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net/portmapper: also send UPnP SSDP query to the SSDP multicast address.

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Fixes #3197

Signed-off-by: David Anderson <danderson@tailscale.com>
(cherry picked from commit 4a65b07e34)
This commit is contained in:
David Anderson 2021-10-27 14:55:59 -07:00 committed by Brad Fitzpatrick
parent 647486dc46
commit d117f77094
1 changed files with 41 additions and 0 deletions
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41
net/portmapper/portmapper.go
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@ -677,6 +677,7 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
pxpAddr := netaddr.IPPortFrom(gw, c.pxpPort()).UDPAddr() pxpAddr := netaddr.IPPortFrom(gw, c.pxpPort()).UDPAddr()
upnpAddr := netaddr.IPPortFrom(gw, c.upnpPort()).UDPAddr() upnpAddr := netaddr.IPPortFrom(gw, c.upnpPort()).UDPAddr()
upnpMulticastAddr := netaddr.IPPortFrom(netaddr.IPv4(239, 255, 255, 250), c.upnpPort()).UDPAddr()
// Don't send probes to services that we recently learned (for // Don't send probes to services that we recently learned (for
// the same gw/myIP) are available. See // the same gw/myIP) are available. See
@ -694,7 +695,47 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
if c.sawUPnPRecently() { if c.sawUPnPRecently() {
res.UPnP = true res.UPnP = true
} else if !DisableUPnP { } else if !DisableUPnP {
// Strictly speaking, you discover UPnP services by sending an
// SSDP query (which uPnPPacket is) to udp/1900 on the SSDP
// multicast address, and then get a flood of responses back
// from everything on your network.
//
// Empirically, many home routers also respond to SSDP queries
// directed at udp/1900 on their LAN unicast IP
// (e.g. 192.168.1.1). This is handy because it means we can
// probe the router directly and likely get a reply. However,
// the specs do not _require_ UPnP devices to respond to
// unicast SSDP queries, so some conformant UPnP
// implementations only respond to multicast queries.
//
// In theory, we could send just the multicast query and get
// all compliant devices to respond. However, we instead send
// to both a unicast and a multicast addresses, for a couple
// of reasons:
//
// First, some LANs and OSes have broken multicast in one way
// or another, so it's possible for the multicast query to be
// lost while the unicast query gets through. But we still
// have to send the multicast query to also get a response
// from strict-UPnP devices on multicast-working networks.
//
// Second, SSDP's packet dynamics are a bit weird: you send
// the SSDP query from your unicast IP to the SSDP multicast
// IP, but responses are from the UPnP devices's _unicast_ IP
// to your unicast IP. This can confuse some less-intelligent
// stateful host firewalls, who might block the responses. To
// work around this, we send the unicast query first, to teach
// the firewall to expect a unicast response from the router,
// and then send our multicast query. That way, even if the
// device doesn't respond to the unicast query, we've set the
// stage for the host firewall to accept the response to the
// multicast query.
//
// See https://github.com/tailscale/tailscale/issues/3197 for
// an example of a device that strictly implements UPnP, and
// only responds to multicast queries.
uc.WriteTo(uPnPPacket, upnpAddr) uc.WriteTo(uPnPPacket, upnpAddr)
uc.WriteTo(uPnPPacket, upnpMulticastAddr)
} }
buf := make([]byte, 1500) buf := make([]byte, 1500)