tests with 2.6.25+SNAT showed massive issues with
connections from the same internal address to different destinations getting
NATed to different addresses in the pool. Which breaks, for example, ICQ quite
badly.
Problems like these were the reason why I wrote the SAME target in the
first place. I NAT'ed a few hundred students behind a small range of
ipaddresses and with normal SNAT they had extreme problems with ICQ and
online banking sites which require you to have the same source ip even if
you move between servers (diffrent destinations) etc.
After I wrote SAME I didn't get a single complaint.
One bonus feature that many students really liked was that they always
had the same external ipaddress as long as they had the same internal
ipaddress and the external range of ipaddresses remained the same.
This feature isn't as important as having the same source ip for all
current sessions when using online banking services etc.
I didn't think anyone was still using SAME... guess I was wrong.
The uneven distribution of the patch in question can't be worse than the
SAME distributions which, iirc, just does something like:
first_ip_in_snat_range + (client_ip % num_ips_in_snat_range)
And given many clients behind the SNAT the distribution should be quite
even.
Patrick, how about making the ipaddress selection based on only client
ipaddress behaviour selectable with an SNAT parameter if the problem with
the patch is that the distribution can be uneven for a small number of
clients?
/Martin
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