Port 3268 AD Global Catalog · TCP

What runs on port 3268

Active Directory Global Catalog (LDAP) on 3268, and over TLS on 3269: a read-only partial replica of every domain in the forest. Applications that search users across domains must use it instead of 389, which only sees one domain.

Software that uses it: Active Directory domain controllers (GC role)

Should port 3268 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.

• Logon failures for users in another domain of the forest often mean the application is pointed at 389 instead of 3268.

Check port 3268

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :3268'
sudo lsof -i :3268

# from another machine: is it reachable?
nc -zv host 3268
nmap -p 3268 host
curl -v telnet://host:3268      # 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 3268

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 3268 proto tcp
sudo ufw deny 3268/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 3268 accept
nft add rule inet filter input tcp dport 3268 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 3268 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 3268 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=3268 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 3268" with no source is just "open".

Related ports

• 389 LDAP — Directory services; plaintext/STARTTLS
• 636 LDAPS — LDAP over TLS
• 88 Kerberos — Authentication (Active Directory)

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