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Port 77 (any private RJE service)

IANA assigns port 77 to any private RJE service.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
- TCP any private RJE service Jon Postel -
- UDP any private RJE service Jon Postel -

Check what is listening on port 77

Linux: is anything listening on TCP 77?
sudo ss -tlnp 'sport = :77'
Linux: UDP 77
sudo ss -ulnp 'sport = :77'
Linux / macOS: which process owns the port
sudo lsof -nP -i :77
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':77 '
Windows (Command Prompt)
netstat -ano | findstr :77
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 77 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 77
From Windows
Test-NetConnection HOST -Port 77
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 77

Block port 77 in the firewall

ufw
sudo ufw deny 77/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 77 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 77 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 77' -Direction Inbound -Protocol TCP -LocalPort 77 -Action Block
ufw
sudo ufw deny 77/udp
iptables
sudo iptables -A INPUT -p udp --dport 77 -j DROP
nftables
sudo nft add rule inet filter input udp dport 77 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 77' -Direction Inbound -Protocol UDP -LocalPort 77 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.