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fix(@libp2p/mdns): do not send TXT records that are too long (#2014)
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The data field of mDNS TXT records has a [hard limit of 255 characters](https://agnat.github.io/node_mdns/user_guide.html#txt_records) - some multiaddrs can be longer than this so filter them out before sending, otherwise remote peers can fail to parse our mDNS response.

We should also only be sending [link-local addresses](https://github.com/libp2p/specs/blob/master/discovery/mdns.md#issues) in mDNS responses so filter any non-link-local addresses out too, though go-libp2p includes loopback addresses even though it probably shouldn't (according to the mDNS spec which conflicts with our mDNS Peer Discovery spec) so we continue to do so as well.

Fixes #2012
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@achingbrain
achingbrain authored Sep 1, 2023
1 parent 3282563 commit 4f19234
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Showing 5 changed files with 159 additions and 44 deletions.
1 change: 1 addition & 0 deletions packages/peer-discovery-mdns/package.json
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,7 @@
"@libp2p/interface": "^0.1.2",
"@libp2p/logger": "^3.0.2",
"@libp2p/peer-id": "^3.0.2",
"@libp2p/utils": "^4.0.2",
"@multiformats/multiaddr": "^12.1.5",
"@types/multicast-dns": "^7.2.1",
"dns-packet": "^5.4.0",
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22 changes: 13 additions & 9 deletions packages/peer-discovery-mdns/src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ import * as query from './query.js'
import { stringGen } from './utils.js'
import type { PeerDiscovery, PeerDiscoveryEvents } from '@libp2p/interface/peer-discovery'
import type { PeerInfo } from '@libp2p/interface/peer-info'
import type { Startable } from '@libp2p/interface/src/startable.js'
import type { AddressManager } from '@libp2p/interface-internal/address-manager'

const log = logger('libp2p:mdns')
Expand All @@ -23,7 +24,7 @@ export interface MulticastDNSComponents {
addressManager: AddressManager
}

class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDiscovery {
class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDiscovery, Startable {
public mdns?: multicastDNS.MulticastDNS

private readonly broadcast: boolean
Expand All @@ -50,9 +51,10 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc
this.port = init.port ?? 5353
this.components = components
this._queryInterval = null
this._onPeer = this._onPeer.bind(this)
this._onMdnsQuery = this._onMdnsQuery.bind(this)
this._onMdnsResponse = this._onMdnsResponse.bind(this)
this._onMdnsWarning = this._onMdnsWarning.bind(this)
this._onMdnsError = this._onMdnsError.bind(this)
}

readonly [peerDiscovery] = this
Expand All @@ -76,6 +78,8 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc
this.mdns = multicastDNS({ port: this.port, ip: this.ip })
this.mdns.on('query', this._onMdnsQuery)
this.mdns.on('response', this._onMdnsResponse)
this.mdns.on('warning', this._onMdnsWarning)
this.mdns.on('error', this._onMdnsError)

this._queryInterval = query.queryLAN(this.mdns, this.serviceTag, this.interval)
}
Expand Down Expand Up @@ -113,14 +117,12 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc
}
}

_onPeer (evt: CustomEvent<PeerInfo>): void {
if (this.mdns == null) {
return
}
_onMdnsWarning (err: Error): void {
log.error('mdns warning', err)
}

this.dispatchEvent(new CustomEvent<PeerInfo>('peer', {
detail: evt.detail
}))
_onMdnsError (err: Error): void {
log.error('mdns error', err)
}

/**
Expand All @@ -135,6 +137,8 @@ class MulticastDNS extends EventEmitter<PeerDiscoveryEvents> implements PeerDisc

this.mdns.removeListener('query', this._onMdnsQuery)
this.mdns.removeListener('response', this._onMdnsResponse)
this.mdns.removeListener('warning', this._onMdnsWarning)
this.mdns.removeListener('error', this._onMdnsError)

if (this._queryInterval != null) {
clearInterval(this._queryInterval)
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56 changes: 46 additions & 10 deletions packages/peer-discovery-mdns/src/query.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
import { logger } from '@libp2p/logger'
import { peerIdFromString } from '@libp2p/peer-id'
import { multiaddr, type Multiaddr } from '@multiformats/multiaddr'
import { isPrivate } from '@libp2p/utils/multiaddr/is-private'
import { multiaddr, type Multiaddr, protocols } from '@multiformats/multiaddr'
import type { PeerInfo } from '@libp2p/interface/peer-info'
import type { Answer, StringAnswer, TxtAnswer } from 'dns-packet'
import type { MulticastDNS, QueryPacket, ResponsePacket } from 'multicast-dns'
Expand All @@ -9,7 +10,7 @@ const log = logger('libp2p:mdns:query')

export function queryLAN (mdns: MulticastDNS, serviceTag: string, interval: number): ReturnType<typeof setInterval> {
const query = (): void => {
log('query', serviceTag)
log.trace('query', serviceTag)

mdns.query({
questions: [{
Expand Down Expand Up @@ -40,6 +41,16 @@ export function gotResponse (rsp: ResponsePacket, localPeerName: string, service
}
})

