.. _phk_barriers:
============================
Security barriers in Varnish
============================
Security is a very important design driver in Varnish, more likely than not,
if you find yourself thinking "Why did he do _that_ ? the answer has to
do with security.
The Varnish security model is based on some very crude but easy to understand
barriers between the various components::
.-->- provides ->---------------------------------------.
| | |
(ADMIN)--+-->- runs ----->---. | |
| | | |
|-->- cli_req -->---| v v
'--<- cli_resp -<---| VCL MODULE
| | |
(OPER) | |reads |
| | | |
|runs | | |
| .-<- create -<-. | .->- fork ->-. v |
v |->- check -->-|-- MGT --| |-- VCC <- loads -|
VSM |-<- write --<-' | '-<- wait -<-' | |
TOOLS | | | |
^ | .-------------' | |
| | | |writes |
|reads | |->- fork ----->-. | |
| | |->- cli_req -->-| | |
VSM ----' |-<- cli_resp -<-| v |
| '-<- wait -----<-| VCL.SO |
| | | |
| | | |
|---->----- inherit --->------|--<-- loads -------' |
|---->----- reads ---->------| |
'----<----- writes ----<------|--<-- loads --------------------'
|
|
|
.--->-- http_req --->--. | .-->-- http_req --->--.
(ANON) --| |-- CLD --| |-- (BACKEND)
'---<-- http_resp --<--' '--<-- http_resp --<--'
(ASCII-ART rules!)
The really Important Barrier
============================
The central actor in Varnish is the Manager process, "MGT", which is the
process the administrator "(ADMIN)" starts to get web-cache service.
Having been there myself, I do not subscribe to the "I feel cool and important
when I get woken up at 3AM to restart a dead process" school of thought, in
fact, I think that is a clear sign of mindless stupidity: If we cannot
get a computer to restart a dead process, why do we even have them ?
The task of the Manager process is therefore not cache web content,
but to make sure there always is a process which does that, the
Child "CLD" process.
That is the major barrier in Varnish: All management happens in
one process all actual movement of traffic happens in another, and
the Manager process does not trust the Child process at all.
The Child process is in a the totally unprotected domain: Any
computer on the InterNet "(ANON)" can connect to the Child process
and ask for some web-object.
If John D. Criminal manages to exploit a security hole in Varnish, it is
the Child process he subverts. If he carries out a DoS attack, it is
the Child process he tries to fell.
Therefore the Manager starts the Child with as low priviledge as practically
possible, and we close all filedescriptors it should not have access to and
so on.
There are only three channels of communication back to the Manager
process: An exit code, a CLI response or writing stuff into the
shared memory file "VSM" used for statistics and logging, all of
these are well defended by the Manager process.
The Admin/Oper Barrier
======================
If you look at the top left corner of the diagram, you will see that Varnish
operates with separate Administrator "(ADMIN)" and Operator "(OPER)" roles.
The Administrator does things, changes stuff etc. The Operator keeps an
eye on things to make sure they are as they should be.
These days Operators are often scripts and data collection tools, and
there is no reason to assume they are bugfree, so Varnish does not
trust the Operator role, that is a pure one-way relationship.
(Trick: If the Child process us run under user "nobody", you can
allow marginally trusted operations personel access to the "nobody"
account (for instance using .ssh/authorized_keys2), and they will
be able to kill the Child process, prompting the Manager process to
restart it again with the same parameters and settings.)
The Administrator has the final say, and of course, the administrator
can decide under which circumstances that authority will be shared.
Needless to say, if the system on which Varnish runs is not properly
secured, the Administrator's monopoly of control will be compromised.
All the other barriers
======================
There are more barriers, you can spot them by following the arrows in
the diagram, but they are more sort of "technical" than "political" and
generally try to guard against programming flaws as much as security
compromise.
For instance the VCC compiler runs in a separate child process, to make
sure that a memory leak or other flaw in the compiler does not accumulate
trouble for the Manager process.
Hope this explanation helps understand why Varnish is not just a single
process like all other server programs.
Poul-Henning, 2010-06-28
Henceforth, whatever our philosopher says about Matter will apply to extension and to extension alone. It cannot be apprehended by sight, nor by hearing, nor by smell, nor by taste, for it is neither colour, nor sound, nor odour, nor juice. Neither can it be touched, for it is not a body, but it becomes corporeal on being blended with sensible qualities. And, in a later essay, he describes it as receiving all things and letting them depart again without retaining the slightest trace of their presence.483 Why then, it may be asked, if Plotinus meant extension, could he not say so at once, and save us all this trouble in hunting out his meaning? There were very good reasons why he should not. In the first place, he wished to express himself, so far as possible, in Aristotelian phraseology, and this was incompatible with the reduction of Matter to extension. In the next place, the idea of an infinite void had been already appropriated by the Epicureans, to whose system he was bitterly opposed. And, finally, the extension of ordinary327 experience had not the absolute generality which was needed in order to bring Matter into relation with that ultimate abstraction whence, like everything else, it has now to be derived. That the millionaire was genuine, ¡°in person and not a caricature,¡± as Dick put it, was evident. Both the nurse, his relative, and his wife, were chatting with him as Jeff delivered the heavy packed ball made up of the gum. 233 "I guess not," said Landor, tolerantly, as he turned[Pg 106] his horse over to his orderly; "but, anyway," he added to Ellton, "we had a picnic¡ªof a sort." Si, unable to think of anything better, went with him. The train had stopped on a switch, and seemed likely to rust fast to the rails, from the way other trains were going by in both directions. The bridge gang, under charge of a burly, red-faced young Englishman, was in the rear car, with their tools, equipments, bedding and cooking utensils. THE DEACON HAS SOME EXPERIENCES WITH THE QUADRUPED. "You are not within a mile of the truth. I know it. Look here: I believe that is Gen. Rosecrans's own cow. She's gone, and I got an order to look around for her. I've never seen her, but from the description given me I believe that's she. Who brought her here?" "Deacon, these brothers and sisters who have come here with me to-night are, like myself, deeply interested in the moral condition of the army, where we all have sons or kinsmen. Now, can't you sit right there and tell us of your observations and experiences, as a Christian man and father, from day to day, of every day that you were down there? Tell us everything, just as it happened each day, that we may be able to judge for ourselves." HAS AN ENCOUNTER WITH THE PROVOST-MARSHAL. "Wonder which one o' them is the 200th Injianny's?" said Si to Shorty. "And your mother, and Harry?" The daughter must be the girl who was talking to him now. She sat on a little stool by the fire, and had brought out some sewing. "Over at Grandturzel¡ªcan't see wot's burning from here. Git buckets and come!" These things, however, gave little concern to the worthy who commanded the Kentish division. Tyler, though an excellent blacksmith, possessed few of the qualities requisite for forming a good general. Provided there was no very sensible diminution in the number of his followers, he cared not a straw for the score or two who, after quarrelling, or perhaps fighting, withdrew in such disgust that they vowed rather to pay the full tax for ever than submit to the insolence of the rebels. One man could fight as well as another, reasoned he; and, provided he was obeyed, what mattered it by whom. Dick went and Tom came¡ªit was sure to be all one in the end. But this burst of indignation soon passed away, and upon the suggestion of the prudent Sir Robert Hailes, he sent an evasive answer, with a command that the Commons should attend him at Windsor on the Sunday following. That it was a stratagem to gain entrance to the Tower, was the opinion of several, but, after much discussion, it was decided that the man should be admitted, and that the monk should be exhibited merely to intimidate the rebels, until the result of this promised communication should be known. HoMEŮͬÐÔÁµcbcb
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