17. Encryption

17. Encryption

Overview

Zabbix supports encrypted communications between Zabbix server, Zabbix proxy, Zabbix agent, zabbix_sender and zabbix_get utilities using Transport Layer Security (TLS) protocol v.1.2. Encryption is supported starting with Zabbix 3.0. Certificate-based and pre-shared key-based encryption is supported.

Encryption is optional and configurable for individual components (e.g. some proxies and agents can be configured to use certificate-based encryption with the server, while others can use pre-shared key-based encryption, and yet others continue with unencrypted communications as before).

Server (proxy) can use different encryption configurations for different hosts.

Zabbix daemon programs use one listening port for encrypted and unencrypted incoming connections. Adding an encryption does not require opening new ports on firewalls.

Limitations

Private keys are stored in plain text in files readable by Zabbix components during startup.
Pre-shared keys are entered in Zabbix frontend and stored in Zabbix database in plain text.
Built-in encryption does not protect communications:

   * between web server running Zabbix frontend and user web browser,
          * between Zabbix frontend and Zabbix server,
          * between Zabbix server (proxy) and Zabbix database.
       * Currently each encrypted connection opens with a full TLS handshake, no session caching and tickets are implemented.
       * Adding encryption increases time of checks and actions, depending on network latency.\\ For example, if packet delay is 100ms then opening a TCP connection and sending unencrypted request takes around 200ms.\\ With encryption about 1000 ms are added for establishing TLS connection.\\ Timeouts may need to be increased, otherwise some items and actions running remote scripts on agents may work with unencrypted connections but fail with timeout with encrypted.
       * Encryption is not supported by [[/manual/discovery/network_discovery|network discovery]]. Zabbix agent checks performed by network discovery will be unencrypted and if Zabbix agent is configured to reject unencrypted connections such checks will not succeed.

Compiling Zabbix with encryption support

To support encryption Zabbix must be compiled and linked with one of three crypto libraries:

mbed TLS (formerly PolarSSL)(version 1.3.9 and later 1.3.x). mbed TLS 2.x is not currently supported, it is not a drop-in replacement for 1.3 branch, Zabbix will not compile with mbed TLS 2.x.
GnuTLS (from version 3.1.18)
OpenSSL (from version 1.0.1)

The library is selected by specifying an option to “configure” script:

--with-mbedtls[=DIR]
--with-gnutls[=DIR]
--with-openssl[=DIR]

For example, to configure the sources for server and agent with OpenSSL you may use something like:
./configure —enable-server —enable-agent —with-mysql —enable-ipv6 —with-net-snmp —with-libcurl —with-libxml2 —with-openssl

Different Zabbix components may be compiled with different crypto libraries (e.g. a server with OpenSSL, an agent with GnuTLS).

If you plan to use pre-shared keys (PSK) consider using GnuTLS or mbed TLS libraries in Zabbix components using PSKs. GnuTLS and mbed TLS libraries support PSK ciphersuites with Perfect Forward Secrecy. OpenSSL library (versions 1.0.1, 1.0.2c) does support PSKs but available PSK ciphersuites do not provide Perfect Forward Secrecy.

Connection encryption management

Connections in Zabbix can use:

There are two important parameters used to specify encryption for connections between Zabbix components:

TLSConnect
TLSAccept

TLSConnect specifies what encryption to use for outgoing connections and can take one of 3 values (unencrypted, PSK, certificate). TLSConnect is used in configuration files for Zabbix proxy (in active mode, specifies only connections to server) and Zabbix agentd (for active checks). In Zabbix frontend the TLSConnect equivalent is Connections to host field in Configuration→Hosts→<some host>→Encryption tab and Connections to proxy field in Administration→Proxies→<some proxy>→Encryption tab. If the configured encryption type for connection fails, no other encryption types will be tried.

TLSAccept specifies what types of connections are allowed for incoming connections. Connection types are: unencrypted, PSK, certificate. One or more values can be specified. TLSAccept is used in configuration files for Zabbix proxy (in passive mode, specifies only connections from server) and Zabbix agentd (for passive checks). In Zabbix frontend the TLSAccept equivalent is Connections from host field in Configuration→Hosts→<some host>→Encryption tab and Connections from proxy field in Administration→Proxies→<some proxy>→Encryption tab.

