CLDAP, Portmapper and other open UDP services: reflection attacks and how to close them

Security·· 3 views· 5 min read

For VPS and dedicated server customers with root access. The short answer: some services that speak UDP answer a small request with a much bigger reply. UDP does not check who is asking, so attackers forge the source address (the victim’s) and use your server to flood the victim’s connection: a reflection attack. The ones a national incident response team (CERT) reports most are CLDAP (from Windows servers running Active Directory) and Portmapper (rpcbind, which comes with NFS). You close them like this: switch off what you do not use, make what you do use listen internally only, and let only the addresses that need it through the firewall.

The services these reports name

Service Port What to do
CLDAP (Active Directory) UDP 389 A domain controller should not answer the internet. Limit the inbound rules to your own network or VPN.
Portmapper (rpcbind) TCP and UDP 111 If you do not use NFS, switch it off. If you do, allow only the servers that mount the shares.
Memcached UDP and TCP 11211 Listen on 127.0.0.1 only and turn UDP off.
NTP (ntpd serving time) UDP 123 The server can set its own clock without serving time to others: block inbound. chrony, by default, serves time to nobody.
SNMP UDP 161 Switch it off unless you monitor the server over SNMP. If you do, allow only the monitoring system’s IP.
SSDP, WS-Discovery, NetBIOS UDP 1900, 3702, 137 Local network services: there is no reason for them to answer the internet. Switch off or block.
DNS resolver UDP 53 Has its own article: an open DNS resolver.

Finding and closing them

1 List what listens on UDP: sudo ss -ulnp
rpcinfo -p Anything on 0.0.0.0 or [::] listens on every address; 127.0.0.1 only internally. On a Windows VPS, in an administrator command prompt: netstat -ano -p udp.
2 Before switching anything off, check who uses it: cat /etc/exports
findmnt -t nfs,nfs4 The first shows what this server shares over NFS; the second, the NFS shares it mounts. If both are empty and you know of no other use, rpcbind is not needed.
3 Portmapper, if you do not use NFS: sudo systemctl disable --now rpcbind.service rpcbind.socket
sudo systemctl mask rpcbind.service rpcbind.socket The .socket unit restarts the service on the first request, so both go. mask stops another package from turning it back on; undo it with sudo systemctl unmask rpcbind.service rpcbind.socket.
4 CLDAP on a Windows VPS: if the server does not need to be a domain controller, do not make it one. If it does, open Windows Defender Firewall with Advanced Security, find the Active Directory LDAP inbound rules and, on the scope tab, limit the remote address to your private network or VPN. Names vary with the Windows version and language. See also making Remote Desktop safer.
5 Memcached: on Debian and Ubuntu, in /etc/memcached.conf (on the Red Hat family, in the OPTIONS line of /etc/sysconfig/memcached): -l 127.0.0.1
-U 0 then sudo systemctl restart memcached.
6 A firewall that refuses all inbound traffic and opens only what serves the public. Let SSH through before changing the default rule: sudo ufw allow OpenSSH
sudo ufw default deny incoming
sudo ufw status verbose If you moved SSH to another port, allow that port instead of OpenSSH. Without ufw, see the rules with sudo nft list ruleset. Guide: setting up ufw without locking yourself out.
7 Test from another machine: sudo nmap -sU -p 111,389,11211 YOUR_SERVER_IP (nmap may not be installed). In a UDP scan, “open|filtered” is normal for a port the firewall blocks; “open” means the service still answers.
8 Reply to the support ticket in Meu Interweb, within the notice’s deadline (short, often a single day), with a short report: the cause, what you changed and how you checked that the port no longer answers from outside. There is a template in writing the reply to an abuse notice. With no reply, or if the problem carries on, the server may be switched off to protect the network, with your data left intact: why a server is switched off and how to bring it back.
Switching services off blindly can break things: rpcbind is needed to mount NFS shares, and Active Directory by the machines that depend on it. Check first (step 2) and, on a domain controller, limit rather than switch off. A wrong firewall rule can lock you out: On a VPS, root access is yours: we do not log in to the server and cannot change a firewall that runs inside it. If you lose SSH, tell us in the support ticket.
These notices do not mean the server was hacked: the service is abused from outside, just as it is. The rule that prevents the next notice is simple: refuse everything inbound and open only what must be open. See the six that matter.

Not sure which program opened one of these ports? Send us the step 1 output in the support ticket.

Open a support ticket

SEE ALSO

An abuse notice about your server: what it means, the deadline and what we need from you

Writing the reply to an abuse notice: cause, what you removed and how it will not happen again

An open DNS resolver on your VPS: why it is used in attacks and how to turn off recursion

Setting up the ufw firewall on Ubuntu without locking yourself out

Keeping your VPS or dedicated server secure: the six that matter

Making Remote Desktop on a Windows VPS safer

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