Synology DSM6 with 1.14.6 or 1.16.0 fails to use TUN

[bradfitz]

We've seen a few reports now that Synology DSM6 fails to use TUN. It was part of #2922 but it was also in a support request I just got over email:

2021-10-10 01:40:41.201018012 -0500 -0500: Program starting: v1.16.0-t86bcf6a96-gf787a582e, Go 1.17.1-ts097d1284f9: []string{"/volume1/@appstore/Tailscale/bin/tailscaled", "--state=/var/packages/Tailscale/etc/tailscaled.state", "--socket=/var/packages/Tailscale/etc/tailscaled.sock", "--port=41641"}
2021-10-10 01:40:41.201068267 -0500 -0500: LogID: 55df50fa9f7ead63b90a8e27954c617d7aad3291505e83e1bfd7cf7a52a7de9b
2021-10-10 01:40:41.201103071 -0500 -0500: logpolicy: using $STATE_DIRECTORY, "/var/packages/Tailscale/etc"
2021-10-10 01:40:41.203127701 -0500 -0500: wgengine.NewUserspaceEngine(tun "tailscale0") ...
2021-10-10 01:40:41.240300058 -0500 -0500: router: v6nat = false
2021-10-10 01:40:41.267145609 -0500 -0500: dns: [rc=unknown ret=direct]
2021-10-10 01:40:41.267408585 -0500 -0500: dns: using dns.directManager
...
2021-10-10 01:40:41.331871966 -0500 -0500: wgengine.NewUserspaceEngine(tun "tailscale0") error: running "ip link set dev tailscale0 up" failed: fork/exec /sbin/ip: invalid argument
2021-10-10 01:40:41.331992582 -0500 -0500: wgengine.NewUserspaceEngine(tun "userspace-networking") ...
2021-10-10 01:40:41.332040991 -0500 -0500: using fake (no-op) tun device
2021-10-10 01:40:41.33208304 -0500 -0500: using fake (no-op) OS network configurator
2021-10-10 01:40:41.332119423 -0500 -0500: using fake (no-op) DNS configurator
2021-10-10 01:40:41.332389545 -0500 -0500: dns: using dns.noopManager
2021-10-10 01:40:41.332576855 -0500 -0500: link state: interfaces.State{defaultRoute=eth0 ifs={docker0:[172.17.0.1/16] eth0:[xxxx]} v4=true v6=true}
2021-10-10 01:40:41.332907751 -0500 -0500: magicsock: unable to bind udp4 port 41641: listen udp4 :41641: bind: address already in use
2021-10-10 01:40:41.333225624 -0500 -0500: magicsock: unable to bind udp6 port 41641: listen udp6 :41641: bind: address already in use
2021-10-10 01:40:41.334437517 -0500 -0500: magicsock: disco key = d:0256cc50a305f206

/cc @maisem

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[bradfitz]

@maisem, user reports that downgrading to 1.12.1 works again.

[maisem]

Dupe of #2922