core/commands/dns: Add 'ipfs dns ...' for resolving DNS references

This lets users resolve (recursively or not) DNS links without pulling
in the other protocols.  That makes an easier, more isolated target
for alternative implemenations, since they don't need to understand
IPNS, proquint, etc. to handle these resolutions.

core/commands/dns.go

+package commands
+
+import (
+	"io"
+	"strings"
+
+	cmds "github.com/ipfs/go-ipfs/commands"
+	namesys "github.com/ipfs/go-ipfs/namesys"
+	util "github.com/ipfs/go-ipfs/util"
+)
+
+var DNSCmd = &cmds.Command{
+	Helptext: cmds.HelpText{
+		Tagline: "DNS link resolver",
+		ShortDescription: `
+Multihashes are hard to remember, but domain names are usually easy to
+remember.  To create memorable aliases for multihashes, DNS TXT
+records can point to other DNS links, IPFS objects, IPNS keys, etc.
+This command resolves those links to the referenced object.
+`,
+		LongDescription: `
+Multihashes are hard to remember, but domain names are usually easy to
+remember.  To create memorable aliases for multihashes, DNS TXT
+records can point to other DNS links, IPFS objects, IPNS keys, etc.
+This command resolves those links to the referenced object.
+
+For example, with this DNS TXT record:
+
+  ipfs.io. TXT "dnslink=/ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy ..."
+
+The resolver will give:
+
+  > ipfs dns ipfs.io
+  /ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy
+
+And with this DNS TXT record:
+
+  ipfs.ipfs.io. TXT "dnslink=/dns/ipfs.io ..."
+
+The resolver will give:
+
+  > ipfs dns ipfs.io
+  /dns/ipfs.io
+  > ipfs dns --recursive
+  /ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy
+`,
+	},
+
+	Arguments: []cmds.Argument{
+		cmds.StringArg("domain-name", true, false, "The domain-name name to resolve.").EnableStdin(),
+	},
+	Options: []cmds.Option{
+		cmds.BoolOption("recursive", "r", "Resolve until the result is not a DNS link"),
+	},
+	Run: func(req cmds.Request, res cmds.Response) {
+
+		recursive, _, _ := req.Option("recursive").Bool()
+		name := req.Arguments()[0]
+		var resolver namesys.DNSResolver
+
+		depth := 1
+		if recursive {
+			depth = namesys.DefaultDepthLimit
+		}
+		output, err := resolver.ResolveN(req.Context().Context, name, depth)
+		if err != nil {
+			res.SetError(err, cmds.ErrNormal)
+			return
+		}
+		res.SetOutput(&ResolvedPath{output})
+	},
+	Marshalers: cmds.MarshalerMap{
+		cmds.Text: func(res cmds.Response) (io.Reader, error) {
+			output, ok := res.Output().(*ResolvedPath)
+			if !ok {
+				return nil, util.ErrCast()
+			}
+			return strings.NewReader(output.Path.String()), nil
+		},
+	},
+	Type: ResolvedPath{},
+}

core/commands/root.go

@@ -41,6 +41,7 @@ ADVANCED COMMANDS
     daemon        Start a long-running daemon process
     mount         Mount an ipfs read-only mountpoint
     name          Publish or resolve IPNS names
+    dns           Resolve DNS links
     pin           Pin objects to local storage
     repo gc       Garbage collect unpinned objects
 
@@ -84,6 +85,7 @@ var rootSubcommands = map[string]*cmds.Command{
 	"config":    ConfigCmd,
 	"dht":       DhtCmd,
 	"diag":      DiagCmd,
+	"dns":       DNSCmd,
 	"get":       GetCmd,
 	"id":        IDCmd,
 	"log":       LogCmd,