Normally you configure only one type of encryption for incoming encryptions. But you may want to switch encryption type, e.g. from unencrypted to certificate-based with minimum downtime and rollback possibility.
To achieve this you can set TLSAccept=unencrypted,cert in agentd configuration file and restart Zabbix agent.
Then you can test connection with zabbix_get to the agent using certificate. If it works, you can reconfigure encryption for that agent in Zabbix frontend in Configuration→Hosts→<some host>→Encryption tab by setting Connections to host to “Certificate”.
When server configuration cache gets updated (and proxy configuration is updated if the host is monitoring by proxy) then connections to that agent will be encrypted.
If everything works as expected you can set TLSAccept=cert in agent configuration file and restart Zabbix agent.
Now the agent will be accepting only encrypted certificate-based connections. Unencrypted and PSK-based connections will be rejected.

In a similar way it works on server and proxy. If in Zabbix frontend in host configuration Connections from host is set to “Certificate” then only certificate-based encrypted connections will be accepted from agent (active checks) and zabbix_sender (trapper items).

Most likely you will configure incoming and outgoing connections to use the same encryption type or no encryption at all. But technically it is possible to configure it asymmetrically, e.g. certificate-based encryption for incoming and PSK-based for outgoing connections.

For overview, encryption configuration for each host is displayed in Zabbix frontend Configuration→Hosts on the right side, in column AGENT ENCRYPTION. Configuration display examples:

Connections TO host	Allowed connections FROM host	Rejected connections FROM host
Unencrypted	Unencrypted	Encrypted certificate and PSK-based
Encrypted, certificate-based	Encrypted certificate-based	Unencrypted and PSK-based
Encrypted, PSK-based	Encrypted PSK-based	Unencrypted and certificate-based
Encrypted, PSK-based	Unencrypted and PSK-based encrypted	Certificate-based
Encrypted, certificate-based	Unencrypted, PSK or certificate-based encrypted	-

Default is unencrypted connections. Encryption must be configured for each host and proxy individually.

zabbix_get and zabbix_sender with encryption

See man-pages zabbix_get and zabbix_sender for using them with encryption.

Ciphersuites

Ciphersuites are configured internally during Zabbix startup and depend on crypto library, currently they are not user-configurable.

Configured ciphersuites by library type in order from higher to lower priority:

Library	Certificate ciphersuites	PSK ciphersuites
mbed TLS (PolarSSL) 1.3.9	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256 TLS-ECDHE-RSA-WITH-AES-128-CBC-SHA256 TLS-ECDHE-RSA-WITH-AES-128-CBC-SHA TLS-RSA-WITH-AES-128-GCM-SHA256 TLS-RSA-WITH-AES-128-CBC-SHA256 TLS-RSA-WITH-AES-128-CBC-SHA	TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA256 TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA TLS-PSK-WITH-AES-128-GCM-SHA256 TLS-PSK-WITH-AES-128-CBC-SHA256 TLS-PSK-WITH-AES-128-CBC-SHA
GnuTLS 3.1.18	TLS_ECDHE_RSA_AES_128_GCM_SHA256 TLS_ECDHE_RSA_AES_128_CBC_SHA256 TLS_ECDHE_RSA_AES_128_CBC_SHA1 TLS_RSA_AES_128_GCM_SHA256 TLS_RSA_AES_128_CBC_SHA256 TLS_RSA_AES_128_CBC_SHA1	TLS_ECDHE_PSK_AES_128_CBC_SHA256 TLS_ECDHE_PSK_AES_128_CBC_SHA1 TLS_PSK_AES_128_GCM_SHA256 TLS_PSK_AES_128_CBC_SHA256 TLS_PSK_AES_128_CBC_SHA1
OpenSSL 1.0.2c	ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA AES128-GCM-SHA256 AES128-SHA256 AES128-SHA	PSK-AES128-CBC-SHA
OpenSSL 1.1.0	ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA AES128-GCM-SHA256 AES128-CCM8 AES128-CCM AES128-SHA256 AES128-SHA	ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA PSK-AES128-GCM-SHA256 PSK-AES128-CCM8 PSK-AES128-CCM PSK-AES128-CBC-SHA256 PSK-AES128-CBC-SHA

Cipher suites using certificates:

	TLS server
TLS client	mbed TLS (PolarSSL)	GnuTLS	OpenSSL 1.0.2
mbed TLS (PolarSSL)	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256
GnuTLS	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256
OpenSSL 1.0.2	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256	TLS-ECDHE-RSA-WITH-AES-128-GCM-SHA256

Cipher suites using PSK:

	TLS server
TLS client	mbed TLS (PolarSSL)	GnuTLS	OpenSSL 1.0.2
mbed TLS (PolarSSL)	TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA256	TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA256	TLS-PSK-WITH-AES-128-CBC-SHA
GnuTLS	TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA256	TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA256	TLS-PSK-WITH-AES-128-CBC-SHA
OpenSSL 1.0.2	TLS-PSK-WITH-AES-128-CBC-SHA	TLS-PSK-WITH-AES-128-CBC-SHA	TLS-PSK-WITH-AES-128-CBC-SHA

User-configured ciphersuites

The built-in ciphersuite selection criteria can be overridden with user-configured ciphersuites.

