How to debug SSL handshake using cURL

Question

I would like to troubleshoot per directory authentication with client certificate  I would specially like to find out which acceptable client certificates does server send   How do I debug SSL handshake  preferably with cURL   Thanks in advance

User · Answer

I have used this command to troubleshoot client certificate negotiation:

openssl s_client -connect www.test.com:443 -prexit

The output will probably contain "Acceptable client certificate CA names" and a list of CA certificates from the server, or possibly "No client certificate CA names sent", if the server doesn't always require client certificates.

User · Answer

curl probably does have some options for showing more information but for things like this I always use openssl s client  With the -debug option this gives lots of useful information  Maybe I should add that this also works with non HTTP connections   So if you are doing  https   try the curl commands suggested below   If you aren t or want a second option openssl s client might be good

User · Answer

Actually openssl command is a better tool than curl for checking and debugging SSL  Here is an example with openssl  openssl s client -showcerts -connect stackoverflow com 443  lt   dev null  and  lt   dev null is for adding EOL to the STDIN otherwise it hangs on the Terminal   But if you liked  you can wrap some useful openssl commands with curl  as I did with curly  and make it more human readable like so    check if SSL is valid  gt  gt  gt  curly --ssl valid -d stackoverflow com Verify return code  0  ok  issuer C   US O   Let s Encrypt CN   R3 subject CN     stackexchange com  option  ssl action  valid status  OK     check how many days it will be valid   gt  gt  gt  curly --ssl date -d stackoverflow com Verify return code  0  ok  from  Tue Feb  9 16 13 16 UTC 2021 till  Mon May 10 16 13 16 UTC 2021 days total   89 days passed  8 days left    81  option  ssl action  date status  OK     check which names it supports curly --ssl name -d stackoverflow com   askubuntu com   blogoverflow com   mathoverflow net   meta stackexchange com   meta stackoverflow com   serverfault com   sstatic net   stackexchange com   stackoverflow com   stackoverflow email   superuser com askubuntu com blogoverflow com mathoverflow net openid stackauth com serverfault com sstatic net stackapps com stackauth com stackexchange com stackoverflow blog stackoverflow com stackoverflow email stacksnippets net superuser com  option  ssl action  name status  OK     check the CERT of the SSL  gt  gt  gt  curly --ssl cert -d stackoverflow com -----BEGIN CERTIFICATE----- MIIG9DCCBdygAwIBAgISBOh5mcfyJFrMPr3vuAuikAYwMA0GCSqGSIb3DQEBCwUA MDIxCzAJBgNVBAYTAlVTMRYwFAYDVQQKEw1MZXQncyBFbmNyeXB0MQswCQYDVQQD EwJSMzAeFw0yMTAyMDkxNjEzMTZaFw0yMTA1MTAxNjEzMTZaMB4xHDAaBgNVBAMM Eyouc3RhY2tleGNoYW5nZS5jb20wggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEK AoIBAQDRDObYpjCvb2smnCP UUpkKdSr6nVsIN8vkI6YlJfC4xC72bY2v38lE2xB LCaL9MzKhsINrQZRIUivnEHuDOZyJ3Xwmxq3wY0qUKo2c963U7ZJpsIFsj37L1Ac