to File > Preferences > Additional Boards Manager URLs
Tools > Board > Boards Manager
Wait for downloads to complete.
Click "Close" (don't install the ESP8266 boards, you already have them installed)
The ESP8266 boards will now appear in your Tools > Board menu again.
Note that the above Boards Manger URL can not be used to install the package because it relies on files at arduino.esp8266.com but if it was previously installed it will allow you to get the boards back.
I just noticed you commented on that github issue and had already solved your problem. It would have been courteous for you to mention that here so I didn't waste my time finding a solution that you already knew about.
Fel vid nerladdning av http://arduino.esp8266.com/win32-xtensa-lx106-elf-gb404fb9-2.tar.gz
java.lang.RuntimeException: java.lang.Exception: Fel vid nerladdning av http://arduino.esp8266.com/win32-xtensa-lx106-elf-gb404fb9-2.tar.gz
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$1(ContributionManagerUI.java:176)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.Exception: Fel vid nerladdning av http://arduino.esp8266.com/win32-xtensa-lx106-elf-gb404fb9-2.tar.gz
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:131)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:69)
at cc.arduino.contributions.packages.ContributionInstaller.install(ContributionInstaller.java:110)
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$1(ContributionManagerUI.java:173)
... 1 more
Caused by: java.net.UnknownHostException: arduino.esp8266.com
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:527)
at sun.net.www.http.HttpClient.(HttpClient.java:211)
at sun.net.www.http.HttpClient.New(HttpClient.java:308)
at sun.net.www.http.HttpClient.New(HttpClient.java:326)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1202)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1181)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1032)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:966)
at cc.arduino.utils.network.FileDownloader.downloadFile(FileDownloader.java:170)
at cc.arduino.utils.network.FileDownloader.download(FileDownloader.java:128)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:129)
... 4 more
The issue has been reported to the ESP8266 core developers:
I'm sure they will have it fixed shortly.
It's a shame this problem keeps happening over and over again but we can't complain too much since they're providing the core for free. It would almost be worth setting up a mirror. I know it affects a lot of people every time this happens.
If you need a quick fix you can read through the issue report for some workarounds in addition to the one luiz_666 posted. If you use an alternative JSON file URL then be sure to change it back once the problem with the official one is solved because it's likely the unofficial ones will not be maintained and so you will not get a notification when an updated version of the ESP8266 core is available.
Open preferences.txt from C:\Users\AppData\Local\Arduino15 ( this path may vary depending on your
OS) and remove value for "boardsmanager.additional.urls"
Delete all .tmp files from this folder (C:\Users\AppData\Local\Arduino15)
Copy & paste URL in File->preferences
Goto Board-> Manager board... now you dont see any error and ESP8266 should be there in the list
ashok99:
This workaround worked for me. Though the URL was returning expected JSON it was returning error "Error downloading http://arduino.esp8266.com/stable/package_esp8266com_index.json" in IDE. I tried following and now I'm able to resolve this issue.
Open preferences.txt from C:\Users\AppData\Local\Arduino15 ( this path may vary depending on your
OS) and remove value for "boardsmanager.additional.urls"
Delete all .tmp files from this folder (C:\Users\AppData\Local\Arduino15)
Copy & paste URL in File->preferences
Goto Board-> Manager board... now you dont see any error and ESP8266 should be there in the list
ashok99:
This workaround worked for me. Though the URL was returning expected JSON it was returning error "Error downloading http://arduino.esp8266.com/stable/package_esp8266com_index.json" in IDE. I tried following and now I'm able to resolve this issue.
Open preferences.txt from C:\Users\AppData\Local\Arduino15 ( this path may vary depending on your
OS) and remove value for "boardsmanager.additional.urls"
Delete all .tmp files from this folder (C:\Users\AppData\Local\Arduino15)
Copy & paste URL in File->preferences
Goto Board-> Manager board... now you dont see any error and ESP8266 should be there in the list
This problem with the libraries is an ongoing mess. Sometimes it works, sometimes it doesn't. When it does not the Arduino IDE does not list the ESP8266 boards. Sometimes it works on one computer and fails on another. Sometimes it fails on both using the Arduino IDE. I also have the Visual Micro installed for Visual Studio 2017 and when the Arduino IDE, fails, I can jump onto Visual Micro and continue working. (MOST of the time). I will post a different problem I encountered last weekend with the libaries becoming corrupted.
After digging on the Internet, I finally found a page that described how to manually install the boards and libraries from GITHUB without using the Arduino IDE Preferences and boards manager.
The problem with that URL only works for the 2.5.0-beta2 release. If you use that URL, you will not get notifications for future updates or be able to update to newer updates than 2.5.0-beta2. I have seen the use of these URLs cause problems for people several times in the past where it caused people to be using a very outdated version while thinking that was the latest one available.
If there is a situation where the standard URL simply can't be made to work and the version-specific URL will work, then I guess it's a reasonable choice. However, I can't imagine how that would ever occur. Certainly, if you're going to recommend the use of one of these URLs, you should also warn people of the consequences and recommend that they regularly check for new versions. This is especially important with this release since it's a beta version and thus is more likely to have bugs. These beta releases are not really intended for general use. They should only be used by people who are interested in helping the development work by doing beta testing and reporting any bugs they find.
I have solved this in a bit of a tedious operation,
opening "%appdata%\local\arduino15\staging\packages" and for each error downloading given in the arduino IDE, i downloaded the ZIP file manually, and copy to the above directory, after the fifth zip was copied. all was fine.
I know this is not a sustainable solution. but for the time being it solved my problem