network protocols handbook.pdf

118Protocols GuideSecurity and VPN - Other Security ProtocolsOther Security ProtocolsProtocol NameSOCKS v5: Protocol for sessionstraversal across firewallsecurelyProtocol DescriptionThe SOCKS protocol provides a framework for client-server applicationsin both the TCP and UDP domains to convenientlyand securely use the services of a network firewall. The protocolis conceptually a “shim-layer” between the application layer andthe transport layer, and as such does not provide network layergateway services, such as forwarding of ICMP messages.The use of network firewalls, systems that effectively isolate anorganizations internal network structure from an exterior network,such as the Internet is becoming increasingly popular.These firewall systems typically act as application-layer gatewaysbetween networks, usually offering controlled TELNET,FTP, and SMTP access. SOCKS provides a general frameworkfor these protocols to transparently and securely traverse a firewall.SOCKS version 5, also, provides strong authentication of suchtraversal, while SOCKS Version 4 provides only unsecured firewalltraversal for TCP-based client-server applications, includingTELNET, FTP, and protocols such as HTTP, WAIS and GO-PHER. SOCKS version 5 extends the SOCKS Version 4 modelto include UDP, and extends the framework to include provisionsfor generalized strong authentication schemes. It also adaptsthe addressing scheme to encompass domain-name and IPv6addresses.The implementation of the SOCKS protocol typically involvesthe recompilation or relinking of TCP-based client applicationsto use the appropriate encapsulation routines in the SOCKS library.Protocol StructureSOCKS v5 has a few messages with different formats.The method selection message:The reply message:1 byte 1 byteVersionMethod1 byte 1 byte Value of 0 1 byte Variable 2 bytesVersion REP RSV ATYPUDP request header:BNDaddrBND Port2 bytes 1 byte 1 byte Variable 2 bytes VariableRSV FRAG ATYP DST Addr DST Port DataRelated protocolsTCP, UDP, ICMP, HTTP, Gopher, TELNET, FTPSponsor SourceSOCKS is defined by IETF (http://www.ietf.org) in RFC 1928.Referencehttp://www.javvin.com/protocol/rfc1928.pdfSOCKS Protocol Version 5Version identifier/method selection message:1 byte 1 byte 1-225 bytesVersion NMethods MethodsThe SOCKS request message:1 byte 1 byte Value of 0 1 byte Variable 2 bytesVersion CMD Rsv ATYP DST addr DST Port