Ghast

Ghasts are very large jellyfish-like mobs that float through the air and shoot fireballs from their mouths. Their bodies are 4 by 4 by 4 block cubes with 9 tentacle-like limbs hanging from the bottom. They first appeared in the Halloween Update. Ghasts are able to spawn anywhere in the Nether, as long as there is sufficient space and the game is not in Peaceful mode.

Behavior
Ghasts normally float around with their eyes and mouth closed. They open their eyes and mouth when attacking. They make a chirp-like noise when shooting and scream loudly when taking damage. The sounds that Ghasts make have a minimum intensity regardless of their distance, so they often sound closer when they are actually far away.

Unlike most other aggressive mobs, the Ghast does not have a pursuit mode - it does not attempt to draw closer to the player when at long range (and likewise is not particularly likely to float away). Compared to other mobs, the Ghast has a very long search range and can target a player up to 100 blocks away. When the player comes within about sixteen blocks, the Ghast will open range and attempt to gain altitude.

If a Ghast's fireball hits a portal (or close to it) the portal will be closed, requiring re-ignition. If the player manages to get a Ghast to fire at the portal while it is closed, it can be reignited using the fireball. Blocks originally ignited once struck, adding an extra hazard to fighting Ghasts; however, the fire is extinguished immediately afterwards because of a bug.

The minimum block resistance required to absorb all blast force of the Ghast's fireballs is 20.17, meaning that fences and other less resistant blocks will be destroyed in a fireball explosion.

Ghasts will not fire at the player without line-of-sight, and this is blocked by glass, leaves, portals, vines, or webs making safe scenic paths through the Nether a viable possibility. When not moving, Ghasts within firing range will face the player, giving the impression that they can see through transparent materials. Ghasts can draw line-of-sight through curtains of lava, and their fireballs can pass through it meaning players near a pillar of lava can be struck without seeing the source.

Ghasts drop Gunpowder when killed, but this can be hard to collect - as it is the only mob that flies, the gunpowder may fall into lava or an inaccessible area.

Fighting Ghasts
Ghasts are often considered the most difficult mob to kill owing to their extreme attack range, explosive attack power, mobility and flight. They are also quite common; players in any exposed area will have to deal with them sooner or later and possibly from the moment they step out of a portal. Ghast fireballs have a profound effect on Nether architecture, making a compelling case to use only materials resistant to their explosions for outer walls and exterior fittings in places where their fire may be drawn. As a result of their large size and the extreme unpleasantness of an indoor Ghast spawn, the primary defense mechanism for buildings in the Nether becomes enclosure rather than lighting.

The Ghast's fireball attack has a straight trajectory unaffected by gravity and an unlimited or very long range limited in practice only by the confines of the Nether's caverns. Their explosions can often be heard without the impact being seen. At close range, it is the second most powerful mob attack in Minecraft, second only to the blast of a Creeper. A direct hit from point blank range will deal three and a half hearts of damage but damage decreases with distance and is less if the player is simply caught in the blast radius. Additionally, the projectile is fairly slow and aims slightly above the player's head. A Ghast's firepower is highly inaccurate at most ranges if one is moving. Players are most vulnerable when they are building in the Nether, when they may well not be moving and possibly standing above a long fall. Ghasts can also break blocks from under the player, often dropping them into lava or causing death through falling damage.

The player can reflect fireballs by hitting them with anything before they impact, even bare hands. Because of their low speed, spamming mouseclicks is often enough to deflect any given attack. It is not possible to kill Ghasts by reflecting their attacks back at them even considering their low health and high attack damage, even though you can still hear the Ghast's "hurt" clip being played when hit. You can make a fireball easily reflect in the direction you want by shooting an arrow, fishing rod, or snowball at the fireball. The fishing rod, when released, slightly affects the fireball's trajectory. This relationship goes both ways and a Ghast's constant stream of fireballs may well interfere with the firing arc of your bow.

Most Ghast kills are made with a bow and arrows for obvious reasons but a Ghast can be killed with melee weapons if the player can bring it to bear before the Ghast can draw away. Ghasts are highly vulnerable when caught on the ground since their attacks can be deflected and their health is quite low - a single blow from a diamond sword will destroy one. It is possible to use a fishing rod to draw Ghasts in near enough to hit with a sword. A close Ghast is also easier to hit with a bow, though reaction time against its fireballs is likewise lessened. When aiming your bow be sure to lead your shots as Ghasts can quickly change trajectory and move quite quickly. Depending on the Ghast's range and relative altitude, it is usually necessary to also aim at a high trajectory to gain extra range. Where possible, having the advantage in altitude such is shooting from the topmost layer of the Nether, is of enormous help. If ammunition is not a concern, it is adviseable to shoot many arrows at differing trajectories in order to find range. The Ghast's hitbox is mainly its tentacles, and aiming too high may cause a shot to go through the Ghast's body dealing no damage. However, it is perfectly practical to shoot the hitbox through the ephemeral body from above using plunging fire.