User-configured ciphersuites is a feature intended for advanced users who understand TLS ciphersuites, their security and consequences of mistakes, and who are comfortable with TLS troubleshooting.

The built-in ciphersuite selection criteria can be overridden using the following parameters:

Override scope	Parameter	Value	Description
Ciphersuite selection for certificates	TLSCipherCert13	Valid OpenSSL 1.1.1 cipher strings for TLS 1.3 protocol (their values are passed to the OpenSSL function SSL_CTX_set_ciphersuites()).	Certificate-based ciphersuite selection criteria for TLS 1.3 Only OpenSSL 1.1.1 or newer.
Ciphersuite selection for certificates	TLSCipherCert	Valid OpenSSL cipher strings for TLS 1.2 or valid GnuTLS priority strings. Their values are passed to the SSL_CTX_set_cipher_list() or gnutls_priority_init() functions, respectively.	Certificate-based ciphersuite selection criteria for TLS 1.2/1.3 (GnuTLS), TLS 1.2 (OpenSSL)
Ciphersuite selection for PSK	TLSCipherPSK13	Valid OpenSSL 1.1.1 cipher strings for TLS 1.3 protocol (their values are passed to the OpenSSL function SSL_CTX_set_ciphersuites()).	PSK-based ciphersuite selection criteria for TLS 1.3 Only OpenSSL 1.1.1 or newer.
Ciphersuite selection for PSK	TLSCipherPSK	Valid OpenSSL cipher strings for TLS 1.2 or valid GnuTLS priority strings. Their values are passed to the SSL_CTX_set_cipher_list() or gnutls_priority_init() functions, respectively.	PSK-based ciphersuite selection criteria for TLS 1.2/1.3 (GnuTLS), TLS 1.2 (OpenSSL)
Combined ciphersuite list for certificate and PSK	TLSCipherAll13	Valid OpenSSL 1.1.1 cipher strings for TLS 1.3 protocol (their values are passed to the OpenSSL function SSL_CTX_set_ciphersuites()).	Ciphersuite selection criteria for TLS 1.3 Only OpenSSL 1.1.1 or newer.
Combined ciphersuite list for certificate and PSK	TLSCipherAll	Valid OpenSSL cipher strings for TLS 1.2 or valid GnuTLS priority strings. Their values are passed to the SSL_CTX_set_cipher_list() or gnutls_priority_init() functions, respectively.	Ciphersuite selection criteria for TLS 1.2/1.3 (GnuTLS), TLS 1.2 (OpenSSL)

To override the ciphersuite selection in zabbix_get and zabbix_sender utilities - use the command-line parameters:

--tls-cipher13
--tls-cipher

The new parameters are optional. If a parameter is not specified, the internal default value is used. If a parameter is defined it cannot be empty.

If the setting of a TLSCipher* value in the crypto library fails then the server, proxy or agent will not start and an error is logged.

It is important to understand when each parameter is applicable.

Outgoing connections

The simplest case is outgoing connections:

For outgoing connections with certificate - use TLSCipherCert13 or TLSCipherCert
For outgoing connections with PSK - use TLSCipherPSK13 and TLSCipherPSK
In case of zabbix_get and zabbix_sender utilities the command-line parameters --tls-cipher13 and --tls-cipher can be used (encryption is unambiguously specified with a --tls-connect parameter)

Incoming connections

It is a bit more complicated with incoming connections because rules are specific for components and configuration.

