Check supported TLS versions and ciphers

Reading what was negotiated #

Every successful handshake reports the version and suite that were actually agreed. That is the ground truth — everything else on this page is a way of constraining the negotiation to find the edges:

openssl s_client -connect example.com:443 -servername example.com </dev/null 2>/dev/null \
| grep -E "^ *(Protocol|Cipher) *:"

Protocol  : TLSv1.3
Cipher    : TLS_AES_256_GCM_SHA384

Probing each version #

Ask for one version at a time. A version the server accepts completes the handshake; one it has disabled fails:

for v in tls1_3 tls1_2 tls1_1 tls1; do
printf '%-8s ' "$v"
openssl s_client -"$v" -connect example.com:443 -servername example.com </dev/null 2>&1 \
  | grep -qE "^ *Cipher *: *[A-Z0-9]" && echo "accepted" || echo "refused"
done

tls1_3   accepted
tls1_2   accepted
tls1_1   refused
tls1     refused

To test those two honestly, lower your own client’s policy for the duration of the probe:

openssl s_client -tls1 -cipher 'DEFAULT@SECLEVEL=0' \
-connect example.com:443 -servername example.com </dev/null 2>&1 | head -20

A server still accepting TLS 1.0 in 2026 is a finding, not a configuration to preserve.

Cipher suites, and the flag that catches everyone #

TLS 1.3 replaced the suite namespace entirely, and OpenSSL kept them on separate switches:

Passing a TLS 1.3 suite to -cipher is accepted and does nothing, which reads as the server ignoring your request. Start from what your own build offers:

openssl ciphers -s -tls1_3 -v

openssl ciphers -v 'ECDHE+AESGCM'

Then test whether the server will accept a specific suite:

openssl s_client -tls1_2 -cipher 'ECDHE-RSA-AES128-GCM-SHA256' \
-connect example.com:443 -servername example.com </dev/null 2>&1 \
| grep -E "^ *Cipher *:"

no ciphers available or a handshake failure alert means the server declined that suite.

Enumerating everything the server accepts #

Walk your build’s list and record which ones survive. This is slow — one handshake per suite — so narrow the list before running it:

for c in $(openssl ciphers -s -tls1_2 'ECDHE' | tr ':' ' '); do
openssl s_client -tls1_2 -cipher "$c" \
  -connect example.com:443 -servername example.com </dev/null 2>&1 \
  | grep -qE "^ *Cipher *: *$c" && echo "accepted  $c"
done

ALPN and OCSP stapling #

Two other properties of the same handshake, worth checking while you are here. HTTP/2 requires successful ALPN negotiation:

openssl s_client -alpn h2 -connect example.com:443 -servername example.com </dev/null 2>/dev/null \
| grep -i "ALPN"

ALPN protocol: h2

And stapling, which saves every visitor an OCSP round trip:

openssl s_client -status -connect example.com:443 -servername example.com </dev/null 2>/dev/null \
| grep -A3 "OCSP response"

no response sent by server means stapling is off or the server’s fetch from the CA is failing — see OCSP vs CRL for what that costs.

Related checks #

To fetch and decode the certificate itself rather than interrogate the connection, see get a certificate from a server. If the handshake never starts and OpenSSL reports a wrong version number, the port is probably not speaking TLS at all — that is a different problem.