Sentry ice protects a server not by preventing or restricting access, but instead by attacking any intruder that comes near it, either by "tagging" the intruder to be tracked down later, attacking the intruder's software, or even inflicting direct, painful, harm. There are numerous sentries that can harm the unwary Runner, and they include:
Sentries inflict pain through spike processes. The more power a spike process can use, the harder it can hit. The really clever ones - such as Cortex Lock - borrow the runner’s own memory to iterate locally.
Flare is an NBN sentry that can trace back to the Runner's rig and trigger an overheating of the rig's power supply, potentially causing a console meltdown: A bright light blossomed, and then the console went dark. That's when she smelled smoke.
Guard is one of the few sentries that can prevent access; it "blocks" the Runner from moving any further, giving it some advantages over barrier and code gate access prevention methods: Out of the corner of his vision, a flash of light. He felt a thud as his grip on cyberspace loosened. Spider didn't like this. He should have bypassed the outer layer of ice by jacking in from the internal server. Another thud, and his whole frame shook with the resounding shockwave. He desperately reached for his shuriken...
As reported cases of brain damage in veterans rise, mind/machine interface devices are subject to increased public scrutiny. That certain programs can cause irreparable harm to users has gone from fringe theory to accepted truth. One such program is the oft-rumored Fenris Haas-Bioroid sentry.
Haas-Bioroid naturally employs bioroid ice that aggressively hunts down intruders, thus acting as sentries, such as Ichi 1.0, which always issues a warning to its victims: "My reputation would precede me, if any could speak of it."
There are rumors of a corporate program called Grim that destroys the software of Runners that encounter it, as described by the Professor: "The Grim of legend was one of may so-called 'black dog' myths common to Gaelic and English-speaking communities. What's fascinating is that it has propagated to the network, where it lives on as a program that, or so the story goes, hunts for the unwary."
There are several sentries that, instead of attacking intruders, capitalize on the intrusion to make their servers run more efficiently. One such example is Weyland's Errand Boy. After all, recycling the energy entering the system via a run is both responsible and economical.
Another such example is NBN's Matrix Analyzer: Analyzing was great. Delegating commands turned out to be even better.