Much like using neutral or enemy mobs as shields against arrows fired by skeletons in the regular world, it is feasible to use zombie pigmen as shields against Ghasts in the Nether. It is possible to injure or kill zombie pigmen in this fashion without ever making them hostile to the player.

An effective method of fighting ghasts in the nether is to make a shelter, preferably out of brick, cobblestone, obsidian, or other explosion-resistant materials. Make a 1x1 hole in the roof and add ladders leading up to it, in such a way that you can poke the upper half of your body through the roof. When a ghast is near, climb the ladder, shoot an arrow at the ghast and quickly descend again into your explosion-proof shelter. This should mean that you can kill ghasts easily with minimal health loss. To improve this even further, add a small 3x3 roof 1 block above the hole in the main roof. Most of the ghasts' fireballs will now hit this roof rather than you, meaning that you should be safe against most ghast attacks.

Resistance to Fire
Ghasts may also be set on fire, but fire and lava don't actually damage them. A similar effect is seen with zombie pigmen. When a Ghast is caught in lava, its movement is impeded, and then they can be easily targeted by arrows. At around 23:09 in this video, a Ghast impeded by a lavafall can be seen, and again, but closer and for longer, at 23:59.

It is disputed that Ghasts can be killed by lava or fire if continually re-exposed. This is achieved by a Ghast floating in and out of lava. There have been reports of zombie pigmen dying while in lava, due to the fact they "bob" in and out of lava, which raises doubt that Ghasts and zombie pigmen are completely lava/fireproof.

Despite being fireproof, Ghasts and Zombie Pigmen are capable of drowning if under water or lava for too long. This means they, like the player and other mobs, require air to live.

Trivia

 * Ghasts can "cancel" their fireballs if the targeted player has an obstruction between the Ghast and themselves.
 * Until Beta 1.7(?), Ghasts aimed at the camera, not the player
 * Ghasts can cause other monsters to attack it but it will only attempt to attack the player.
 * It is possible to kill Zombie Pigmen by deflecting Ghasts' fireballs at them.
 * Ghasts are not affected by light, and thus appear the same brightness anywhere.
 * Ghasts' eyes look like they have tears.
 * When a Ghast shoots a fireball, it looks like a flaming snowball.
 * If the cursor is over the Ghast's fireball the box around the equipped item will light up.
 * Ghasts are the biggest mob, being eight times as big as the biggest variation of slime.
 * If a Ghast is trapped in a cube, it will shoot fireballs through the cube. This is because like all other things that shoot, it spawns the projectile one block in front of it.
 * When a player in an SMP server dies near Ghasts or is teleported out of the Nether to the surface world (not via a portal) there is a chance the Ghasts will follow them to their new location in the surface world. These Ghasts may act like normal Ghasts but could also behave radically differently than normal. Changes in teleported Ghasts include the Ghast-less explosions and Ghasts without explosions mentioned above. The teleported Ghast may also freeze in one location or "stalk" a certain player (moving closer to a player until they touch like a normal hostile mob) but both of these are extremely rare.
 * Due to the explosive behavior of Ghast's attacks, they can be used to mine blocks in the Nether.
 * Ghasts can actually fly much higher than y-coordinate 128.
 * As of the 1.8 beta update, Ghasts are more easily killed than in previous versions, due to the reworking of bow mechanics that came with the update.

Bugs

 * Some times if there is a 1-4 space between a platform they will be trapped, unable to move. They will continue to fire at you.
 * Their hitbox data seems out of place. Objects will pass through the top of the Ghast, however, hitting their tentacles will hurt it.
 * Arrows will sometimes pass through the Ghast during server lag in SMP.
 * In SMP, their fireballs often move back and forth, similar to arrows.
 * If spawned in the Overworld (by using hacks) in a glass cage, they will float and stare at the player, but Ghasts will sometimes unexpectedly shoot a fireball in the direction of the player even though there is a glass barrier between them and a player. This bug appears to be absent in the Nether.
 * A fireball will not disappear if in flight when switching to peaceful.
 * Ghasts' explosions are less "connected" to them in SMP than in SSP. For this reason in SMP mobs damaged by a Ghast's fireballs will not attempt to attack the Ghast and Ghasts that are either spawned or follow a player into the surface world (see below) can become divorced from their explosions and become an essentially peaceful mob. Similarly, Ghast explosions can sometimes follow a player into the real world, without any nearby ghasts, but these normally only appear in the form of apparently random explosion noises not connected to actual damage causing explosions.
 * Arrows look bigger when it is stuck to the ghast.