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Added more comments, plus renamed vars with better names

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This commit is contained in:
Juan Font Alonso 2021-10-10 12:34:55 +02:00
parent 9a0c9768ad
commit d70c3d6189
1 changed files with 29 additions and 12 deletions
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41
dns.go
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@ -8,9 +8,26 @@ import (
"tailscale.com/util/dnsname"
)
// generateMagicDNSRootDomains generates a list of DNS entries to be included in the
// routing for DNS in the MapResponse struct. This list of DNS instructs the OS
// on what domains the Tailscale embedded DNS server should be used for.
// generateMagicDNSRootDomains generates a list of DNS entries to be included in `Routes` in `MapResponse`.
// This list of reverse DNS entries instructs the OS on what subnets and domains the Tailscale embedded DNS
// server (listening in 100.100.100.100 udp/53) should be used for.
//
// Tailscale.com includes in the list:
// - the `BaseDomain` of the user
// - the reverse DNS entry for IPv6 (0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa., see below more on IPv6)
// - the reverse DNS entries for the IPv4 subnets covered by the user's `IPPrefix`.
// In the public SaaS this is [64-127].100.in-addr.arpa.
//
// The main purpose of this function is then generating the list of IPv4 entries. For the 100.64.0.0/10, this
// is clear, and could be hardcoded. But we are allowing any range as `IPPrefix`, so we need to find out the
// subnets when we have 172.16.0.0/16 (i.e., [0-255].16.172.in-addr.arpa.), or any other subnet.
//
// How IN-ADDR.ARPA domains work is defined in RFC1035 (section 3.5). Tailscale.com seems to adhere to this,
// and do not make use of RFC2317 ("Classless IN-ADDR.ARPA delegation") - hence generating the entries for the next
// class block only.
// From the netmask we can find out the wildcard bits (the bits that are not set in the netmask).
// This allows us to then calculate the subnets included in the subsequent class block and generate the entries.
func generateMagicDNSRootDomains(ipPrefix netaddr.IPPrefix, baseDomain string) (*[]dnsname.FQDN, error) {
base, err := dnsname.ToFQDN(baseDomain)
if err != nil {
@ -26,20 +43,20 @@ func generateMagicDNSRootDomains(ipPrefix netaddr.IPPrefix, baseDomain string) (
netRange := ipPrefix.IPNet()
maskBits, _ := netRange.Mask.Size()
// lastByte is the last IP byte covered by the mask
lastByte := maskBits / 8
// lastOctet is the last IP byte covered by the mask
lastOctet := maskBits / 8
// unmaskedBits is the number of bits not under the mask in the byte lastByte
unmaskedBits := 8 - maskBits%8
// wildcardBits is the number of bits not under the mask in the lastOctet
wildcardBits := 8 - maskBits%8
// min is the value in the lastByte byte of the IP
// max is basically 2^unmaskedBits - i.e., the value when all the unmaskedBits are set to 1
min := uint(netRange.IP[lastByte])
max := uint((min + 1<<uint(unmaskedBits)) - 1)
// min is the value in the lastOctet byte of the IP
// max is basically 2^wildcardBits - i.e., the value when all the wildcardBits are set to 1
min := uint(netRange.IP[lastOctet])
max := uint((min + 1<<uint(wildcardBits)) - 1)
// here we generate the base domain (e.g., 100.in-addr.arpa., 16.172.in-addr.arpa., etc.)
rdnsSlice := []string{}
for i := lastByte - 1; i >= 0; i-- {
for i := lastOctet - 1; i >= 0; i-- {
rdnsSlice = append(rdnsSlice, fmt.Sprintf("%d", netRange.IP[i]))
}
rdnsSlice = append(rdnsSlice, "in-addr.arpa.")