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Version 49 (modified by Antoine Martin, 17 months ago) (diff)





Version 1.0 also supports SSL (see #1252) which can be used for authentication using certificates (see #1252).

When using ssh to connect to a server, encryption and authentication can be skipped (by default the unix domain sockets used by ssh do not use authentication).

Xpra's authentication modules can be useful for:

  • securing socket connections
  • making the unix domain socket accessible to other users safely
  • using the Proxy Server mode

For more information on the different types of connections, see wiki/Network.


Starting with version 4.0, the preferred way of specifying authentication is within the socket option itself, ie for TCP:

xpra start --start=xterm -d auth

So that multiple sockets can use different authentication modules, and those modules can more easily be chained:

xpra start --start=xterm -d auth \
    --bind-tcp=,auth=hosts,auth=file:filename=password.txt --bind 

For older versions, the authentication modules used are specified using

  • --auth=MODULE for unix domain sockets and named pipes
  • --tcp-auth=MODULE for TCP sockets
  • --vsock-auth=MODULE for vsock (#983)


For more information on the different modules, f. e. ssl, see wiki/Network/.

With version 2.3 and later, you may specify more than one module. ie: --tcp-auth=hosts --tcp-auth=sqlite will verify "hosts" (aka "TCP Wrappers") before checking the sqlite database. (see #1728 for details)

Here are the modules that can be used:

Module Result Purpose Version requirements
allowalways allows the user to login, the username used is the one supplied by the clientdangerous / only for testing
nonealways allows the user to login, the username used is the one the server is running asdangerous / only for testing
failalways fails authentication, no password requireduseful for testing
rejectalways fails authentication, pretends to ask for a passworduseful for testing
envmatches against an environment variable (XPRA_PASSWORD by default)alternative to file module
passwordmatches against a password given as a module option, ie: auth=password:value=mysecretalternative to file module
multifilematches usernames and passwords against an authentication fileproxy: see below
filecompares the password against the contents of a password file, see belowsimple password authentication
pamlinux PAM authenticationLinux system authentication
win32win32security authenticationMS Windows system authentication
syssystem authenticationvirtual module which will choose win32 or pam authentication automatically
sqlitesqlite database authenticationsee ticket:1488#comment:37 >=2.1
peercredSO_PEERCRED authenticationsee r15886 >=2.1
hostsTCP Wrappersee #1730 >=2.3
execDelegates to an external commandsee ticket:1690#comment:4 >=2.3
kerberos-passwordUses kerberos to authenticate a username + passwordsee ticket:1691#comment:4 >=2.3
kerberos-ticketUses a kerberos ticket to authenticate a clientsee ticket:1691#comment:4 >=2.3
gssUses a GSS ticket to authenticate a clientsee ticket:1691#comment:4 >=2.3
ldapUses ldap via python-ldapsee #1791 >=2.3
ldapUses ldap via python-ldap3see #1791 >=2.3
u2fUniversal 2nd Factorsee #1789 >=2.3


The username can be specified in the connection files you can save from the launcher, or in the client connection string, ie for tcp:

xpra attach tcp://username:password@host:port/

When an authentication module is used to secure a single session, many modules will completely ignore the username part and it can be omitted from the connection string. ie for tcp:

xpra attach tcp://:password@host:port/

Or even replaced with any string of your liking, ie 'foobar':

xpra attach tcp://foobar:password@host:port/

Only the following modules will make use of both the username and password to authenticate against their respective backend: kerberos-password, ldap, ldap3, sys (pam and win32), sqlite, multifile and u2f. In this case, using an invalid username will cause the authentication to fail.

The username is more important when authenticating against the wiki/ProxyServer (see authentication details there).

Module Options

The same module may be used with different types of sockets (tcp-auth vs auth), each with a different set of options. For more details see ticket:1159#comment:1.

Some examples:

  • XPRA_PASSWORD=mysecret xpra start --auth=env
  • SOME_OTHER_ENV_VAR_NAME=mysecret xpra start --auth=env:name=SOME_OTHER_ENV_VAR_NAME
  • xpra start --auth=password:value=mysecret
  • xpra start --auth=file:filename=/path/to/mypasswordfile.txt
  • xpra start --auth=sqlite:filename=/path/to/userlist.sdb

Beware when mixing environment variables and password files as the latter may contain a trailing newline character whereas the former often do not.

Password File

"file" vs "multifile":

  • "file" contains a single password, the whole file is the password
  • "multifile" contains a list of authentication values, see proxy server file authentication - this module is deprecated in favour of "sqlite" #1488 which is easier to configure.

Authentication Process

The steps below assume that the client and server have been configured to use authentication:

  • If the server is not configured for authentication, the client connection should be accepted and a warning will be printed
  • If the client is not configured for authentication, a password dialog may show up, and the connection will fail with an authentication error if the correct value is not supplied


  • This information applies to all clients: regular GUI clients as well as command line clients like "xpra info"
  • Each authentication module specifies the type of password hashing it supports (usually HMAC)
  • Some authentication modules (pam, win32, kerberos-password, ldap and ldap3) require the actual password to be sent across to perform the authentication on the server - they therefore use the weak "xor" hashing
  • You must use wiki/Encryption to be able to use "xor" hashing so that the password is protected during the exchange: the system will refuse to send a "xor" hashed password unencrypted
  • Encryption is processed before authentication

For more information on packets, see wiki/NetworkProtocol.


  • All clients first send a hello packet to the server. If the client expects the server to request authentication for the connection, the client packet may omit most of the regular configuration information since a second packet will need to be sent. Until the server accepts the connection with its own hello packet response, the only packets that will be accepted by the clients are challenge and set_deflate (used to control packet compression). The client will exit unless the server responds within the XPRA_SOCKET_TIMEOUT delay + 10 seconds.
  • The server sends back a challenge packet containing a random salt and the digest method to use as specified by the authentication module. If the client does not respond within the XPRA_SOCKET_TIMEOUT delay (defaults to 10 seconds), it is disconnected. The only packets that will be accepted by the server until the client has successfully authenticated are hello and disconnect.
  • The client generates its own random salt and responds with a new "hello" packet containing the challenge response (the hashed password and the client salt used), the client and server salts are combined before hashing the password. (so there is no way for the server or client to predict the actual salt used)
  • When the server receives the hello packet containing the challenge response, it hands it over to the authentication module, which can then:
    • with hmac: verify that it obtains the same hash by combining its own password value with the two salts
    • with xor: xor the password again with the two salts to retrieve the original password value
  • these steps may be repeated if there is more than one authentication module configured (though not all authentication modules require a challenge exchange)

Security Considerations

  • when used over TCP sockets, password authentication is vulnerable to man-in-the-middle attacks where an attacker could intercept the initial exchange and use the stolen authentication challenge response to access the session, Encryption prevents that
  • the client does not verify the authenticity of the server, Encryption does
  • Enabling auth wiki/Logging may leak some authentication information
  • if you are concerned about security, use SSH as transport instead

Salt handling is important