Network services - SNMP

By Kurt Seifried [email protected]



SNMP (Simple Network Management Protocol) was designed to let heterogeneous systems and equipment talk to each other, report data and allow modifications to there settings over a TCP-IP network. For example an SNMP enabled device (such as a Cisco router) can be monitored/configured from an SNMP client, and you can easily write scripts to, say, alert you if denied packets/second rises above 20. Unfortunately SNMP has no security built into it. SNMPv1, originally proposed in RFC 1157 (May 1990) and section 8 (Security Considerations) reads thusly: "Security issues are not discussed in this memo.". I think that about sums it up. In 1992/1993 SNMPv2 was released, and did contain security considerations however these security considerations were dropped later on when they were shown to be completely broken. Thus we end up today with SNMPv2 and no security. 

Currently the only way to protect your SNMP devices consists of setting the community name to something hard to guess (but it is very easy to sniff the wire and find the name), and firewall/filter SNMP so that only the hosts that need to talk to each other can (which leaves you open to spoofing). Brute force community name attacks are easy to do and usually effective, and there are several tools specifically for monitoring SNMP transmissions and cracking open an SNMP community, it is a pretty dangerous world out there. 

These risks are slightly mitigated by the usefulness of SNMP, if properly supported and implemented it can make network administration significantly easier. In almost every SNMP implementation the default community name is "public" (this goes for Linux, NT, etc), you must change this, to something obscure (your company name is a bad idea). Once a person has your community name they can conduct an "snmpwalk" and take over your network. SNMP runs over UDP on ports 161 and 162; block this at all entrances to your network (the backbone, the dialup pool, etc). If a segment of network does not have SNMP enabled devices or an SNMP console you should block SNMP to and from that network. This is your only real line of defense with SNMP. 

Additionally the use of IPSec (or other VPN software) can greatly reduce the risk from sniffing. The RFC's for SNMPv3 however go extensively into security (especially RFC 2274, Jan 1998) so there is hope for the future. If you are purchasing new SNMP aware/enabled products make sure they support SNMPv3, as you then have a chance at real security.

There are no specific problems with cu-snmpd per se, apart from the general SNMP problems I have covered. The cu-snmp tools and utilities only support SNMPv1 and SNMPv2, so remember to be careful when using them on or across untrusted networks as your main line of security (the community name) will be out in the open for anyone to see.

ipfwadm -I -a accept -P udp -S -D 161:162
ipfwadm -I -a accept -P udp -S -D 161:162
ipfwadm -I -a deny -P udp -S -D 161:162


ipchains -A input -p udp -j ACCEPT -s -d 161:162
ipchains -A input -p udp -j ACCEPT -s -d 161:162
ipchains -A input -p udp -j DENY -s -d 161:162

SNMP server software



Last updated on 1/9/2001

Copyright Kurt Seifried 2001 [email protected]