For Zabbix agent:

Agent connection setup	Cipher configuration
TLSConnect=cert	TLSCipherCert, TLSCipherCert13
TLSConnect=psk	TLSCipherPSK, TLSCipherPSK13
TLSAccept=cert	TLSCipherCert, TLSCipherCert13
TLSAccept=psk	TLSCipherPSK, TLSCipherPSK13
TLSAccept=cert,psk	TLSCipherAll, TLSCipherAll13

For Zabbix server and ** proxy**:

Connection setup	Cipher configuration
Outgoing connections using PSK	TLSCipherPSK, TLSCipherPSK13
Incoming connections using certificates	TLSCipherAll, TLSCipherAll13
Incoming connections using PSK if server has no certificate	TLSCipherPSK, TLSCipherPSK13
Incoming connections using PSK if server has certificate	TLSCipherAll, TLSCipherAll13

Some pattern can be seen in the two tables above:

TLSCipherAll and TLSCipherAll13 can be specified only if a combined list of certificate- and PSK-based ciphersuites is used. There are two cases when it takes place: server (proxy) with a configured certificate (PSK ciphersuites are always configured on server, proxy if crypto library supports PSK), agent configured to accept both certificate- and PSK-based incoming connections
in other cases TLSCipherCert* and/or TLSCipherPSK* are sufficient

The following tables show the TLSCipher* built-in default values. They could be a good starting point for your own custom values.

Parameter	GnuTLS 3.6.12
TLSCipherCert	NONE:+VERS-TLS1.2:+ECDHE-RSA:+RSA:+AES-128-GCM:+AES-128-CBC:+AEAD:+SHA256:+SHA1:+CURVE-ALL:+COMP-NULL:+SIGN-ALL:+CTYPE-X.509
TLSCipherPSK	NONE:+VERS-TLS1.2:+ECDHE-PSK:+PSK:+AES-128-GCM:+AES-128-CBC:+AEAD:+SHA256:+SHA1:+CURVE-ALL:+COMP-NULL:+SIGN-ALL
TLSCipherAll	NONE:+VERS-TLS1.2:+ECDHE-RSA:+RSA:+ECDHE-PSK:+PSK:+AES-128-GCM:+AES-128-CBC:+AEAD:+SHA256:+SHA1:+CURVE-ALL:+COMP-NULL:+SIGN-ALL:+CTYPE-X.509

Parameter	OpenSSL 1.1.1d ¹
TLSCipherCert13
TLSCipherCert	EECDH+aRSA+AES128:RSA+aRSA+AES128
TLSCipherPSK13	TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256
TLSCipherPSK	kECDHEPSK+AES128:kPSK+AES128
TLSCipherAll13
TLSCipherAll	EECDH+aRSA+AES128:RSA+aRSA+AES128:kECDHEPSK+AES128:kPSK+AES128

1 Default values are different for older OpenSSL versions (1.0.1, 1.0.2, 1.1.0), for LibreSSL and if OpenSSL is compiled without PSK support.

** Examples of user-configured ciphersuites **

See below the following examples of user-configured ciphersuites:

Testing cipher strings and allowing only PFS ciphersuites
Switching from AES128 to AES256

Testing cipher strings and allowing only PFS ciphersuites

To see which ciphersuites have been selected you need to set ‘DebugLevel=4’ in the configuration file, or use the -vv option for zabbix_sender.

Some experimenting with TLSCipher* parameters might be necessary before you get the desired ciphersuites. It is inconvenient to restart Zabbix server, proxy or agent multiple times just to tweak TLSCipher* parameters. More convenient options are using zabbix_sender or the openssl command. Let’s show both.

1. Using zabbix_sender.

Let’s make a test configuration file, for example /home/zabbix/test.conf, with the syntax of a zabbix_agentd.conf file:

  Hostname=nonexisting
         ServerActive=nonexisting
         TLSConnect=cert
         TLSCAFile=/home/zabbix/ca.crt
         TLSCertFile=/home/zabbix/agent.crt
         TLSKeyFile=/home/zabbix/agent.key
         TLSPSKIdentity=nonexisting
         TLSPSKFile=/home/zabbix/agent.psk

You need valid CA and agent certificates and PSK for this example. Adjust certificate and PSK file paths and names for your environment.

If you are not using certificates, but only PSK, you can make a simpler test file:

  Hostname=nonexisting
         ServerActive=nonexisting
         TLSConnect=psk
         TLSPSKIdentity=nonexisting
         TLSPSKFile=/home/zabbix/agentd.psk

The selected ciphersuites can be seen by running zabbix_sender (example compiled with OpenSSL 1.1.d):

