mirror of
https://github.com/juanfont/headscale.git
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74 lines
2.9 KiB
Go
74 lines
2.9 KiB
Go
package headscale
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import (
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"fmt"
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"strings"
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"inet.af/netaddr"
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"tailscale.com/util/dnsname"
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)
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// generateMagicDNSRootDomains generates a list of DNS entries to be included in `Routes` in `MapResponse`.
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// This list of reverse DNS entries instructs the OS on what subnets and domains the Tailscale embedded DNS
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// server (listening in 100.100.100.100 udp/53) should be used for.
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//
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// Tailscale.com includes in the list:
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// - the `BaseDomain` of the user
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// - 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)
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// - the reverse DNS entries for the IPv4 subnets covered by the user's `IPPrefix`.
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// In the public SaaS this is [64-127].100.in-addr.arpa.
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//
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// The main purpose of this function is then generating the list of IPv4 entries. For the 100.64.0.0/10, this
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// is clear, and could be hardcoded. But we are allowing any range as `IPPrefix`, so we need to find out the
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// subnets when we have 172.16.0.0/16 (i.e., [0-255].16.172.in-addr.arpa.), or any other subnet.
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//
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// How IN-ADDR.ARPA domains work is defined in RFC1035 (section 3.5). Tailscale.com seems to adhere to this,
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// and do not make use of RFC2317 ("Classless IN-ADDR.ARPA delegation") - hence generating the entries for the next
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// class block only.
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// From the netmask we can find out the wildcard bits (the bits that are not set in the netmask).
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// This allows us to then calculate the subnets included in the subsequent class block and generate the entries.
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func generateMagicDNSRootDomains(ipPrefix netaddr.IPPrefix, baseDomain string) (*[]dnsname.FQDN, error) {
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base, err := dnsname.ToFQDN(baseDomain)
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if err != nil {
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return nil, err
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}
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// TODO(juanfont): we are not handing out IPv6 addresses yet
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// and in fact this is Tailscale.com's range (note the fd7a:115c:a1e0: range in the fc00::/7 network)
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ipv6base := dnsname.FQDN("0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa.")
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fqdns := []dnsname.FQDN{base, ipv6base}
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// Conversion to the std lib net.IPnet, a bit easier to operate
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netRange := ipPrefix.IPNet()
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maskBits, _ := netRange.Mask.Size()
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// lastOctet is the last IP byte covered by the mask
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lastOctet := maskBits / 8
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// wildcardBits is the number of bits not under the mask in the lastOctet
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wildcardBits := 8 - maskBits%8
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// min is the value in the lastOctet byte of the IP
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// max is basically 2^wildcardBits - i.e., the value when all the wildcardBits are set to 1
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min := uint(netRange.IP[lastOctet])
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max := uint((min + 1<<uint(wildcardBits)) - 1)
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// here we generate the base domain (e.g., 100.in-addr.arpa., 16.172.in-addr.arpa., etc.)
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rdnsSlice := []string{}
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for i := lastOctet - 1; i >= 0; i-- {
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rdnsSlice = append(rdnsSlice, fmt.Sprintf("%d", netRange.IP[i]))
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}
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rdnsSlice = append(rdnsSlice, "in-addr.arpa.")
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rdnsBase := strings.Join(rdnsSlice, ".")
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for i := min; i <= max; i++ {
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fqdn, err := dnsname.ToFQDN(fmt.Sprintf("%d.%s", i, rdnsBase))
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if err != nil {
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continue
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}
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fqdns = append(fqdns, fqdn)
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}
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return &fqdns, nil
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}
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