bandwidth limits. A badly configured open proxy can also allow access to a private subnetwork or DMZ: this is a high security concern for any company or home network because computers that usually are out of risk or firewalled can be directly attacked. Many open proxies run very slowly, sometimes below 14.4 kbit/s, or even below 300 bit/s, while other times the speed may change from fast to slow every minute. Some, such as PlanetLab proxies, run faster and were intentionally set up for public use. Because open proxies are often implicated in abuse, a number of methods have been developed to detect them and to refuse service to them.
IRC networks with strict usage policies automatically test client systems for known types of open proxies. Likewise, a mail server may be configured to automatically test mail senders for open proxies, using software such as proxycheck. Increasingly, mail servers are configured out of the box to consult various DNSBL servers in order to block spam; some of those DNSBLs also list open proxies.