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diff --git a/docs-xml/Samba3-HOWTO/TOSHARG-Securing.xml b/docs-xml/Samba3-HOWTO/TOSHARG-Securing.xml new file mode 100644 index 00000000000..21218ea9da4 --- /dev/null +++ b/docs-xml/Samba3-HOWTO/TOSHARG-Securing.xml @@ -0,0 +1,448 @@ +<?xml version="1.0" encoding="iso-8859-1"?> +<!DOCTYPE chapter PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc"> +<chapter id="securing-samba"> + +<chapterinfo> + &author.tridge; + &author.jht; + <pubdate>May 26, 2003</pubdate> +</chapterinfo> + +<title>Securing Samba</title> + +<sect1> +<title>Introduction</title> + +<para> +<indexterm><primary>security</primary></indexterm> +<indexterm><primary>direct internet access</primary></indexterm> +<indexterm><primary>firewall</primary></indexterm> +<indexterm><primary>private network</primary></indexterm> +<indexterm><primary>barriers</primary></indexterm> +<indexterm><primary>deterents</primary></indexterm> +<indexterm><primary>secured networks</primary></indexterm> +The information contained in this chapter applies in general to all Samba installations. Security is +everyone's concern in the information technology world. A surprising number of Samba servers are being +installed on machines that have direct internet access, thus security is made more critical than it would have been had the +server been located behind a firewall and on a private network. Paranoia regarding server security is causing +some network administrators to insist on the installation of robust firewalls even on servers that are located +inside secured networks. This chapter provides information to assist the administrator who understands +how to create the needed barriers and deterents against <quote>the enemy</quote>, no matter where [s]he may +come from. +</para> + +<blockquote> +<para> +A new apprentice reported for duty to the chief engineer of a boiler house. He said, <quote>Here I am, +if you will show me the boiler I'll start working on it.</quote> Then engineer replied, <quote>You're leaning +on it!</quote> +</para> +</blockquote> + +<para> +Security concerns are just like that. You need to know a little about the subject to appreciate +how obvious most of it really is. The challenge for most of us is to discover that first morsel +of knowledge with which we may unlock the secrets of the masters. +</para> + +</sect1> + +<sect1> +<title>Features and Benefits</title> + +<para> +<indexterm><primary>moderately secure</primary></indexterm> +<indexterm><primary>perimeter firewall</primary></indexterm> +<indexterm><primary>host security</primary></indexterm> +<indexterm><primary>Samba security</primary></indexterm> +There are three levels at which security principles must be observed in order to render a site +at least moderately secure. They are the perimeter firewall, the configuration of the host +server that is running Samba, and Samba itself. +</para> + +<para> +Samba permits a most flexible approach to network security. As far as possible Samba implements +the latest protocols to permit more secure MS Windows file and print operations. +</para> + +<para> +<indexterm><primary>host-based protection</primary></indexterm> +<indexterm><primary>interface-based exclusion</primary></indexterm> +<indexterm><primary>resource-based exclusion</primary></indexterm> +Samba can be secured from connections that originate from outside the local network. This can be done using +<emphasis>host-based protection</emphasis>, using Samba's implementation of a technology known as +<quote>tcpwrappers,</quote> or it may be done be using <emphasis>interface-based exclusion</emphasis> so +&smbd; will bind only to specifically permitted interfaces. It is also possible to set specific share- or +resource-based exclusions, for example, on the <smbconfsection name="[IPC$]"/> autoshare. The <smbconfsection +name="[IPC$]"/> share is used for browsing purposes as well as to establish TCP/IP connections. +</para> + +<para> +<indexterm><primary>Access Control Entries</primary><see>ACE</see></indexterm> +<indexterm><primary>ACL</primary></indexterm> +<indexterm>controls<primary></primary></indexterm> +Another method by which Samba may be secured is by setting Access Control Entries (ACEs) in an Access +Control List (ACL) on the shares themselves. This is discussed in +<link linkend="AccessControls">File, Directory, and Share Access Controls</link>. +</para> + +</sect1> + +<sect1> +<title>Technical Discussion of Protective Measures and Issues</title> + +<para> +The key challenge of security is that protective measures suffice at best +only to close the door on known exploits and breach techniques. Never assume that +because you have followed these few measures, the Samba server is now an impenetrable +fortress! Given the history of information systems so far, it is only a matter of time +before someone will find yet another vulnerability. +</para> + + <sect2> + <title>Using Host-Based Protection</title> + + <para> +<indexterm><primary>outside threat</primary></indexterm> +<indexterm><primary>insecure</primary></indexterm> +<indexterm><primary>Internet</primary></indexterm> + In many installations of Samba, the greatest threat comes from outside + your immediate network. By default, Samba accepts connections from + any host, which means that if you run an insecure version of Samba on + a host that is directly connected to the Internet, you can be + especially vulnerable. + </para> + + <para> +<indexterm><primary>allow access</primary></indexterm> +<indexterm><primary>range of hosts</primary></indexterm> + One of the simplest fixes in this case is to use the <smbconfoption name="hosts allow"/> and + <smbconfoption name="hosts deny"/> options in the Samba &smb.conf; configuration file to + allow access to your server only from a specific range of hosts. An example might be: + <smbconfblock> + <smbconfoption name="hosts allow">127.0.0.1 192.168.2.0/24 192.168.3.0/24</smbconfoption> + <smbconfoption name="hosts deny">0.0.0.0/0</smbconfoption> + </smbconfblock> + </para> + + <para> +<indexterm><primary>localhost</primary></indexterm> +<indexterm><primary>private networks</primary></indexterm> +<indexterm><primary>called name</primary></indexterm> + The above will allow SMB connections only from <constant>localhost</constant> (your own + computer) and from the two private networks 192.168.2 and 192.168.3. All other + connections will be refused as soon as the client sends its first packet. The refusal + will be marked as <literal>not listening on called name</literal> error. + </para> + + </sect2> + + <sect2> + <title>User-Based Protection</title> + + <para> + If you want to restrict access to your server to valid users only, then the following + method may be of use. In the &smb.conf; <smbconfsection name="[global]"/> section put: + <smbconfblock> + <smbconfoption name="valid users">@smbusers, jacko</smbconfoption> + </smbconfblock> + </para> + + <para> +<indexterm><primary>smbusers</primary></indexterm> + This restricts all server access either to the user <emphasis>jacko</emphasis> + or to members of the system group <emphasis>smbusers</emphasis>. + </para> + + </sect2> + + <sect2> + + <title>Using Interface Protection</title> + + <para> +<indexterm><primary>network interface</primary></indexterm> +<indexterm><primary>accept connections</primary></indexterm> +<indexterm><primary>Internet</primary></indexterm> + By default, Samba accepts connections on any network interface that + it finds on your system. That means if you have an ISDN line or a PPP + connection to the Internet then Samba will accept connections on those + links. This may not be what you want. + </para> + + <para> + You can change this behavior using options like this: + <smbconfblock> + <smbconfoption name="interfaces">eth* lo</smbconfoption> + <smbconfoption name="bind interfaces only">yes</smbconfoption> + </smbconfblock> + </para> + + <para> +<indexterm><primary>interfaces</primary></indexterm> +<indexterm><primary>loopback interface</primary></indexterm> +<indexterm><primary>Ethernet adapters</primary></indexterm> +<indexterm><primary>listen for connections</primary></indexterm> + This tells Samba to listen for connections only on interfaces with a name starting with + <constant>eth</constant> such as <constant>eth0</constant> or <constant>eth1</constant>, plus on the loopback interface called + <constant>lo</constant>. The name you will need to use depends on what OS you are using. In the above, I used + the common name for Ethernet adapters on Linux. + </para> + + <para> +<indexterm><primary>PPP</primary></indexterm> +<indexterm><primary>SMB</primary></indexterm> +<indexterm><primary>cracker</primary></indexterm> +<indexterm><primary>confirm address</primary></indexterm> + If you use the above and someone tries to make an SMB connection to your host over a PPP interface called + <constant>ppp0</constant>, then [s]he will get a TCP connection refused reply. In that case, no Samba code + is run at all, because the operating system has been told not to pass connections from that interface to any + Samba process. However, the refusal helps a would-be cracker by confirming that the IP address provides + valid active services. + </para> + + <para> +<indexterm><primary>ignore connection</primary></indexterm> +<indexterm><primary>refusing connection</primary></indexterm> +<indexterm><primary>exploitation</primary></indexterm> +<indexterm><primary>denial of service</primary></indexterm> +<indexterm><primary>firewall</primary></indexterm> + A