mirror of
https://github.com/syncthing/syncthing.git
synced 2024-11-16 18:41:59 -07:00
277 lines
8.4 KiB
Go
277 lines
8.4 KiB
Go
/*
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This is the local node discovery protocol. In principle we might be better
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served by something more standardized, such as mDNS / DNS-SD. In practice, this
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was much easier and quicker to get up and running.
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The mode of operation is to periodically (currently once every 30 seconds)
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broadcast an Announcement packet to UDP port 21025. The packet has the
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following format:
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Magic Number (0x20121025) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Port Number | Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of NodeID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ NodeID (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of IP |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ IP (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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This is the XDR encoding of:
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struct Announcement {
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unsigned int Magic;
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unsigned short Port;
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string NodeID<>;
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}
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(Hence NodeID is padded to a multiple of 32 bits)
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The sending node's address is not encoded in local announcement -- the Length
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of IP field is set to zero and the address is taken to be the source address of
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the announcement. In announcement packets sent by a discovery server in
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response to a query, the IP is present and the length is either 4 (IPv4) or 16
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(IPv6).
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Every time such a packet is received, a local table that maps NodeID to Address
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is updated. When the local node wants to connect to another node with the
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address specification 'dynamic', this table is consulted.
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For external discovery, an identical packet is sent every 30 minutes to the
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external discovery server. The server keeps information for up to 60 minutes.
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To query the server, and UDP packet with the format below is sent.
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Magic Number (0x19760309) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of NodeID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ NodeID (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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This is the XDR encoding of:
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struct Announcement {
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unsigned int Magic;
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string NodeID<>;
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}
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(Hence NodeID is padded to a multiple of 32 bits)
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It is answered with an announcement packet for the queried node ID if the
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information is available. There is no answer for queries about unknown nodes. A
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reasonable timeout is recommended instead. (This, combined with server side
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rate limits for packets per source IP and queries per node ID, prevents the
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server from being used as an amplifier in a DDoS attack.)
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*/
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package discover
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import (
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"fmt"
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"log"
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"net"
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"sync"
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"time"
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)
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const (
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AnnouncementPort = 21025
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AnnouncementMagic = 0x20121025
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QueryMagic = 0x19760309
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)
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type Discoverer struct {
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MyID string
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ListenPort int
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BroadcastIntv time.Duration
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ExtListenPort int
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ExtBroadcastIntv time.Duration
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conn *net.UDPConn
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registry map[string]string
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registryLock sync.RWMutex
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extServer string
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}
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// We tolerate a certain amount of errors because we might be running on
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// laptops that sleep and wake, have intermittent network connectivity, etc.
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// When we hit this many errors in succession, we stop.
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const maxErrors = 30
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func NewDiscoverer(id string, port int, extPort int, extServer string) (*Discoverer, error) {
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local4 := &net.UDPAddr{IP: net.IP{0, 0, 0, 0}, Port: AnnouncementPort}
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conn, err := net.ListenUDP("udp4", local4)
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if err != nil {
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return nil, err
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}
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disc := &Discoverer{
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MyID: id,
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ListenPort: port,
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BroadcastIntv: 30 * time.Second,
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ExtListenPort: extPort,
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ExtBroadcastIntv: 1800 * time.Second,
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conn: conn,
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registry: make(map[string]string),
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extServer: extServer,
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}
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go disc.recvAnnouncements()
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if disc.ListenPort > 0 {
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disc.sendAnnouncements()
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}
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if len(disc.extServer) > 0 && disc.ExtListenPort > 0 {
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disc.sendExtAnnouncements()
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}
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return disc, nil
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}
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func (d *Discoverer) sendAnnouncements() {
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remote4 := &net.UDPAddr{IP: net.IP{255, 255, 255, 255}, Port: AnnouncementPort}
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buf := EncodePacket(Packet{AnnouncementMagic, uint16(d.ListenPort), d.MyID, nil})
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go d.writeAnnouncements(buf, remote4, d.BroadcastIntv)
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}
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func (d *Discoverer) sendExtAnnouncements() {
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extIP, err := net.ResolveUDPAddr("udp", d.extServer+":22025")
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if err != nil {
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log.Printf("discover/external: %v; no external announcements", err)
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return
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}
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buf := EncodePacket(Packet{AnnouncementMagic, uint16(d.ExtListenPort), d.MyID, nil})
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go d.writeAnnouncements(buf, extIP, d.ExtBroadcastIntv)
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}
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func (d *Discoverer) writeAnnouncements(buf []byte, remote *net.UDPAddr, intv time.Duration) {
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var errCounter = 0
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var err error
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for errCounter < maxErrors {
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_, _, err = d.conn.WriteMsgUDP(buf, nil, remote)
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if err != nil {
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log.Println("discover/write: warning:", err)
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errCounter++
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} else {
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errCounter = 0
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}
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time.Sleep(intv)
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}
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log.Println("discover/write: %v: stopping due to too many errors:", remote, err)
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}
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func (d *Discoverer) recvAnnouncements() {
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var buf = make([]byte, 1024)
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var errCounter = 0
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var err error
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for errCounter < maxErrors {
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n, addr, err := d.conn.ReadFromUDP(buf)
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if err != nil {
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errCounter++
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time.Sleep(time.Second)
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continue
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}
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pkt, err := DecodePacket(buf[:n])
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if err != nil || pkt.Magic != AnnouncementMagic {
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errCounter++
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time.Sleep(time.Second)
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continue
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}
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errCounter = 0
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if pkt.ID != d.MyID {
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nodeAddr := fmt.Sprintf("%s:%d", addr.IP.String(), pkt.Port)
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d.registryLock.Lock()
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if d.registry[pkt.ID] != nodeAddr {
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d.registry[pkt.ID] = nodeAddr
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}
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d.registryLock.Unlock()
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}
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}
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log.Println("discover/read: stopping due to too many errors:", err)
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}
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func (d *Discoverer) externalLookup(node string) (string, bool) {
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extIP, err := net.ResolveUDPAddr("udp", d.extServer+":22025")
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if err != nil {
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log.Printf("discover/external: %v; no external lookup", err)
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return "", false
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}
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var res = make(chan string, 1)
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conn, err := net.DialUDP("udp", nil, extIP)
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if err != nil {
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log.Printf("discover/external: %v; no external lookup", err)
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return "", false
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}
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_, err = conn.Write(EncodePacket(Packet{QueryMagic, 0, node, nil}))
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if err != nil {
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log.Printf("discover/external: %v; no external lookup", err)
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return "", false
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}
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log.Println("query", extIP)
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go func() {
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var buf = make([]byte, 1024)
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n, err := conn.Read(buf)
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if err != nil {
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log.Printf("discover/external/read: %v; no external lookup", err)
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return
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}
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pkt, err := DecodePacket(buf[:n])
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if err != nil {
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log.Printf("discover/external/read: %v; no external lookup", err)
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return
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}
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if pkt.Magic != AnnouncementMagic {
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log.Printf("discover/external/read: bad magic; no external lookup", err)
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return
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}
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res <- fmt.Sprintf("%s:%d", ipStr(pkt.IP), pkt.Port)
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}()
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select {
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case r := <-res:
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return r, true
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case <-time.After(5 * time.Second):
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return "", false
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}
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}
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func (d *Discoverer) Lookup(node string) (string, bool) {
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d.registryLock.Lock()
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addr, ok := d.registry[node]
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d.registryLock.Unlock()
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if ok {
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return addr, true
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} else if len(d.extServer) != 0 {
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// We might want to cache this, but not permanently so it needs some intelligence
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return d.externalLookup(node)
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}
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return "", false
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}
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