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docs: Add troubleshooting entry for TLS issue on Android 7.0.

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Learned this thanks to a user report and some debugging:
  https://chat.zulip.org/#narrow/stream/48-mobile/near/782174
This commit is contained in:
Greg Price
2019-08-26 13:03:00 -07:00
parent f1b91e577e
commit 3c07c054b6
1 changed files with 47 additions and 0 deletions
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docs/production/ssl-certificates.md
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@@ -139,3 +139,50 @@ service nginx reload
```
[desktop-certs]: https://zulipchat.com/help/custom-certificates
## Troubleshooting
### The Android app can't connect to the server
This is most often caused by an incomplete certificate chain. See
discussion in the [Manual install](#manual-install) section above.
### The Android app connects to the server on some devices but not others
An issue on Android 7.0 ([report][android7.0-tls-issue],
[description][android7.0-tls-issue-so]) in the system TLS/SSL stack,
which the Zulip app relies on, makes it finicky about the server's TLS
configuration.
[android7.0-tls-issue]: https://issuetracker.google.com/issues/37122132#comment13
[android7.0-tls-issue-so]: https://stackoverflow.com/questions/39133437/sslhandshakeexception-handshake-failed-on-android-n-7-0/42047877#42047877
The issue is that Android 7.0 supports only the curve `secp256r1` when
doing elliptic-curve cryptography for TLS, and not other curves like
`secp384r1` or `secp512r1`. If your server's TLS/SSL configuration
offers only other curves, then Android 7.0 clients will be unable to
connect.
By default `nginx` (and therefore a Zulip server) offers the
`secp256r1` curve among others, and so everything works. You can
control the offered curves with `ssl_ecdh_curve` in the `nginx`
configuration on your server. See [nginx docs][nginx-doc-curve] for
details.
[nginx-doc-curve]: http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_ecdh_curve
Two signs for diagnosing this issue in contrast to some other root
cause:
* This issue affects only Android 7.0; it's fixed in Android 7.1.1 and
later.
* If your server is reachable from the public Internet, use the [SSL
Labs tester][ssllabs-tester]. Under "Cipher Suites" you may see
lines beginning with `TLS_ECDHE`, for cipher suites which use
elliptic-curve cryptography. These lines will have further text
like `ECDH secp256r1` or `ECDH secp384r1`, which identifies specific
elliptic curves your server offers to use. This issue applies if
your server does not offer `secp256r1`.