Re: use of smtp(d)_tls_CAfile with opportunistic TLS?
I'm wondering about the usefulness of smtp(d)_tls_CAfile(path) when using opportunistic encryption in both incoming and outgoing connections. The TLS_README suggests that certificate and key files be left empty for opportunistic smtp processes, but it doesn't talk specifically about smtp_tls_CAfile(path). For the SMTP server, you should NOT leave the cert file empty, as many clients won't support aNULL ciphers. Oops, sincere apologies. I meant the docs suggest empty cert and key for SMTP *client* only. I was originally asking only about the client, then changed my inquiry but forgot to change this. Anyway, sorry. Rather, you need to set a self-signed cert, if one of the usual CAs is not suitable. Right. Check. For the SMTP server, since you probably won't ask for client certs, you never need a CAfile or CApath. OK, got it. For the SMTP client, indeed, generally, your key and cert should be set empty. On the other hand, it is a good idea in most cases to have a CAfile and/or CApath with a few trusted roots. Am I correct to infer that both smtp(d)_tls_CAfile settings only serve a purpose when you want to verify client/server certificates? If that's the case, why does the example at the bottom of TLS_README use both the CAfile settings with only opportunistic encryption? This reduces log noise, and improves the audit trail. Hmm, OK, not to imply these things are not important, but are these the only reasons you'd have a CAfile or CApath? Our system seems to work without any CAfile/CApath settings under opportunistic encryption both incoming and outgoing. Is there a performance or security difference between using them or not? You should probably throw in a few trusted root CAs. 1) Is there a place to get a file with the usual suspects already in it? 2) Does postfix add new CAs to it when it sees a new one from a client? 3) Does it make much difference between CApath or CAfile? I suppose using CApath only makes sense if the answer to question 2 is yes? (File probably sufficient if it is static and not that big)
Re: use of smtp(d)_tls_CAfile with opportunistic TLS?
On 4/12/2011 2:17 AM, email builder wrote: Am I correct to infer that both smtp(d)_tls_CAfile settings only serve a purpose when you want to verify client/server certificates? If that's the case, why does the example at the bottom of TLS_README use both the CAfile settings with only opportunistic encryption? This reduces log noise, and improves the audit trail. Hmm, OK, not to imply these things are not important, but are these the only reasons you'd have a CAfile or CApath? With opportunistic TLS you don't gain any extra security by verifying the remote cert. This is what makes self-signed certificates adequate for opportunistic TLS. Our system seems to work without any CAfile/CApath settings under opportunistic encryption both incoming and outgoing. Is there a performance or security difference between using them or not? You should probably throw in a few trusted root CAs. 1) Is there a place to get a file with the usual suspects already in it? Most OS's have a package of common root certs available. For example, FreeBSD provides the security/ca_root_nss port. 2) Does postfix add new CAs to it when it sees a new one from a client? No. The CA file/path is a trust list. It would be inappropriate for a program to add trust automatically. 3) Does it make much difference between CApath or CAfile? I suppose using CApath only makes sense if the answer to question 2 is yes? (File probably sufficient if it is static and not that big) Performance may be better with CApath for a large number of certificates, but mostly this is about how the certs are bundled for you. -- Noel Jones
Re: use of smtp(d)_tls_CAfile with opportunistic TLS?
On 4/12/2011 2:17 AM, email builder wrote: Am I correct to infer that both smtp(d)_tls_CAfile settings only serve a purpose when you want to verify client/server certificates? If that's the case, why does the example at the bottom of TLS_README use both the CAfile settings with only opportunistic encryption? This reduces log noise, and improves the audit trail. Hmm, OK, not to imply these things are not important, but are these the only reasons you'd have a CAfile or CApath? With opportunistic TLS you don't gain any extra security by verifying the remote cert. This is what makes self-signed certificates adequate for opportunistic TLS. Our system seems to work without any CAfile/CApath settings under opportunistic encryption both incoming and outgoing. Is there a performance or security difference between using them or not? You should probably throw in a few trusted root CAs. 1) Is there a place to get a file with the usual suspects already in it? Most OS's have a package of common root certs available. For example, FreeBSD provides the security/ca_root_nss port. 2) Does postfix add new CAs to it when it sees a new one from a client? No. The CA file/path is a trust list. It would be inappropriate for a program to add trust automatically. 3) Does it make much difference between CApath or CAfile? I suppose using CApath only makes sense if the answer to question 2 is yes? (File probably sufficient if it is static and not that big) Performance may be better with CApath for a large number of certificates, but mostly this is about how the certs are bundled for you. Thank you and Victor.
Re: use of smtp(d)_tls_CAfile with opportunistic TLS?
On Fri, Apr 08, 2011 at 11:09:00PM -0700, email builder wrote: I'm wondering about the usefulness of smtp(d)_tls_CAfile(path) when using opportunistic encryption in both incoming and outgoing connections. The TLS_README suggests that certificate and key files be left empty for opportunistic smtp processes, but it doesn't talk specifically about smtp_tls_CAfile(path). For the SMTP server, you should NOT leave the cert file empty, as many clients won't support aNULL ciphers. Rather, you need to set a self-signed cert, if one of the usual CAs is not suitable. For the SMTP server, since you probably won't ask for client certs, you never need a CAfile or CApath. For the SMTP client, indeed, generally, your key and cert should be set empty. On the other hand, it is a good idea in most cases to have a CAfile and/or CApath with a few trusted roots. Am I correct to infer that both smtp(d)_tls_CAfile settings only serve a purpose when you want to verify client/server certificates? If that's the case, why does the example at the bottom of TLS_README use both the CAfile settings with only opportunistic encryption? This reduces log noise, and improves the audit trail. Our system seems to work without any CAfile/CApath settings under opportunistic encryption both incoming and outgoing. Is there a performance or security difference between using them or not? You should probably throw in a few trusted root CAs. -- Viktor.
