tcp:: or quic:: fail to bind 22000 at all if only one of ipv4 or ipv6 is enabled

[jackwasey]

actual:

If ipv6 is disabled at system level, e.g. kernel command line or sysctl, then setting tcp:// or quic:// will fail for all protocols, giving the OP's reported behavior, although without a relevant message from kernel or syncthing that I can identify. Switch to tcp4...,/quic4 eliminated this error for me.

syncthing --version
# syncthing v1.3.1 "Fermium Flea" (go1.13.3 linux-amd64) deb@build.syncthing.net 2019-10-07 11:30:25 UTC
cat /proc/sys/net/ipv6/conf/*/disable_ipv6
1
1
1
...
# then the syslog shows only the following, with IP address redacted
Listen (BEP/quic): listen udp a.b.c.d:22000: bind: cannot assign requested address

expected:

syncthing listens on either ipv4 or ipv6, with notice that the other was not possible.

grep syncthing.*listener /var/log/syslog
# gives me ... INFO: TCP listener (a.b.c.d:22000) starting

Version Information

Syncthing Version: v1.3.1
OS Version: Ubuntu Eoan 19.10 5.3.0-19-generic

[calmh]

Doesn't reproduce for me, but on the other hand IPv6 doesn't seem to disable fully either (Ubuntu 18.04).

jb@test:~ $ sysctl -a 2>/dev/null | grep disable_ipv6
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.ens33.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
jb@test:~ $ syncthing -home asd
...
[start] 13:04:09 INFO: syncthing v1.3.1 "Fermium Flea" (go1.13.3 linux-amd64) deb@build.syncthing.net 2019-10-07 11:30:25 UTC
[start] 13:04:09 INFO: Generating ECDSA key and certificate for syncthing...
[start] 13:04:09 INFO: Default folder created and/or linked to new config
[start] 13:04:09 INFO: Default config saved. Edit /home/jb/asd/config.xml to taste (with Syncthing stopped) or use the GUI
...
[QPPMM] 13:04:10 INFO: TCP listener ([::]:22000) starting
[QPPMM] 13:04:10 INFO: QUIC listener ([::]:22000) starting
...

(no errors)

jb@test:~ $ netstat -anp | grep 22000
tcp6       0      0 :::22000                :::*                    LISTEN      4648/syncthing
udp6       0      0 :::22000                :::*                                4648/syncthing

(still udp6/tcp6)

Then again, the tcp4/quic4 schemes sound like they were made for your use case.

[AudriusButkevicius]

I don't think this is true.

By default tcp in go means dual stack and go tries both stacks internally, and succeeds if any of them succeed.

Can you provide strace output for bind/listen calls failing?