“Come on,” said Harry dully. “With Umbridge policing the fires and reading all our mail?”
Umbridge started policing the fires after Harry first talked to Sirius in Year 5.
Prior to Umbridge, could anyone simply floo to Hogwarts (the way Sirius did at the start of the book to Gryffindor common room)?
If not, what does the canon say about any security around Flooing to the castle?
I believe Hogwarts was generally disconnected from the Floo network, although connections could be made if necessary (or by force).
In particular, Floo connections directly to student common rooms would be problematic from a child protection aspect. Some members of staff might have theirs connected, but I don’t recall instances when staff used them. In general, I can’t think of many instances when the staff leave Hogwarts during term (Dumbledore aside, and he has methods other than Floo).
In Half-Blood Prince, students return to the school by Floo at Christmas, but this is described as a special case.
Late in the afternoon, a few days after New Year, Harry, Ron, and Ginny lined up beside the kitchen fire to return to Hogwarts. The Ministry had arranged this one-off connection to the Floo Network to return students quickly and safely to the school.
Pottermore adds weight to this theory (in spoiler tags if you haven’t got to this part):
The fireplaces of Hogwarts are not generally connected, although there have been occasions when one or more has been tampered with, often without the staff’s knowledge.
In both instances where the Floo network is used within Hogwarts, it’s only used for a person’s head/as a communication tool, not for full bodily transfer. I would guess that the security measures prevent you entering or leaving the grounds via Floo, just like apparition (special exceptions aside). The relevant passages are quoted below.
Undoubtedly, head-only transport isn’t as much of a risk as, say, allowing Death Eaters direct physical passage into the school, but it’s still a security hole. When Amos Diggory uses Floo to reach the Weasley’s, we see that this head-only transport still allows physical objects to be transferred:
Mrs. Weasley took a piece of buttered toast from a stack on the kitchen table, put it into the fire tongs, and transferred it into Mr. Diggory’s mouth. “Fanks,” he said in a muffled voice, and then, with a small pop, vanished.
Whether this is an intentional omission or an oversight, I don’t believe is addressed in the canon.
In Goblet of Fire:
Harry looked back into the flames, and jumped. Sirius’s head was sitting in the fire. If Harry hadn’t seen Mr. Diggory do exactly this back in the Weasleys’ kitchen, it would have scared him out of his wits.
There’s no evidence that Harry or Hermione tampered with the Gryffindor fireplace to allow Sirius to Floo in that evening, but clearly he was able to. This is an apparent hole in the Floo network security.
We can use this to surmise that Hogwarts Floo traffic is not monitored, as there is never any indication that staff or Ministry officials were aware of Sirius’s presence. If they were, it could probably have been used to trace his location.
In Order of the Phoenix, when Harry uses the Floo network without detection:
It was one of the most curious sensations he had ever experienced. He had traveled by Floo powder before, of course, but then it had been his entire body that had spun around and around in the flames through the network of wizarding fireplaces that stretched over the country. This time, his knees remained firm upon the cold floor of Umbridge’s office, and only his head hurtled through the emerald fire…
As far as we know, Umbridge never knows about this conversation. (When she catches Harry the second time, it’s because of Stealth Sensoring Spells used to prevent Niffler attacks, not a notification from the Floo authority.)
Sirius and Dumbledore are both writing to each other during Goblet of Fire:
“You are not Sirius’s only correspondent,” said Dumbledore. “I have also been in contact with him ever since he left Hogwarts last year. It was I who suggested the mountainside cave as the safest place for him to stay.”
If Dumbledore knew that Sirius wanted to speak to Harry, he could have suggested or supported using the fireplace in the Gryffindor common room, and opened access to the Floo network for those specific instances. Otherwise, the network could be completely shut at all times.
Since I don’t have canon information about whether this theory is true, I’m marking it as speculation.
I believe that Hogwarts most likely had some sort of enchantment or similar effect to stop people from transporting into and out of Hogwarts, or at the very least monitor it in some form. In fact Sirius was only able to talk to Harry Potter in year 5, which is why Umbridge was policing the fires to make sure there was nobody trying to get in or out of Hogwarts unnoticed or trying to communicate with people outside of the school.
http://harrypotter.wikia.com/wiki/Floo_Network
One may also use the Floo Network to speak to someone in another location by merely putting their head through the green fire, as Sirius Black did to talk to Harry in latter's fourth and fifth years at Hogwarts.
As Sirius Black could use the fire to talk to Harry in fourth and fifth year, but in Harry's sixth year, a special arrangement had to be made from the Ministry to enable students to arrive at Hogwarts, it seems that head-only transport is possible even if human transportation is not. Otherwise, intruders, including Sirius Black, could simply use the fire in the common room to circumvent the castle's defences.
The only time it actually is shown as transporting to Hogwarts was during Christmas in the sixth year.
Also it could be noted that if the Floo Network was able to sneak people in, then why would Draco Malfoy take all the trouble to fix the Vanishing Cabinet to transport Death Eaters into the school if they could simply Floo into a fireplace.
I do believe you can "floo" into Hogwarts because, in Harry Potter and the Goblet of Fire, Harry Potter actually meets Sirius Black. But there may be something like a charm that hits people when they go through to the castle. But in the book Harry Potter and the Order of the Phoenix, Harry travels with floo powder to Grimmauld Place. They did that because Mr. Weasley had gotten seriously injured by Nagini (Voldemort's pet snake/Horcrux). So yes, you can just "floo" into and out of Hogwarts.
The evidence indicates that Umbridge was merely policing the fires, not that they couldn't be used.
When Dumbledore sends Harry and the Weasleys to Grimmauld Place following the attack on Mr. Weasley, we have the following dialogue:
“How’re we going?” asked Fred, looking shaken. “Floo powder?”
“No,” said Dumbledore, “Floo powder is not safe at the moment, the Network is being watched. You will be taking a Portkey.”
Note that Dumbledore does not say that the Floo Network is blocked; he just says that it's being watched. This implies that it would be perfectly possible to use it to travel, but it would be detected.
During the climactic scene in Dumbledore's office, Umbridge mentions the help they were receiving from Marietta's mother who worked in the Floo Network office:
Marietta’s mother, Minister,” she added, looking up at Fudge, “is Madam Edgecombe from the Department of Magical Transportation. Floo Network office — she’s been helping us police the Hogwarts fires, you know.”
If travel was blocked they wouldn't need any help policing it. It is precisely because the Network can still be used that they need someone from the Floo Network office to be policing it.
Later, when Umbridge interrogates Harry about Sirius's whereabouts she mentions this again:
“Very well, Potter, I will take your word for it this time, but be warned: The might of the Ministry stands behind me. All channels of communication in and out of this school are being monitored. A Floo Network Regulator is keeping watch over every fire in Hogwarts — except my own, of course.
Once again, if travel was blocked there would be no need for someone to be watching over every fire. They are watching because the Network can still be used. The difference between Umbridge's fire and all the others is not that hers is the only one that can be used; it is that hers is the only one not being watched.
On the other hand, if Floo travel wasn't blocked, why didn't they in fact block it? We know from the beginning of Goblet of Fire that fires can be connected to and disconnected from the Network, as Mr. Weasley tells the Dursleys:
I had your fireplace connected to the Floo Network, you see — just for an afternoon, you know, so we could get Harry. Muggle fireplaces aren't supposed to be connected, strictly speaking — but I've got a useful contact at the Floo Regulation Panel and he fixed it for me.
So they could have simply disconnected Hogwarts from the network (maybe leave one fire open for emergencies, with additional security around it)?
Additionally, in Prisoner of Azkaban when everyone was trying to figure out how Sirius was getting into the castle, no one suggested that he might have been using the Floo Network.
Perhaps it was blocked during Prisoner of Azkaban and it was unblocked once Sirius was deemed to no longer be a major threat to Hogwarts?