  $ zabbix_sender -vv -c /home/zabbix/test.conf -k nonexisting_item -o 1 2>&1 | grep ciphersuites
         zabbix_sender [41271]: DEBUG: zbx_tls_init_child() certificate ciphersuites: TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA AES128-GCM-SHA256 AES128-CCM8 AES128-CCM AES128-SHA256 AES128-SHA
         zabbix_sender [41271]: DEBUG: zbx_tls_init_child() PSK ciphersuites: TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA PSK-AES128-GCM-SHA256 PSK-AES128-CCM8 PSK-AES128-CCM PSK-AES128-CBC-SHA256 PSK-AES128-CBC-SHA
         zabbix_sender [41271]: DEBUG: zbx_tls_init_child() certificate and PSK ciphersuites: TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA AES128-GCM-SHA256 AES128-CCM8 AES128-CCM AES128-SHA256 AES128-SHA ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA PSK-AES128-GCM-SHA256 PSK-AES128-CCM8 PSK-AES128-CCM PSK-AES128-CBC-SHA256 PSK-AES128-CBC-SHA

Here you see the ciphersuites selected by default. These default values are chosen to ensure interoperability with Zabbix agents running on systems with older OpenSSL versions (from 1.0.1).

With newer systems you can choose to tighten security by allowing only a few ciphersuites, e.g. only ciphersuites with PFS (Perfect Forward Secrecy). Let’s try to allow only ciphersuites with PFS using TLSCipher* parameters.

The result will not be interoperable with systems using OpenSSL 1.0.1 and 1.0.2, if PSK is used. Certificate-based encryption should work.

Add two lines to the test.conf configuration file:

  TLSCipherCert=EECDH+aRSA+AES128
         TLSCipherPSK=kECDHEPSK+AES128

and test again:

  $ zabbix_sender -vv -c /home/zabbix/test.conf -k nonexisting_item -o 1 2>&1 | grep ciphersuites            
         zabbix_sender [42892]: DEBUG: zbx_tls_init_child() certificate ciphersuites: TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA        
         zabbix_sender [42892]: DEBUG: zbx_tls_init_child() PSK ciphersuites: TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA        
         zabbix_sender [42892]: DEBUG: zbx_tls_init_child() certificate and PSK ciphersuites: TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA AES128-GCM-SHA256 AES128-CCM8 AES128-CCM AES128-SHA256 AES128-SHA ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA PSK-AES128-GCM-SHA256 PSK-AES128-CCM8 PSK-AES128-CCM PSK-AES128-CBC-SHA256 PSK-AES128-CBC-SHA

The “certificate ciphersuites” and “PSK ciphersuites” lists have changed - they are shorter than before, only containing TLS 1.3 ciphersuites and TLS 1.2 ECDHE-* ciphersuites as expected.

2. TLSCipherAll and TLSCipherAll13 cannot be tested with zabbix_sender; they do not affect “certificate and PSK ciphersuites” value shown in the example above. To tweak TLSCipherAll and TLSCipherAll13 you need to experiment with the agent, proxy or server.

So, to allow only PFS ciphersuites you may need to add up to three parameters

  TLSCipherCert=EECDH+aRSA+AES128
         TLSCipherPSK=kECDHEPSK+AES128
         TLSCipherAll=EECDH+aRSA+AES128:kECDHEPSK+AES128

to zabbix_agentd.conf, zabbix_proxy.conf and zabbix_server_conf if each of them has a configured certificate and agent has also PSK.

If your Zabbix environment uses only PSK-based encryption and no certificates, then only one:

  TLSCipherPSK=kECDHEPSK+AES128

Now that you understand how it works you can test the ciphersuite selection even outside of Zabbix, with the openssl command. Let’s test all three TLSCipher* parameter values:

  $ openssl ciphers EECDH+aRSA+AES128 | sed 's/:/ /g'
         TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA
         $ openssl ciphers kECDHEPSK+AES128 | sed 's/:/ /g'
         TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA
         $ openssl ciphers EECDH+aRSA+AES128:kECDHEPSK+AES128 | sed 's/:/ /g'
         TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-RSA-AES128-GCM-SHA256 ECDHE-RSA-AES128-SHA256 ECDHE-RSA-AES128-SHA ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA

You may prefer openssl ciphers with option -V for a more verbose output:

