RHEL 5 lftp with FTPES (SSL/TLS) fails Certificate verification when server certificate Signature Algorithm is sha256WithRSAEncryption

Solution Verified - Updated -


  • Using lftp to connect to a particular ftp site while forcing ssl (set ftp:ssl-force yes) with cert-validation (set ssl:verify-certificate yes) works in RHEL 6, but not in RHEL 5. We have confirmed that the certificate chain is intact and trusted by using openssl s_client -starttls ftp -connect FTP.EXAMPLE.COM:21. We have even tried pointing lftp directly to a simplified CA file with the set ssl:ca-file option.

  • Example: the trust chain for dropbox.redhat.com is good ...

    [rhel5]$ openssl s_client -starttls ftp -connect dropbox.redhat.com:21 <<<"" | grep 'Verify return code:'
    depth=2 /C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
    verify return:1
    depth=1 /C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
    verify return:1
    depth=0 / Organization/ East Davie St./postalCode=27601/C=US/ST=North Carolina/L=Raleigh/O=Red Hat Inc./CN=dropbox.redhat.com
    verify return:1
        Verify return code: 0 (ok)

    However, connecting with ssl:verify-certificate enabled fails in RHEL 5:

    [rhel5]$ file=$RANDOM$RANDOM$RANDOM
    [rhel5]$ touch $file
    [rhel5]$ lftp -e "set ftp:ssl-force yes; set ssl:verify-certificate yes; cd incoming; put $file" anonymous:ftp@dropbox.redhat.com
    cd: Fatal error: Certificate verification: Not trusted
    put: Fatal error: Certificate verification: Not trusted

    Disabling cert-verification allows it to work:

    [rhel5]$ lftp -e "set ftp:ssl-force yes; set ssl:verify-certificate no; cd incoming; put $file" anonymous:ftp@dropbox.redhat.com
    cd ok, cwd=/incoming
    lftp anonymous@dropbox.redhat.com:/incoming> exit


  • FTP client

    • Red Hat Enterprise Linux 5
    • lftp (compiled against gnutls)
      • with ftp:ssl-force and ssl:verify-certificate enabled
  • FTP server

    • SSL certificate where Signature Algorithm is sha256WithRSAEncryption

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