derp,magicsock: add debug envknobs for HTTP and derp server name (#7744)

Make developing derp easier by:

1. Creating an envknob telling clients to use HTTP to connect to derp
servers, so devs don't have to acquire a valid TLS cert.

2. Creating an envknob telling clients which derp server to connect
to, so devs don't have to edit the ACLs in the admin console to add a
custom DERP map.

3. Explaining how the -dev and -a command lines args to derper
interact.

To use this:

1. Run derper with -dev.

2. Run tailscaled with TS_DEBUG_USE_DERP_HTTP=1 and
TS_DEBUG_USE_DERP_ADDR=localhost

This will result in the client connecting to derp via HTTP on port
3340.

Fixes #7700

Signed-off-by: Val <valerie@tailscale.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/wgengine/magicsock/debugknobs.go b/wgengine/magicsock/debugknobs.go
index c8ec0e0c0..adfff17d2 100644
--- a/wgengine/magicsock/debugknobs.go
+++ b/wgengine/magicsock/debugknobs.go
@@ -33,13 +33,19 @@ var (
 	debugReSTUNStopOnIdle = envknob.RegisterBool("TS_DEBUG_RESTUN_STOP_ON_IDLE")
 	// debugAlwaysDERP disables the use of UDP, forcing all peer communication over DERP.
 	debugAlwaysDERP = envknob.RegisterBool("TS_DEBUG_ALWAYS_USE_DERP")
+	// debugDERPAddr sets the derp address manually, overriding the DERP map from control.
+	debugUseDERPAddr = envknob.RegisterString("TS_DEBUG_USE_DERP_ADDR")
+	// debugDERPUseHTTP tells clients to connect to DERP via HTTP on port 3340 instead of
+	// HTTPS on 443.
+	debugUseDERPHTTP = envknob.RegisterBool("TS_DEBUG_USE_DERP_HTTP")
 	// debugEnableSilentDisco disables the use of heartbeatTimer on the endpoint struct
 	// and attempts to handle disco silently. See issue #540 for details.
 	debugEnableSilentDisco = envknob.RegisterBool("TS_DEBUG_ENABLE_SILENT_DISCO")
-	// DebugSendCallMeUnknownPeer sends a CallMeMaybe to a
-	// non-existent destination every time we send a real
-	// CallMeMaybe to test the PeerGoneNotHere logic.
+	// debugSendCallMeUnknownPeer sends a CallMeMaybe to a non-existent destination every
+	// time we send a real CallMeMaybe to test the PeerGoneNotHere logic.
 	debugSendCallMeUnknownPeer = envknob.RegisterBool("TS_DEBUG_SEND_CALLME_UNKNOWN_PEER")
+	// Hey you! Adding a new debugknob? Make sure to stub it out in the debugknob_stubs.go
+	// file too.
 )
 
 // inTest reports whether the running program is a test that set the