HTTP/3 and QUIC Setup

HTTP/3 is the latest version of the HTTP protocol, built on QUIC (a UDP-based transport). It improves page load times — especially on mobile or unreliable connections — by eliminating head-of-line blocking and reducing connection setup time.

LiteSpeed supports HTTP/3 natively. No additional modules needed.

Prerequisites

• LiteSpeed Web Server (standard on all RemarkableCloud Cloud Cubes)
• A valid SSL certificate on the domain (HTTP/3 requires HTTPS)
• Port 443 UDP open in the firewall (in addition to TCP 443)

Opening UDP port 443

By default, firewalls only open TCP 443. HTTP/3 uses UDP 443. Open it:

# CSF firewall
csf -a UDP_IN 443
# Or edit /etc/csf/csf.conf: add 443 to UDP_IN and UDP_OUT, then restart CSF
csf -r

# iptables directly
iptables -I INPUT -p udp --dport 443 -j ACCEPT

Enabling HTTP/3 in LiteSpeed WebAdmin

1. Open the WebAdmin console at https://YOUR_IP:7080
2. Go to Configuration → Listeners → Default (HTTPS) → Edit
3. Find QUIC section
4. Set Enable QUIC to Yes
5. Save and Graceful Restart LiteSpeed

Enabling HTTP/3 per virtual host (cPanel)

For cPanel servers, LiteSpeed manages virtual hosts per account. Enable QUIC at the server level and it applies to all domains:

1. WebAdmin → Server → General → HTTP/3 (QUIC)
2. Set to Enabled
3. Save and restart

Verifying HTTP/3 is active

Use Chrome DevTools → Network tab → Protocol column. HTTP/3 connections show as h3.

Or from the command line:

curl -I --http3 https://yourdomain.com
# Should show: HTTP/3 200

Or use an online checker: http3check.net

The Alt-Svc header

LiteSpeed automatically adds the Alt-Svc header to responses, advertising HTTP/3 support to browsers:

alt-svc: h3=":443"; ma=2592000

On the first visit, browsers use HTTP/2. When they see this header, they upgrade to HTTP/3 on subsequent requests (or the next visit). The ma=2592000 value means the browser caches this for 30 days.

Performance expectations

HTTP/3 improvements are most noticeable:

• On mobile networks — high latency, packet loss → QUIC's multiplexing helps significantly
• On first page load — QUIC reduces the number of round trips to establish a connection
• On pages with many resources — QUIC eliminates head-of-line blocking between requests

For users on fast wired connections, the difference is minimal. For mobile users, HTTP/3 can reduce load time by 20–30%.