coredns/plugin/pkg/parse/host.go

package parse

import (
	"errors"
	"fmt"
	"net"
	"os"
	"strings"

	"github.com/coredns/coredns/plugin/pkg/transport"

	"github.com/miekg/dns"
)

// ErrNoNameservers is returned by HostPortOrFile if no servers can be parsed.
var ErrNoNameservers = errors.New("no nameservers found")

// Strips the zone, but preserves any port that comes after the zone
func stripZone(host string) string {
	if strings.Contains(host, "%") {
		lastPercent := strings.LastIndex(host, "%")
		newHost := host[:lastPercent]
		return newHost
	}
	return host
}

// HostPortOrFile parses the strings in s, each string can either be a
// address, [scheme://]address:port or a filename. The address part is checked
// and in case of filename a resolv.conf like file is (assumed) and parsed and
// the nameservers found are returned.
func HostPortOrFile(s ...string) ([]string, error) {
	var servers []string
	for _, h := range s {
		trans, host := Transport(h)
		if len(host) == 0 {
			return servers, fmt.Errorf("invalid address: %q", h)
		}

		if trans == transport.UNIX {
			servers = append(servers, trans+"://"+host)
			continue
		}

		addr, _, err := net.SplitHostPort(host)

		if err != nil {
			// Parse didn't work, it is not a addr:port combo
			host = strings.Trim(host, "[]")
			hostNoZone := stripZone(host)
			if net.ParseIP(hostNoZone) == nil {
				ss, err := tryFile(host)
				if err == nil {
					servers = append(servers, ss...)
					continue
				}
				return servers, fmt.Errorf("not an IP address or file: %q", host)
			}
			var ss string
			switch trans {
			case transport.DNS:
				ss = net.JoinHostPort(host, transport.Port)
			case transport.TLS:
				ss = transport.TLS + "://" + net.JoinHostPort(host, transport.TLSPort)
			case transport.QUIC:
				ss = transport.QUIC + "://" + net.JoinHostPort(host, transport.QUICPort)
			case transport.GRPC:
				ss = transport.GRPC + "://" + net.JoinHostPort(host, transport.GRPCPort)
			case transport.HTTPS:
				ss = transport.HTTPS + "://" + net.JoinHostPort(host, transport.HTTPSPort)
			}
			servers = append(servers, ss)
			continue
		}

		if net.ParseIP(stripZone(addr)) == nil {
			ss, err := tryFile(host)
			if err == nil {
				servers = append(servers, ss...)
				continue
			}
			return servers, fmt.Errorf("not an IP address or file: %q", host)
		}
		servers = append(servers, h)
	}
	if len(servers) == 0 {
		return servers, ErrNoNameservers
	}
	return servers, nil
}

// Try to open this is a file first.
func tryFile(s string) ([]string, error) {
	c, err := dns.ClientConfigFromFile(s)
	if errors.Is(err, os.ErrNotExist) {
		return nil, fmt.Errorf("failed to open file %q: %w", s, err)
	} else if err != nil {
		return nil, err
	}

	servers := []string{}
	for _, s := range c.Servers {
		servers = append(servers, net.JoinHostPort(stripZone(s), c.Port))
	}
	return servers, nil
}

// HostPort will check if the host part is a valid IP address, if the
// IP address is valid, but no port is found, defaultPort is added.
func HostPort(s, defaultPort string) (string, error) {
	addr, port, err := net.SplitHostPort(s)
	if port == "" {
		port = defaultPort
	}
	if err != nil {
		if net.ParseIP(s) == nil {
			return "", fmt.Errorf("must specify an IP address: `%s'", s)
		}
		return net.JoinHostPort(s, port), nil
	}

	if net.ParseIP(addr) == nil {
		return "", fmt.Errorf("must specify an IP address: `%s'", addr)
	}
	return net.JoinHostPort(addr, port), nil
}