Port 3128 Squid proxy · TCP

What runs on port 3128

Squid's default port: the classic HTTP forward proxy and cache, used for corporate egress control, content filtering, lab environments without direct internet access and as a transparent cache. Clients send it absolute URLs or CONNECT requests for HTTPS. An open proxy on 3128 is abused immediately.

Software that uses it: Squid · Privoxy (8118) · Tinyproxy (8888) · Zscaler/Blue Coat (proxy appliances)

Should port 3128 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• curl -x http://proxy:3128 https://example.com -v shows whether CONNECT is allowed.
• http_access deny all after your allow rules is the line that keeps it from being open.

Check port 3128

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :3128'
sudo lsof -i :3128

# from another machine: is it reachable?
nc -zv host 3128
nmap -p 3128 host
curl -v telnet://host:3128      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 3128

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 3128 proto tcp
sudo ufw deny 3128/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 3128 accept
nft add rule inet filter input tcp dport 3128 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 3128 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 3128 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=3128 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 3128" with no source is just "open".

Related ports

• 1080 SOCKS — Proxy
• 8080 HTTP alt — Tomcat, proxies, dev servers
• 8118 Privoxy — Filtering proxy, often in front of Tor

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions