Add support for IPv6-only environments

Previously, the proxy would attempt to bind to an IPv6 + IPv4 enabled socket.
On failure, it would try to bind to an IPv4 only socket. If that failed, the
proxy would fail to start. This update makes it so that the proxy also tries
to bind to an IPv6-only socket, which is necessary in environments where IPv4
is disabled.

In short, the proxy will try binding in the following order, only moving
to the next step when binding fails:
IPv6 + IPv4 -> IPv4 only -> IPv6 only -> proxy fails to start.

1 files changed, 15 insertions, 3 deletions

diff --git a/mitmproxy/net/tcp.py b/mitmproxy/net/tcp.py
index 2496d47c..07cee466 100644
--- a/mitmproxy/net/tcp.py
+++ b/mitmproxy/net/tcp.py
@@ -558,7 +558,7 @@ class TCPServer:
         self.socket = None
 
         try:
-            # First try to bind an IPv6 socket, with possible IPv4 if the OS supports it.
+            # First try to bind an IPv6 socket, attempting to enable IPv4 support if the OS supports it.
             # This allows us to accept connections for ::1 and 127.0.0.1 on the same socket.
             # Only works if self.address == ""
             self.socket = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
@@ -572,8 +572,20 @@ class TCPServer:
             self.socket = None
 
         if not self.socket:
-            # Binding to an IPv6 socket failed, lets fall back to IPv4.
-            self.socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+            try:
+                # Binding to an IPv6 + IPv4 socket failed, lets fall back to IPv4 only.
+                self.socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+                self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+                self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
+                self.socket.bind(self.address)
+            except socket.error:
+                if self.socket:
+                    self.socket.close()
+                self.socket = None
+
+        if not self.socket:
+            # Binding to an IPv4 only socket failed, lets fall back to IPv6 only.
+            self.socket = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
             self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
             self.socket.bind(self.address)