  $ openssl ciphers -V EECDH+aRSA+AES128:kECDHEPSK+AES128
                   0x13,0x02 - TLS_AES_256_GCM_SHA384  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(256) Mac=AEAD
                   0x13,0x03 - TLS_CHACHA20_POLY1305_SHA256 TLSv1.3 Kx=any      Au=any  Enc=CHACHA20/POLY1305(256) Mac=AEAD
                   0x13,0x01 - TLS_AES_128_GCM_SHA256  TLSv1.3 Kx=any      Au=any  Enc=AESGCM(128) Mac=AEAD
                   0xC0,0x2F - ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(128) Mac=AEAD
                   0xC0,0x27 - ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA256
                   0xC0,0x13 - ECDHE-RSA-AES128-SHA    TLSv1 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA1
                   0xC0,0x37 - ECDHE-PSK-AES128-CBC-SHA256 TLSv1 Kx=ECDHEPSK Au=PSK  Enc=AES(128)  Mac=SHA256
                   0xC0,0x35 - ECDHE-PSK-AES128-CBC-SHA TLSv1 Kx=ECDHEPSK Au=PSK  Enc=AES(128)  Mac=SHA1

Similarly, you can test the priority strings for GnuTLS:

  $ gnutls-cli -l --priority=NONE:+VERS-TLS1.2:+ECDHE-RSA:+AES-128-GCM:+AES-128-CBC:+AEAD:+SHA256:+CURVE-ALL:+COMP-NULL:+SIGN-ALL:+CTYPE-X.509
         Cipher suites for NONE:+VERS-TLS1.2:+ECDHE-RSA:+AES-128-GCM:+AES-128-CBC:+AEAD:+SHA256:+CURVE-ALL:+COMP-NULL:+SIGN-ALL:+CTYPE-X.509
         TLS_ECDHE_RSA_AES_128_GCM_SHA256                        0xc0, 0x2f      TLS1.2
         TLS_ECDHE_RSA_AES_128_CBC_SHA256                        0xc0, 0x27      TLS1.2
         Protocols: VERS-TLS1.2
         Ciphers: AES-128-GCM, AES-128-CBC
         MACs: AEAD, SHA256
         Key Exchange Algorithms: ECDHE-RSA
         Groups: GROUP-SECP256R1, GROUP-SECP384R1, GROUP-SECP521R1, GROUP-X25519, GROUP-X448, GROUP-FFDHE2048, GROUP-FFDHE3072, GROUP-FFDHE4096, GROUP-FFDHE6144, GROUP-FFDHE8192
         PK-signatures: SIGN-RSA-SHA256, SIGN-RSA-PSS-SHA256, SIGN-RSA-PSS-RSAE-SHA256, SIGN-ECDSA-SHA256, SIGN-ECDSA-SECP256R1-SHA256, SIGN-EdDSA-Ed25519, SIGN-RSA-SHA384, SIGN-RSA-PSS-SHA384, SIGN-RSA-PSS-RSAE-SHA384, SIGN-ECDSA-SHA384, SIGN-ECDSA-SECP384R1-SHA384, SIGN-EdDSA-Ed448, SIGN-RSA-SHA512, SIGN-RSA-PSS-SHA512, SIGN-RSA-PSS-RSAE-SHA512, SIGN-ECDSA-SHA512, SIGN-ECDSA-SECP521R1-SHA512, SIGN-RSA-SHA1, SIGN-ECDSA-SHA1

Switching from AES128 to AES256

Zabbix uses AES128 as the built-in default for data. Let’s assume you are using certificates and want to switch to AES256, on OpenSSL 1.1.1.

This can be achieved by adding the respective parameters in zabbix_server.conf:

  TLSCAFile=/home/zabbix/ca.crt
         TLSCertFile=/home/zabbix/server.crt
         TLSKeyFile=/home/zabbix/server.key
         TLSCipherCert13=TLS_AES_256_GCM_SHA384
         TLSCipherCert=EECDH+aRSA+AES256:-SHA1:-SHA384
         TLSCipherPSK13=TLS_CHACHA20_POLY1305_SHA256
         TLSCipherPSK=kECDHEPSK+AES256:-SHA1
         TLSCipherAll13=TLS_AES_256_GCM_SHA384
         TLSCipherAll=EECDH+aRSA+AES256:-SHA1:-SHA384

Although only certificate-related ciphersuites will be used, TLSCipherPSK* parameters are defined as well to avoid their default values which include less secure ciphers for wider interoperability. PSK ciphersuites cannot be completely disabled on server/proxy.

And in zabbix_agentd.conf:

  TLSConnect=cert
         TLSAccept=cert
         TLSCAFile=/home/zabbix/ca.crt
         TLSCertFile=/home/zabbix/agent.crt
         TLSKeyFile=/home/zabbix/agent.key
         TLSCipherCert13=TLS_AES_256_GCM_SHA384
         TLSCipherCert=EECDH+aRSA+AES256:-SHA1:-SHA384