diff --git a/Cargo.lock b/Cargo.lock
index e062511..2a4a2f6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1209,6 +1209,7 @@ dependencies = [
  "fltk",
  "quic_demo",
  "quinn",
+ "reqwest",
  "structopt",
  "tokio",
  "tracing",
@@ -1500,9 +1501,9 @@ dependencies = [
 
 [[package]]
 name = "reqwest"
-version = "0.11.2"
+version = "0.11.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bf12057f289428dbf5c591c74bf10392e4a8003f993405a902f20117019022d4"
+checksum = "246e9f61b9bb77df069a947682be06e31ac43ea37862e244a69f177694ea6d22"
 dependencies = [
  "base64",
  "bytes",
diff --git a/prototypes/ptth_quic_client_gui/Cargo.toml b/prototypes/ptth_quic_client_gui/Cargo.toml
index 7281233..995a913 100644
--- a/prototypes/ptth_quic_client_gui/Cargo.toml
+++ b/prototypes/ptth_quic_client_gui/Cargo.toml
@@ -12,6 +12,7 @@ anyhow = "1.0.38"
 fltk = "1.1.1"
 quic_demo = { path = "../quic_demo" }
 quinn = "0.7.2"
+reqwest = "0.11.4"
 structopt = "0.3.20"
 tokio = { version = "1.8.1", features = ["full"] }
 tracing-subscriber = "0.2.16"
diff --git a/prototypes/ptth_quic_client_gui/src/main.rs b/prototypes/ptth_quic_client_gui/src/main.rs
index 4cfdfda..e11bfbe 100644
--- a/prototypes/ptth_quic_client_gui/src/main.rs
+++ b/prototypes/ptth_quic_client_gui/src/main.rs
@@ -25,6 +25,8 @@ struct Opt {
 	relay_addr: Option <String>,
 	#[structopt (long)]
 	client_id: Option <PeerId>,
+	#[structopt (long)]
+	cert_url: Option <String>,
 }
 
 #[derive (Clone, Copy)]
@@ -103,8 +105,15 @@ fn main () -> anyhow::Result <()> {
 	wind.show ();
 	
 	let connection_p2_p3 = rt.block_on (async move {
-		let server_cert = tokio::fs::read ("quic_server.crt").await?;
-		let relay_addr = opt.relay_addr.unwrap_or_else (|| String::from ("127.0.0.1:30380")).parse ()?;
+		let server_cert = match opt.cert_url.as_ref () {
+			Some (url) => reqwest::get (url).await?.bytes ().await?,
+			None => tokio::fs::read ("quic_server.crt").await?.into (),
+		};
+		
+		let relay_addr = opt.relay_addr
+		.unwrap_or_else (|| String::from ("127.0.0.1:30380"))
+		.parse ()
+		.context ("relay_addr should be like 127.0.0.1:30380")?;
 		let endpoint = make_client_endpoint ("0.0.0.0:0".parse ()?, &[&server_cert])?;
 		
 		trace! ("Connecting to relay server");
@@ -114,7 +123,8 @@ fn main () -> anyhow::Result <()> {
 		let quinn::NewConnection {
 			connection,
 			..
-		} = protocol::p2_connect_to_p3 (&endpoint, &relay_addr, &client_id).await?;
+		} = protocol::p2_connect_to_p3 (&endpoint, &relay_addr, &client_id).await
+		.context ("P2 can't connect to P3")?;
 		
 		Ok::<_, anyhow::Error> (connection)
 	})?;