Vixie Enterprises

Non-BGP Multihoming for Web Hosts

Paul Vixie

Table of Contents

• Stan Barber's Notes
• February 1997 NANOG Home Page

Stan Barber's Notes

Every form of publication known to man has not really successful until it is used to publish pornography.

Mutihomed Server Implications

• Has only provider assigned address space
• Needs multiple full routing tables
• Depends on DNS "Round Robin"
• Motives: performance or reliability?

If you want to load balance, you can use multiple A records and it works until one of the provides goes down. Then, only ½ get through (unless the client is bright about trying all addresses, which some are).

Outbound Route Balancing

• TCP is ACK-timed -> path symmetry is good.
• Symmetry is the intermediate is really hard. We really have "hot potato routing".
• First hop symmetry is most of what you want anyway.
• BGP path selection is hard to defeat. Typically, this means that one connection will be used a lot more than the other.

Per-interface Default Route

• BSD TCP binds outbound route to PCB on SYN-ACK
• Our trick: remember the inbound interface identity from the SYN
• Each interface has its own "default route"
• For outbound TCP and all UDP, a normal default is also needed.

Observed results

• Incoming connections rotate per DNS round robin.
• Users stopped complaining about slow/hung connection
• Providers like it better than sending a full BGP table

Complications

• Used socket utility to forward telnet
• Some squidversions leak memory, need periodic restarts
• A full NAT Solution would be easier to manage
• DEC PAIX may be the only place where this can be used

Future work

• Round Robin for outgoing TCP, QoS is a problem, but may not be worth doing.
• Do more thinking about UDP, round robin is probably not a good idea
• Get a WWW server vendor interested
• Probably a dead end for Vixie

ftp://ftp.vix.com/pub/vixie/ifdefault for the software. Diffs are small.

Questions & Answers

Matt Butak from InterNex. Have you tried to use some other DNS to do real load balancing DNS?

No, not really. I think that using DNS for policy is a bad idea. With DYNUpdate, there can be a way to take an A records off line when those addresses are unreachable.