// according to the spec, peer details should be in the additional records,
// not the answers though it seems go-libp2p at least ignores this?
// https://github.com/libp2p/specs/blob/master/discovery/mdns.md#response
rsp.additionals.forEach((answer) => {
switch (answer.type) {
case 'TXT': txtAnswers.push(answer); break
default: break
}
})

if (answerPTR == null ||
answerPTR?.name !== serviceTag ||
txtAnswers.length === 0 ||
Expand All @@ -63,7 +74,7 @@ export function gotResponse (rsp: ResponsePacket, localPeerName: string, service

return {
id: peerIdFromString(peerId),
multiaddrs,
multiaddrs: multiaddrs.map(addr => addr.decapsulateCode(protocols('p2p').code)),
protocols: []
}
} catch (e) {
Expand Down Expand Up @@ -92,20 +103,45 @@ export function gotQuery (qry: QueryPacket, mdns: MulticastDNS, peerName: string
data: peerName + '.' + serviceTag
})

multiaddrs.forEach((addr) => {
// spec mandates multiaddr contains peer id
if (addr.getPeerId() != null) {
multiaddrs
// mDNS requires link-local addresses only
// https://github.com/libp2p/specs/blob/master/discovery/mdns.md#issues
.filter(isLinkLocal)
.forEach((addr) => {
const data = 'dnsaddr=' + addr.toString()

// TXT record fields have a max data length of 255 bytes
// see 6.1 - https://www.ietf.org/rfc/rfc6763.txt
if (data.length > 255) {
log('multiaddr %a is too long to use in mDNS query response', addr)
return
}

// spec mandates multiaddr contains peer id
if (addr.getPeerId() == null) {
log('multiaddr %a did not have a peer ID so cannot be used in mDNS query response', addr)
return
}

answers.push({
name: peerName + '.' + serviceTag,
type: 'TXT',
class: 'IN',
ttl: 120,
data: 'dnsaddr=' + addr.toString()
data
})
}
})
})

log('responding to query')
log.trace('responding to query')
mdns.respond(answers)
}
}

function isLinkLocal (ma: Multiaddr): boolean {
// match private ip4/ip6 & loopback addresses
if (isPrivate(ma)) {
return true
}

return false
}
24 changes: 15 additions & 9 deletions packages/peer-discovery-mdns/test/compliance.spec.ts
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
/* eslint-env mocha */

import { CustomEvent } from '@libp2p/interface/events'
import { isStartable } from '@libp2p/interface/startable'
import tests from '@libp2p/interface-compliance-tests/peer-discovery'
import { createEd25519PeerId } from '@libp2p/peer-id-factory'
import { multiaddr } from '@multiformats/multiaddr'
Expand Down Expand Up @@ -32,16 +33,21 @@ describe('compliance tests', () => {
})

// Trigger discovery
const maStr = '/ip4/127.0.0.1/tcp/15555/ws/p2p-webrtc-star/p2p/QmcgpsyWgH8Y8ajJz1Cu72KnS5uo2Aa2LpzU7kinSooo2d'

// @ts-expect-error not a PeerDiscovery field
intervalId = setInterval(() => discovery._onPeer(new CustomEvent('peer', {
detail: {
id: peerId2,
multiaddrs: [multiaddr(maStr)],
protocols: []
const maStr = '/ip4/127.0.0.1/tcp/15555/ws/p2p-webrtc-star'

intervalId = setInterval(() => {
if (isStartable(discovery) && !discovery.isStarted()) {
return
}
})), 1000)

discovery.dispatchEvent(new CustomEvent('peer', {
detail: {
id: peerId2,
multiaddrs: [multiaddr(maStr)],
protocols: []
}
}))
}, 1000)

return discovery
},
Expand Down
100 changes: 84 additions & 16 deletions packages/peer-discovery-mdns/test/multicast-dns.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -38,25 +38,25 @@ describe('MulticastDNS', () => {
])

aMultiaddrs = [
multiaddr('/ip4/127.0.0.1/tcp/20001'),
multiaddr('/ip4/192.168.1.142/tcp/20001'),
multiaddr('/dns4/webrtc-star.discovery.libp2p.io/tcp/443/wss/p2p-webrtc-star'),
multiaddr('/dns4/discovery.libp2p.io/tcp/8443')
]

