Date: Mon, 12 Dec 2005 08:01:09 -0800 From: "Ted Mittelstaedt" <tedm@toybox.placo.com> To: "Yance Kowara" <yance_kowara@yahoo.com> Cc: freebsd-questions@freebsd.org Subject: RE: FreeBSD router two DSL connections Message-ID: <LOBBIFDAGNMAMLGJJCKNIEAAFDAA.tedm@toybox.placo.com> In-Reply-To: <20051212075707.5314.qmail@web30312.mail.mud.yahoo.com>
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>-----Original Message----- >From: Yance Kowara [mailto:yance_kowara@yahoo.com] >Sent: Sunday, December 11, 2005 11:57 PM >To: Ted Mittelstaedt >Subject: RE: FreeBSD router two DSL connections > > >Ted, > >Thanks for the advice. > >A friend of mine has just acquired an Internet Cafe. >The previous owner connected the lan to 2 different >ADSL (two different ISPs) one is a back up he said. > >So, two ADSL routers with half the Lan connected to >one router and another half to the other router. > Most likely the trick used was to setup 2 independent routers, one on each DSL line, and set half of the machines to use one router as their default gateway, and half of the systems to use the other. If they really did use separate physical networks that is a dumb idea, because you now have problems copying update files and such in between systems in the Cafe. It is a very crude form of redundancy but this is NOT a load-sharing scenario. Keep in mind the real need of an Internet Cafe is redundancy, not bandwidth, so although crude, this solution is one of the few solutions that is available on a shoestring that is really effective. >I am just thingking of a way to optimise the >connection and came accross Steven's article. I >thought I could do something similar with *BSD + pf. > >There is such thing as Dual Wan ADSL router: >http://www.infosmart.com.tw/p-ndr3024.htm > And they do NOT work to combine bandwidth. What these devices do is they split the NAT translation table and whichever DSL line is unused gets the next translation slot allocated. However the restriction is each translation slot still only gets the bandwidth available for that DSL line. Thus if your web-surfing and 1 DSL line is busy, you get shunted to the next, but you cannot get the bandwidth available from both lines at the same time, on the same PC. Now, if you happened to open 2 separate FTP sessions on your PC, and if the load-sharer was sophisticated enough, it might be able to put 1 session on 1 DSL line, and the other on the other. But each session is still limited to the top speed of the DSL line. To the uninitiated, however, that might APPEAR to work as a bandwidth load balancer. The challenge I have always posed to the proponents of this trick was to post results of downloading the latest FreeBSD iso file that show they got the iso file in half the time. Never been met, of course. These devices also have a lot of trouble detecting when one of the DSL lines is having a problem. For example you could have 1 DSL line going very, very slow, the router thinks that circuit is still up because all it can do is decide if a DSL line is up or not - but traffic going through this is dog-slow. If for example one of those Internet Cafe PC's got infected with a mass-mailing virus, it would cause exactly that scenario. Would you rather have 1/2 of the PC's in the Internet cafe that are using the slow DSL line as their default gateway just get dog-slow, and the other 1/2 continue to work normally, or would you rather have every single PC in the Cafe become intermittently slow when one of the DSL lines gets slow? >However, they are quite pricey compare to setting up a >*BSD box (using old readily available hardware). > The NAT software in FreeBSD (and indeed, in any UNIX os) does not have the notion of separate route tables and cannot do this. In fact, just about all Cisco or other high-end routers cannot deal with multiple, independent route tables in the same box. > >So, if this load balancing idea does not work, any >other thing I can do to optimise two DSLs? > >I also came accross this (linux way): >http://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.rpdb.multiple- >links.html > >Is this worth trying? > It is the same issue - would you rather have half the PCs in the Cafe get slow if there's a problem, or all of them become intermittently slow? I know about that Linux howto. It came about a few years or so ago when the bozo that wrote it, who had no understanding of networking, posted exactly the same question you posted on one of the major networking mailing lists, and when he was told it wasn't possible, he got so pisssed off he was going to show those upity mucks that he knew better than they did. The result is a scheme that appeared to work enough to satisfy this guy's ego, he never of course has posted any followup as to how well it works when presented with the kinds of failure scenarios (fiber-seeking backhoe, etc.) that are common in real life. It's easier for the proctor of the Internet Cafe to simply tell the customer if one PC is acting up to go to another one that isn't. Also keep in mind that unless both DSL lines are coming in on completely separate wiring plants, you really don't have true redundancy. If your going to do this on the cheap, it would be more effective to use 1 DSL line for some of the machines, and a cable modem for the other. Like the other guy said, if your friend wants more bandidth, buy a business-class DSL line for more money. :-) Ted
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