// Copyright 2019 The go-ethereum Authors // This file is part of the go-ethereum library. // // The go-ethereum library is free software: you can redistribute it and/or modify // it under the terms of the GNU Lesser General Public License as published by // the Free Software Foundation, either version 3 of the License, or // (at your option) any later version. // // The go-ethereum library is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU Lesser General Public License for more details. // // You should have received a copy of the GNU Lesser General Public License // along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>. package dnsdisc import ( "bytes" "context" "errors" "fmt" "math/rand" "net" "strings" "sync" "time" "github.com/ethereum/go-ethereum/common/lru" "github.com/ethereum/go-ethereum/common/mclock" "github.com/ethereum/go-ethereum/crypto" "github.com/ethereum/go-ethereum/log" "github.com/ethereum/go-ethereum/p2p/enode" "github.com/ethereum/go-ethereum/p2p/enr" "golang.org/x/sync/singleflight" "golang.org/x/time/rate" ) // Client discovers nodes by querying DNS servers. type Client struct { cfg Config clock mclock.Clock entries *lru.Cache[string, entry] ratelimit *rate.Limiter singleflight singleflight.Group } // Config holds configuration options for the client. type Config struct { Timeout time.Duration // timeout used for DNS lookups (default 5s) RecheckInterval time.Duration // time between tree root update checks (default 30min) CacheLimit int // maximum number of cached records (default 1000) RateLimit float64 // maximum DNS requests / second (default 3) ValidSchemes enr.IdentityScheme // acceptable ENR identity schemes (default enode.ValidSchemes) Resolver Resolver // the DNS resolver to use (defaults to system DNS) Logger log.Logger // destination of client log messages (defaults to root logger) } // Resolver is a DNS resolver that can query TXT records. type Resolver interface { LookupTXT(ctx context.Context, domain string) ([]string, error) } func (cfg Config) withDefaults() Config { const ( defaultTimeout = 5 * time.Second defaultRecheck = 30 * time.Minute defaultRateLimit = 3 defaultCache = 1000 ) if cfg.Timeout == 0 { cfg.Timeout = defaultTimeout } if cfg.RecheckInterval == 0 { cfg.RecheckInterval = defaultRecheck } if cfg.CacheLimit == 0 { cfg.CacheLimit = defaultCache } if cfg.RateLimit == 0 { cfg.RateLimit = defaultRateLimit } if cfg.ValidSchemes == nil { cfg.ValidSchemes = enode.ValidSchemes } if cfg.Resolver == nil { cfg.Resolver = new(net.Resolver) } if cfg.Logger == nil { cfg.Logger = log.Root() } return cfg } // NewClient creates a client. func NewClient(cfg Config) *Client { cfg = cfg.withDefaults() rlimit := rate.NewLimiter(rate.Limit(cfg.RateLimit), 10) return &Client{ cfg: cfg, entries: lru.NewCache[string, entry](cfg.CacheLimit), clock: mclock.System{}, ratelimit: rlimit, } } // SyncTree downloads the entire node tree at the given URL. func (c *Client) SyncTree(url string) (*Tree, error) { le, err := parseLink(url) if err != nil { return nil, fmt.Errorf("invalid enrtree URL: %v", err) } ct := newClientTree(c, new(linkCache), le) t := &Tree{entries: make(map[string]entry)} if err := ct.syncAll(t.entries); err != nil { return nil, err } t.root = ct.root return t, nil } // NewIterator creates an iterator that visits all nodes at the // given tree URLs. func (c *Client) NewIterator(urls ...string) (enode.Iterator, error) { it := c.newRandomIterator() for _, url := range urls { if err := it.addTree(url); err != nil { return nil, err } } return it, nil } // resolveRoot retrieves a root entry via DNS. func (c *Client) resolveRoot(ctx context.Context, loc *linkEntry) (rootEntry, error) { e, err, _ := c.singleflight.Do(loc.str, func() (interface{}, error) { txts, err := c.cfg.Resolver.LookupTXT(ctx, loc.domain) c.cfg.Logger.Trace("Updating DNS discovery root", "tree", loc.domain, "err", err) if err != nil { return rootEntry{}, err } for _, txt := range txts { if strings.HasPrefix(txt, rootPrefix) { return parseAndVerifyRoot(txt, loc) } } return rootEntry{}, nameError{loc.domain, errNoRoot} }) return e.(rootEntry), err } func parseAndVerifyRoot(txt string, loc *linkEntry) (rootEntry, error) { e, err := parseRoot(txt) if err != nil { return e, err } if !e.verifySignature(loc.pubkey) { return e, entryError{typ: "root", err: errInvalidSig} } return e, nil } // resolveEntry retrieves an entry from the cache or fetches it from the network // if it isn't cached. func (c *Client) resolveEntry(ctx context.Context, domain, hash string) (entry, error) { // The rate limit always applies, even when the result might be cached. This is // important because it avoids hot-spinning in consumers of node iterators created on // this client. if err := c.ratelimit.Wait(ctx); err != nil { return nil, err } cacheKey := truncateHash(hash) if e, ok := c.entries.Get(cacheKey); ok { return e, nil } ei, err, _ := c.singleflight.Do(cacheKey, func() (interface{}, error) { e, err := c.doResolveEntry(ctx, domain, hash) if err != nil { return nil, err } c.entries.Add(cacheKey, e) return e, nil }) e, _ := ei.