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Port 105 (cso, csnet-ns)

IANA assigns port 105 to cso, csnet-ns: CCSO name server protocol.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
cso TCP CCSO name server protocol Martin Hamilton -
csnet-ns TCP Mailbox Name Nameserver Marvin Solomon -
cso UDP CCSO name server protocol Martin Hamilton -
csnet-ns UDP Mailbox Name Nameserver Marvin Solomon -

Check what is listening on port 105

Linux: is anything listening on TCP 105?
sudo ss -tlnp 'sport = :105'
Linux: UDP 105
sudo ss -ulnp 'sport = :105'
Linux / macOS: which process owns the port
sudo lsof -nP -i :105
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':105 '
Windows (Command Prompt)
netstat -ano | findstr :105
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 105 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 105
From Windows
Test-NetConnection HOST -Port 105
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 105

Block port 105 in the firewall

ufw
sudo ufw deny 105/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 105 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 105 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 105' -Direction Inbound -Protocol TCP -LocalPort 105 -Action Block
ufw
sudo ufw deny 105/udp
iptables
sudo iptables -A INPUT -p udp --dport 105 -j DROP
nftables
sudo nft add rule inet filter input udp dport 105 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 105' -Direction Inbound -Protocol UDP -LocalPort 105 -Action Block

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