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Port 5 (rje)

IANA assigns port 5 to rje: Remote Job Entry.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
rje TCP Remote Job Entry Jon Postel -
rje UDP Remote Job Entry Jon Postel -

Check what is listening on port 5

Linux: is anything listening on TCP 5?
sudo ss -tlnp 'sport = :5'
Linux: UDP 5
sudo ss -ulnp 'sport = :5'
Linux / macOS: which process owns the port
sudo lsof -nP -i :5
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':5 '
Windows (Command Prompt)
netstat -ano | findstr :5
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 5 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

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

Block port 5 in the firewall

ufw
sudo ufw deny 5/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 5 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 5 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 5' -Direction Inbound -Protocol TCP -LocalPort 5 -Action Block
ufw
sudo ufw deny 5/udp
iptables
sudo iptables -A INPUT -p udp --dport 5 -j DROP
nftables
sudo nft add rule inet filter input udp dport 5 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 5' -Direction Inbound -Protocol UDP -LocalPort 5 -Action Block

Related ports

1tcpmux7echo9discard

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.