Port 88 Kerberos · TCP and UDP

What runs on port 88

Kerberos: the ticket-based authentication protocol behind Active Directory and MIT/Heimdal realms. Clients talk to the KDC on 88 to obtain tickets (AS and TGS exchanges), then present tickets to services. It needs accurate clocks (a skew of more than five minutes breaks it) and working DNS, including the reverse zone.

Software that uses it: Active Directory domain controllers · MIT Kerberos · Heimdal · FreeIPA · Samba AD

Should port 88 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.

• "Clock skew too great" means NTP first, Kerberos second.
• UDP is tried first; large tickets (many group memberships) fall back to TCP, so allow both.

Check port 88

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :88'
ss -lunp 'sport = :88'
sudo lsof -i :88

# from another machine: is it reachable?
nc -zv host 88
nmap -p 88 host
nc -zuv host 88
sudo nmap -sU -p 88 host
curl -v telnet://host:88      # 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 88

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 88 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 88 proto udp
sudo ufw deny 88/tcp
sudo ufw deny 88/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 88 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 88 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 88 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 88 -j DROP
iptables -A INPUT -p udp --dport 88 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 88 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=88 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 88" with no source is just "open".

Related ports

• 464 Kerberos passwd
• 389 LDAP — Directory services; plaintext/STARTTLS
• 636 LDAPS — LDAP over TLS
• 123 NTP — Time synchronisation

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