bMultiaddrs = [
multiaddr('/ip4/127.0.0.1/tcp/20002'),
multiaddr('/ip6/::1/tcp/20002'),
multiaddr('/ip4/192.168.1.143/tcp/20002'),
multiaddr('/ip6/2604:1380:4602:5c00::3/tcp/20002'),
multiaddr('/dnsaddr/discovery.libp2p.io')
]

cMultiaddrs = [
multiaddr('/ip4/127.0.0.1/tcp/20003'),
multiaddr('/ip4/127.0.0.1/tcp/30003/ws'),
multiaddr('/ip4/192.168.1.144/tcp/20003'),
multiaddr('/ip4/192.168.1.144/tcp/30003/ws'),
multiaddr('/dns4/discovery.libp2p.io')
]

dMultiaddrs = [
multiaddr('/ip4/127.0.0.1/tcp/30003/ws')
multiaddr('/ip4/192.168.1.145/tcp/30003/ws')
]
})

Expand Down Expand Up @@ -110,7 +110,8 @@ describe('MulticastDNS', () => {
await pWaitFor(() => peers.has(expectedPeer))
mdnsA.removeEventListener('peer', foundPeer)

expect(peers.get(expectedPeer).multiaddrs.length).to.equal(3)
// everything except loopback
expect(peers.get(expectedPeer).multiaddrs.length).to.equal(2)

await stop(mdnsA, mdnsB, mdnsD)
})
Expand Down Expand Up @@ -141,15 +142,6 @@ describe('MulticastDNS', () => {
await stop(mdnsC)
})

it('should start and stop with go-libp2p-mdns compat', async () => {
const mdnsA = mdns({
port: 50004
})(getComponents(pA, aMultiaddrs))

await start(mdnsA)
await stop(mdnsA)
})

it('should not emit undefined peer ids', async () => {
const mdnsA = mdns({
port: 50004
Expand Down Expand Up @@ -219,4 +211,80 @@ describe('MulticastDNS', () => {

await stop(mdnsA, mdnsB)
})

it('only includes link-local addresses', async function () {
this.timeout(40 * 1000)

// these are not link-local addresses
const publicAddress = '/ip4/48.52.76.32/tcp/1234'
const relayDnsAddress = `/dnsaddr/example.org/tcp/1234/p2p/${pD.toString()}/p2p-circuit`
const dnsAddress = '/dns4/example.org/tcp/1234'

// this address is too long to fit in a TXT record
const longAddress = `/ip4/192.168.1.142/udp/4001/quic-v1/webtransport/certhash/uEiDils3hWFJmsWOJIoMPxAcpzlyFNxTDZpklIoB8643ddw/certhash/uEiAM4BGr4OMK3O9cFGwfbNc4J7XYnsKE5wNPKKaTLa4fkw/p2p/${pD.toString()}/p2p-circuit`

// these are link-local addresses
const relayAddress = `/ip4/192.168.1.142/tcp/1234/p2p/${pD.toString()}/p2p-circuit`
const localAddress = '/ip4/192.168.1.123/tcp/1234'
const localWsAddress = '/ip4/192.168.1.123/tcp/1234/ws'

// these are not link-local but go-libp2p advertises loopback addresses even
// though you shouldn't for mDNS
const loopbackAddress = '/ip4/127.0.0.1/tcp/1234'
const loopbackAddress6 = '/ip6/::1/tcp/1234'

const mdnsA = mdns({
broadcast: false, // do not talk to ourself
port: 50005,
ip: '224.0.0.252'
})(getComponents(pA, aMultiaddrs))

const mdnsB = mdns({
port: 50005, // port must be the same
ip: '224.0.0.252' // ip must be the same
})(getComponents(pB, [
multiaddr(publicAddress),
multiaddr(relayAddress),
multiaddr(relayDnsAddress),
multiaddr(localAddress),
multiaddr(loopbackAddress),
multiaddr(loopbackAddress6),
multiaddr(dnsAddress),
multiaddr(longAddress),
multiaddr(localWsAddress)
]))

await start(mdnsA, mdnsB)

const { detail: { id, multiaddrs } } = await new Promise<CustomEvent<PeerInfo>>((resolve) => {
mdnsA.addEventListener('peer', resolve, {
once: true
})
})

expect(pB.toString()).to.eql(id.toString())

;[
publicAddress,
relayDnsAddress,
dnsAddress,
longAddress
].forEach(addr => {
expect(multiaddrs.map(ma => ma.toString()))
.to.not.include(addr)
})

;[
relayAddress,
localAddress,
localWsAddress,
loopbackAddress,
loopbackAddress6
].forEach(addr => {
expect(multiaddrs.map(ma => ma.toString()))
.to.include(addr)
})

await stop(mdnsA, mdnsB)
})
})

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