Port 113 ident · TCP

What runs on port 113

ident: a server asks the client's machine "which user owns this TCP connection?". IRC servers still query it, and some mail servers used to, which is why a firewall that silently drops 113 can make IRC connects slow (the server waits for a timeout). Rejecting with a TCP RST is the usual fix.

Software that uses it: oidentd · identd · IRC servers (querying side)

Should port 113 be open?

Disable it. This is a legacy or insecure protocol. The recommendation is not to firewall it but to turn the service off and use the modern replacement.

• Reject rather than drop inbound 113 to avoid connection delays to IRC and some SMTP servers.

Check port 113

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :113'
sudo lsof -i :113

# from another machine: is it reachable?
nc -zv host 113
nmap -p 113 host
curl -v telnet://host:113      # 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 113

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 113 proto tcp
sudo ufw deny 113/tcp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 tcp dport 113 accept
nft add rule inet filter input tcp dport 113 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 113 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 113 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=113 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 113" with no source is just "open".

Related ports

• 194 IRC
• 79 finger

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