Based on the recent found weaknesses in DH key exchange, http://weakdh.org/ I increased ssl_dh_parameters_length to 2048 bits, and found waited for 5+ minutes for dovecot to come back online after a restart. Unless you got a fast machine, the initialization of DH parameters can exceed your patience. Regeneration may not be a problem (if ssl_parameters_regenerate=0 or if Dovecot uses old parameters until regeneration finishes), but for cold starts, the server can be tied up for a few minutes creating DH parameters while clients queue up. I ran "openssl dhparam 2048" and got wildly varying run times of 1m45s, 11m56s, 0.4s, 2m19s, 3h23s. Most of the time was spent testing primality of candidate p *and* (p-1)/2 -- so called "safe prime". If you're unlucky, this can take a long time. However, it appears "safe" primes are not what they're cracked up to be -- they offer some guarantees, but are not safer than non-safe primes. Creating DH parameters without requiring primality of (p-1)/2 (i.e. what "openssl dhparam -dsaparam" does) results in much lower run-time bounds. I cribbed some OpenSSL code to create this (untested) patch. -------------------------------------------------------------------------------- --- iostream-openssl-params.c~ Tue Nov 3 16:08:38 2015 +++ iostream-openssl-params.c Tue Nov 3 15:43:39 2015 @@ -6,5 +6,2 @@ -/* 2 or 5. Haven't seen their difference explained anywhere, but 2 is the - default.. */ -#define DH_GENERATOR 2 @@ -14,2 +11,3 @@ DH *dh; + DSA *dsa; unsigned char *p; @@ -17,3 +15,13 @@ - dh = DH_generate_parameters(bitsize, DH_GENERATOR, NULL, NULL); + dsa = DSA_generate_parameters(bitsize, NULL, 0, NULL, NULL, NULL, NULL); + if (dsa == NULL) { + *error_r = t_strdup_printf( + "DSA_generate_parameters(bits=%d) failed: %s", + bitsize, openssl_iostream_error()); + return -1; + } + + dh = DSA_dup_DH(dsa); + DSA_free(dsa); + if (dh == NULL) { @@ -20,4 +28,4 @@ *error_r = t_strdup_printf( - "DH_generate_parameters(bits=%d, gen=%d) failed: %s", - bitsize, DH_GENERATOR, openssl_iostream_error()); + "DSA_dup_DH() failed: %s", + openssl_iostream_error()); return -1; -------------------------------------------------------------------------------- The other way to prevent long startup times is to pre-compute the DH parameter using "openssl dhparam -out dh2048.pem 2048". I can contribute a patch to do this (read file, convert it into ssl-parameters.dat, then set/behave like ssl_parameters_regenerate=0), but I couldn't figure out the best place to do this. ssl_params_if_unchanged()? Joseph Tam