Qp4pubyyKPxTeFAzKbpfwhNml633Ao78Cy l sYjNFhMPoBN4LYBX7 WJNIfc3UZ niMfh230NE2dwoXGqA0MnkPQyFKlIwHcmMb ZI5T8TziYq0WQiYUY3ssOEu1CI5n wh0 BTAwpx7XBUe5Z B9SrFp8BUDYWcWuVEIh2btYvo763mrr lmm8PP23XKkE4f 287Iwlfg IqxxIxKv9smFoPkyZcFAgMBAAGjggQWMIIEEjAOBgNVHQ8BAf8EBAMC BaAwHQYDVR0lBBYwFAYIKwYBBQUHAwEGCCsGAQUFBwMCMAwGA1UdEwEB wQCMAAw HQYDVR0OBBYEFMnjX41T J1bbLgG9TjR 4CvHLv MB8GA1UdIwQYMBaAFBQusxe3 WFbLrlAJQOYfr52LFMLGMFUGCCsGAQUFBwEBBEkwRzAhBggrBgEFBQcwAYYVaHR0 cDovL3IzLm8ubGVuY3Iub3JnMCIGCCsGAQUFBzAChhZodHRwOi8vcjMuaS5sZW5j ci5vcmcvMIIB5AYDVR0RBIIB2zCCAdeCDyouYXNrdWJ1bnR1LmNvbYISKi5ibG9n b3ZlcmZsb3cuY29tghIqLm1hdGhvdmVyZmxvdy5uZXSCGCoubWV0YS5zdGFja2V4 Y2hhbmdlLmNvbYIYKi5tZXRhLnN0YWNrb3ZlcmZsb3cuY29tghEqLnNlcnZlcmZh dWx0LmNvbYINKi5zc3RhdGljLm5ldIITKi5zdGFja2V4Y2hhbmdlLmNvbYITKi5z dGFja292ZXJmbG93LmNvbYIVKi5zdGFja292ZXJmbG93LmVtYWlsgg8qLnN1cGVy dXNlci5jb22CDWFza3VidW50dS5jb22CEGJsb2dvdmVyZmxvdy5jb22CEG1hdGhv dmVyZmxvdy5uZXSCFG9wZW5pZC5zdGFja2F1dGguY29tgg9zZXJ2ZXJmYXVsdC5j b22CC3NzdGF0aWMubmV0gg1zdGFja2FwcHMuY29tgg1zdGFja2F1dGguY29tghFz dGFja2V4Y2hhbmdlLmNvbYISc3RhY2tvdmVyZmxvdy5ibG9nghFzdGFja292ZXJm bG93LmNvbYITc3RhY2tvdmVyZmxvdy5lbWFpbIIRc3RhY2tzbmlwcGV0cy5uZXSC DXN1cGVydXNlci5jb20wTAYDVR0gBEUwQzAIBgZngQwBAgEwNwYLKwYBBAGC3xMB AQEwKDAmBggrBgEFBQcCARYaaHR0cDovL2Nwcy5sZXRzZW5jcnlwdC5vcmcwggEE BgorBgEEAdZ5AgQCBIH1BIHyAPAAdgBElGUusO7Or8RAB9io ijA2uaCvtjLMbU  0zOWtbaBqAAAAXeHyHI8AAAEAwBHMEUCIQDnzDcCrmCPdfgcb ojY0WJV1rCj uE hCiQi0 4fBP9lgIgSI5mwEqBmVcQwRfKikUzhkH0w6K 6wq0e 1zJA0j5a4AdgD2 XJQv0XcwIhRUGAgwlFaO400TGTO 3wwvIAvMTvFk4wAAAXeHyHIoAAAEAwBHMEUC IHd0ZLB3j0b31Sh D3RIfF8C31NxIRSG6m BFSCGlxSWAiEAvYlgPjrPcBZpX4Xm SdkF39KbVicTGnFOSAqDpRB3IJwwDQYJKoZIhvcNAQELBQADggEBABZ 2WXyP4w  A jJtBgKTZQsA5VhUCabAFDEZdnlWWcV3WYrz4iuJjp5v6kL4MNzAvAVzyCTqD1T m7EUn usz59m02mZF82 ELLW6Mqix8krYZTpYt7Hu3Znf6HxiK3QrjEIVlwSGkjV XMCzOHdALreTkB UJaL6bEs1sB 9h20zSnZAKrPokGL XwgxUclXIQXr1uDAShJB Ts0yjoSY9D687W9sjhq BIjNYIWg1n9NJ7HM48FWBCDmV3NlCR0Zh1Yx15pXCUhb UqWd6RzoSLmIfdOxgfi9uRSUe0QTZ9o Fs4YoMi5K50tfRycLKW BoYDgde37As5 0pCUFwVVH2E  -----END CERTIFICATE-----  option  ssl action  cert status  OK

User · Answer

openssl s client -connect 127 0 0 1 6379 -state CONNECTED 00000003  SSL connect before SSL initialization SSL connect SSLv3 TLS write client hello

User · Answer

curl -iv https   your domain io   That will give you cert and header output if you do not wish to use openssl command

User · Answer

For TLS handshake troubleshooting please use openssl s client instead of curl  -msg does the trick  -debug helps to see what actually travels over the socket  -status OCSP stapling should be standard nowadays    openssl s client -connect example com 443 -tls1 2 -status -msg -debug -CAfile  lt path to trusted root ca pem gt  -key  lt path to client private key pem gt  -cert  lt path to client cert pem gt     Other useful switches -tlsextdebug -prexit -state  https   www openssl org docs man1 0 2 man1 s client html

[curl] How to debug SSL handshake using cURL?

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