commit 492549864e51fdc30195f1be467af6e463ba6fb2 Author: teor teor@torproject.org Date: Wed Feb 5 10:55:04 2020 +1000

Prop 312: Add libevent DNS API

As suggested by Nick Mathewson.

Part of 33073. --- proposals/312-relay-auto-ipv6-addr.txt | 32 ++++++++++++++++++++++++++++---- 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/proposals/312-relay-auto-ipv6-addr.txt b/proposals/312-relay-auto-ipv6-addr.txt index 31a5dd7..ed5ebac 100644 --- a/proposals/312-relay-auto-ipv6-addr.txt +++ b/proposals/312-relay-auto-ipv6-addr.txt @@ -1070,7 +1070,7 @@ Ticket: #33073 method will find the IP address of the default route, in most cases (see section 3.2.5).

-3.5.9. Add IPv6 Support Using gethostbyname2() +3.5.9. Add IPv6 Support via Other DNS APIs

We propose these optional changes, to add IPv6 support to hostname resolution on older OSes. These changes affect: @@ -1082,8 +1082,14 @@ Ticket: #33073 Directory authorities do not use this address detection method to discover their own addresses, for security reasons.

- Use gethostbyname2() to add IPv6 support to hostname resolution on older - OSes, which don't support getaddrinfo(). + Tor currently uses getaddrinfo() on most systems, which supports IPv6 DNS. + But tor also supports the legacy gethostbyname() DNS API, which does not + support IPv6. + + There are two alternative APIs we could use for IPv6 DNS, if getaddrinfo() + is not available: + * libevent DNS API, and + * gethostbyname2().

But this change may be unnecessary, because: * Linux has used getaddrinfo() by default since glibc 2.20 (2014) @@ -1092,7 +1098,23 @@ Ticket: #33073 getaddrinfo() in a similar timeframe * Windows has supported getaddrinfo() since Windows Vista; tor's minimum supported Windows version is Vista. - See [Tor Supported Platforms] for more details. + See [Tor Supported Platforms] for more detai + + If a large number of systems do not support getaddrinfo(), we propose + implementing one of these alternatives: + + The libevent DNS API supports IPv6 DNS, and tor already has a dependency on + libevent. Therefore, we should prefer the libevent DNS API. (Unless we find + it difficult to implement.) + + We could also use gethostbyname2() to add IPv6 support to hostname + resolution on older OSes, which don't support getaddrinfo(). + + Handling multiple addresses: + + When looking up hostnames using libevent, the DNS callbacks provide a list + of all addresses received. Therefore, we should ignore any private + addresses, and then choose the first address in the list.

When looking up hostnames using gethostbyname() or gethostbyname2(), if the first address is a private address, we may want to look at the entire list @@ -1105,6 +1127,8 @@ Ticket: #33073 (On OSes that support getaddrinfo(), tor searches the list of addresses for a publicly routable address.)

+ Alternative change: remove gethostbyname(): + As an alternative, if we believe that all supported OSes have getaddrinfo(), we could simply remove the gethostbyname() code, rather than trying to modify it to work with IPv6.