This is related to [#702384].

As per comment#2, the issue looks similar to Bug ID -- 702384, so merging into the same. Please un-merge if not the case.
Thank You.

702384 is a tracking bug for feature development.  This is a bug report, unmerging.

This is probably because HeadlessURLRequestContextGetter::GetURLRequestContext doesn't set up the proxy_service.  Looking at ProfileIOData::Init this is done:

 main_request_context_storage_->set_proxy_service(
      ProxyServiceFactory::CreateProxyService(
          io_thread->net_log(), main_request_context_.get(),
          network_delegate.get(),
          std::move(profile_params_->proxy_config_service), command_line,
          io_thread->WpadQuickCheckEnabled(),
          io_thread->PacHttpsUrlStrippingEnabled()));


We should do something similar in HeadlessURLRequestContextGetter.

One wrinkle, ProxyServiceFactory::CreateProxyService lives in the chrome/ layer (fortunately the proxy implementation lives in net/) which we (probably) can't use.  I expect we'd need to duplicate the file :(

Actually can we confirm if this is with --headless or not?

Yes it is with --headless.

 Issue 732499  has been merged into this issue.

#8 is wrong.  The proxy service is set up, not sure what I was thinking ;)

I wonder if this is something entirely different, exchequer598@ if you're listening is your --proxy_server flag a simple URL or do you have something more complex in there?

The --proxy_server flag supported by headless is currently just a URL, but normal chrome supports more complex syntax (which seems to be) parsed by ProxyConfig::ProxyRules::ParseFromString.  Maybe headless should use that too?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/094472afa242fa5ec156b2b744b774e1dfd0a046

commit 094472afa242fa5ec156b2b744b774e1dfd0a046
Author: Alex Clarke <alexclarke@chromium.org>
Date: Tue Jul 11 14:59:32 2017

Headless: support more complex --proxy_server strings

Previously in headless mode --proxy_server only supported a URL where as
chrome supports a richer syntax allowing various proxy fallbacks.  This patch
makes headless mode --proxy_server work the same way.

Bug:  732154 
Change-Id: I850c6ea5596ffc7e528b3a005a585c209d3af8e5
Reviewed-on: https://chromium-review.googlesource.com/567008
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485630}
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/app/headless_shell.cc
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/lib/browser/headless_browser_context_impl.cc
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/lib/browser/headless_browser_context_options.cc
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/lib/browser/headless_browser_context_options.h
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/lib/browser/headless_url_request_context_getter.cc
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/lib/browser/headless_url_request_context_getter.h
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/lib/headless_browser_browsertest.cc
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/public/headless_browser.cc
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/public/headless_browser.h
[modify] https://crrev.com/094472afa242fa5ec156b2b744b774e1dfd0a046/headless/public/headless_browser_context.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/203e89c4f1912c51bb837e22c7574af1604b05c1

commit 203e89c4f1912c51bb837e22c7574af1604b05c1
Author: Alex Clarke <alexclarke@chromium.org>
Date: Wed Jul 12 10:40:00 2017

Headless: Add a test of DevTools request interception with an Auth Proxy

There have been some reports of these features not working together. This 
test tries to ensure that they do work.

Bug:  732154 
Change-Id: Ieccd55603d35a92342a868fc7753f3af98a4aed7
Reviewed-on: https://chromium-review.googlesource.com/567505
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485917}
[modify] https://crrev.com/203e89c4f1912c51bb837e22c7574af1604b05c1/headless/lib/headless_devtools_client_browsertest.cc
[modify] https://crrev.com/203e89c4f1912c51bb837e22c7574af1604b05c1/headless/test/headless_browser_test.cc
[modify] https://crrev.com/203e89c4f1912c51bb837e22c7574af1604b05c1/headless/test/headless_browser_test.h

Well it appears there's nothing fundamental in the net stack that prevents interception working with a proxy.  The test I just added spins up a simple Auth proxy and it sets up a HeadelssBrowserContext with the same code path that --proxy_server in HeadlessShell uses, and it tests that devtools interception works.  Currently I'm puzzled why this isn't working for the bug reporter.

@Alex,

Thanks for the test. I've noted it in a slightly related thread (see https://bugs.chromium.org/p/chromium/issues/detail?id=702384#c14), but installing Wireshark fixed the issue for me. I'm guessing there was some dependency I was missing on my box - Ubuntu Xenial's base Docker image.

Thanks for closing the loop. I guess we can close this too :)

@Alex,

Thanks a ton for your work on the interception API. Would you mind helping figure out the dependency that was missing? Install wireshark increases my Docker image's size considerably, so I'd rather install only the Chome interception-specific ones.

 Issue 681521  has been merged into this issue.