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, 2 insertions, 2 deletions

diff --git a/cmd/derper/derper.go b/cmd/derper/derper.go
index 48efaba40..02736b6be 100644
--- a/cmd/derper/derper.go
+++ b/cmd/derper/derper.go
@@ -36,8 +36,8 @@ import (
 )
 
 var (
-	dev        = flag.Bool("dev", false, "run in localhost development mode")
-	addr       = flag.String("a", ":443", "server HTTPS listen address, in form \":port\", \"ip:port\", or for IPv6 \"[ip]:port\". If the IP is omitted, it defaults to all interfaces.")
+	dev        = flag.Bool("dev", false, "run in localhost development mode (overrides -a)")
+	addr       = flag.String("a", ":443", "server HTTP/HTTPS listen address, in form \":port\", \"ip:port\", or for IPv6 \"[ip]:port\". If the IP is omitted, it defaults to all interfaces. Serves HTTPS if the port is 443 and/or -certmode is manual, otherwise HTTP.")
 	httpPort   = flag.Int("http-port", 80, "The port on which to serve HTTP. Set to -1 to disable. The listener is bound to the same IP (if any) as specified in the -a flag.")
 	stunPort   = flag.Int("stun-port", 3478, "The UDP port on which to serve STUN. The listener is bound to the same IP (if any) as specified in the -a flag.")
 	configPath = flag.String("c", "", "config file path")