(entry) return e, err } // doResolveEntry fetches an entry via DNS. func (c *Client) doResolveEntry(ctx context.Context, domain, hash string) (entry, error) { wantHash, err := b32format.DecodeString(hash) if err != nil { return nil, fmt.Errorf("invalid base32 hash") } name := hash + "." + domain txts, err := c.cfg.Resolver.LookupTXT(ctx, hash+"."+domain) c.cfg.Logger.Trace("DNS discovery lookup", "name", name, "err", err) if err != nil { return nil, err } for _, txt := range txts { e, err := parseEntry(txt, c.cfg.ValidSchemes) if errors.Is(err, errUnknownEntry) { continue } if !bytes.HasPrefix(crypto.Keccak256([]byte(txt)), wantHash) { err = nameError{name, errHashMismatch} } else if err != nil { err = nameError{name, err} } return e, err } return nil, nameError{name, errNoEntry} } // randomIterator traverses a set of trees and returns nodes found in them. type randomIterator struct { cur *enode.Node ctx context.Context cancelFn context.CancelFunc c *Client mu sync.Mutex lc linkCache // tracks tree dependencies trees map[string]*clientTree // all trees // buffers for syncableTrees syncableList []*clientTree disabledList []*clientTree } func (c *Client) newRandomIterator() *randomIterator { ctx, cancel := context.WithCancel(context.Background()) return &randomIterator{ c: c, ctx: ctx, cancelFn: cancel, trees: make(map[string]*clientTree), } } // Node returns the current node. func (it *randomIterator) Node() *enode.Node { return it.cur } // Close closes the iterator. func (it *randomIterator) Close() { it.cancelFn() it.mu.Lock() defer it.mu.Unlock() it.trees = nil } // Next moves the iterator to the next node. func (it *randomIterator) Next() bool { it.cur = it.nextNode() return it.cur != nil } // addTree adds an enrtree:// URL to the iterator. func (it *randomIterator) addTree(url string) error { le, err := parseLink(url) if err != nil { return fmt.Errorf("invalid enrtree URL: %v", err) } it.lc.addLink("", le.str) return nil } // nextNode syncs random tree entries until it finds a node. func (it *randomIterator) nextNode() *enode.Node { for { ct := it.pickTree() if ct == nil { return nil } n, err := ct.syncRandom(it.ctx) if err != nil { if errors.Is(err, it.ctx.Err()) { return nil // context canceled. } it.c.cfg.Logger.Debug("Error in DNS random node sync", "tree", ct.loc.domain, "err", err) continue } if n != nil { return n } } } // pickTree returns a random tree to sync from. func (it *randomIterator) pickTree() *clientTree { it.mu.Lock() defer it.mu.Unlock() // First check if iterator was closed. // Need to do this here to avoid nil map access in rebuildTrees. if it.trees == nil { return nil } // Rebuild the trees map if any links have changed. if it.lc.changed { it.rebuildTrees() it.lc.changed = false } for { canSync, trees := it.syncableTrees() switch { case canSync: // Pick a random tree. return trees[rand.Intn(len(trees))] case len(trees) > 0: // No sync action can be performed on any tree right now. The only meaningful // thing to do is waiting for any root record to get updated. if !it.waitForRootUpdates(trees) { // Iterator was closed while waiting. return nil } default: // There are no trees left, the iterator was closed. return nil } } } // syncableTrees finds trees on which any meaningful sync action can be performed. func (it *randomIterator) syncableTrees() (canSync bool, trees []*clientTree) { // Resize tree lists. it.syncableList = it.syncableList[:0] it.disabledList = it.disabledList[:0] // Partition them into the two lists. for _, ct := range it.trees { if ct.canSyncRandom() { it.syncableList = append(it.syncableList, ct) } else { it.disabledList = append(it.disabledList, ct) } } if len(it.syncableList) > 0 { return true, it.syncableList } return false, it.disabledList } // waitForRootUpdates waits for the closest scheduled root check time on the given trees. func (it *randomIterator) waitForRootUpdates(trees []*clientTree) bool { var minTree *clientTree var nextCheck mclock.AbsTime for _, ct := range trees { check := ct.nextScheduledRootCheck() if minTree == nil || check < nextCheck { minTree = ct nextCheck = check } } sleep := nextCheck.Sub(it.c.clock.Now()) it.c.cfg.Logger.Debug("DNS iterator waiting for root updates", "sleep", sleep, "tree", minTree.loc.domain) timeout := it.c.clock.NewTimer(sleep) defer timeout.Stop() select { case <-timeout.C(): return true case <-it.ctx.Done(): return false // Iterator was closed. } } // rebuildTrees rebuilds the 'trees' map. func (it *randomIterator) rebuildTrees() { // Delete removed trees. for loc := range it.trees { if !it.lc.isReferenced(loc) { delete(it.trees, loc) } } // Add new trees. for loc := range it.lc.backrefs { if it.trees[loc] == nil { link, _ := parseLink(linkPrefix + loc) it.trees[loc] = newClientTree(it.c, &it.lc, link) } } }