better response would be to ignore the connection (from, for example, ppp0) altogether. The + advantage of ignoring the connection attempt, as compared with refusing it, is that it foils those who + probe an interface with the sole intention of finding valid IP addresses for later use in exploitation + or denial of service attacks. This method of dealing with potential malicious activity demands the + use of appropriate firewall mechanisms. + </para> + + </sect2> + + <sect2 id="firewallports"> + <title>Using a Firewall</title> + + <para> +<indexterm><primary>deny access</primary></indexterm> +<indexterm><primary>exposed</primary></indexterm> +<indexterm><primary>firewall active</primary></indexterm> + Many people use a firewall to deny access to services they do not want exposed outside their network. This can + be a good idea, although I recommend using it in conjunction with the above methods so you are protected even + if your firewall is not active for some reason. + </para> + + <para> + If you are setting up a firewall, you need to know what TCP and UDP ports to allow and block. Samba uses + the following: +<indexterm><primary>Port 135/TCP</primary></indexterm> +<indexterm><primary>Port 137/UDP</primary></indexterm> +<indexterm><primary>Port 138/UDP</primary></indexterm> +<indexterm><primary>Port 139/TCP</primary></indexterm> +<indexterm><primary>Port 445/TCP</primary></indexterm> + </para> + + <simplelist> + <member>Port 135/TCP - used by smbd</member> + <member>Port 137/UDP - used by nmbd</member> + <member>Port 138/UDP - used by nmbd</member> + <member>Port 139/TCP - used by smbd</member> + <member>Port 445/TCP - used by smbd</member> + </simplelist> + + <para> +<indexterm><primary>firewall setups</primary></indexterm> + The last one is important because many older firewall setups may not be aware of it, given that this port + was only added to the protocol in recent years. + </para> + + <para> +<indexterm><primary>configuring a firewall</primary></indexterm> +<indexterm><primary>high order ports</primary></indexterm> +<indexterm><primary>block incoming packets</primary></indexterm> + When configuring a firewall, the high order ports (1024-65535) are often used for outgoing connections and + therefore should be permitted through the firewall. It is prudent to block incoming packets on the high order + ports except for established connections. + </para> + + </sect2> + + <sect2> + <title>Using IPC$ Share-Based Denials </title> + + <para> +<indexterm><primary>IPC$</primary></indexterm> +<indexterm><primary>deny</primary></indexterm> +<indexterm><primary>security hole</primary></indexterm> + If the above methods are not suitable, then you could also place a more specific deny on the IPC$ share that + is used in the recently discovered security hole. This allows you to offer access to other shares while + denying access to IPC$ from potentially untrustworthy hosts. + </para> + + <para> + To do this you could use: + <smbconfblock> + <smbconfsection name="[IPC$]"/> + <smbconfoption name="hosts allow">192.168.115.0/24 127.0.0.1</smbconfoption> + <smbconfoption name="hosts deny">0.0.0.0/0</smbconfoption> + </smbconfblock> + </para> + + <para> +<indexterm><primary>IPC$</primary></indexterm> +<indexterm><primary>protection against attackers</primary></indexterm> +<indexterm><primary>valid username/password</primary></indexterm> + This instructs Samba that IPC$ connections are not allowed from anywhere except the two listed network + addresses (localhost and the 192.168.115 subnet). Connections to other shares are still allowed. Because the + IPC$ share is the only share that is always accessible anonymously, this provides some level of protection + against attackers who do not know a valid username/password for your host. + </para> + + <para> +<indexterm><primary>access denied</primary></indexterm> +<indexterm><primary>IPC$</primary></indexterm> +<indexterm><primary>browse shares</primary></indexterm> + If you use this method, then clients will be given an <literal>`access denied'</literal> reply when they try + to access the IPC$ share. Those clients will not be able to browse shares and may also be unable to access + some other resources. This is not recommended unless for some reason you cannot use one of the other methods + just discussed. + </para> + + </sect2> + + <sect2> + <title>NTLMv2 Security</title> + + <para> +<indexterm><primary>NTLMv2</primary></indexterm> + To configure NTLMv2 authentication, the following registry keys are worth knowing about: + </para> + + <para> + <screen> + [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa] + "lmcompatibilitylevel"=dword:00000003 + </screen> + </para> + + <para> + The value 0x00000003 means to send NTLMv2 response