How do I configure the proxy settings so that Eclipse can download new plugins?

Question

I had the same problem. I installed Eclipse 3.7 into a new folder, and created a new workspace. I launch Eclipse with a -data argument to reference the new workspace.

When I attempt to connect to the marketplace to get the SVN and Maven plugins, I get the same issues described in OP.

After a few more tries, I cleared the proxy settings for SOCKS protocol, and I was able to connect to the marketplace.

So the solution for me was to configure the manual settings for HTTP and HTTPS proxy, clear the settings for SOCKS, and restart Eclipse.

Manual + disable SOCKS didn't work for me (still tried to use SOCKS and my company proxy refused it),
Native + changed eclipse.ini worked for me

-Dorg.eclipse.ecf.provider.filetransfer.excludeContributors=org.eclipse.ecf.provider.filetransfer.httpclient
-Dhttp.proxyHost=myproxy
-Dhttp.proxyPort=8080
-Dhttp.proxyUser=mydomain\myusername
-Dhttp.proxyPassword=mypassword
-Dhttp.nonProxyHosts=localhost|127.0.0.1

These settings require IDE restart (sometimes with -clean -refresh command line options).
https://bugs.eclipse.org/bugs/show_bug.cgi?id=281472

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Java8, Eclipse Neon3, slow proxy server:

-Dorg.eclipse.ecf.provider.filetransfer.excludeContributors=org.eclipse.ecf.provider.filetransfer.httpclient4
-Dhttp.proxyHost=<proxy>
-Dhttp.proxyPort=8080
-Dhttps.proxyHost=<proxy>
-Dhttps.proxyPort=8080
-DsocksProxyHost=
-DsocksProxyPort=
-Dhttp.proxyUser=<user>
-Dhttp.proxyPassword=<pass>
-Dhttp.nonProxyHosts=localhost|127.0.0.1
-Dorg.eclipse.equinox.p2.transport.ecf.retry=5
-Dorg.eclipse.ecf.provider.filetransfer.retrieve.connectTimeout=15000
-Dorg.eclipse.ecf.provider.filetransfer.retrieve.readTimeout=1000
-Dorg.eclipse.ecf.provider.filetransfer.retrieve.retryAttempts=20
-Dorg.eclipse.ecf.provider.filetransfer.retrieve.closeTimeout=1000
-Dorg.eclipse.ecf.provider.filetransfer.browse.connectTimeout=3000
-Dorg.eclipse.ecf.provider.filetransfer.browse.readTimeout=1000

There is an eclipse.ini (sts.ini) parameter that can help:

-Djava.net.useSystemProxies=true

A lot of effort wasted on this trivial setting each time I change the work environment... See one of the related bugs on eclipse bugzilla.

finally work for me !

In Eclipse, Window > Preferences > General > Network Connections,
set Active Provider to Native
add to eclipse.ini :

-Djava.net.useSystemProxies=true
-Dhttp.proxyPort=8080
-Dhttp.proxyHost=myproxy
-Dhttp.proxyUser=mydomain\myusername
-Dhttp.proxyPassword=mypassword
-Dhttp.nonProxyHosts=localhost|127.0.0.1|192.168.*|10.*
-Dorg.eclipse.ecf.provider.filetransfer.excludeContributors=org.eclipse.ecf.provider.filetransfer.httpclient4

Just to add to the thread as a POSSIBLE solution, I faced a similar issue when developing on a Linux system that was behind a company firewall. However, using a Windows XP machine, Eclipse was able to access different update sites just fine as both the manual and native network connection providers worked just fine using the company proxy.

After stumbling around for some time, I came across a discussion about using NTLMv2 and an implementation to be found at http://cntlm.sourceforge.net/. To whomever posted this, I give much credit to as it helped me get past the issue running on Linux. As a side note, I was using Eclipse 3.6.2 / Helios on both the Linux and Windows distros.

Best of luck on finding a solution!