SoC/2017

Improve libcli/dns

Samba comes with its own asynchronous DNS parser framework developed for the internal DNS server. Basic calls have been implemented for a client-side library as well, but a more fleshed out implementation would be needed. The goal of this project is to implement more high-level calls handling DNS requests, such as UDP/TCP switchover and client-side GSS-TSIG cryptography. A test suite excercising all the functions is required and can be used to cross-check and complement the existing DNS server tests already shipped by Samba. This testsuite should use cmocka.

• Difficulty: Medium
• Language(s): C
• Mentors: Kai Blin, David Disseldorp
• Student: Dimitris Gravanis

Project Information

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Client-side DNS call handling with GSS-TSIG

Unix SMB/CIFS implementation

Dimitrios Gravanis (C) 2017

Based on the existing work by Samba Team

About

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For the Samba AD DC, libcli/dns is a library that allows the handling of DNS calls (send/receive requests) and generates GSS-TSIG type encryption signature for signed packets, to accomodate encrypted client-server communication.

The project goal was to enhance client-server communication, by implementing TCP request send/receive handling and sign client-side packets with GSS-TSIG signatures, to provide security.

It consists of its respective function and structure libraries, that provide definitions for client-side functionality.

Project libcli/dns structure:

• cli-fn/
  • README.md
  • client_crypto.c
  • dns_tcp.c
  • dns_udp.c
• cmocka-tests/
  • test-fn
    • cli_crypto_test.c
    • dns_tcp_test.c
    • dns_udp_test.c
    • wscript
  • README.md
  • cli_tests.c
  • wscript_build
• README.md
• cli_dns.c
• dns.h
• libtcp.h
• libudp.h
• libtsig.h
• libwrap.h
• wrap_cli.c
• wscript_build

For more information on the project goals, read the GSoC proposal here.

The project timeline and development journal is documented in its dedicated blogspot.

Repositories

• Individual project repository (requires Samba source code for integration - NOT STANDALONE): link
• Samba GitHub mirror: link
• Personal samba-team/samba fork with integrated changes in libcli/dns: link

DNS Client (with wrapper support)

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Handles TCP and UDP requests.

The client may use either TCP or UDP protocols to send a DNS name request to the server, then handle the reception of the appropriate server response.

Features:

• UDP request send/receive
• TCP request send/receive
• GSS-TSIG generation
• DNS name packet parsing and signing

The library consists of cli_dns.c, that includes functions, and dns.h, libtcp.h, libtsig.h, libudp.h, that provide declarations, definitions and structures.

Wrapping

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wrap_cli.c provides multiple wrapping of the above functionality, to hide buffer creation, DNS packet parsing and signature generation. Definitions of the wrapped functions are provided in libwrap.h.

Test suite

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In cmocka-tests, cli_tests.c provides a test suite for the complete client-side functionality, as defined by the functions in libcli/dns/cli_dns.c. The API used for unit testing is Cmocka.

In cmocka-tests/test-fn, there are individual unit tests for every feature library in libcli/dns. All of these tests are incorporated in cmocka-tests/cli_tests.c These tests can be built by using waf-samba and the intended configuration in cmocka-tests/test-fn/wscript. The purpose of these test suites is to facilitate future additions and features in Samba client-side code, without the necessity to integrate them directly to cli_dns.c, thus making changes easier to test and encourage future contributions.