[dns] "buffer size too small" error

[rade]

--- FAIL: TestDNSServer (1.53s)
    util.go:69: Unexpected error: dns: buffer size too small
        goroutine 74 [running]:
        github.com/zettio/weave/testing.stackTrace(0xa23700, 0x0, 0x0)
            /home/matthias/go/src/github.com/zettio/weave/testing/util.go:78 +0x88
        github.com/zettio/weave/testing.StackTrace(0x0, 0x0)
            /home/matthias/go/src/github.com/zettio/weave/testing/util.go:73 +0x35
        github.com/zettio/weave/testing.Fatalf(0xc20806cd80, 0x863650, 0x14, 0xc2080c3de8, 0x1, 0x1)
            /home/matthias/go/src/github.com/zettio/weave/testing/util.go:69 +0x27
        github.com/zettio/weave/testing.AssertNoErr(0xc20806cd80, 0x2aafcb1cf0f8, 0xc20800a340)
            /home/matthias/go/src/github.com/zettio/weave/testing/util.go:12 +0xea
        github.com/zettio/weave/nameserver.TestDNSServer(0xc20806cd80)
            /home/matthias/go/src/github.com/zettio/weave/nameserver/server_test.go:75 +0xb79
        testing.tRunner(0xc20806cd80, 0xa20338)
            /usr/local/go/src/testing/testing.go:447 +0xbf
        created by testing.RunTests
            /usr/local/go/src/testing/testing.go:555 +0xa8b
DEBUG: 2015/01/23 11:25:53.989234 [zonedb] Looking for address: [10 0 2 1]
FAIL
exit status 1
FAIL    github.com/zettio/weave/nameserver  2.847s

This doesn't fail consistently, so clearly it depends on environmental conditions.

[rade]

@bboreham mentioned something about our dns server not implementing TCP fallback. Does that mean the above failure may be encountered in ordinary deployments and cause problems for our users?

[bboreham]

Weavedns is using dns.Exchange which applies a buffer size of 512 bytes. We could call via a Client object to increase this number; as long as we stay under 1400 bytes it should work in most circumstances.

However, it is still technically possible to have a ginormous DNS reply that requires TCP fallback.

[rade]

This should have been closed when #361 was merged. Doing so now.