Introduction to Computer Security Benoit Donnet Academic Year

Transcription

1 Introduction to Computer Security Benoit Donnet Academic Year Agenda Networking Chapter 1: Firewalls Chapter 2: Proxy Chapter 3: Intrusion Detection System Chapter 4: Network Attacks Chapter 5: Spam Chapter 6: Virology 2

2 Agenda Chapter 2: Proxy - Definition - Features - Types of Proxies - Reverse Proxies 3 Agenda Chapter 2: Proxy - Definition - Features - Types of Proxies - Reverse Proxies 4

3 Definition Proxies are application relays They play the role of - server for the client - client for the server client TCP IP link phys. client TCP IP link phys. client TCP IP link phys. 5 Agenda Chapter 2: Proxies - Definition - Features - Types of Proxies - Reverse Proxies 6

4 Features Benefits of using proxies - they prevent direct connections from an internal network towards the Internet choke point possibly authentication - they can analyse data within the application's context and filter if required URL or DNS blacklists keyword filtering... - they can reformat pages for smartphones, tablets,... - they can be used to test a system from outside 7 Features (2) Cache features - the proxy can keep a local copy of all documents that it has fetched - when another client asks for the same document, it can deliver directly the local copy Advantages - transfer much faster increase in surf comfort - bandwidth saving limit cost 8

5 Features (3) Anonymous surf - user IP address hidden - open proxy 9 Agenda Chapter 2: Proxy - Definition - Features - Types of Proxies Transparent Proxy FTP Proxy SMTP Proxy DNS Proxy SOCKS Proxy HTTPS Proxy - Reverse Proxies 10

6 Transparent Proxy Transparent proxy - a proxy that does not modify the request or response beyond what is required by the proxy authentication and identification - [RFC2616] Any traffic targeting at a certain port (e.g., 80) is automatically redirected towards the proxy by the firewall Typical usage - force the usage of a proxy - load balancing Limit - does not work if server does not use standard port 11 FTP Proxy FTP - File Transfer Protocol - network protocol used to transfer files from one computer to another - [RFC959] FTP establishes 2 connections between both hosts - command connection used for exchanging command and replies between client and server based on telnet - data connection data is transferred over this connection can be directed towards the client active mode can be directed towards the server passive mode 12

7 FTP Proxy (2) FTP Proxy using HTTP - browsers allow specifying URLs such as FTP:// my.server.com/toto.txt - if the browser is configured to use a HTTP proxy, it will ask the proxy for the URL - the HTTP proxy carries out the FTP transfer and provides the document as part of the HTTP reply User HTTP connection to the proxy proxy get ftp://my.server.com/toto.txt HTTP/1.0 FTP connection USER anonymous PASS toto RETR toto.txt 13 FTP Proxy (3) User@ FTP Proxy - the User@ server behaves like a standard FTP server - it can be used by any FTP client - how to access a remote server Bob with user name Joe provide Joe@Bob as username to the proxy the proxy connects to the server and relays the passwords, commands, and data the two connections can use active or passive mode independently User proxy FTP connection to proxy bob.com FTP connection to bob USER [email protected] PASS maison2 RETR toto.txt USER joe PASS maison2 RETR toto.txt 14

8 SMTP Proxy SMTP - Simple Mail Transfer Protocol - protocol for transmission conceived to relay hop by hop - [RFC821, RFC5321] An SMTP server can act as a proxy - outbound (forward path) the proxy is specified as SMTP server for the outgoing mail in the mail client - inbound (reverse path) the proxy has to be registered in the DNS as the official server for that domain the proxy has to be configured to forward all mails to the internal server that should receive the mails 15 DNS Proxy DNS - Domain Name System - hierarchical distributed naming system for computers mainly ensures domain names IP addresses translation requests are retransmitted from server to server - defined in [RFC882] and [RFC883] DNS servers can work as proxies DNS servers have a cache to limit traffic and reduce response time Good idea to define a DNS proxy to direct all requests towards a bigger server - to take advantage of a bigger cache 16

9 SOCKS Proxy SOCKS proxy is a general proxy for TCP and UDP connections It accepts a client's connection and opens another one towards the server - the data is then transferred between the two connections Advantage - SOCKS allows any protocol to pass via a proxy Drawback - SOCKS allows any protocol to pass via a proxy 17 HTTPS Proxy HTTPS is a secure version of HTTP - authentication - encryption HTTPS proxies are not a secure version of HTTP proxies HTTPS encrypts and authenticates end-to-end - if the proxy was able to create the connection to the server, we would loose all advantages HTTPS proxy does no more than transparently relaying data between client's connection and server's connection - pretty much like SOCKS proxy 18

10 HTTPS Proxy (2) Implementation - HTTPS proxy uses the HTTP command connect that indicates the server address - it replies with a status and becomes transparent connect HTTP/1.0 User proxy GET secret.txt HTTP/1.0 GET secret.txt HTTP/ HTTPS Proxy (3) Security issues - HTTPS proxy allows relaying any type of protocol - to limit abuses, the available ports are often limited to 443 (HTTPS) and 563 (NNTPS) - to allow any protocol to cross a firewall, it is sufficient to run the server on port 443 and pass through an HTTPS proxy 20

11 Agenda Chapter 2: Proxy - Definition - Features - Types of Proxies - Reverse Proxies Principle Reverse HTTP Proxy Reverse HTTPS Proxy Protocol Translation 21 Principle In the forward path, the client knows he must pass through a proxy - he can adapt his requests accordingly In the return path, the client does not know if he is talking to a server or to a proxy The proxy must behave like a server For protocols that do not support relaying (e.g., HTTP, FTP), the reverse proxy can relay only to one server 22

12 Reverse HTTP Reverse HTTP proxy allows for - filtering of requests i.e., blocking exploits see Part 3 of the course - authenticating clients before they speak to the server you can't attack the server, unless you are authenticated - accelerating servers Server accelerators - reverse proxies work just like caches - the proxy provides static documents the server is in charge of building dynamic documents - workload dispatcher 23 Reverse HTTPS HTTPS proxies are used as encryption accelerators - they can reduce the workload of servers by taking care of the encryption and the authentication - the proxy can have a hardware accelerator for HTTPS The connection between the proxy and the server is of HTTP - not HTTPS 24

13 Protocol Translation The proxy can use different protocols each side - a web mail application can accept HTTPS requests from the Internet generate IMAP requests towards the mail server - an e-commerce application can accept HTTPS requests from the Internet generate CORBA or SQL requests towards the servers The protocol diversity strongly limits the chances of exploiting a vulnerability across a proxy 25