only. Clients will use NTLMv2 authentication; + use NTLMv2 session security if the server supports it. Domain controllers accept LM, + NTLM, and NTLMv2 authentication. + </para> + + <para> + <screen> + [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0] + "NtlmMinClientSec"=dword:00080000 + </screen> + </para> + + <para> + The value 0x00080000 means permit only NTLMv2 session security. If either NtlmMinClientSec or + NtlmMinServerSec is set to 0x00080000, the connection will fail if NTLMv2 + session security is negotiated. + </para> + </sect2> +</sect1> + +<sect1> +<title>Upgrading Samba</title> + +<para> +<indexterm><primary>updates</primary></indexterm> +<indexterm><primary>important announcements</primary></indexterm> +<indexterm><primary>security vulnerability</primary></indexterm> +Please check regularly on <ulink noescape="1" url="http://www.samba.org/">http://www.samba.org/</ulink> for +updates and important announcements. Occasionally security releases are made, and it is highly recommended to +upgrade Samba promptly when a security vulnerability is discovered. Check with your OS vendor for OS-specific +upgrades. +</para> + +</sect1> + +<sect1> +<title>Common Errors</title> + +<para> +If all Samba and host platform configurations were really as intuitive as one might like them to be, this +chapter would not be necessary. Security issues are often vexing for a support person to resolve, not because +of the complexity of the problem, but because most administrators who post what turns out to be a security +problem request are totally convinced that the problem is with Samba. +</para> + + <sect2> + <title>Smbclient Works on Localhost, but the Network Is Dead</title> + + <para> + This is a common problem. Linux vendors tend to install a default firewall. + With the default firewall in place, only traffic on the loopback adapter (IP address 127.0.0.1) + is allowed through the firewall. + </para> + + <para> + The solution is either to remove the firewall (stop it) or modify the firewall script to + allow SMB networking traffic through. See <link linkend="firewallports">the Using a + Firewall</link> section. + </para> + + </sect2> + + <sect2> + <title>Why Can Users Access Other Users' Home Directories?</title> + + <para> + <quote> +<indexterm><primary>mapping home directory</primary></indexterm> +<indexterm><primary>own home directory</primary></indexterm> + We are unable to keep individual users from mapping to any other user's home directory once they have + supplied a valid password! They only need to enter their own password. I have not found any method to + configure Samba so that users may map only their own home directory. + </quote> + </para> + + <para><quote> + User xyzzy can map his home directory. Once mapped, user xyzzy can also map anyone else's home directory. + </quote></para> + + <para> +<indexterm><primary>security flaw</primary></indexterm> +<indexterm><primary>defined shares</primary></indexterm> + This is not a security flaw, it is by design. Samba allows users to have exactly the same access to the UNIX + file system as when they were logged on to the UNIX box, except that it only allows such views onto the file + system as are allowed by the defined shares. + </para> + + <para> +<indexterm><primary>UNIX home directories</primary></indexterm> +<indexterm><primary>permissions</primary></indexterm> + If your UNIX home directories are set up so that one user can happily <command>cd</command> + into another user's directory and execute <command>ls</command>, the UNIX security solution is to change file + permissions on the user's home directories so that the <command>cd</command> and <command>ls</command> are denied. + </para> + + <para> +<indexterm><primary>security policies</primary></indexterm> +<indexterm><primary>permissions</primary></indexterm> + Samba tries very hard not to second guess the UNIX administrator's security policies and + trusts the UNIX admin to set the policies and permissions he or she desires. + </para> + + <para> + Samba allows the behavior you require. Simply put the <smbconfoption name="only user">%S</smbconfoption> + option in the <smbconfsection name="[homes]"/> share definition. + </para> + + <para> + The <smbconfoption name="only user"></smbconfoption> works in conjunction with the <smbconfoption name="users">list</smbconfoption>, + so to get the behavior you require, add the line: + <smbconfblock> + <smbconfoption name="users">%S</smbconfoption> + </smbconfblock> + This is equivalent to adding + <smbconfblock> + <smbconfoption name="valid users">%S</smbconfoption> + </smbconfblock> + to the definition of the <smbconfsection name="[homes]"/> share, as recommended in + the &smb.conf; man page. + </para> + </sect2> + +</sect1> +</chapter> |